Meesmann dystrophy is characterized by distinct tiny bubble-like, round-to-oval punctate opacities that form in the central corneal epithelium and to a lesser extent in the peripheral cornea of both eyes during infancy 789 (Figure 1). These opacities caused by intraepithelial cysts appear as transparent dew drops in retroillumination and they are extremely difficult to see without slit lamp biomicroscopy (Figure 2). They usually fail to stain after topically applied fluorescein because they seldom open to the surface. MECD often remains asymptomatic until about middle age, when, intermittent, mild ocular irritation, photophobia, transient blurred vision, and irregular astigmatism develop. At that time, the entire corneal epithelium contains the intraepithelial opacities with some breaking through the epithelial surface. In severe cases, subepithelial scarring produces a slight grayish central corneal opacification. Corneal sensitivity is normal.

Histopathologically, MECD is characterized by intraepithelial cysts at different levels in the corneal epithelium, which is irregular in thickness. Degenerated cellular debris within the intraepithelial microcysts manifests autofluorescence in ultraviolet (UV) light and it stains with the Hale colloidal iron technique for negatively charged substances such as glycosaminoglycans (GAGs) 10111213. It is also periodic acid-Schiff (PAS)-positive and diastase- and neuraminidase-resistant 11. The epithelial basement membrane is variably thickened, but this is a common nonspecific finding in many disorders of the corneal epithelium. Bowman layer and the corneal stroma are unremarkable. Transmission electron microscopy (TEM) discloses focal aggregations of electron dense fibrillogranular keratin within the cytoplasm of the corneal epithelium (the "peculiar substance" of Kuwabara and Ciccarelli) 12, which is surrounded by cytoplasmic filaments and vacuoles (Figures 3 and 4).

MECD is caused by a mutation in either one of the pair of genes (KRT3 or KRT12) that encode the two units of cytokeratin in the corneal epithelium 1415161718. The mutations have been in extremely conserved keratin boundary motifs. For example, in cytokeratin 12 they involve the helix termination or initiation motif. Dominant mutations affecting this part of the molecule in other keratins severely impair cytoskeletal function. The epithelial cells normally become displaced to the surface of the cornea discharging the defective cells and cysts. Stocker-Holt corneal dystrophy 1920 is a variant of MECD caused by a p. Arg19Leu amino acid change in the cytokeratin 12 21.

MECD persists throughout life. Removal of the abnormal corneal epithelium has been used as a treatment modality, but it is not curative. Even with removal of the pathologic epithelium, this dystrophy recurs in the regenerated epithelium.

Symmetrical reticular opacities form in the superficial central cornea of both eyes at about 4–5 years of age in RBCD a corneal disorder that was first described by Reis 22 and later by Bücklers 23 (Figure 5). The opacities assume an irregularly ring-shaped pattern of discrete spots and lines that focally elevate the corneal epithelium. RBCD remains asymptomatic until epithelial erosions precipitate acute episodes of ocular hyperemia, pain, and photophobia. Visual acuity eventually becomes reduced during the second and third decades of life following a progressive superficial haze and an irregular corneal surface. RBCD becomes symptomatic earlier and with a higher frequency of recurrent erosions than patients with other variants of GCD. The superficial corneal stroma in RBCD contains deposits of mutated transforming growth factor beta induced protein that are indistinguishable from those of various forms of GCD 24252627 (Figures 6 and 7), hence the synonym GCD type III.

All well-documented cases of RBCD are caused by a specific mutation in the TGFBI gene (p. Arg124Leu). Haplotype analyses of different families have provided evidence of multiple origins of this mutation. Other mutations in the TGFBI gene have been reported in subjects alleged to have RBCD or an atypical variant (p. Gly623Asp) based on the clinical recognition of geographic opacities in the anterior to mid corneal stroma, but without histopathologic confirmation 28. Some of these cases may have TBCD, which is impossible to differentiate from RBCD without transmission electron microscopy (TEM) of the affected cornea 2930. The RBCD phenotype has been detected in Sardinians with a Δ F540 mutation in TGFBI without histopathologic confirmation 31. Reports of a p. Arg555Gln mutation in TGFBI in patients alleged to have RBCD are unacceptable because these individuals were not characterized by appropriate typical light microscopic and TEM findings 1.

In RBCD a progressive deterioration of vision follows ocular discomfort and pain from recurrent epithelial erosions and the condition remains symptomatic in the absence of therapy. In advanced cases of RBCD, a superficial keratectomy, phototherapeutic keratectomy (PTK) or lamellar keratoplasty (LKP) may improve vision, but a penetrating keratoplasty (PK) is rarely necessary because the pathologic changes only involve the superficial cornea.

In this type of corneal dystrophy, sub-epithelial corneal opacities form a honeycomb-shaped pattern in the superficial cornea 32. A clear zone is present at the corneoscleral limbus. Corneal erosions start in the first and second decade and cause ocular discomfort and pain. The erosions recur and vision gradually becomes impaired.

On histology, the corneal epithelium varies in thickness and contains nonspecific degenerative changes. The epithelial basal lamina and Bowman layer display variable degenerative changes. There is also irregular subepithelial collagenous tissue. TEM discloses pathognomonic short, curled filaments measuring (about 8–10 nm in diameter) interspersed among normal collagen fibrils in Bowman zone and the contiguous superficial corneal stroma 33. In advanced stages of TBCD, the anterior stromal collagen and Bowman layer may be markedly disorganized and replaced by numerous aggregates of these filaments. Histopathologically, subepithelial fibrous tissue accumulates in a wave-like configuration. The characteristic subepithelial "curly" fibers can only be identified by TEM. Laminin and bullous pemphigoid antigen have been localized in a piebald mosaic distribution within the aberrant subepithelial fibrous tissue suggesting a primarily epithelial disease with the peculiar curly material paralleling the distribution of attachment proteins. Except for abnormalities in the superficial cornea the remainder of the cornea is unremarkable.

TBCD has an autosomal dominant mode of inheritance and was mapped to chromosome 5 (5q31). It was later found to be associated with a p. Arg555Gln mutation in TGFBI 2934, which may be diagnostic, but more families with the characteristic ultrastructural abnormalities need to be documented to establish this with certainty. Many reports documenting the p. Arg555Gln mutation in the TGFBI gene have based the diagnosis solely on the clinical phenotype and have not taken into account the necessary requirement of a tissue diagnosis. Genetic heterogeneity seems to exist and another locus for TBD has also been identified on chromosome 10 (10q23–q24) 35.

Like many other inherited corneal disorders TBCD may recur in the graft following therapeutic methods that ablate the abnormal cornea.

Multiple prominent milky-white gelatinous mulberry-shaped nodules form beneath the corneal epithelium during the first decade of life GDCD 3637 (Figures 8 and 9). Other features are severe photophobia, tearing, a corneal foreign body sensation and a severe progressive loss of vision. Fusiform deposits similar to those in lattice corneal dystrophy (LCD) may also form in the deeper stroma. In GDCD multiple nodules of amyloid deposit in the subepithelial corneal tissue of both corneas (Figures 10 to 12). The amyloid contains lactoferrin, but the disease is not linked to the lactoferrin gene 38.

More than 20 mutations in the TACSTD2 (formerly M1S1, TROP2, GA733-1) gene that encodes tumor-associated calcium signal transducer 2 (gastrointestinal tumor-associated antigen 1) cause GDCD. The p. Gln118X mutation has been detected most often. Some affected individuals have been found not to have mutations in TACSTD2 suggesting the existence of genetic heterogeneity in this autosomal recessive disease 39.

In GDCD, the response to both LKP and PK as well as to a superficial keratectomy is unsatisfactory as amyloid recurs in the graft within about 5 years.

LECD is characterized by feather shaped opacities and microcysts in the corneal epithelium that are arranged in a band-shaped and sometimes whorled pattern. Painless blurred vision sometimes begins after sixty years of life 4041. The corneal epithelium has a bubbly vacuolization with optically empty spaces.

The epithelial cells in parts of the cornea that correspond to the clinically evident whorl-like opacity have a diffuse cytoplasmic vacuolization. By TEM, the vacuoles are mainly empty but contain scant nonspecific osmiophilic material 404243. The identity of the content of the vacuoles remains unknown.

The gene for LECD has been mapped to the short arm of the X chromosome (Xp22.3) at a maximum likelihood of odds (LOD) score of 2.93 41. As expected because of the mode of inheritance it is not linked to the KRT3 and KRT12 genes.

The epithelial opacities are slowly progressive and may lead to visual deterioration in LECD. Cellular debridement of the corneal epithelium has been tried as a therapeutic modality, but after this treatment LECD recurs.

This type of corneal dystrophy is characterized by recurrent episodes of epithelial erosions from childhood in the absence of associated diseases 4445464748. The erosions begin spontaneously or are precipitated by minor trauma, dust or smoke. The condition may become apparent by 6 months of age, but as a rule it only starts at 4 to 6 years of age. The erosions may be accompanied by a subepithelial haze or blebs and subepithelial opacities, apparently due to fibrosis or keloid-like nodules may develop.

Specific morphologic abnormalities have not been identified in ERED.

The etiology is not completely understood. The gene for ERED remains to be mapped to a specific chromosomal locus.

ERED can be treated medically with effort devoted towards healing the epithelial defect and protecting the loosely adherent epithelium. A topical antibiotic, cycloplegic and pressure patch are valuable. A lubricating ointment is useful at night. Hypertonic saline and bandage contact lens therapy may also have a role.

In ERED the intensity and frequency of the recurrent epithelial erosions tend to diminish with time and usually cease by the end of the fourth decade. When present, subepithelial opacities continue to enlarge.

Frequent recurrent corneal erosions in the first decade of life characterizes SMCD and in the only publication on this condition older individuals developed subepithelial opacities and a corneal haze 49 (Figures 13 and 14).

A striking morphologic feature of corneal tissue with SMCD is the presence of subepithelial mucinous material. It is associated with a band in Bowman layer of GAGs that have been identified as chondroin-4-sulfate and dermatan sulfate histochemically (Figures 15 to 17).

The gene for SMCD has not been mapped to a particular chromosomal locus.

Initially when recurrent epithelial erosions of the cornea are present, treatment can be as in ERED (see above). As this dystrophy has only been reported in one family only, the ideal therapy remains uncertain. SMCD eventually progresses over time leading to corneal opacities.

Ill-defined cloudy regions usually first appear within a hazy stroma of both corneas during adolescence, but the opacities may become apparent in early infancy or as late as the sixth decade (Figure 18). Over time, the non-transparent areas progressively merge as the entire corneal stroma gradually becomes cloudy, causing severe visual impairment usually before the fifth decade. The bilateral corneal opacities progressively extend through the entire thickness of the central and peripheral corneal stroma. The corneal stroma is thinner than normal. Most patients with MCD do not have detectable keratan sulfate in the serum (MCD types I and IA), but in some have normal antigenic keratan sulfate levels in the serum (MCD type II) 50515253. These immunophenotypes can not be distinguished from each other clinically and are of no clinical significance.

MCD has been identified throughout the world, but in most populations it is rare. It is most prevalent in India, Saudi Arabia, Iceland and parts of the USA. At one time MCD was the most frequent indication for a penetrating keratoplasty in Iceland 54.

Despite its traditional inclusion as a stromal dystrophy in classifications of the corneal dystrophies, MCD involves Descemet membrane and the corneal endothelium in addition to the corneal stroma. The histopathology of MCD is characteristic. Intracytoplasmic accumulations occur within the keratocytes and corneal endothelium, but the corneal epithelium is spared. The accumulations stain positively with histochemical stains for glycosaminoglycans, such as periodic acid-Schiff, alcian blue, metachromatic dyes, and possess an affinity for colloidal iron 55 (Figure 19). The accumulations also stain with the periodic acid-Schiff/thiocarbohyrazide/silver proteinate and the periodic acid-methenamine silver techniques. Intracytoplasmic vacuoles are a distinct ultrastructural feature of the keratocytes and with appropriate tissue fixation delicate fibrillogranular material can be discerned within the vesicles (Figures 20 to 22). Some corneal endothelial cells contain similar material (Figures 23 to 25). Numerous electron-lucent lacunae are randomly distributed throughout corneas and some lacunae are filled with clusters of abnormal sulfated chondroitinase ABC non-susceptible proteoglycan filaments.

The collagen fibrils have a normal diameter, but the interfibrillar spacing of collagen fibrils in affected corneas is less than that in the normal cornea. This close packing of collagen fibrils seems to be responsible for the reduced corneal thickness in MCD. The anterior banded portion of Descemet membrane which forms in utero is of normal thickness and has an unremarkable ultrastructure, whereas the posterior layer usually contains numerous corneal guttae. The latter, together with intervening parts of the posterior layer of Descemet membrane, are studded with numerous vacuoles giving a honeycombed appearance (Figure 26). The electron-lucent areas presumably contained extracellular deposits of GAGs which dissolved during tissue processing. The stroma of MCD corneas contains congregations of various sized cuprolinic blue stained filaments which vary both in size and in electron density. Histopathologically, MCD is typified by an intracellular storage of glycosaminoglycans (GAGs) within keratocytes and the corneal endothelium combined with an extracellular deposition of similar material in the corneal stroma and Descemet membrane. Guttae are common on Descemet membrane. Numerous electron-transparent lacunae are randomly distributed throughout MCD corneas. The collagen fibrils have a normal diameter but their packing differs between specimens.

Three immunophenotypes of MCD are recognized 515356: one has no detectable keratan sulfate (KS) in the serum or cornea (MCD type I) (Figures 27 and 28), another has normal amounts of KS in the serum and cornea (MCD type II) (Figure 29) and a third lacks detectable antigenic keratan sulfate in the serum, but has stainable KS in the keratocytes (MCD type IA). While deposits similar to those in the cornea have not been noted in cartilage, the chondrocytes and extracellular matrix of the nasal cartilage do not react with a monoclonal antibody against a sulfated epitope on KS and the KS content of the cartilage is at least 800 times lower normal. Patients with MCD whose corneas fail to react with anti-KS antibodies also lack detectable corneal KS, while those with immunohistochemically detectable KS have normal levels of serum KS.

Mutations in the CHST6 gene are responsible for most cases of MCD 5758, but all cases of MCD can not be explained by mutations in the coding region of CHST6, by major deletions or insertions in the upstream region, or by splice site mutations which create or destroy signals for exon-intron splicing 59. Marked allelic heterogeneity in CHST6 has been documented in different populations throughout the world. The most frequent abnormalities are missense and nonsense single nucleotide polymorphisms (SNPs) in CHST6 that alter a conserved amino acid. Insertional or deletional defects in the region between the CHST5 and CHST6 genes account for some cases. More than 125 CHST6 mutations have been identified in subjects with MCD from different countries (Britain, France, Iceland, India, Italy, Japan, Saudi Arabia, USA, and Vietnam) 459. Heterozygous mutations have been detected in exon 3 of CHST6 in most families and almost all of them are associated with another heterozygous mutation in the coding region of CHST6 on the other chromosome. Other MCD causing mutations are nucleotide insertions or deletions in the coding region of CHST6 that cause frameshift changes as well as a few deletions or substitutions upstream between CHST5 and CHST6. The molecular basis for the different immunophenotypes remains to be determined and a molecular genetic study of MCD in Saudi Arabia found identical CHST6 mutations in families with MCD type I, IA and II 60. A recent study has recognized several atypical immunophenotypes and found no relationship to specific mutations in CHST6 61.

In MCD, vision can be restored by corneal grafting in MCD, but the disease may recur in the graft after many years. Because the condition affects the entire corneal stroma, Descemet membrane and the corneal endothelium a lamellar keratoplasty will not excise all of the pathologic tissue.

Multiple small white discrete irregular-shaped sharply demarcated spots that resemble bread crumbs or snowflakes become apparent beneath Bowman zone in the superficial central corneal stroma (Figures 30 and 31). They initially appear within the first decade of life and may be evident by 3 years of age. The opaque spots are often arranged in lines and with time they gradually enlarge and become more numerous. In children the external corneal surface is smooth, but in adults it often becomes uneven. While some patients have only a few corneal granules, others eventually have many and the cornea becomes markedly opaque. Visual acuity is more or less normal. By the end of a second decade, many opacities are present in the central and superficial cornea but rarely in the deep stroma. Intervening tissue between the opacities and in the peripheral 2–3 mm of the cornea usually remains crystal clear. The opaque spots eventually extend throughout the central two-thirds of the cornea. The stromal deposits do not move after they form. Subtle differences in the clinical appearance of the discrete corneal opacities permit two types of GCD to be recognized: GCD type I (GCD1) and GCD type II (GCD2). GCD2 tends to have fewer corneal deposits that GCD1 and the corneal deposits in GCD2 sometimes resemble a combination of GCD and LCD.

GCD has been extensively studied in Denmark by Møller 62. GCD1 seems to be most prevalent in Europe, but GCD2 is more common in Japan, Korea and the USA. In Korea GCD2 is estimated to have prevalence of at least 5.52 affected persons/10,000 individuals [Kim EK. Personal communication 2008, unpublished]. The ancestry of some families with GCD2 have been traced to the Avellino district of Italy (hence the synonym Avellino corneal dystrophy).

The corneal opacities in GCD are readily visualized in the excised cornea (Figure 32). The light microscopic and TEM appearance and staining attributes of the corneal deposits in GCD are diagnostic. Eosinophilic lesions deposit in the cornea in GCD (Figure 33). The corneal opacities consist predominantly of an extracellular deposition of mutant transforming growth factor beta induced protein (TGFBIp), which stains a brilliant red with the Masson trichrome stain 63 (Figures 34 and 35). With the Wilder reticulin stain, the accumulations contain tangles of argyrophilic fibers. The deposits react with histochemical methods for protein as well as with antibodies to TGFBIp (Figure 36). The granules stain positively with luxol fast blue and are reported to stain positively with antibodies to microfibrillar protein.

By TEM characteristic electron dense, discrete, rod-shaped or trapezoid bodies are evident 646566 (Figure 37). Cross-sectional profiles of the corneal deposits are usually irregularly-shaped, but sometimes hexagonal measuring 100–500 nm in diameter. Clusters of these elongated bodies occur particularly in the superficial corneal stroma and they may be present in the epithelial intercellular space or within degenerated basal epithelial cells. Some rod-shaped structures appear homogeneous without a discernible inner structure; others, however, are composed of an orderly array of closely packed filaments (70–100 nm in width) orientated parallel to their long axis, while others appear moth-eaten with variable-shaped cavities containing fine filaments (Figure 38). Some superficial and most deep stromal deposits do not all possess the rod-shaped configuration. Descemet membrane and the corneal endothelium are unremarkable, and so is the cornea between the deposits.

Most histopathologically confirmed recurrences have been superficial to Bowman layer in subepithelial non-vascularized fibrocollagenous tissue with fibroblasts almost certainly of host origin. The rare characteristic deposits within the peripheral stroma of the donor cornea probably result from an invasion of the graft by corneal fibroblasts (keratocytes) of the recipient.

GCD usually has an autosomal dominant mode of inheritance, but rarely occurs sporadically due to de novo mutations or perhaps even mosaicism. In some families the mutant gene is completely penetrant, but in others the penetrance is incomplete. The mutation rate for GCD has been estimated to be about 0.3/1,000,000 in the Danish population 67. Inter-familial differences and intra-familial similarities occur. When both parents have GCD their offspring may be homozygous for the TGFBI mutation and develop an unusually severe corneal dystrophy with larger corneal opacities and an earlier onset than heterozygous cases. One family study suggests that persons heterozygous and homozygous for the TGFBI gene are phenotypically identical, but genetic mutations were not performed in this instance. Numerous mutations in TGFB1 have been found in clinically and histopathologically distinct phenotypes, but GCD1 results from a p. Arg555Trp mutation 686970, whereas GCD2 is the effect of a p. Arg124His mutation in the TGFBI gene 7172.

In most cases of GCD, visual acuity remains good until late in the course of the disease. After a penetrating keratoplasty, the graft usually remains free of recurrence for at least 30 months, but the opacities may recur in the grafts within a year, usually superficial to the donor tissue, even with lamellar grafts, or at the host-graft interface.

Many individuals with GCD never require corneal grafting because vision is usually not sufficiently impaired. However, in some cases visual impairment may be marked and multiple therapeutic procedures may be necessary because the opacities can recur within after keratoplasty, sometimes within a year. Until relatively recently, a penetrating keratoplasty has been the traditional method for treating GCD, but postoperative recurrent disease can be detected in the donor tissue and even along the suture tracts within several years, particularly in GCD2 73. Phototherapeutic keratectomy has been advocated as an initial therapy for GCD, but recurrent disease is still a common complication 74. Since laser-assisted in situ keratomileusis (LASIK) became popular in the treatment of myopia, the typical corneal opacities of GCD have been first become evident following the refractive surgery in persons with GCD2 75. GCD2 may also be exacerbated by laser epithelial keratomileusis (LASEK) 76. GCD (type not determined) has also become manifest after a radial keratotomy (RK) 77. LASIK, LASEK and other forms of refractive surgery are hence contraindicated in individuals with GCD2.

In most cases of GCD, visual acuity remains good until late in the course of the disease. After a penetrating keratoplasty, the graft usually remains free of recurrence for at least 30 months, but the opacities may recur in the grafts within a year, usually superficial to the donor tissue, even with lamellar grafts, or at the host-graft interface.

A network of delicate interdigitating branching filamentous opacities form within the cornea in two genetically distinct inherited disorders (Figure 39). One with no systemic manifestations is caused by specific mutations in the TGFBI gene (LCD type I and its variants) (LCD1); the other resulting from a mutation in the GSN gene (LCD type II) (LCD2) has systemic manifestations. LCD1 usually becomes apparent in both eyes towards the end of the first decade of life, but occasionally it begins in middle life and rarely in infancy. Linear and other shaped opaque areas accumulate particularly within the central corneal stroma, while the peripheral cornea remains relatively transparent. Corneal sensation is often diminished and the interwoven linear opaque filaments have some resemblance to nerves, but may not be evident in all affected members of families with LCD1. Recurrent corneal erosions may precede the corneal opacities and even appear in individuals lacking recognizable stromal disease. Both corneas are usually symmetrically involved, but sometimes one cornea remains clear or has discrete rather than linear opacities.

In LCD2 systemic amyloidosis (familial amyloid polyneuropathy type IV, Finnish or Meretoja type; FAP type IV; Meretoja syndrome, MIM #105120) both corneas contain randomly scattered short fine glassy lines, which are less numerous, more delicate and more radially oriented than those in LCD1. The peripheral cornea is chiefly affected and the central cornea is almost spared. Corneal sensitivity is reduced. The cornea has fewer amorphous deposits than LCD1 and epithelial erosions are not a feature. The condition first becomes apparent after 20 years of age. In persons homozygous for the relevant mutation in the GSN gene the disorder begins earlier. Vision does not usually become significantly impaired before the age of 65 years. LCD2 can be mistaken for LCD1 both clinically and histopathologically.

The corneal abnormalities are accompanied by a progressive bilateral cranial and peripheral neuropathy, dysarthria, a dry and extremely lax itchy skin with amyloid deposits. A characteristic "mask-like" facial expression, protruding lips with impaired movement, pendulous ears and blepharochalasis are also features.

LCD1 is one of the more common corneal dystrophies in the Western world, but cases have been recognized throughout the world including Bulgaria 78, Spain 70 and China 79. LCD2 is most common in Finland, where the disease was first discovered and most extensively studied 80.

Like nerves the linear deposits of LCD1 are argyrophilic (Figure 40) in silver impregnated preparations, but nerves have not been identified in relation to the eosinophilic amyloid deposits (Figures 41 to 44). Amyloid deposits occur throughout the corneal stroma 81 and coinciding with the lattice pattern of lines and other opacities 82. The amyloid seems to react mainly with antibodies to the N-terminal sequence of TGFBIp and not with those to the C-terminal portion 83. The amyloid in LCD type II is composed of a mutated 71 amino acid long fragment of gelsolin and it accumulates in the corneal stroma and between the epithelium and Bowman layer (Figure 45). In LCD2 amyloid deposits in the cornea (Figure 46), but also in scleral, choroidal and adnexal blood vessels as well as in the lacrimal gland and perineurium of ciliary nerves. The amyloid is also found in the heart, kidney, skin, nerves, wall of arteries, and other tissues 84. The amyloid within the cornea in LCD type II reacts with the anti-gelsolin antibody 85, but not with the antibodies produced to the amino and carboxy terminals of gelsolin 86.

SCD usually becomes apparent early in life with corneal clouding or with crystals within the corneal stroma. Over time, an initially unremarkable corneal stroma acquires small white opacities and a diffuse haze (Figure 47). In approximately 50% of cases, crystals are not evident clinically (Schnyder crystalline dystrophy sine crystals). Typically, a ring-shaped yellow-white opacity composed of innumerable fine needle-shaped crystals forms in Bowman layer and the adjacent anterior stroma of the central cornea. The crystals usually remain in the anterior third of the cornea. The remaining stroma is unremarkable initially, but with time it may acquire small white opacities and a diffuse haze. While sometimes appearing dull white, the crystals are frequently scintillating with variegated red and green hues. The corneal epithelium and endothelium as well as Descemet membrane are spared. SCD is usually bilateral, but one eye may become affected prior to the other.

Most cases of SCD lack an obvious systemic disorder, but hypercholesterolemia is common (approximately 40% of the affected patients), and so are associated manifestations, such as arcus lipoides (Figure 48) and xanthelasma. Genu valgum has been documented in < 4% of patients 88.

The ancestry of four American families with SCD have been traced to Southwestern Finland 89.

On histology, birefringent cholesterol crystals and associated neutral fats accumulate within keratocytes and extracellularly, corresponding to the crystals observed clinically (Figures 49 to 50). The lipid is also present in Bowman layer, between the superficial corneal lamellae and dispersed within the stroma midst the collagen fibrils. The lipid deposits in SCD comprise mainly multilamellar vesicles containing unesterified cholesterol and phospholipids, with a lesser contribution of cholesteryl ester lipid droplets 90. The lipid deposits within the cornea are predominantly phospholipids and cholesterol (esterified and unesterified) 9091. The predominant phospholipid is sphingomyelin 91. Apolipoproteins A-I, A-II, and E are present, but not apolipoprotein B 92. An ultrastructural study of a skin biopsy and cultured fibroblasts from an affected person has disclosed lipid containing membrane-bound spherical vacuoles. Moreover, skin fibroblasts from one patient showed abnormal cytoplasmic deposits that were fluorescent after staining with filipin, a reagent specific for unesterified cholesterol 93. The above mentioned observations on skin fibroblasts taken together with a frequent finding of hyperlipidemia in patients with SCD strongly suggests the presence of a systemic disorder of lipid metabolism. However, one study failed to find evidence of lipid deposition in fibroblasts from the conjunctiva or skin in a patient with SCD 94.

SCD is caused by one of numerous mutations in the UBIAD1 gene 9596.

Visual acuity is usually good in SCD and, as a rule, the course is stationary after childhood, but corneal opacification may progress over time and form a dense central disc shaped corneal opacity. Excellent scotopic vision continues until middle age, but most affected individuals need a penetrating keratoplasty before the seventh decade 88. An opaque disc of corneal crystals or other lipids may diminish vision sufficiently in both eyes to result in the need for a corneal graft.

FCD is characterized by multiple asymptomatic, non-progressive symmetric minute opacities disseminated throughout the corneal stroma. One type of opacity consists of numerous small, oval, round, wreath-like or semicircular-shaped flattened opacities with distinct borders ("flecks") in the central and peripheral cornea with intervening portions of the cornea being normal (Figures 51 and 52). Other opacities resemble snowflakes or clouds and consist of small grayish aggregations with ill-defined margins and occur particularly in the central third of the cornea (central cloudy phenotype). They are less numerous in the anterior and peripheral stroma and are sometimes most dense in the deeper stroma near Descemet membrane, on which they are occasionally localized. These seemingly different opacities, which usually involve both corneas symmetrically, have been observed in the same family and even in the same individual. Unilateral cases have been reported. FCD affects males and females equally and has been observed throughout life and even in children as young as 2 years. The corneal epithelium, Bowman layer, and Descemet membrane are unremarkable. Corneal sensation is usually normal.

Corneal tissue with FCD has rarely been examined, but some keratocytes contain fibrillogranular material within intracytoplasmic vacuoles or pleomorphic electron-dense and membranous intracytoplasmic inclusions 979899. The stored material reacts positively with alcian blue, colloidal iron, Sudan black B and oil red O stains and is partially sensitive to hyaluronidase and β-galactosidase. It has the histochemical attributes of GAGs and lipids. Extracellular alterations are rare, but foci of broad spaced collagen have been observed. By TEM single membrane-limited inclusions containing fine granular material are evident in the affected cells. Some keratocytes contain pleomorphic electron-dense and membranous intracytoplasmic inclusions.

FCD is due to a mutation in the PIP5K3 gene 100.

FCD is non-progressive, does not affect vision and is usually asymptomatic and does not require treatment, but mild photophobia has been reported. In one patient who underwent a penetrating keratoplasty for keratoconus, there was no clinical evidence of recurrent FCD within the donor tissue after a 10 year follow-up 99.

CSCD is a non-progressive disorder characterized by numerous opaque flaky or feathery clouding of the corneal stroma. The flakes and spots become more numerous with age and eventually prevent a clinical evaluation of corneal endothelium (Figure 53). Corneal erosions, photophobia and corneal vascularization are absent. Some affected individuals have strabismus or primary open angle glaucoma.

CSCD is extremely rare; only 4 families have been reported. One large family of CSCD is known to have descendants in Germany and France. Affected individuals with CSCD have been extensively studied in large French and Norwegian families.

In CSCD, the morphologic abnormalities include a peculiar arrangement of tightly packed lamellae having highly aligned collagen fibrils of an unusually small diameter 101102 (Figure 54). The cornea is of normal thickness and both Descemet membrane and the corneal endothelium are relatively normal. Nothing is known about the biochemical alterations, but the abnormally small stromal collagen fibrils and disordered lamellae suggest a disturbance in collagen fibrogenesis.

A Norwegian family with CSCD has a 1 bp deletion in the DCN gene 101 which encodes the core protein of decorin, whereas another Belgian family has a DCN frame shift mutation 103.

PACD is the designation given by Chapel et al. 104 for a corneal disorder characterized clinically by irregular "amorphous" sheet-like opacities in the corneal stroma, predominantly posterior, and Descemet membrane. In keeping with the current notion that this is a developmental disorder, the abnormalities have been observed in infancy and childhood and, in contrast to the traditional corneal dystrophies, non-corneal manifestations have been reported including abnormalities of the iris (iridocorneal adhesions, corectopia, and pseudopolycoria). Transparent corneal stroma may intervene between opacities, which sometimes indent Descemet membrane and the corneal endothelium, which may have focal abnormalities. Centroperipheral and peripheral forms of PACD are recognized. The centroperipheral type extends to the corneoscleral limbus and is accompanied by corneal thinning and a flattened corneal curvature. Visual acuity is usually minimally impaired.

Families with PACD have been reported in the USA and apparent isolated cases of this entity have documented in Britain 105 and Turkey 106.

Insufficient tissue specimens have been examined to characterize the histopathology of PACD. Disorganized posterior stromal collagen lamellae, and an attenuated corneal endothelium have been observed 104107. A zone of collagen fibers may interrupt Descemet membrane beneath the anterior banded layer.

The chromosomal locus of the gene responsible for the autosomal dominant PACD has not been determined.

PACD tend to be non-progressive or slowly progressive. Visual acuity is usually minimally impaired, but may be severe enough to warrant a penetrating keratoplasty.

FECD is a common bilateral slowly progressive corneal disorder of aging 108122123. As a rule, it presents clinically during the fifth or sixth decade of life and seldom at an earlier age. The characteristic clinical findings of FECD are excrescences on a thickened Descemet membrane (cornea guttae), generalized corneal edema and decreased visual acuity. Initially FECD is asymptomatic, but corneal guttae form in the central cornea and they are frequently surrounded by a fine dusting of pigment dots. The guttae have a glittering golden brown appearance on slit lamp biomicroscopy and when viewed by retroillumination they appear as small dew drops. Corneal endothelial abnormalities can be detected clinically several years before the patient becomes symptomatic. Vision becomes hazy and glare begins as the corneal stroma and epithelium become edematous. A loss of corneal clarity is followed by painful episodes of recurrent corneal erosions, and a severe impairment of visual acuity and sometimes even blindness in the elderly population (Figure 55).

At first, stromal edema produces a blue-gray haze anterior to Descemet membrane. The entire corneal stroma eventually thickens and develops a ground-glass appearance, while Descemet membrane wrinkles. Epithelial edema results in a characteristic fine pigskin texture designated "bedewing". Fluid accumulates between the epithelial cells and in a subepithelial location (bullous keratopathy) and it bursts through the epithelium causing painful corneal erosions. Eventually, the subepithelial edema and discomfort diminishes, but visual acuity continues to deteriorate as connective tissue replaces it. The corneal abnormalities start centrally, but spread towards the corneoscleral limbus. FECD is commonly associated with cataracts.

In advanced cases of FECD, abnormalities are found in the all layers of the cornea, but consistent abnormalities involve the corneal endothelium and Descemet membrane, its basement membrane. The underlying abnormality in FECD results in a decline in the number of functional endothelial cells, premature corneal endothelial cell degeneration and apoptosis. Melanosomes are often found within the endothelial cells (Figures 56 to 58). The corneal endothelium is attenuated over the guttate excrescences and abnormal endothelial cells have widened intercellular spaces, swollen mitochondria, a dilated rough endoplasmic reticulum, and melanosomes. The Na-K ATPase pump site density in endothelial cells is decreased in FECD. Adherent to the posterior surface of Descemet membrane some cells possess morphological features of fibroblasts. The endothelial alterations precede the epithelial changes.

Multiple excrescences (guttae) of differing size and shape are evident on the posterior surface of Descemet membrane and protrude into the anterior chamber (Figures 59 to 63). Some guttae are mushroom or anvil shaped. Others are multilaminar warts or are buried in the multilaminar extracellular matrix. In FECD, the guttae are typically more confluent and more centrally located than the guttae of aging, which characteristically involve predominantly the peripheral cornea (Hassall-Henle warts). Fissures within some guttae are penetrated by cellular debris. The distribution of the guttae can be visualized in flat preparations of Descemet membrane using phase-contrast microscopy or scanning electron microscopy. Descemet membrane is also multilayered and often irregularly thickened (two to four times normal) due to an excessive accumulation of collagen especially where the guttae are most abundant. A multilaminar collagenous layer which stains with variable intensity with the periodic acid-Schiff stain is commonly present. Some subjects with FECD have multilaminar connective tissue posterior to Descemet membrane, but without corneal guttae.

When viewed by TEM, Descemet membrane has a normal 3-μm-thick anterior banded layer and a normal non-banded portion, but posteriorly it contains fusiform bundles and sheets of wide-spacing (100-nm) collagen with a macroperiodicity of 55 or 100 nm within amorphous material and manifests sub-bands with a periodicity of about 30–40 nm (Figure 60). Horizontal fibrils run perpendicular to the vertical bands. The wide-spacing collagen forms a hexagonal pattern, which is evident in tangential sections. This pattern is identical to that observed in horizontal sections of the normal anterior-banded Descemet membrane. Tissue specimens in advanced cases of FECD contain a layer consisting of loosely packed thin collagen fibrils (20–30 nm in diameter) with a 64-nm banding scattered within basement membrane-like material. Groups of 10- to 20-nm-diameter collagen fibrils are found next to the wide-spacing collagen and may fuse with the horizontal fibrils of the wide-spacing collagen. The excessive amounts of Descemet membrane are of a type analogous to that assembled in utero. In FECD corneas, Descemet membrane and its adjacent posterior collagenous contain similar collagen types to age-matched controls and immunohistochemical studies have disclosed fibrinogen/fibrin in the posterior collagenous layer in FECD, but not in normal Descemet membrane. Due to abnormal extracellular matrix deposits within Descemet membrane in FECD, an immunohistochemical study on selected cases of FECD as well as in situ hybridization using labeled sense and antisense TGFBI oligonucleotide probes was performed 124. This study disclosed the presence of transforming growth factor beta induced protein in the sub-epithelial corneal matrix and in the posterior collagenous layer of FECD.

Corneal edema is usually most marked in the central and paracentral cornea and this overlies the abnormal corneal endothelium. Initially, edema of the corneal epithelium is restricted to the basal epithelial layer, but the more superficial layers are also involved in more advanced cases and the epithelium becomes separated from its basal lamina, which is fragmented, and Bowman layer (bullous keratopathy) (Figures 61 to 63). This is associated with a loss of hemidesmosomes. Bowman layer usually remains intact, but focal breaks become traversed by subepithelial connective tissue.

The cause of FECD is unknown, but it seems to be a heterogenous complex inherited disorder caused by the interaction of genetic products and environmental factors. Although most patients with FECD lack a positive family history, blood relatives sometimes manifest corneal guttae. FECD may also affect siblings and two or more successive generations apparently as an autosomal dominant disorder having incomplete penetrance, but a simple autosomal dominant pattern is unlikely. For reasons that remain to be determined, FECD is expressed much more frequently in females. Rare young onset cases of FECD have been associated with mutations in the COL8A2 gene 120. Other cases of FECD have been mapped to chromosome 13 (13pTe1-3q12.13) 125 and 18 (18q21.2–q21.32) 126. Two families with an early-onset autosomal dominant variant of FECD and PPCD have recently been found to have a p. Gln455Lys mutation in the COL8A2 gene suggesting that these two disorders are related to each other. A p. Leu450Trp mutant in COL8A2 has also been found in early-onset FECD 127. Heterozygous mutation in the SLC4A11 gene have been detected in 4 of 89 cases of late-onset FECD 109, but the significance of these findings remains to determined as one would expect all parents of persons with CHED2 to have FECD. A single novel mutation in the TCF8 gene, which causes PPCD type 3, was only found in one of 74 Chinese patients with FECD 121.

The clinical course of FECD usually spans 10–20 years and is characterized by a progressive edema of the corneal stroma, epithelium and subepithelial tissue (bullous keratopathy) and an eventual subepithelial fibrosis. Cataracts are common in individuals with FECD and cataract extraction accelerates the corneal decompensation. Microbial keratitis and corneal neovascularization are extremely rare complications of FECD.

Most patients with FECD ultimately require a penetrating keratoplasty or one of the more recent procedures for repairing the posterior surface of the cornea, such as a deep lamellar endothelial keratoplasty (DLEK), Descemet stripping endothelial keratoplasty (DSEK), or Descemet stripping automated endothelial keratoplasty (DSAEK).

Small aggregates of apparent vesicles bordered by a gray haze and gray geographic areas appear clinically at the level of Descemet membrane in PPCD 128129. These abnormalities occasionally appear nodular and contain round or elliptical vesicular zones creating a pattern that resembles Swiss cheese. Broad bands with more or less parallel edges and gray sheets appear as thickenings of Descemet membrane. These abnormalities exhibit a refractile quality on retroillumination. Adhesions sometimes unite the iris with the posterior surface of the peripheral cornea giving rise to glaucoma. These adhesions differ in appearance from the fine strands that attach iris to a prominent Schwab ring in the Axenfeld and Rieger anomalies. PPCD is usually bilateral but the abnormalities may be asymmetrical and sometimes only one cornea is apparently affected. Most patients remain asymptomatic and corneal edema is usually absent, presumably because the corneal endothelium is able to maintain a normal state of corneal hydration in most affected individuals. However, sometimes edema of the corneal epithelium and stroma are present even at birth and the anterior cornea may become scarred and develop calcific band keratopathy. A single example of keratoglobus with PPCD has been reported 130.

Unlike FECD, corneal guttae are not present on Descemet membrane. Sometimes edema of the corneal epithelium and stroma occur, and the anterior cornea may become scarred and develop calcific band keratopathy. Corneal tissue has only been examined in cases of PPCD with glaucoma or those that are severe enough to require a penetrating keratoplasty. A major morphologic feature of PPCD is the replacement of corneal endothelial with cells having epithelial attributes. Instead of an endothelial monolayer, the posterior cornea is lined by variable numbers of stratified squamous epithelial cells having tonofilaments, cytokeratin and desmosomes 131132 (Figure 64 to 68). These epithelial cells have numerous microvilli, but unlike normal corneal epithelium, microplicae are not a feature. Descemet membrane is multilaminar and irregularly thickened and occasionally with focal nodular excrescences, but they differ from the corneal guttae that characterize FECD. TEM of Descemet membrane has disclosed an anterior 3 μm thick, 110 nm banded layer and an extremely thin posterior non-banded portion with numerous delicate collagen fibrils (10–20 nm in diameter with a normal cross-striational periodicity) as well as long spacing collagen (with a banding of 55–110 nm) interspersed with fine granular homogeneous basement membrane-like material. That the banded anterior layer of Descemet membrane is present and of normal thickness suggests that the abnormality of the corneal endothelium in PPCD does not become manifest until late in gestation. The abnormal endothelial cells show strong immunostaining for a wide variety of cytokeratins, but express predominantly cytokeratin 7 and cytokeratin 19 133. Edema of the corneal epithelium and stroma may be present in advanced cases.

PPCD is an autosomal dominant genetically heterogenous entity with extremely variable expression. Three genes have been implicated in PPCD (VSX1, COL8A2, TCF8), but the evidence implicating VSX1 and COL8A2 is questionable. p. Leu159Met and p. Gly160Asp mutations in VSX1 have also been reported 134, but an analysis of two large families in the Czech republic has shown that the locus excludes the VSXI gene 135. A missense p. Gln455Lys mutation in the COL8A2 has also been identified in PPCD, but a tissue diagnosis in that family was not documented 120. Evidence for TCF8, which encodes transcription factor 8, is much more convincing 136.

A yet to be identified gene for autosomal dominant PPCD has also been mapped to the pericentromeric region of human chromosome 20 (20q11) 137, where a gene for CHED1 is also located. While PPCD and CHED1 may be due to different genes one gene in this region could be responsible for both corneal dystrophies. The latter seems more likely because blood relatives of individuals with PPCD may have CHED1 138139. PPCD shares developmental, morphological and clinical similarities with CHED1, and one variant of PPCD is probably related to CHED1.

PPCD is usually asymptomatic and hence most cases do not require treatment. However, those that do usually warrant a penetrating keratoplasty, or one of the current therapeutic procedures advocated for FECD. PPCD can recur in the graft following a penetrating keratoplasty. The corneal endothelial changes often remain stationary for years, but the disorder may progress slowing with time leading to corneal endothelial decompensation and corneal edema.

CHED is characterized by a diffuse ground-glass appearance of both corneas and markedly thickened (2–3 times thicker than normal) corneas from birth or infancy (Figure 69). Two types of CHED are recognized: CHED1 (autosomal dominant) and CHED2 (autosomal recessive) 140141. CHED1 becomes manifest during the first two years of life with photophobia and tearing, but in contrast to CHED2 nystagmus is absent. In CHED2, affected individuals are born with corneas having a diffuse ground glass appearance and this is accompanied by nystagmus. CHED2 is sometimes associated with receptive deafness (Harboyan syndrome) 142.

The cornea in CHED is swollen due to extensive stromal edema (Figure 70) with some enlargement of collagen fibrils. These changes, together with the decreased fibril density, scatter light and result in a ground-glass appearance of the cornea. The endothelial cells are scant or degenerated when present. The corneal endothelium presumably functions normally in utero as evidenced by the thin embryonic Descemet membrane, which consists of a normal anterior 110 nm banded portion and a narrow zone of posterior non-banded material (Figure 71). In addition, there is a fibrous connective tissue layer posterior to Descemet membrane, which is composed of an admixture of fibrils measuring 20–40 nm in diameter and small amounts of basement membrane-like material. In some foci, the latter becomes densely packed and resembles an additional layer of Descemet membrane. The keratocytes and Bowman layer are usually unremarkable. The histopathology of CHED1 and CHED2 are similar, but subtle differences in the thickness of collagen in Descemet membrane have been described. In CHED1, a posterior collagenous layer of fibrillary collagen contributes to the thickness of Descemet membrane. CHED2 cases show an increased tendency for the abnormal endothelium to synthesize a homogenous, posterior, non-banded Descemet. As with PPCD, the corneal endothelial cells in CHED express cytokeratin in contrast to normal corneal endothelium 143, but unlike PPCD the endothelial layer less often becomes multilayered 144. Subepithelial amyloidosis has been observed in cases of CHED2 145146 and it remains to be determined if this is a manifestation of CHED2 or an associated GDCD in persons with two different dystrophies.

The gene responsible for CHED1 has been mapped to the pericentromeric region of chromosome 20 (20p11.2-q11.2) in an area overlapping a gene for one type of PPCD 147 and this distinct from the locus for CHED2 116. Most cases of CHED2 are caused by homozygous mutations in the SLC4A11 gene 111112113114115118119142 which encodes bicarbonate transporter-related protein 1 that reportedly regulates the intracellular boron concentration. A high degree of mutational heterogeneity has been detected in CHED2 114 and genetic heterogeneity may exist as no mutations in SLC4A11 or in its promoter region have been detected in some families 111112148.

In CHED where bilateral corneal opacification is severe, a penetrating keratoplasty is the only hope for improved vision. CHED1 progresses slowly over 5–10 years, but CHED2 remains stationary throughout life.

X-linked endothelial corneal dystrophy manifests as a congenital ground glass corneal clouding or a diffuse corneal haze 149. In males this is often associated with blurred vision. Advanced cases have a subepithelial band keratopathy associated with endothelial changes that resemble moon craters. Males are affected more severely than females and the corneal opacification may be severe and associated with nystagmus. Females are asymptomatic, but have moon crater-like endothelial abnormalities. In keeping with an X-linked mode of inheritance, affected males transmit the disorder to their daughters, but not to their sons.

Light microscopy and TEM have disclosed focal discontinuities and degenerative changes in the corneal endothelial cells in XECD accounting for the moon crater-like changes seen clinically 150. Moreover, Descemet membrane is irregularly thickened with small pits and excavations as well as abnormal anterior and posterior banded zones. The posterior non-banded zone of Descemet membrane is absent. The corneal epithelium and Bowman zone may be irregularly thinned. In contrast to PPCD, epithelial cells with desmosomes and bundles of tonofilaments have not been found to line Descemet membrane.

XECD has been mapped to the long arm of the X-chromosome (Xq25) 150. The critical interval contains 72 genes of which 7 code for putative transcription factors.

A penetrating keratoplasty is sometimes indicated in XECD and the graft may remain clear for as long as 30 years. Because so few cases have been treated, the optimum mode of therapy is uncertain. The course in XECD is slowly progressive with intermittent corneal clouding and a subepithelial band keratopathy develops in adulthood starting in the peripheral cornea.

Corneal guttae are not specific for FECD and may arise as part of corneal aging or as a response to interstitial keratitis. They are also a feature of MCD. Because cataracts are commonly associated with FECD, the differential diagnosis also includes aphakic bullous keratopathy and pseudophakic bullous keratopathy. In FECD, the guttae are typically more confluent and more centrally located than the guttae of aging, which characteristically involve predominantly the peripheral cornea (Hassall-Henle warts). The dome-shape, fissured appearance and lack of a multilaminar pattern of Hassall-Henle bodies usually distinguish them from the guttae of FECD. Fissures similar to those in Hassall-Henle bodies are occasionally found in the guttae of FECD, but they are less plentiful and not as prominent.

A diffuse corneal edema in PPCD may simulate CHED1, but the corneas are thicker in the latter condition. In contrast to PPCD corneas epithelial cells do not replace endothelial cells on the posterior surface of Descemet membrane in CHED1. PPCD needs to be differentiated from an acquired posterior corneal opacification that follows recurrent uveitis and keratitis (posterior polymorphous keratopathy) 151. PPCD needs to be distinguished from other disorders in which the corneal endothelium is replaced in part by a stratified squamous epithelium, such as the iridocorneal endothelial (ICE) syndrome and an epithelial ingrowth that follows a penetrating wound at the corneoscleral limbus.

CHED can be confused with other congenital causes of corneal opacification, such as CSCD and Peters anomaly. As mentioned above CHED also needs to be distinguished from PPCD.