Sir,

A 27-year-old woman complained of decreased vision, photophobia and monocular diplopia. She has been a vegan for 7 years. Her best-corrected visual acuity was 20/60 OD and 20/20 OS. The corneas displayed a diffuse epithelial punctate keratopathy with intraepithelial cyst-like relucent lesions (Figures 1a and b). Superficial keratectomy was performed; despite an initial improvement in vision, the lesions reappeared.

Figure 1
figure 1

Clinical features of corneal epithelium before and after vitamin supplementation. (a) Diffuse illumination slit-lamp photograph of corneal epithelial lesions that caused diffuse superficial corneal haze (arrowhead). (b) Retroillumination image of intraepithelial cyst-like lesions scattered throughout the visual axis. (c) Resolution of corneal haze after vitamin supplementation. (d) The cyst-like lesions disappeared after vitamin supplementation.

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Epithelial biopsy detected clusters of degenerating cells (either dyskeratotic or apoptotic; Figure 2a, inset). There were myriad cytoplasmic vacuoles and isolated, swollen, lighter-staining cells beneath the surface parakeratotic layer (Figure 2b). Electron microscopy disclosed cytoplasmic vacuoles among basilar and suprabasilar cells, above which were more electron-lucent cells (Figure 2c). The vacuoles had a floccular content and a two-layered, limiting membrane with an occasional, preserved projecting crista (Figure 2d), establishing that they were degenerating mitochondria.

Figure 2
figure 2

Pathological features of corneal epithelial biopsies. (a) An undulating strip of corneal epithelium displays two to three layers of parakeratotic, hyperstaining surface cells (pre-keratinization, arrows). The inset shows small clusters of dyskeratotic or apoptotic cells and a clear cell (asterisk; 1 μ plastic section, toluidine blue, × 80, inset × 120). (b) High-power fields from each of the two biopsies showing basilar and suprabasilar (wing), frequently pale cells with cytoplasmic vacuoles that often indent the nuclei (arrows; 1 μ sections, toludine blue, × 180). (c) Electron micrograph of a full-thickness portion of epithelium with degenerative changes consisting of myriad basilar and suprabasilar cell vacuoles, apical surface cells with dissolution of their cell membranes (arrow), and fragmentation of their nuclei (crossed arrow). The inset displays preservation of surface microplicae in another region and subjacent desmosomes (designated ‘d’). BM, a portion of included Bowman s membrane; N, nuclei of basilar and suprabasilar cells ( × 2600). (d) The numerous cytoplasmic vacuoles contain a fibrillo-flocculent material. The inset demonstrates a double membrane (arrows) delimiting the vacuoles, indicating that they are distorted mitochondria, as does a crista (crossed arrow) projecting centrally. Profiles of rough endoplasmic reticulum envelop the mitochondria. N, nuclei of suprabasilar cells; Nl nucleolus. ( × 4600, × 25000 inset).

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The patient was referred to a metabolic geneticist and found to have elevated serum homocysteine levels to 60.7 μmol/l (normal <10). Serum folate levels were decreased to 2.9 ng/ml (normal >4 ng/ml) and vitamin B12 levels to 190 pg/ml (normal >211). DNA mutation analysis revealed a homozygous mutation for the C677T of the methylenetetrahydrofolate gene. The patient's diet was supplemented with B12 (1 mg) and folic acid (1 mg). After 6 months, serum levels of homocysteine, vitamin B12, and folate had normalized and the epithelial lesions and symptoms had resolved (Figures 1c and d).

In our patient, methylenetetrahydrofolate deficiency was compounded by nutritional deficits of folate and vitamin B12, which resulted in the emergence of a corneal epitheliopathy due to a mitochondriopathy that was reversible with dietary supplements.1, 2 Vitamin A deficiency, on the other hand, causes bundling of cytoplasmic tonofilaments to form keratofibrils, increases the numbers of desmosomes, and leads to disappearance of surface microplicae, features absent in our case.3 In cytosol and mitochondria, vitamin B12 is modified by a series of reactions into cobalamine.4 Cobalamine is a coenzyme necessary for methionine synthase, which is involved in homocysteine metabolism, and methylmalonyl-CoA mutase, which is involved in the catabolism of branched chain amino acids.4, 5 The patient's strict veganism together with methylenetetrahydrofolate deficiency resulted in critically low levels of cobalamine, abolition of the normal breakdown of homocysteine (reflected in the abnormally elevated plasma levels of homocysteine), and interference in the catabolism of branched chain amino acids (hence the mitochondrial abnormalities). This case exemplifies a model of how an endogenous genetic predisposition interacted with an exogenous dietary insufficiency to create a previously unreported corneal entity.