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Editor,—Posterior polymorphous dystrophy (PPD) is a hereditary corneal dystrophy that is typically asymptomatic and non-progressive. It rarely results in severe visual dysfunction due to corneal decompensation and/or glaucoma. 1 We examined the anterior chamber angle of a PPD patient with corneal oedema and broad iridocorneal adhesion by using ultrasound biomicroscopy (UBM). The examination indicated a unique iridocorneal adhesion that could not be seen in gonioscopy.
A 39 year old woman was examined for progressive loss of vision (30/200) in her left eye and increased foreign body sensation. Slit lamp examination of the left eye revealed diffuse corneal oedema and bullous keratopathy (Fig 1). Broad based iridocorneal adhesion extending anteriorly to the Schwalbe’s line was found by gonioscopy from 3 o’clock to 8 o’clock and 9 o’clock to 1 o’clock. Intraocular pressure was 14 mm Hg in the right eye and 12 mm Hg in the left eye. Pupillary distortion and glassy membrane were found but no iridial hole was noticed in the left eye (Fig 1). The right eye was almost normal except for three very tiny vesicular lesions on the posterior cornea. Specular microscopy of the right central cornea revealed diffuse endothelial changes with pleomorphism and polymegathism. The number of endothelial cells in the right eye was 500/mm2 but in the left eye, the measurement was not possible. Her daughter’s corneas demonstrated thickening of Descemet’s membrane, band-like figures on posterior corneal surface and endothelial cell loss (1500/mm2).
By UBM (Zeiss-Humphrey, San Leandro, CA, USA), the anterior iris was broadly adhered to the posterior corneal surface but the trabecular meshwork was not fully covered (Fig 2). The iridial surface adhered to the protruded Schwalbe’s line, and the glassy membrane pulled the anterior iris to the cornea but the position of posterior iris was rather normal. UBM could not distinguish the glassy membrane from the iris.
Patients with PPD usually demonstrate normal vision, but endothelial decompensation and/or glaucoma can develop, resulting in visual loss. Intraocular pressure (IOP) elevation occurs in approximately 15% of PPD patients.1 It seems that extent of synechial closure is not fully correlated with IOP.1 In this case, three quarters of the anterior chamber angle especially in the superior and inferior quadrants was closed by gonioscopy. However, the aqueous outflow pathway was preserved because of tunnels of open trabecular meshwork below the synechiae as shown in Figure 2. The UBM findings explain why her IOP was not elevated in spite of the broad iridocorneal adhesion. Furthermore, the anterior layer of the iris were elevated to the cornea but those of the posterior layer was not. This finding suggests that only the anterior layer might be atrophic because of the maldevelopment of the iris and/or iris ischaemia.1 2
Glaucoma in PPD is believed to result from the aqueous outflow obstruction caused by synechial closure of the trabecular meshwork by metaplastic corneal endothelial cells and membranes.1 3-6However, our findings do not support this hypothesis. IOP elevation in PPD may be explained not only by synechial closure but also by abnormalities of neural crest cell differentiation or the basement membrane.1 3-6 Recording of UBM images on more cases of PPD is needed to clarify mechanism of IOP elevation in this disease.