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Sir,

Pellucid marginal degeneration (PMD) is a rare bilateral corneal disorder characterised by thinning in the peripheral portion of the inferior cornea with marked steepening just superior to the thinned zone.1 It is differentiated from other peripheral corneal thinning disorders such as Terrien's marginal degeneration and Mooren's ulcer by the absence of vascularisation, lipid infiltration, or corneal ulceration.

Unilateral isolated PMD is extremely unusual, and corneal topography provides valuable clues in suspected cases. Described here is such a case, in which the other eye was normal.

Case report

A 46-year-old healthy Indian man presented with painless progressive diminution of vision in the right eye for 6 years. There was no history of redness, pain, or use of ophthalmic medication during this time. He was otherwise healthy.

Ophthalmic examination revealed best-corrected visual acuities of 6/18 OD with −1.0/−13.50 D×110° and 6/6 OS unaided. On slit-lamp biomicroscopy, the cornea in the right eye showed an irregular contour with a thin band inferiorly, approximately 1.5 mm in width and 2 mm from the limbus, with bulging above the thinned zone (Figure 1, top). The portion between the thinned area and limbus was normal in thickness. There was no evidence of iron lines, lipid deposition, or vascularisation. The rest of the anterior and posterior segment was normal. The intraocular pressure by applanation tonometry was normal in both eyes. The left eye was normal, with no evidence of corneal thinning, striae, or abnormal protrusion (Figure 1, bottom).

Figure 1
figure 1

Top: slit-lamp photograph of the right eye showing inferior steepening (arrow), with a clear zone between limbus and steep band. Bottom: slit-lamp photograph of the left eye showing normal corneal curvature.

Keratometric readings were 50.00 D×20°/41.00 D× 111° in the right eye, and 43.00 D×95°/44.00 D×179° in the left eye. Corneal topography in both eyes was assessed by computerised videokeratoscopy using Holladay Diagnostic Summary analysis maps. In the right eye, there was a bow-tie-shaped against-the-rule astigmatism measuring +9.30 D at 23° (Figure 2, top), while the left eye showed a regular cornea with astigmatism measuring +0.97 D at 17° (Figure 2, bottom). This clinical picture was consistent with a diagnosis of pellucid marginal degeneration of the right eye.

Figure 2
figure 2

Top: videokeratograph of the right eye showing the characteristic bow-tie appearance of corneal steepening (red zones) and midperipheral inferior corneal flattening. Bottom: videokeratograph of the left eye showing normal corneal contour.

During the follow-up period in the last 30 months, repeated contact lens trials with different sizes of gas-permeable contact lenses have been unsuccessful. Sequential keratometric readings at 6-month intervals revealed a fairly regular against-the-rule astigmatism with a trend towards increasing steepening around the 20o meridian in the right eye, with no change in the left eye. The patient is presently on a regular follow-up, and corrected to 6/24 with spectacles in the right eye. The left eye does not show any feature of peripheral corneal thinning.

Comment

Pellucid marginal degeneration is a bilateral, slowly progressive condition classically described between the second and fourth decades. This rare entity has been postulated to be an abnormality of the connective tissue,2,3 but the exact pathogenesis is still unknown.

Topographic analysis in our case revealed a characteristic bow-tie appearance of marked against-the-rule astigmatism oblique-inferiorly, without peripheral steepening. Stromal thinning is known to cause corneal flattening over the area of tissue loss, and steepening at the border of unaffected tissue.4 This results in a relatively steep contour approximately 90° away.4,5 The topographic pattern described is distinctly different from that seen in keratoconus, in which a small area of high corneal power is surrounded by concentric bands of low corneal power.6

These characteristic features help to differentiate this condition from other noninflammatory corneal thinning disorders such as keratoconus, posterior keratoconus, and keratoglobus. It also needs to be differentiated from peripheral corneal disorders associated with inflammation such as Terrien's marginal degeneration, Mooren's ulceration, and ulcers associated with connective tissue disorders.

The case reported highlights the rare occurrence of unilateral PMD. Though bilateral conditions do present asymmetrically in degree or time period, the left eye in this case has not shown any sign of the disease 11 years after the right eye started becoming symptomatic. Two cases of unilateral isolated PMD have been reported so far by Wagenhorst7 and Basak et al8. The first was in an elderly black patient, while the second was an Indian patient similar to the case described. Another series by Biswas et al 9 describes 16 patients with pellucid marginal degeneration, of which it was unilateral in three patients.

To the best of our knowledge, this is the sixth such patient reported so far. Peripheral corneal thinning disorders are a group of distinct entities with differing implications for management. They need to be recognised and differentiated from each other. Topographical analysis is an invaluable tool that may help confirm a clinical suspicion.