Superior keratoconus with inferior paracentral corneal thinning and inferior peripheral pellucid marginal degeneration

Sir,

Herein is a description of an uncommon case of bilateral superior keratoconus with inferior paracentral corneal thinning and inferior peripheral pellucid marginal degeneration.

Case report

A 30-year-old male patient was presented with poor vision in both eyes. His visual acuity was 20/40 OD with plano — 6.0 × 30° and 20/100 OS with plano — 6.5 × 170°. Slit-lamp examination revealed a superior corneal protrusion and inferior paracentral corneal thinning in both eyes. An Orbscan, a three-dimensional slit-scan topography system, was utilized (Figure 1a and b). A well-defined superior steep area, with the rule astigmatism and an asymmetric bow-tie configuration, was noted (Figure 1a and b, left lower). The cone apex (the point of maximal reading on the anterior elevation map) was 1.8 mm above the corneal vertex on the 150 meridian in the right eye and 1.7 mm above the corneal vertex on the 25 meridian in the left eye (Figure 1a and b, left upper). The thinnest point of cornea was 309 μm, 2.3 mm below the corneal vertex on the 290 Meridian in the right eye and 311 μm, 2.2 mm below the corneal vertex on the 260 Meridian in the left eye (Figure 1a and b, right lower). There was disparity between the anterior and posterior corneal surface elevations (Figure 1a and b, left and right upper). The distance between the apex and the thinnest point was 3.57 mm in the right eye and 3.47 mm in the left eye. Superior keratoconus with inferior paracentral corneal thinning was diagnosed.

Figure 1

(a and b) The Orbscan topography in the right eye (a) and left eye (b). Disparity was noted between the anterior and posterior corneal surface elevations: anterior corneal elevation was located superiorly (left upper) and posterior corneal elevation was located inferiorly (right upper). The thinnest point was near the posterior elevation area (right lower). In keratometric map (left lower), there was a well-defined superior steep area, with the rule astigmatism and an asymmetric bow-tie configuration in upper $\frac{2}{3}$ cornea, and a relatively flat area surrounded by relatively steep circle in the lower $\frac{1}{3}$ cornea.

Moreover, below the paracentral corneal thinning, there is a bend and thick region and below the bend, there is a thin region near the limbus. The Orbscan showed there was a flat area surrounded by steep circle in the inferior peripheral zone of the cornea (Figure 1a and b, left lower). Hence, the patient was suspected with early pellucid marginal degeneration (PMD).

Discussion

The patient had three rare conditions in his eyes: superior keratoconus, a very long distance between keratoconus apex and the thinnest point; and two different corneal ectatic diseases in the same eyes. Although each condition was reported before, no case with the three conditions occurring simultaneously in the same eyes were reported.

The relationship between the apex and the thinnest point of keratoconus had been reported by Auffarth et al¹ that study, the apex (the highest) and the thinnest point were usually close, but sometimes there were large distances between them (0.917±0.729 mm, range 3.364–0.068 mm). In our case, the distance between the apex and the thinnest point (3.57 mm in the right eye and 3.47 mm in the left eye) is longer than reported, and the thinnest point was found to be nearly outside the cone.

There is a superior corneal ectasia with inferior paracentral corneal thinning in the patient's upper $\frac{2}{3}$ cornea. Corneal ectasia above the zone of thinning is characterized by PMD, so PMD and its superior variation were the most closely logical differential diagnosis in our patient's upper $\frac{2}{3}$ cornea. The superior variation of PMD described the superior corneal protrusion located below the superior and peripheral corneal thinning.^{2, 3, 4} The topography of PMD reveals against-the-rule astigmatism, inferior loop cylinder, and a unique butterfly pattern of horizontal sagging. In the patient, topographic findings demonstrate with the rule astigmatism and an asymmetric bow-tie configuration, and the thinnest point is located below the cone. Hence, in the patient’s upper $\frac{2}{3}$ cornea, it was superior keratoconus, and PMD or its superior variation was not likely.

However, there was quiet a different condition in the patient's inferior peripheral cornea. Under the slit-lamp examination, there is a bend below the inferior paracentral corneal thinning. Below the bend, there is a thin region near the limbus. Topography reveals a flat area surrounded by steep circle in the inferior peripheral zone of cornea. Hence, early PMD in the peripheral cornea was suspected in our case.

Keratoconus combined with PMD was reported before, but it is rare.⁵ Kayazawa et al⁵ has ever reported only 17 cases with PMD in 1625 Japanese keratoconus patients, and the prevalence in Caucasians is lower than that. In their series, no superior keratoconus with PMD was found.

There are other diseases that may result in superior corneal ectasia, but the patient did not have corneal inflammation, vascularization, scarring, or lipid deposition or other signs of Terrien's degeneration, Mooren's ulcer, or systemic autoimmune disease. The absence of senile arcus and juxalimbal corneal thinning also ruled out senile marginal degeneration. However, the patient was young and without ptosis and there was a real corneal thinning, so ptosis induced superior keratoconus like topographic appearance was unlikely.