Visual Rehabilitation of a PMD Patient

BY ROBERT CAMPBELL, M.D. & PATRICK CAROLINE, C.O.T., F.A.A.O.
MAY 1996

Pellucid marginal degeneration (PMD) is an uncommon, non-ulcerative thinning disorder of the inferior peripheral cornea. The term was coined by Schlaeppi in 1957 to describe a marked steepening of the inferior cornea superior to a narrow, clear band of corneal thinning concentric to the inferior limbus.

Classically, the band of thinning extends from the 4 o'clock to the 8 o'clock meridian. There is a 1mm to 2mm wide region of uninvolved normal cornea between the thinned zone and the limbus (Fig. 1). PMD occurs in both sexes, usually between the ages of 20 and 40. Subjective symptoms are strictly visual with significant increases in against-the-rule astigmatism and a loss of best corrected spectacle acuity secondary to irregular astigmatism.

FIG. 1: SLIT LAMP PHOTO SHOWS INFERIOR THINNING AND A 1-2MM ZONE OF NORMAL CORNEA BETWEEN THE THINNING AND THE LIMBUS.

PMD can manifest many of the same features as other ectatic conditions such as keratoconus and Terriens corneal degeneration, making diagnosis and differentiation difficult, especially in the early stages.

As with all ectatic conditions, contact lenses are the suggested mode of treatment for visual rehabilitation. Surgical intervention (a kidney-shaped penetrating keratoplasty or an inferior lamellar patch graft) has been successful in reducing the inferior thinning.

Our case involves a 31-year-old male with an eight-year history of keratoconus. (Many patients carry the diagnosis of keratoconus until the condition advances and the PMD slit lamp and topography findings become more evident.) Photokeratoscopy demonstrated elongation of the central mires consistent with high against-the-rule astigmatism. The close approximation of the inferior mid-peripheral mires indicated steepening just superior to the zone of peripheral thinning. Corneal mapping illustrated inferior mid-peripheral zones of corneal steepening at 4 o'clock and 8 o'clock (Fig. 2). This produced a "butterfly wing-like" pattern quite diagnostic of PMD. Central keratometric readings were: OD 48.25 @ 148/41.25 @ 58; OS 44.87 @ 178/40.75 @ 88. Manifest refraction: OD -0.75 -5.50 x 68, 20/40; OS -1.50 -3.50 x 73, 20/25.

FIG. 2: CORNEAL MAP SHOWS THE TYPICAL "BUTTERFLY WING" PATTERN ALONG THE STEEP 4 & 8 O'CLOCK MERIDIANS.

We used quantitative photokeratoscopy to determine the mid-peripheral corneal curvature. In this situation, the distance from the center of the keratograph to the inside edge of the ninth ring OD was 18.2mm, or 7.83mm curvature.

Our clinical experience has shown that the aspheric Boston Envision design provides the most acceptable lens fit on these highly asymmetric, against-the-rule corneas. Therefore, we prescribed Envision lenses, OD 7.80 -3.25 9.6, OS 8.00 -3.50 9.6 (Fig. 3).

FIG. 3: FLUORESCEIN PATTERN OF THE BOSTON ENVISION DESIGN ON THE PATIENT'S RIGHT CORNEA.

The lenses provided the typical fluorescein pattern seen in PMD patients (i.e., superior touch in the area of the flat superior cornea, and inferior and peripheral touch over the paracentral ectasia).

Final visual acuities were OD 20/30, OS 20/20, with a wearing time of 14 hours a day. CLS

Dr. Campbell is medical director of the Park Nicollet Contact Lens Clinic & Research Center, Minnetonka, Minn. Patrick Caroline is an assistant professor of optometry at Pacific University, Forest Grove, Ore., and director of contact lens research at Oregon Health Sciences University, Portland, Ore.