Pellucid marginal degeneration (PMD)

Pellucid marginal degeneration, or PMD, is often confused with keratoconus, and indeed it is a type of irregularity affecting the cornea which results in visual impairment. PMD is caused by a thinning of a narrow band of the cornea between 4 and 8 o’clock, approximately 2mm in from the edge of the cornea and the white of the eye. 

The position of this arc of thinning means that the cornea tends to bulge out at the bottom, rather like a “beer-belly”. Because of the position, vision can sometimes be corrected quite well in the early stages with spectacles. As the condition progresses it becomes more difficult to correct, and the best option is contact lenses. These too can be difficult to fit as the bulge tends to occur lower down than in traditional keratoconus, which makes it difficult to fit a lens that bridges the bulge and still manages to rest on the white of the eye beneath.

Specialist manufacturers can now make lenses that are more curved in one meridian, to match this difference in curvature. These “quadrant-specific” design lenses are available as regular gas permeable or scleral designs. Soft lenses may be used in the early stages of the disease but as the condition develops gas permeable and scleral lenses tend to give better results.

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