Pellucid Marginal Degeneration

Pellucid Marginal Degeneration (PMD), is a degenerative corneal condition, often confused with keratoconus. It is typically characterized by a bilateral thinning (ectasia) in the peripehry of the cornea, that is non-inflammatory, but unilateral cases have been reported. The term was first coined in 1957 by the ophthalmologist Schalaeppi.

Incidence
Incidence of PMD is rare, but it is unknown how many cases occur each year, due in part to the confusion of the condition with keratoconus.

Symptoms
Unlike keratoconus, pain is not normally present, and aside from the visual deterioration, no symptoms accompany the condition. Normally, PMD does not present with vascularization of the cornea, scarring, or any deposits of lipid.

Signs and diagnosis
Upon examination by an ophthalmologist, the patient exhibits normal corneal thickness in the center, with an intact central epithelium, but the inferior cornea exhibits a peripheral band of thinning.

Epidemiology
No identifiable factors have been found to link to race, sex, or age, but most cases present early in life, in the age group of 20-40. No known cause for the disease has been found.

Prognosis
Visual function declines as a result of the irregular corneal shape, resulting in astigmatism, and causing a distortion in vision. Deterioration can become severe over time.

Like keratoconus, PMD rules a patient out for refractive surgery. Due to the thinning of the cornea, procedures such as LASIK and PRK would make the patient a poor candidate.

Contact lenses
Visual acuity can not usually be corrected with the use of corrected lenses, but success has been shown with the use of rigid contact lenses combined with over-refraction. Patients wearing contacts report increased problems with glare and contrast sensitivity, but it is not clear if this is due to the corneal disease, or the contact lenses themselves.

Intacs
The use of intacs implants has been tested as a treatment for PMD, with slight improvement in visual acuity noted after eleven months, and intacs have been used with keratoconus with success.