Pellucid Marginal Degeneration, Differential Diagnosis Of Keratoconus

Pellucid marginal degeneration is a progressive, non-inflammatory disease of the lower cornea. It is often confused with keratoconus due to clinical, histological and topographical similarities. 

The aim of our work is to study the clinical and topographical features of the disease, its differential diagnosis and therapeutic strategies through observation and a review of the literature.

We report the case of a 46-year-old female patient with a personal history of: 
      - a post-traumatic cataract operated 12 years ago on the right eye, and family history: 
      - a sister who had undergone transfixing keratoplasty

 the patient presented to ophthalmology with a 6-month history of progressive visual acuity loss in the right eye, with no associated redness or pain.

Clinical examination found:

reduced visual acuity at MDD of OD and 2/10 in OG. 
Biomicroscopy showed a band of cornea thinned inferiorly, separated from the limbus by a crescent of normal cornea, and a central cornea protruding above the thinning zone; its appearance was much more advanced at OD than OG.

The topography of the right eye shows corneal thinning, which predominates in the inferior position on the pachymetric map.
 Elevation maps show a corneal protrusion above the thinning zone. 
The axial map shows a flattening of the upper vertical hemi meridians. Anterior astigmatism of 17 D at 87° was noted for the OD, and 10 D at 93° for the OG. 
Treatment consisted in cross-linking the OD and inserting a scleral lens.

Diagnosis of the disease is primarily clinical: 
 - functional signs include decreased visual acuity or worsening of significant irregular astigmatism, 
 - Biomicroscopy shows a band of thinned inferior cornea.

Topography confirms the diagnosis, and distinguishes MPD from other non-inflammatory ectatic diseases of the horn, such as keratoconus, posterior keratoconus and keratoglobe, by classically showing a very peripheral inferior ectasia with a "crab claw" appearance.

Treatment is based on : 
     - in the early stages, glasses, contact lenses or intra-corneal rings. 
     - In advanced stages, more invasive treatments are required, such as corneal resections and transplants.

PMD is an idiopathic condition, rarely unilateral and responsible for irregular astigmatism. 
Corneal topography plays a key role in its diagnosis. 
Several surgical procedures have been used to try and improve visual acuity when glasses and contact lenses fail to provide adequate visual correction.