H18.711-713 Pellucid Marginal Degeneration

Pellucid marginal degeneration (PMD) is a non-inflamatory thinning of the peripheral cornea.  It is usually bilateral, but is often asymmetric.  The area of corneal thinning is usually inferior and the distribution of the corneal degeneration is arcuate-shaped like a “crab’s claw” or “kissing dove” appearance.

Corneal Topography Characteristics Abnormal corneal shape Vertical flattening superiorly Horizontal steepening centrally Vertical steepening with horizontal flattening inferiorly Inferior corneal steepening extending from 4 to 7 o’clock positions Inferior area of corneal ectasia-shaped like a “crab’s claw”

Pellucid marginal degeneration is an idiopathic corneal disease that is not associated with any systemic conditions.

Structural Damage to the Eye

• Possible increase in mucopolysaccharides in the corneal stroma
• Thinning of the cornea in the peripheral region
• The area of thinning is usually 1-2 mm wide and 1-2 mm from the limbus
• Steepening of the cornea (corneal ectasia) usually occurs in the inferior region 
• Corneal protrusion may occur just above the area of corneal thinning
• Breaks in the posterior cornea lead to severe stromal edema – although rare (acute corneal hydrops)
• Breaks in the anterior cornea lead to corneal perforation (very rare) 

Functional Damage to the Eye 

• Decreased vision
• Glare
• Halos around lights
• Monocular diplopia
• Ghost images
• Multiple unsatisfactory attempts in obtaining good visual acuity with eyeglasses or contact lenses

The main goal of the diagnostic evaluation in a patient with pellucid marginal degeneration is to accomplish the following:

1.  Determine the correct diagnosis with corneal topography 

• Corneal topography reveals an arcuate-shaped area of corneal ectasia
• Central corneal region measures normal thickness
• Inferior,  peripheral corneal region reveals corneal thinning

2.  Determine if the patient can function with eyeglasses

3.  If necessary, prescribe contact lenses to improve visual function

4.  In rare cases of significant progression, prescribe surgical treatment to improve visual function

Patient History

Patients with pellucid marginal degeneration may present with any of the following signs and symptoms:

• None
• Photophobia
• Halos around lights
• Monocular diplopia
• Variable and inconsistent refractions
• History of multiple spectacle prescriptions in a short period of time
• History of unsuccessful contact lens wear 
• Complaints of distortion rather than blurred vision and both distance and near

External Ocular Examination with Biomicroscopy

Clinical Appearance of the Cornea No loss of stromal transparency Distorted retinoscopic reflexes Increase in curvature of the slit lamp beam inferiorly indicates the area of corneal ectasia

• Document the shape and physical features of the anterior corneal surface
• Identification and follow-up for pellucid marginal degeneration
• The distribution of the degeneration is arcuate or crescent-shaped
• Corneal thinning results in advanced against-the-rule astigmatism
  

Corneal Pachymetry

• Measurement of corneal thickness
• Pellucid marginal degeneration is suspected in any cornea where the peripheral corneal region is thinner than the central region

There is no classification system in place for pellucid marginal degeneration.

The main differential diagnosis with pellucid marginal degeneration lies in its similarity to keratoconus.  The following table marks the major differences between the diseases.

Treatment with Eyeglasses

• Patients with mild pellucid marginal degeneration may obtain functional vision with eyeglasses
• As the disease progresses, patients may need frequent changes in their eyeglass prescriptions

Treatment with Contact Lenses 

• Soft contact lenses
• Gas permeable lenses
• Hybrid lenses
• Scleral lenses
• Kerasoft IC contact lenses
• Piggyback lens system

		Pellucid Marginal Degeneration 50-year-old woman 20/400 acuity with best eyeglass prescription 20 year history of part-time contact lens wear
		TruPMD Gas Permeable Contact Lens TruPMD from Truform Optics is designed so that the inferior portion of the lens can be steepened and prism ballasted to align with the flat and steep zones of the cornea “Flat/Steep” lens design is used for corneal irregularities that vary significantly in curvature from one quadrant to the other
		Intra-Limbal Gas Permeable Lenses Slightly larger than traditional gas permeable lenses (approx. 11.2 mm diameter) Better comfort by reducing contact lens edge stand-off Eliminate contact lens displacement and/or expulsion Patients will have contact lens awareness with an incorrect base curve or diameter The base curve on this diagnostic lens is too flat
		Intra-Limbal Gas Permeable Lenses Evaluate the cornea-contact lens relationship based on fluorescein pattern, retinoscopic reflex, lens movement, and lens orientation The base curve on this diagnostic lens is 0.2 mm steeper than in the previous image

1.  Rasheed K.  Pellucid marginal degeneration.  July 2012.  http://emedicine.medscape.com/article/1196382-overview.  Last accessed March 30, 2014.