treatment
plan
Treating
PMD with Contact Lenses
BY
WILLIAM L. MILLER, OD, PHD, FAAO
Pellucid
marginal degeneration is a corneal degenerative ectasia. Although rare, a
prevalence rate for PMD is unknown. It's likely under-diagnosed owing in part to
its similarities with keratoconus.
It occurs mostly between ages 20 and 40 with no prevalence differences between gender
and race/ethnicity.
Mostly an inferior corneal disease,
one study found that a small percentage (14.7 percent) may demonstrate superior PMD. This increases the diagnosis complexity by mimicking, at least by location,
Terrien's marginal degeneration. PMD is usually characterized by a band of thinning
inferior to the steepest protrusion that's apparent with either ultrasound pachymetry
or a scanning slit topographer. Corneal topography will typically show a characteristic
mustache-like steepened against-the-rule
corneal pattern (Figure 1).
Unlike keratoconus, PMD doesn't
exhibit Vogt's striae or a Fleischer's ring. Keratoconus and keratoglobus can be
associated with PMD, with a small percentage showing other associated conditions
such as vernal keratoconjunctivitis and Marfan's syndrome. Histopathology has demonstrated
breaks and missing anterior limiting lamina similar to keratoconus patients.
Fitting PMD Patients
At the initial visit prepare
your patient for the sometimes arduous process of achieving a satisfactory fit.
Finding a lens that provides good vision and comfort is the goal and depends on
the severity of the PMD. Fit options include lenses for keratoconic patients, GP
lenses with and without special back surface designs, bitoric, bitoric aspherics,
piggyback lenses and hybrid designs such as the SynergEyes lens (SynergEyes, Inc).
GP lenses tend to position inferiorly, aligning with the placement
of the inferior steepened banded cornea. Liu et al (2005) reported on a reverse
geometry lens (Reverse Aspheric Orthofocus, RAOF) to achieve satisfactory fit and
vision. Others use large diameter lenses to increase lens/lid interaction and move
the lens superiorly. Most patients wear lenses with good visual outcomes and sufficient
lens tolerance.
A 2003 study by Raizada and Sridhar
indicated that like keratoconus, the initial base curve depends on the disease severity.
Base curves in this cohort ranged from 7.3mm to 6.3mm, the latter representing more
severe PMD. They used an average of 3.5 diagnostic lenses to achieve a good fit.
Long-term Outlook
When contact lenses are
no longer a viable option, a peripheral lamellar crescent keratoplasty with simultaneous
or subsequent central penetrating keratoplasty may be performed. Intrastromal ring
segments also work by flattening the corneal protrusion and decreasing the irregular
astigmatism. Tzelikis et al found that surgical intervention is more likely in patients
who have a best corrected visual acuity of worse than 20/100 and a follow-up
period of greater than eight years.
Dr. Miller is
the Director, Cornea and Contact Lens Service, at the University of Houston College
of Optometry. He is a member of the American Optometric Association and serves on
its Journal Review Board. You can reach him at wmiller@uh.edu.
Contact Lens Spectrum, Issue: February 2007