CLINICAL

  ANTERIOR

UNMASK PELLUCID MARGINAL DEGENERATION

HOW TO DISTINGUISH THIS CONDITION FROM KERATOCONUS

MISTAKEN IDENTITY is a theme popularized by movies, such as “North by Northwest,” a 1959 film starring Carey Grant. There are notable real-life examples of mistaken identity as well. In 1979, Father Bernard Pagano was charged with armed robbery. Seven witnesses identified him as the Gentleman Bandit, a robber described as regretful to victims. At his trial, former postal worker Ronald W. Clouser, who closely resembled Father Pagano, confessed he was the true Gentleman Bandit.

A corneal topography from a patient with PMD. Note the bowtie pattern with flattening of the cornea along a vertical axis, which extends into an area of inferior steepening on the topography. You can see what is referred to as a “claw shaped”, “crab claw”, or “doves kissing” pattern on topography as the steepening of the inferior corneal periphery extends into the mid peripheral inferior oblique corneal meridians.

Medicine has its share of mistaken identities. In eye care, pellucid marginal degeneration (PMD) is often mistaken for keratoconus. Both conditions create abnormal corneas that can lead to reduced vision and high amounts of astigmatism.

Here, I discuss how to differentiate PMD from keratoconus. This is particularly important in cases that require surgical intervention.

ETIOLOGY

PMD is the second most common non-inflammatory progressive corneal thinning disease behind keratoconus. As is the case with keratoconus, the condition can negatively affect one’s quality of life because it leads to very poor vision. PMD’s pathophysiology has not been determined, but like keratoconus, it is thought to be secondary to collagen abnormalities.

Some studies show that PMD is a variant of keratoconus, but PMD patients usually don’t present with the typical clinical signs of keratoconus, such as apical scarring, iron rings and Vogt’s striae. Also, keratoconus has more centralized thinning than PMD.

Corneal topography is crucial to identifying this dystrophy, as corneal refractive procedures, such as LASIK, are contraindicated in these patients.

The exact incidence and prevalence of PMD are unknown, probably because the condition is often misdiagnosed as keratoconus. In addition PMD does not have any racial or ethnic associations, it occurs equally between sexes, and patients are usually between the ages of 20 and 40 at the time of diagnosis.

SYMPTOMS

PMD is usually asymptomatic except for reduced VA and blur due to the ectasia causing irregular astigmatism.

CLINICAL SIGNS

• Peripheral band of corneal thinning in the inferior cornea from 4 o’clock to 8 o’clock, 1mm to 2mm from the limbus

• Epithelialized, clear avascular area of thinning without lipid deposition

• Corneal thickness less than 20% of the normal corneal thickness in peripherally thinned-out zones

• Bowtie pattern with flattening of the cornea along a vertical axis, which extends into an area of inferior steepening on corneal topography

• “Crab claw” or “doves kissing”pattern on corneal topography, as the steepening of the inferior corneal periphery extends into the mid peripheral inferior oblique corneal meridians

• Corneal hydrops (rare)

• Protruding cornea, resulting from normal and high IOP

MANAGEMENT OPTIONS

The treatment for PMD is similar to keratoconus:

• Refraction. Prescribe glasses or soft contact lenses to treat the refractive error and blur due to irregular astigmatism. As PMD progresses, increased corneal thinning will occur, causing growing amounts of blur from the irregular astigmatism. This blur can be tough to correct with a phoropter, yet these patients often have great pinhole acuity.

Once vision can no longer be corrected with spectacles or regular soft contact lenses, prescribe RGPs, piggyback or hybrid contact lenses. A caveat: RGP contact lenses used for keratoconus often won’t work for PMD patients because keratoconus corneas are steeper inferiorly vs. PMD, which has steepening of the inferior corneal periphery that extends into the mid peripheral inferior oblique cornea. Instead, prescribe a large diameter RGP because it will cover PMD’s larger abnormal corneal surface from more peripheral ectasia.

• Surgery. Refer patients for surgery when spectacles and contact lenses no longer provide good vision or patients complain of contact lens discomfort. Common surgical options include penetrating keratoplasty (PKP), intracorneal ring segments and corneal collagen crosslinking, all explained in last month’s column, in addition to deep anterior lamellar keratoplasty (DALK), toric posterior chamber IOL (PCIOL) and various combined and staged procedures, such as PKP/toric PCIOL.

With regard to PKP, a larger and more eccentric graft (9mm or greater) is needed when compared with a keratoconus patient receiving a donor tissue. This is because PMD causes ectasia that is more peripheral. Unfortunately, these larger grafts increase the rate of graft rejection and postoperative astigmatism, which can be tough to manage after surgery.

DALK removes the corneal stroma down to Descemet’s membrane. Surgeons like it because the recipient’s Descemet’s membrane and endothelium remain intact. That said, DALK can also decrease VA from opacification at the interface layers.

THAT WRONGLY ACCUSED PRIEST

Mr. Clouser went to prison for the robberies, and Father Pagano became an advocate for those wrongly accused of crimes. To ensure your patients aren’t the victims of mistaken identity, use this article to differentiate PMD from keratoconus. OM

JOSH JOHNSTON, O.D., F.A.A.O., practices at Georgia Eye Partners. He focuses on ocular surface disease and has extensive experience in comanaging cataract and refractive surgery patients. Email him at drj@gaeye partners.com, or visit tinyurl.com/OMcomment to comment.