Difference between Keratoconus and Pellucid Marginal Degeneration

Progressive eyesight deterioration can occur due to keratoconus and pellucid marginal degeneration (PMD). While both are considered cornea ectatic disorders, PMD is characterized by slightly different cornea abnormalities, and does not plateau at around age 40 as typical for keratoconus. According to the American Academy of Ophthalmology, 10 percent of PMD cases are associated with keratoconus. The following describes the similarities and differences between keratoconus and PMD.

Diagnosing Pellucid Marginal Degeneration

Inferonasal or inferotemporal steepening of the cornea is the abnormality typically revealed by tomography in keratoconus-afflicted people. In contrast, tomography in a person with solely PMD typically reveals inferior steepening (at the four o’clock and eight o’clock positions) with superior flattening. While keratoconus often presents with symptoms in adolescence, PMD symptoms often do not appear until between age 30 or later. For this reason, a keratoconus-afflicted person is more likely to be diagnosed at an earlier age than a person afflicted with Pellucid Marginal Degeneration.

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What is Keratoconus?

The hallmark of keratoconus is a cornea that is cone-shaped rather than dome-shaped (per the Mayo Clinic). As the “bulge” progresses over time, the cornea thins – and this can produce both worsened nearsightedness and corneal pain. In addition, keratoconus causes an astigmatism (eyesight problem of blurriness) that worsens over time as the abnormal corneal elongation progresses. An article in the Journal of Ophthalmology notes that first-degree relatives of someone with keratoconus are at much higher risk for keratoconus than the general public. However, a recognized hereditary link does not exist for most people diagnosed with keratoconus. Meanwhile, a research article published in 2018 reported that evidence has not shown any hereditary link to PMD. Unlike keratoconus, PMD is almost always painless.

Why LASIK Surgery is Not Recommended for Keratoconus- or PMD-Afflicted People

According to the American Refractive Surgery Council (ARSC), around 20 percent of all people who seek LASIK surgery are ineligible for this type of surgery that corrects myopia (nearsightedness). The majority of those people deemed ineligible have a cornea ectasia disorder such as keratoconus or PMD. The reason is that a potential complication of LASIK surgery is cornea ectasia, so this can worsen keratoconus and/or PMD symptoms instead of correcting them. There are corrective surgical options for people with keratoconus and/or PMD, which are considered if contact lens wear does not resolve the myopia and astigmatism (or the corneal pain due to keratoconus is persistent).

Corneal Cross-Linking for Keratoconus or PMD

This outpatient surgery involves the use of ultraviolet (UV) light and specific eye drops inclusive of Riboflavin to strengthen the weakened corneal fibers. In this way, the progression of keratoconus and PMD can be halted. For people with keratoconus, corneal cross-linking surgery often occurs following intracorneal ring segment surgery (placement of INTACS) that has not been sufficient to halt the progression of the corneal “bulge”.

Two surgical options considered prior to cross-linking surgery are “epithelium-on” and “epithelium-off” (which refers to the outermost layer of the cornea). Although infrequent, three potential risks of this minimally-invasive procedure are:

  • Corneal infection;
  • Cornea (stromal) scarring;
  • Corneal striae (appearance of fine, white lines in field of vision)

For people diagnosed with PMD but not keratoconus, corneal cross-linking resulted in improvement of visual acuity (per an article in Advanced Biomedical Research). Corneal cross-linking does not reverse vision loss in keratoconus-afflicted people, but has been found to halt corneal pain as well as overall progression of this disorder.

Rigid Gas Permeable (RGP) Contact Lenses

The most common treatment to correct eyesight in people with mild-to-moderate keratoconus and/or PMD are RGP contact lenses, since this type of contact lens can correct both myopia and astigmatism. However, keratoconus-afflicted people with cornea pain may be unable to tolerate small-diameter contact lenses (such as RGP lenses) since these rest on the cornea. According to an article in the British Journal of Ophthalmology, a bitoric contact lens design was found to be the most successful for improving eyesight in PMD-afflicted people. In contrast, this is not as often found to be successful for correcting astigmatism in keratoconus-afflicted people since the astigmatism linked to keratoconus is usually an “irregular” astigmatism.

Consider visiting one of the locations of the Precision Keratoconus Center to learn more about your treatment options if you have keratoconus or are co-disordered with both keratoconus and PMD.

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