They someimes present with acute corneal hydrops and pain or acute decrease in vision, but this is relatively rare with only a few case reports. Patients with PMD typically present asymptomatically except with progressive visual deterioration despite spectacle correction due to the irregular astigmatism. The diagnosis is made clinically as patient's are usually asymptomatic except for progressive visual deterioration. Pellucid gets its name from the meaning "transparent" as the cornea typically appears transparent in this degeneration. Given the corneal thinning and the potential association of eye rubbing with keratoconus and its relationship to PMD, it could be considered to advise patients not to rub their eyes. No preventative strategies exist for PMD. The thin, weakened cornea is hypothesized to protrude as a result of intraocular pressure. The exact pathophysiology of PMD has not been ascertained, but it is thought to be secondary to collagen abnormalities, similar to keratoconus. FLS collagen has also been observed in advanced KC. In cases of acute hydrops, breaks in Descemet's membrane with swelling of the stroma and inflammatory infiltrate is seen. Electron microscopy of the thin regions reveals unusual electron-dense areas of fibrous long-spacing (FLS) collagen with a periodicity of 100 to 110 nm, whereas normal collagen has a periodicity of 60 to 64 nm. Lipid deposits are typically absent and stromal ground substance rich in mucopolysaccharides is present. Classically, PMD is histologically thought to show an area of stromal thinning, normal epithelium, endothelium, and Descemet's membrane, and absent or broken (focal disruption) Bowman's membrane. Histopathologic abnormalities in PMD resemble keratoconus. A recent case showed that PMD was coexistent with corneal plana in an individual with a KERA mutation, thus, potentially implicating KERA in the development of PMD. PMD has no sex or racial predilection, and does not appear to be hereditary, but moderate to high astigmatism has been noted in families with affected patients. PMD typically presents in the second to fifth decade of life. It has not been elucidated whether keratoconus, keratoglobus, and PMD are different diseases or phenotypic variations of the same disease.
#PELLUCID MARGINAL DEGENERATION ECTASIA SERIES#
In series performed in Japan, 17 of 27 cases of unilateral PMD revealed keratoconus or suspected keratoconus in the fellow eye. Ten percent of PMD cases are associated with keratoconus and 13% are associated with keratoglobus. Keratoconus, keratoglobus, and PMD are postulated to be related because these conditions coexist in families. Some people postulate, however, that this prevalence might be underestimated as the condition is often misdiagnosed as keratoconus. PMD, while rare, is the second most common noninflammatory corneal thinning disorder after keratoconus. Also, patients who undergo refractive surgery with PMD can have deleterious results, so it is important to consider prior to refractive surgery. It is important to recognize as it can cause severe deterioration in visual function. PMD is disease of peripheral corneal thinning that is slowly progressive over many years. There is no associated inflammation and the central cornea is of normal thickness. This patient will never need to undergo corneal transplant surgery.PMD is uncommon and is characterized by a crescent-shaped band of inferior corneal thinning approaching 20% of normal thickness that is 1 to 2 mm in height, 6 to 8 mm in horizontal extent, and 1 to 2 mm from the limbus. In addition he has been able to wear his lenses every day with all day lens wear comfortably. His vision has remained at 20/20 with his GVR lenses. Every year this patient has returned to our office for yearly examinations, the latest being today.
In 2008, we fit this patient with GVR Scleral lenses which have provided this gentleman with 20/20 vision in each eye. He was also unable to obtain functional vision with eyeglasses. Over these years he tried many different types of contact lenses without success. A number of doctors told him that his only option was to have corneal transplant surgery in both eyes. Over the years he visited a number of major eye institutions and clinics throughout the United States trying to get help.
When this patient first visited our office, his visual acuity was 20/800 in each eye. Note in the photo below the protrusion or bulging of the upper portion of this cornea. This patient suffers from a very rare form of keratoconus known as “Superior Pellucid Marginal Degeneration.” This type of corneal ectasia affects the upper half of both of his corneas. The patient in the photo with me lives in the Bahamas and first visited our office in 2008.
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