Still, many patients with early and moderate disease can be rehabilitated visually with spectacles or contact lenses, the latter being superior for correction of very high cylinders. In general, the treatment of PMCD is difficult due to advanced thinning and severe protrusion. Management modalities can be classified as conservative/visual rehabilitative treatment and surgical treatment. The pachymetry of PMCD patients shows a reversal from normal i.e., thinner in the periphery (usually inferior) and thicker in the center.Īnterior segment optical coherence tomography and Scheimpflug images might help to assess the thinnest location in cases where points of minimum thickness cannot be picked up by topography.

However, sometimes, these can also be observed in advanced keratoconus hence emphasis must be laid on clinical and pachymetric correlation for a definitive diagnosis. The elevation maps typically show a hot spot (area of elevation) in the inferior periphery. In PMCD, inferior peripheral steepening extending into the mid-peripheral, inferior oblique corneal meridians results in a characteristic “crab-claw,” “butterfly,” or “kissing doves” appearance on the curvature/keratometric map on topography. With a better evaluation of posterior corneal morphology, these diagnostic modalities have made early detection of ectatic disorders possible. Topography machines have evolved from the older videokeratoscopy to new generation slit scanning and Scheimpflug imaging technologies. The gold standard investigation for PMCD and other corneal ectasias is corneal topography. In superior PMCD, a narrow band of 1 to 2 mm (ectasia below thinnest location), located around 1.5 to 2 mm below the superior limbus extending from the 10 o'clock to the 2 o'clock position is seen. Atypical locations in the form of superior, nasal, and temporal PMCD have also been documented in the literature. The presence of Munson or Rizzuti signs may be variable but is not characteristically associated with PMCD.Īlthough PMCD is usually found bilaterally, unilateral cases have also been reported. Concentric Descemet's folds like Vogt's striae have been seen in a few patients. Between the thinned out band and the limbus, there is usually a 1 to 2 mm wide area of uninvolved, normal cornea. As opposed to keratoconus, the area of maximal ectasia lies above the thinnest location giving the cornea an appearance of a beer belly when visualized from the side. Usually, the entire cornea is clear, and no vascularization, Fleischer ring, lipid deposits, or changes in corneal sensitivity are observed. Rarely on presentation, these patients can report a sudden, painful red eye with watering accompanied by a sudden drop in vision and photophobia due to acute hydrops and/or spontaneous corneal perforation.Įxamination reveals stromal thinning of the periphery of the cornea, extending mostly from 4 o'clock to 8 o'clock, in a crescent shape, and has a slow and progressive development. The high against-the-rule astigmatism stems from a paradoxical steepening at 90 degrees. The steepening and ectasia occur at the junction of affected and unaffected tissue, leading to the typical high cylindrical loop. The flattening in the vertical meridian is due to tissue loss and a thin stroma in a crescentic pattern. On evaluation, high irregular, against-the-rule astigmatism is commonly found. But best-corrected visual acuity becomes poor only in advanced stages. Most commonly, a gradual, progressive diminution in vision or longstanding poor visual quality is the presenting feature of these patients. Coexistent retinitis pigmentosa, chronic open-angle glaucoma, scleroderma, retinal lattice degeneration, eczema, and hyperthyroidism have been observed in some patients with PMCD, but no direct causative association has been found to be true. Also, floppy eyelid syndrome found in obese patients may lead to chronic mechanical rubbing of the cornea leading to thinning and ectasia.Ītopic and vernal keratoconjunctivitis has been linked in reports to superior PMCD. The repeated occurrence of hypoxic conditions in these patients may be hypothesized to lead to anaerobic glycolysis and stromal acidosis, and in turn, promote transcription of proinflammatory cytokines, tumor necrosis factor-alpha or interleukin-6, leading to corneal thinning.

Obesity and obstructive sleep apnea have been associated with PMCD in the literature. Similar to keratoconus, electron microscopy of the cornea in PMCD reveals abnormally spaced collagen fibers with a periodicity of 100 nm to 110 nm, as opposed to 60 nm to 64 nm found in normal corneas.

Pellucid marginal degeneration ectasia