These electron-dense areas were found to be islands of fibrous long spacing (FLS) collagen with a 100 to 110 nm periodicity in a sea of normally spaced collagen bundles (60 to 64 nm periodicity). Electron microscopic studies have shown the presence of irregularly arranged stromal collagen bundles along with the presence of extracellular, granular electron-dense deposits. Descemet's membrane might show folds occasionally. An increase in stromal mucopolysaccharides and stromal thinning has been reported. Histopathological studies report an absent or irregular Bowman's membrane with breaks. There also seems to be a co-existence of PMCD and keratoconus in about 10% of patients. Although there is no evidence of genetic inheritance of PMCD, a few authors have reported high astigmatism seen in family members of these patients. Īge groups in the 2nd to 5th decade are usually affected, but isolated reports have been seen even in older individuals. It has been seen to be more common in males and affects all ethnicities with no geographical predisposition. The general consensus is that PMCD is a rare condition. Thus the exact incidence of PMCD is expected to be underestimated. Many authors have proposed that PMCD could be just a peripheral form of keratoconus. 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.
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