vitreous liquefaction

The retina and vitreous are primarily affected in this disorder.  The age of onset is unknown but characteristic signs can be seen early in the second decade of life.   Early changes include thickening of the cortical vitreous and white dots in the superficial layers of the retina.  The latter are minute yellow-white crystalline deposits more common in the peripheral retina.  Many (83%) patients have early onset cataracts.  Corneal guttae are common (80% of patients).  The vitreous undergoes fibrillar degeneration with liquefaction and eventually appears optically empty.  Many patients experience symptoms of floaters.  The vitreous changes most closely resemble that seen in Wagner syndrome (143200) but with important differences.  In the latter disorder the vitreous changes are membranous, the retinal changes are deeper in location, RPE changes are evident, the choroid and RPE are involved, and the risk of retinal detachment is much higher.  Only 21% of patients with snowflake vitreoretinal degeneration have retinal detachments compared with about 50% in Wagner syndrome.  Retinal vasculature change such as perivascular sheathing and attenuation of arterioles may be seen in both disorders but occur far less commonly in snowflake degeneration.

Based on lack of visual symptoms, the photoreceptors are minimally involved.  Electrophysiologic studies reveal an elevated dark adaptation and reduced scotopic B waves.  Most patients retain excellent vision.  However, the optic nerve may have a waxy pallor and frequently appears flat and lacks a visible cup. 

Clinical features are variable in this ocular disorder. Small corneas and shallow anterior chambers have been described in some patients.  Chronic narrow angle glaucoma or frank angle closure glaucoma attacks may occur.  Microphthalmia has been reported but nanophthalmos has not been documented.  Presenile cataracts, nystagmus, and strabismus are sometimes present.  Some patients have normal vision but others have a severe reduction in acuity, even blindness.

The vitreous is often liquefied and some patients have a fibrillary vitreous with pleocytosis.  Preretinal white dots and neovascularization are often seen, even in children.  Peripapillary atrophy may extend to the macula which may have cystic edema.  Peripherally in annular fashion there is often a pigmentary retinopathy extending to an equatorial demarcation line at the posterior border.  The ERG is usually moderately abnormal with evidence of rod and cone dystrophy generally in older patients in which some degree of dyschromatopsia is often present.  Some patients demonstrate a concentric reduction in visual field that progresses with age.  A reduced light/dark ratio has also been documented in several families.  Retinal detachment is a risk.  A posterior staphylomas has been noted in a few patients.