Friday, March 18, 2011

Cone-rod Dystrophy

Cone-rod dystrophy (CRD) is a genetical progressive disease which causes deterioration and death of the retina's cone and rod photoreceptor cells. This eye condition usually leads to early impairment of vision, and blindness in the last stages of the disease. Although cone-rod dystrophy can be found as an autosomal dominant trait, it is usually acquired as autosomal recessive. In extreme cases, cone-rod dystrophy is accompanied by retinal pigmentation and chorioretinal atrophy of the central and peripheral retina.

Symptoms of cone-rod dystrophy are decreased visual acuity in the early stages followed by loss of peripheral vision and poor color vision. The disease is similar to retinitis pigmentosa in this way, but there is no loss of night vision, the rate of rod and cone degeneration is equal, patterns of visual field loss are different, and the rate of rod ERG loss is significantly lower in CRD.

Unfortunately, there is currently no treatment for cone-rod dystrophy. Nevertheless, researchers have identified three genes which may eventually provide answers. Two of these are the CXR and ABCR genes. Mutations in the CXR gene cause interference in the development of embryonic photoreceptor cells (Cell, November 1997). Mutations in the recently-discovered ABCR gene (Allikmet et al.'97 Nat. Gen. 15:236-246) lead to lipofuscin accumulation in the retinal pigment epithelium (RPE). This buildup of fatty waste deposits in the RPE (as in Stargardt's disease) eventually starves the photoreceptor cells (Cell, July 1999).

The fundus exam via ophthalmoscope is essentially normal early on in cone dystrophy, and definite macular changes usually occur well after visual loss. Fluorescein angiography (FA) is a useful adjunct in the workup of someone suspected to have cone dystrophy, as it may detect early changes in the retina that are too subtle to be seen by ophthalmoscope. For example, FA may reveal areas of hyperfluorescence, indicating that the RPE has lost some of its integrity, allowing the underlying fluorescence from the choroid to be more visible. These early changes are usually not detected during the ophthalmoscopic exam. The most common type of macular lesion seen during ophthalmoscopic examination has a bull’s-eye appearance and consists of a doughnut-like zone of atrophic pigment epithelium surrounding a central darker area. In another, less frequent form of cone dystrophy there is rather diffuse atrophy of the posterior pole with spotty pigment clumping in the macular area.