Cone-rod dystrophy (CRD) is a progressive retinal degenerative disease that causes deterioration of the cones and rods in the retina and frequently leads to blindness. Cone-rod dystrophy is also accompanied by amelogenesis imperfecta, an abnormality affecting the teeth.
Cone-rod dystrophy is characterized by all of the following elements: skin pigmentation abnormality; involuntary, rhythmic movements of the eyes (nystagmus); degeneration of vision (optic atrophy); and sensitivity to light (photophobia).
Cone-rod dystrophy can be inherited as either an autosomal dominant or autosomal recessive trait. In its most common form, however, it is usually inherited as an autosomal recessive trait, which means that both parents have one copy of the cone-rod dystrophy gene but do not have the disease. Autosomal recessive cone-rod dystrophy (arCRD) is a genetically heterogeneous disease with changes (mutations) in the ABCR gene. These mutations cause an abnormality in rod outer segment function that ultimately leads to dysfunction or death of the photoreceptor cells in the retina.
The CRX gene has been shown to contain mutations that cause an autosomal dominant form of cone-rod dystrophy. This means that only one parent has to pass on the gene mutation in order for the child to be affected with the disease. This genetic form of CRD is clinically known as CORD2, or cone-rod dystrophy 2. Mutations in the CRX gene interfere in the development process of embryonic photoreceptor cells during the early stages of life. The result is abnormal photoreceptor cells with reduced function.
Inherited retinal degeneration dystrophies have an incidence of approximately one in 4,000 people. Conerod dystrophy is an uncommon entity. The prevalence is estimated to be in the range of one in 10,000 to one in 100,000.
Signs and symptoms
The earliest symptom of CRD is loss of night vision that usually begins after the age of 20. The vision loss is progressive and unrelenting. Over the next decade, loss of all vision begins and by age 50, most people with cone-rod dystrophy have gone completely blind.
Cone-rod dystrophy is occasionally accompanied by amelogenesis imperfecta, which is characterized by abnormally shaped teeth and abnormalities in the tooth enamel.
The earliest symptom of cone-rod dystrophy is decreased visual acuity. However, the diagnosis of conerod dystrophy is usually established with loss of the peripheral visual fields. Cone-rod dystrophy must be distinguished from retinitis pigmentosa (RP). In CRD, rods and cones are lost at approximately the same rate. It is further distinguished from RP by the absence of night blindness as a presenting symptom.
Treatment and management
As of 2001, there are no known treatments or cures for cone-rod dystrophy. It has been suggested, however, that people with cone-rod dystrophy may be able to slow the progression of their blindness by wearing sunglasses and avoiding bright light.
Studies of individuals thought to have cone-rod dystrophy reveal that central vision loss begins in the first decade of life with the onset of night blindness occurring sometime after age 20. Little visual function remains after the age of 50. There is no cure for this syndrome.
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Downes, Susan M., et al. "Autosomal Dominant Cone and Cone-Rod Dystrophy With Mutations in the Guanylate Cyclase Activator 1A Gene-Encoding Guanylate Cyclase Activating Protein-1." Archives of Ophthalmology 119, no. 1 (2001): 96–105.
Foundation Fighting Blindness: <http://www.blindness.org/html/science/wcord2.html>.
Retina Foundation of the Southwest. <http://www.retinafoundation.org/eyeinfo2.html>.
Southeastern Eye Center. <http://www.southeasterneyecenter.com/cases/bulls_eye.htm>.
L. Fleming Fallon, Jr, MD, DrPH