The retina is the light-sensing tissue in the back of the eye. It contains both rods which allow us to see at night, and cones which respond to color. Together these cells allow us to see the world around us in both color and black-and-white.
Mutations in a large number of genes affect the function and health of these cells of the retina and cause disorders such as colorblindness and night blindness. Some gene alterations impact cones and others affect the rods. In many instances, both rods and cones are impacted and that is the case for a group of disorders known as cone-rod dystrophies.
Cone-rod dystrophies begin with damage to the cones and early symptoms include some loss of color perception, a mild decrease in vision, and sensitivity to light (photophobia). Patients often become symptomatic in the first decade of life. Many individuals have some fine involuntary movements of the eyes called nystagmus (dancing eyes). Later, sometime after the age of 20, the rods become damaged as well and patients note increasing difficulties seeing in dim light. The field of vision may contain patches of seeing and non-seeing. Later, side vision becomes constricted sometimes to the point where activities such as walking become seriously impaired. These are progressive disorders and symptoms worsen with age so that many patients eventually become legally blind, usually after the age of 50 and sometimes sooner.
Mutations in at least 17 genes have been associated with cone-rod dystrophies. Some are located on the X chromosome, others require the presence of two mutations and still others cause disease when only one mutation is present. Some types of cone-rod dystrophies occur in association with other disorders but many only lead to eye symptoms.
Your eye doctor can often determine the pattern of inheritance in your family and perhaps determine the risk of recurrence in others.
Only an ophthalmologist can diagnose a cone-rod dystrophy but he or she may suggest a general evaluation to rule out other diseases. A visual field test is usually done along with a complete eye examination to check the appearance of the retina and optic nerve which often show changes. An electroretinogram (ERG) may be done to determine the degree of retinal dysfunction and the type of rod-cone dystrophy present.
There is no treatment for this group of disorders. However, low vision aids are useful in early stages and later when vision becomes worse, mobility training can be of immense help. Some people benefit from red-tinted glasses to reduce the sensitivity to light and may improve vision somewhat.