Cone-Rod Dystrophy with Hearing Loss
Patient Information
Characteristics
Background and History
This recently described condition involves the retina of the eye and the nervous system component of the hearing mechanism.
Clinical Correlations
Symptoms usually have their onset in childhood with a reduction in hearing acuity and difficulty seeing at night. Both hearing and visual signs and symptoms are progressive and older individuals may have severe visual handicaps. Color vision acuity is also reduced. Patients especially notice difficulty seeing well in highly illuminated environments. Irregular eye movements (nystagmus) may be present.
The degree of hearing loss is relatively mild and is due to progressive damage to the nervous system of the inner ear.
Genetics
Inheritance
This is an autosomal recessive disorder which requires mutations in both copies of a specific gene, one inherited from each parent. The clinically normal parents have a single mutation but can expect that 25% of the offspring will inherit this condition when both parents carry the mutation.
Pedigree
Autosomal recessive
In order for autosomal recessive disorders to be expressed, offspring generally must inherit two mutations, one from each carrier parent. Carriers with only one mutation, such as the parents, do not have clinical disease. Note that carrier parents can expect that 1 in 4 children (25%) will inherit both mutations and have the disorder, 2 in 4 children (50%) will be carriers like their parents, while 1 in 4 children (25%) inherit neither mutation.
In order for autosomal recessive disorders to be expressed, offspring generally must inherit two mutations, one from each carrier parent. Carriers with only one mutation, such as the parents, do not have clinical disease. Note that carrier parents can expect that 1 in 4 children (25%) will inherit both mutations and have the disorder, 2 in 4 children (50%) will be carriers like their parents, while 1 in 4 children (25%) inherit neither mutation.
Diagnosis and Prognosis
Since both hearing and seeing seem normal at birth it is unlikely that this condition will be diagnosed at birth. There are many inherited forms of hearing loss and progressive vision loss, so a clinically complete examination by ophthalmologists and hearing specialists is important to rule out other conditions. Specialized testing should include an ERG (electroretinogram) to document the damage to the rods and cones in the retina, a visual field examination, and a detailed examination of the retina.
There are no physical deformities and the impact on longevity is unknown. There is no known treatment for the overall condition but hearing aids and tinted glasses should be helpful. Low vision aids may be especially helpful for educational purposes.
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