Usher Syndrome Type IV
Patient Information
Characteristics
Background and History
Usher syndrome is comprised of a group of diseases with a distinctive combination of hearing and progressive vision loss. It is named for a Scottish ophthalmologist, Charles Howard Usher, who recognized their hereditary nature although he was not the first to describe the condition. Type IV Usher Syndrome is the most recently described of the four known forms.
Clinical Correlations
Five patients from three Yemenite Jewish families have been described. A progressive neurosensory hearing loss with onset in midlife together with night vision difficulties and a slow deterioration of visual acuity is characteristic. The peripheral field of vision is narrowed in older individuals and the appearance of the retina resembles a disorder known as retinitis pigmentosa with narrowing of the retinal vessels and pigmentary changes.
An ERG (electroretinogram) documents the progressive degeneration of the retinal photoreceptors known as rods and cones.
Genetics
Inheritance
This condition follows an autosomal recessive pattern of inheritance. Mutations in both members of a specific pair of genes are responsible. Each clinically normal parent contributes one mutated gene and two such parents can expect that 25 per cent of their offspring will have this condition.
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
Symptoms of visual and hearing difficulties may have their onset at any time but usually not until midlife and no general health problems are known to be associated. Longevity is not impacted. Low vision devices and corrective lenses with special tinting can be helpful, especially in school-age children.
Hearing loss is neurosensory in etiology and may require cochlear implants for improvement.
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