photoreceptor degeneration on ERG

Usher Syndrome Type IV

Clinical Characteristics
Ocular Features: 

Nyctalopia is a complaint in adults in by midlife but most individuals deny visual symptoms prior to the 5th decade.  Perimetry reveals a ring-shaped scotoma extending from the paracentral area to the midperiphery (5 to 30 degrees).  Full-field ERGs show decreased photoreceptor responses by the 5th decade or late with the rods more severely affected than the cones.  Some dyschromatopsia is usually present.  Patients have a significant and progressive loss of visual acuity.  Ring-shaped areas of retinal pigment atrophy may extend from the pericentral area to the temporal arcades with relative sparing of the fovea early but older individuals have foveal degeneration as well.  The Arden EOG ratio is usually lower than normal.

Systemic Features: 

Five individuals in three Yemenite Jewish families have been described with this type of Usher syndrome.  A neurosensory hearing loss is present by midlife but no other systemic signs have been reported. 

Genetics

Homozygosity of a missense mutation in the ARSG gene (17q24.2) is responsible for this condition.

Pedigree: 
Autosomal recessive
Treatment
Treatment Options: 

No specific treatment for this condition is available.  However, assistive hearing devices such as cochlear implants and corrective lenses with appropriate tinting can be beneficial.

References
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