OMIM ID:
Usher Syndrome Type IV
Alternate Names
Defective Genes
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
Inheritance
Homozygosity of a missense mutation in the ARSG gene (17q24.2) is responsible for 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.