OMIM ID:
Retinitis Pigmentosa, Hearing Loss, Ataxia, Cataract, and Polyneuropathy
Alternate Names
Defective Genes
Clinical Characteristics
Ocular Features
Cataracts and a pigmentary retinopathy occur in this condition but only in some, primarily older, patients. The lens opacities progress and may become visually significant by the third decade. Bone-spicule-shaped pigment clumping may be present in the midperiphery while the optic disk is often pale and the retinal vessels are attenuated. The ERG responses are consistent with a rod-cone dystrophy.
Systemic Features
This is a progressive neurological disorder with onset of signs and symptoms in childhood although full expression may not occur until adulthood. Young children can have hyporeflexia, pes cavus, spasticity, and gait ataxia. A sensorineural hearing loss may also be present in childhood but sometimes not until later. Hyperreflexia with extensor plantar responses and Achilles tendon contractures are often present later. The peripheral polyneuropathy is predominantly demyelinating with both sensory and motor components and is present in all adults. Cerebellar atrophy, primarily in the vermis, can be demonstrated on MRI examination. Mental function is usually not impaired. Some patients have dysarthria.
This disorder has some clinical similarities to Refsum disease (266500).
Genetics
Inheritance
This is an autosomal recessive disorder resulting from homozygous mutations in the ABHD12 gene (20p11.21).
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.