hyperreflexia

Retinitis Pigmentosa, Hearing Loss, Ataxia, Cataract, and Polyneuropathy

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

This is an autosomal recessive disorder resulting from homozygous mutations in the ABHD12 gene (20p11.21).

Pedigree: 
Autosomal recessive
Treatment
Treatment Options: 

Treatment is directed at symptoms.  Visually significant cataracts may require removal.  Low vision aids and physical therapy can be helpful.

References
Article Title: 

Mutations in ABHD12 cause the neurodegenerative disease PHARC: An inborn error of endocannabinoid metabolism

Fiskerstrand T, H'mida-Ben Brahim D, Johansson S, M'zahem A, Haukanes BI, Drouot N, Zimmermann J, Cole AJ, Vedeler C, Bredrup C, Assoum M, Tazir M, Klockgether T, Hamri A, Steen VM, Boman H, Bindoff LA, Koenig M, Knappskog PM. Mutations in ABHD12 cause the neurodegenerative disease PHARC: An inborn error of endocannabinoid metabolism. Am J Hum Genet. 2010 Sep 10;87(3):410-7.

PubMed ID: 
20797687

Sandhoff Disease

Clinical Characteristics
Ocular Features: 

Retinal ganglion cells are rendered dysfunctional from the toxic accumulation of intra-lysosomal GM2 ganglioside molecules causing early visual symptoms.  These cells in high density around the fovea centralis create a grayish-white appearance.  Since ganglion cells are absent in the foveolar region, this area retains the normal reddish appearance, producing the cherry-red spot.  Axonal decay and loss of the ganglion cells leads to optic atrophy and blindness. 

Systemic Features: 

Sandhoff disease may be clinically indistinguishable from Tay-Sachs disease even though the same enzyme is defective (albeit in separate subunits A and B that together comprise the functional enzymes).  The presence of hepatosplenomegaly in Sandoff disease may be distinguishing. The infantile form of this lysosomal storage disease seems to be the most severe.  Infants appear to be normal until about 3-6 months of age when neurological development slows and muscles become weak.  Seizures, loss of interest, and progressive paralysis begin after this together with loss of vision and hearing.  An exaggerated startle response is considered an early and helpful sign in the diagnosis.  Among infants with early onset disease, death usually occurs by 3 or 4 years of age.   

Ataxia with spinocerebellar degeneration, motor neuron disease, dementia, and progressive dystonia are more common in individuals with later onset of neurodegeneration.  The juvenile and adult-onset forms of the disease also progress more slowly.  

Genetics

Sandhoff disease results from mutations in the beta subunit of the hexosaminidase A and B enzymes.  It is an autosomal recessive disorder caused by mutations in HEXB (5q13). 

Tay-Sachs disease (272800) can be clinically indistinguishable from Sandoff disease and they are allelic disorders.  However, the mutation in Tay-Sachs (272800) is in HEXA resulting in dysfunction of the alpha subunit of hexosaminidase A enzyme. 

Pedigree: 
Autosomal recessive
Treatment
Treatment Options: 

No specific treatment is available beyond general support with proper nutrition and maintainence of airways.  Anticonvulsants may be helpful in some stages.  Gene therapy in fibroblast cultures has achieved some restoration of  hexosaminidase A activity in Tay-Sachs disease and may have potential in Sandhoff disease as well. 

References
Article Title: 

GM1 Gangliosidosis

Clinical Characteristics
Ocular Features: 

Based on clinical manifestations, three types have been described: type I or infantile form, type II or late-infantile/juvenile form, and type III or adult/chronic form but all are due to mutations in the same gene.  Only the infantile form has the typical cherry red spot in the macula but is present in only about 50% of infants.  The corneal clouding is due to intracellular accumulations of mucopolysaccharides in corneal epithelium and keratan sulfate in keratocytes.  Retinal ganglion cells also have accumulations of gangliosides.  Decreased acuity, nystagmus, strabismus and retinal hemorrhages have been described. 

Systemic Features: 

Infants with type I disease are usually hypotonic from birth but develop spasticity, psychomotor retardation, and hyperreflexia within 6 months.  Early death from cardiopulmonary disease or infection is common.  Hepatomegaly, coarse facial features, brachydactyly, and cardiomyopathy with valvular dysfunction are common.  Dermal melanocytosis has also been described in infants in a pattern some have called Mongolian spots.  Skeletal dysplasia is a feature and often leads to vertebral deformities and scoliosis.  The ears are often large and low-set, the nasal bridge is depressed, the tongue is enlarged and frontal bossing is often striking.  Hirsutism, coarse skin, short digits, and inguinal hernias are common.

The juvenile form, type II, has a later onset with psychomotor deterioration, seizures and skeletal changes apparent between 7 and 36 months and death in childhood.  Visceral involvement and cherry-red spots are usually not present. 

Type III, or adult form, is manifest later in the first decade or even sometime by the 4th decade.  Symptoms and signs are more localized.  Neurological signs are evident as dystonia or speech and gait difficulties.  Dementia, parkinsonian signs, and extrapyramidal disease are late features.  No hepatosplenomegaly, facial dysmorphism, or cherry red spots are present in most individuals. Lifespan may be normal in this type. 

Genetics

This is an autosomal recessive lysosomal storage disease secondary to a mutations in GLB1 (3p21.33).  It is allelic to Morquio B disease (MPS IVB) (253010).  The mutations in the beta-galactosidase-1 gene result in intracellular accumulation of GM1 ganglioside, keratan sulfate, and oligosaccharides.  The production of the enzyme varies among different mutations likely accounting for the clinical heterogeneity. 

Pedigree: 
Autosomal recessive
Treatment
Treatment Options: 

There is no treatment that effectively alters the disease course. 

References
Article Title: 

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