Info

Notes: Size ranges of different alleles (normal, premutation, disease) are subject to change with additional data available.

Notes: Size ranges of different alleles (normal, premutation, disease) are subject to change with additional data available.

myotonic dystrophy. Electrophysiologic testing is helpful in identifying myotonia often not recognized on clinical exam.

2.3. HUNTINGTON'S DISEASE Huntington's disease (HD) is a progressive neurodegenerative disorder affecting 1/10,000 individuals and is characterized by choreic movements, impaired cognition, and personality changes. This disorder is inherited as an autosomal dominant trait with complete penetrance and variable age of onset, with a mean age of onset at approx 40 yr. Personality changes and dementia, followed by chorea, are the presenting symptoms of the disease. Minor motor abnormalities, including clumsiness, hyperreflexia, and eye movement disturbances, are also early manifestations of HD. As the disease progresses, features of bradykinesia, rigidity, dystonia, and epilepsy could become evident.

2.4. FRIEDREICH'S ATAXIA Friedreich's ataxia (FRDA) is a slowly progressive neurodegenerative disorder affecting 1/50,000 individuals in the Caucasian population. This disorder demonstrates autosomal recessive inheritance with a variable age of onset, although most patients present with symptoms before the age of 25. It is characterized by ataxia, loss of deep-tendon reflexes, muscle weakness often associated with scolio-sis, foot deformities, cardiomyopathy, and diabetes in a small fraction (10%) of affected patients.

2.5. SPINOCEREBELLAR ATAXIAS Spinocerebellar ataxias are a heterogeneous group of autosomal dominantly inherited neurologic disorders with varying levels of degeneration of the cerebellum, spinocerebellar tracts, and the brainstem neurons. Ataxia, dysarthria, and bulbar dysfunction are clinical features shared among all the SCAs. Extracerebellar features such as ocular dysfunction, extrapyramidal and pyramidal signs, peripheral neuropathy, intellectual impairment, and seizures are characteristics that are variable in presentation among the various identified ataxias. The clinically distinct Machado-Joseph disease (MJD) and spinocerebellar ataxia type 3 (SCA3) are now recognized to be allelic disorders. MJD and SCA3 are the most prevalent diseases representing 25-30% of all dominantly inherited ataxias. Both are characterized by ataxia with ophthalmoparesis and variable pyramidal-extrapyramidal findings. However, dystonia and facial fasci-culations, which are present in MJD patients, are rarely observed in SCA3 patients.

2.6. X-LINKED SPINAL AND BULBAR MUSCULAR ATROPHY The X-linked recessive disorder spinal-bulbar muscular atrophy (SBMA, Kennedy's disease) is a rare adult-onset neuromuscular disorder affecting approx 1/50,000 males. The disease is characterized by the adult onset of proximal muscle weakness, atrophy, and fasciculations. Affected males often have signs of androgen insensitivity such as gynecomas-tia, reduced fertility, and testicular atrophy; female carriers have few or no symptoms. Diabetes is commonly seen in this disorder. The pathological findings in SBMA include degeneration of anterior horn cells and bulbar motor neurons. SBMA typically progresses slowly, and its clinical course is frequently complicated by the involvement of the bulbar muscles, but life expectancy is typically not shortened.

2.7. DENTATORUBRAL-PALLIDOLUYSIAN ATROPHY Dentatorubral-pallidoluysian atrophy (DRPLA) is a rare neu-rodegenerative disorder characterized by ataxia, choreoathetosis, myoclonus, epilepsy, and dementia. This disease has a variable age of onset that ranges from the first to the seventh decade.

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