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The prevalence of BSMA is 1/30,000 male births. The incidence is 1/526,315 males/year.

Disease onset occurs between 30-60 years of age. Initial clinical manifestations include tremor, muscle cramps, muscle twitching, fatigue and slurred speech. With disease progression patients additionally develop weakness and wasting of the limb and bulbar muscles, manifesting as dysarthria, dysphonia, hanging jaw, tongue wasting, chewing difficulty and impaired mobility. Intellectual decline is minimal to none. In the terminal stages of the disease some patients may be unable to swallow or breathe. Non-neurological manifestations include gynecomastia, hypogonadism (leading to infertility and impotence) and in rare cases Dupuytren's contracture, or groin hernia.

BSMA is caused by an unstable expansion of a CAG triplet repeat (40-62 repeats) in exon 1 of the androgen receptor (AR) gene on chromosome Xq11-12. The abnormally increased repetition of this CAG triplet leads to an expanded stretch of glutamines within the androgen-receptor (AR). Polyglutamine-expansion results in misfolding and proteolysis of the mutated AR, rendering it insensitive to androgen hormones. In the nucleus AR fragments are produced, which aggregate and these aggregates are believed to cause dysregulation of the transcription of various other proteins and consecutively lead to motor neuron degeneration. Without a sufficient number of motor neurons, initiation and maintenance of muscle contractions can no longer occur, leading to progressive muscle wasting. Recently, a BSMA phenotype with distal predominance of limb weakness and wasting has been reported, caused by mutations in a subunit of the dynactin 1 DCTN1 gene.

Diagnosis is established upon medical history, clinical examination, elevated creatine-kinase, testosterone, progesterone, follicle-stimulating hormone, luteinizing hormone, reduced nerve conduction velocities or reduced nerve action potential amplitudes, acute or chronic denervation and re-innervation on electromyography and documentation of the mutation.

Differential diagnoses include hereditary spastic paraplegia, spinocerebellar ataxia (see these terms), other motor neuron diseases, myopathies, neuropathies, lead or aluminum poisoning, and cervical spondylosis.

Antenatal diagnosis is possible for mothers carrying the mutation.

Female mutation carriers usually do not manifest clinically but have a 50% risk to transmit the mutation to their male and female offspring. Affected males do not transmit the disease but 100% of their daughters become mutation carriers.

Symptomatic treatment includes physiotherapy and rehabilitation, agents against tremor and muscle cramps and hormone therapy or surgical treatment of gynecomastia. Recently, treatment of patients with the anti-testosterone leuprorelin was found to be beneficial. In advanced stages of the disease, tube feeding or ventilatory support may be indicated.

Disease progression is slow with only one third of patients requiring a wheelchair 20 years after diagnosis. Prognosis of BSMA is usually fair with only a small decrease in life expectancy.