Spinal and bulbar muscular atrophy

A genetic disorder affecting motor neurons

Androgen receptor

Spinal and bulbar muscular atrophy
Synonyms	Spinobulbar muscular atrophy, bulbo-spinal atrophy, X-linked bulbospinal neuropathy (XBSN), X-linked spinal muscular atrophy type 1 (SMAX1), Kennedy's disease (KD)
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Field	Neurology, Genetics
Symptoms	Muscle cramps, muscle weakness, fasciculations, dysphagia, gynecomastia, infertility, mild tremors
Complications	Progressive muscle wasting, difficulty swallowing, aspiration pneumonia, respiratory insufficiency
Onset	Adulthood, typically between 30 and 50 years of age
Duration	Chronic and progressive
Types	X-linked recessive motor neuron disorder
Causes	Mutation in the androgen receptor (AR) gene
Risks	Being male with the mutated X chromosome; females are typically carriers
Diagnosis	Clinical evaluation, family history, genetic testing, electromyography (EMG)
Differential diagnosis	Amyotrophic lateral sclerosis, other motor neuron diseases, myopathies
Prevention	None; genetic counseling is recommended for at-risk families
Treatment	Supportive care, physical therapy, speech and swallowing therapy, respiratory support
Medication	Symptom-based management; some hormonal therapies are under investigation
Prognosis	Slowly progressive; most patients maintain mobility for decades; life expectancy may be slightly reduced due to respiratory complications
Frequency	Rare; estimated at 1 in 40,000 males
Deaths	Typically related to late-stage complications like respiratory failure

Ideogram of human X chromosome.

Spinal and bulbar muscular atrophy (SBMA), also known as Kennedy's disease, is a rare, X-linked recessive genetic disorder characterized by the progressive degeneration of motor neurons in the spinal cord and brainstem. This condition primarily affects males, with onset typically occurring in adulthood.

Pathophysiology

SBMA is caused by a mutation in the androgen receptor (AR) gene located on the X chromosome. This mutation involves an expansion of a CAG trinucleotide repeat within the gene, leading to the production of an abnormal protein that accumulates in motor neurons, causing their dysfunction and eventual death. The disease is classified as a polyglutamine (polyQ) disorder, similar to Huntington's disease.

Clinical Features

The clinical manifestations of SBMA include progressive muscle weakness and atrophy, particularly affecting the proximal muscles. Patients often experience difficulty with activities such as climbing stairs, lifting objects, and swallowing. Bulbar symptoms, such as dysarthria and dysphagia, are common due to the involvement of the brainstem motor neurons.

Other symptoms may include:

• Gynecomastia
• Testicular atrophy
• Reduced fertility
• Hand tremors
• Muscle cramps

Diagnosis

Diagnosis of SBMA is confirmed through genetic testing, which identifies the expanded CAG repeat in the AR gene. Electromyography (EMG) and nerve conduction studies may also be used to assess the extent of motor neuron involvement.

Management

There is currently no cure for SBMA, and treatment is primarily supportive. Management strategies focus on alleviating symptoms and improving quality of life. These may include:

• Physical therapy to maintain muscle strength and mobility
• Speech therapy for bulbar symptoms
• Nutritional support to address swallowing difficulties
• Hormonal therapy to manage androgen-related symptoms

Prognosis

The progression of SBMA is typically slow, and life expectancy is not significantly reduced. However, the disease can lead to significant disability over time, impacting daily activities and quality of life.

Related pages

• Motor neuron disease
• Genetic disorders
• Neurodegenerative diseases

External links

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