Spinal Bulbar Muscular Atrophy (SBMA) or Kennedy’s Disease is an inherited adult-onset, slowly progressing neuromuscular disorder characterized by the degeneration of lower motor neurons within the spinal cord and brainstem. Kennedy disease is named after William R. Kennedy, MD, who described this condition in an abstract in 1966 and a full report in 1968. It’s also sometimes called bulbospinal muscular atrophy. The adjective bulbar refers to a bulblike structure in the lower part of the brain that contains nerve cells controlling muscles in the face, mouth, and throat.
SBMA is an X-linked disease, therefore it can only be passed on through females carriers of the gene. SBMA results from an abnormal expansion of the CAG polyglutamine encoding repeat within the androgen receptor gene. SBMA is, therefore, one of a group of nine neurodegenerative disorders, that includes Huntington’s disease, all caused by expansion of the CAG repeat within specific genes. The loss of motor neurons in SBMA results in muscle atrophy and weakness, involuntary muscle twitching and mild sensory impairment as well as signs of endocrine abnormalities. Since the disease is ligand (androgen)-dependent, disease manifest primarily in males, and females are usually asymptomatic. Although, there is a widespread expression of the androgen receptor protein, the mechanism of selective motor neuron loss within the brainstem and spinal cord is largely unknown.
Age at onset is usually in the third to fifth decade of life, but earlier involvement has been reported. Kennedy disease affects fewer than 1 in 150,000 males and is very rare in females. Kennedy disease has been diagnosed in the USA, Europe, Asia, South America, and Australia. The Japanese population appears to have a very high prevalence of Kennedy Disease because of a founder effect.
SBMA can affect a wide range of muscles throughout the body. Symptoms include:
- Bulbar signs – The Bulbar muscles are those supplied by the motor nerves coming off the brain stem which control breathing, swallowing, talking and other functions of the throat.
- Dysphagia – Trouble swallowing
- Intention tremor – Hand tremors while trying to do something.
- Normal Babinski – Normal plantar response, ie., when the bottom of the foot is scraped, the toes bend down. An abnormal response would be an upward bending of the toes indicating a problem in the brain itself.
- Lower motor neuropathy – The lower motor nerves are those that run from the spinal cord to the muscles that they stimulate to move. Loss of that nerve leads to weakness and wasting of the muscle.
- Primary Sensory Neuropathy – Numbness over certain areas. Loss of sensation
- Decreased or Absent Deep Tendon Reflexes
- Fasciculations – Twitching of small muscles without purposeful movement that can be seen through the skin.
- Large muscle spasms
- Postural Tremor
- Muscular Atrophy – Wasting and shrinkage of muscles that occurs when the lower motor nerve does not stimulate the muscle adequately.
- Hypertrophied Calves – Calf muscles that become thicker because of cramps.
- Gynecomastia – Enlarged breasts in men.
- Androgen Deficiency – Loss of masculinizing effect.
- Estrogen Excess
- Reduced Infertility
- Testicular Atrophy
The first steps in diagnosis of a neuromuscular disease are usually an in-office physical examination and family history, with tests to distinguish SBMA from similar conditions like ALS. A simple blood test for enzyme creatinine kinase may be ordered, as CK levels are elevated in neuromuscular diseases, indicating muscle damage. However, this is not a confirmatory test. If SBMA is suspected, diagnosis can be confirmed by molecular genetic testing on a blood sample for CAG trinucleotide repeat expansion in the AR gene. Individuals with greater than 36 CAG trinucleotide repeats in the AR gene are diagnosed with the condition.
Currently, there are no known cures or treatments for Kennedy’s Disease. Management is a treatment of manifestations. Physical therapy, occupational therapy, and speech therapy are commonly used to adapt to the progressing disease and maintain an individual’s skills. Braces, walkers, and wheelchairs are used for ambulation. Breast reduction surgery is sometimes used as needed in patients with gynecomastia. Speech therapists are also beneficial for those suffering from dysarthria. Medication is available to reduce muscle cramps and tremors.
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