Spinal and bulbar muscular atrophy

Medical quality assurance by Dr. Albrecht Nonnenmacher, MD at September 29, 2016
StartDiseasesSpinal and bulbar muscular atrophy

Spinal and bulbar muscular atrophy (SBMA) is a genetic disorder that is characterized by the progressive degeneration of voluntary muscle movements, resulting in weakness in the arms, legs, and facial area, along with difficulty speaking and difficulty swallowing (dysphagia). It is also known as Kennedy disease, named after William Kennedy, a physician who first discovered and described it in 1966.

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Definition & Facts

Normally, the signals between the nerve cells in the brain and spinal cord help direct and coordinate movement in the muscles. When the body performs an action, the brain sends a signal that transmits down to the motor neurons, which stimulates the muscles and causes the intended motions. However, with SBMA, the neurons from the spinal cord to the muscles are compromised, resulting in the muscle's inability to contract properly.

Muscles that are not used will begin to waste away (muscle atrophy) and eventually deteriorate completely. SBMA affect fewer than 1 in 150,000 people each year. Due to the nature of the disorder, a vast majority are comprised of males and very rarely affects females.

Symptoms & Complaints

Symptoms typically emerge between the ages of 30 and 50, although it has been diagnosed in males both younger and older than that average range. Females that carry the genetic mutation that causes this disorder may experience muscle cramps and twitching, especially at later ages. Signs and symptoms for males includes the following:

Causes

The abnormal gene is carried on the X chromosome, one of the two sex chromosomes. Normal females have two X chromosomes, while normal males have one X and one Y. Females who inherit one abnormal X will become genetic carriers of SBMA, as they are protected by the second X chromosome from developing symptoms.

Meanwhile, should males inherit the abnormal X chromosome, they will not have a second one to protect them. As a natural consequence, affected fathers will always pass the X chromosome to daughters, who will become a carrier.

Sons will inherit the Y chromosome, which will bear no direct consequence to SBMA. On the other hand, a mother carrying the affected chromosome will have 50% chance of passing the abnormal gene onto sons and 50% chance of passing it on to daughters.

Although the specific mechanisms that cause the neuromuscular degeneration are not identified, SBMA is generally acknowledged to be the result of a mutation in the genetic coding that instructs the use of a protein, theandrogen receptor. Normally, androgen receptor binds to androgen, a hormone that not only contributes to male sexual development and libido, but also affects muscle mass and muscle strength.

Researchers believe that the dysfunction of this androgen receptor may be related to the onset of SBMA. It is also believed that testosterone plays an important role in facilitating the expression of androgen receptors. Because females have a much lower count of testosterones circulating throughout the body, it may help with keeping the androgen receptors from regularly activating.

Diagnosis & Tests

SBMA diagnosis generally works through several steps. The first is a blood test to check for elevated serum creatine kinase (CPK). In an average human body, there are low levels of creatine kinase circulating through the blood. Although elevated CPK in the blood is not harmful, it is a reliable indication of whether there are potential muscle damages in the body.

If SBMA is suspected, a molecular genetic testing can be ordered. The genetic test also requires a blood sample, but it will generate an accurate diagnosis. Other test including a nerve conduction velocity test can help facilitate an accurate diagnosis by further assessing possible nerve damages that could be caused by SBMA.

Because SBMA is rare, it is not unusual to be overlooked and misdiagnosed with other motor neuron diseases, such as amyotrophic lateral sclerosis (ALS or Lou Gehrig's disease). However, there are contrasting differences between the two, as those diagnosed with SBMA have a more normal expected lifespan over those diagnosed with ALS. It is worth taking additional tests to determine the most accurate conclusion.

Treatment & Therapy

There are currently no available treatment or cure for SBMA. There are, however, therapies available to help relieve and adapt to living with SBMA. Physical therapy may encourage specific exercises and stretching to stimulate the muscles and motor neurons and keep them functioning longer and deteriorate at a slower pace.

Specialized stretching can also reduce muscle cramping and manage any discomfort or pain. Speech therapy is also recommended, as the reduced function of swallowing properly may carry life threatening consequences, especially if food and liquid enters the windpipe (trachea) instead of the esophagus into the digestive system and causes choking.

Speech therapists also specializes in restoring and coping with any speech disorders that may arise from weakened facial muscles. It may also be worthwhile to consult an occupational therapist if difficulties with daily living arise as a consequence of SBMA.

Occupational therapy can help facilitate new lifestyle changes by reintroducing new methods of performing everyday tasks such as dressing, eating, and bathing, thereby encouraging an independent livelihood. Because the risk of falls are not uncommon in SBMA patients, certain assistive devices such as walkers and leg braces may be discussed.

Prevention & Prophylaxis

Because SBMA is a disorder resulting from genetic inheritance, there are currently limited prevention methods that can be undertaken. If a parent is a suspected genetic carrier of SBMA, he or she may opt to have genetic testing and genetic counseling to assess their risk.

For those who may be at risk for developing SBMA, it is important to work with the provider to observe any potential signs and monitor disease progression.