Spinal and bulbar muscular atrophy is a disorder of specialized
nerve cells that control muscle movement (motor neurons). This
condition, which mainly affects males, is characterized by muscle
weakness and wasting (atrophy) that usually begins in adulthood
and worsens slowly over time. Muscle wasting in the arms and legs
results in cramping, difficulty walking, and a tendency to fall.
Certain muscles in the face and throat (bulbar muscles) are also
affected, which causes progressive problems with swallowing and
speech. Additionally, muscle twitches (fasciculations) are common.
Some men with the disorder experience unusual breast development (gynecomastia)
and may be unable to father a child (infertile).
How common is spinal and bulbar muscular atrophy?
This condition affects fewer than 1 in 50,000 males and is very
rare in females.
What genes are related to spinal and bulbar muscular atrophy?
Mutations in the AR gene cause spinal and bulbar muscular atrophy.
This disorder is caused by a mutation in which a DNA segment,
known as a CAG triplet repeat, is abnormally expanded within the
androgen receptor (AR) gene. Normally, this DNA segment is
repeated up to about 36 times. In people with spinal and bulbar
muscular atrophy, the CAG segment is repeated at least 38 times,
and may be two or three times its usual length. The abnormally
expanded CAG triplet repeat changes the structure of the protein
made by the AR gene, disrupting the normal function of motor
neurons in the brain and spinal cord. These nerve cells gradually
die, leading to the muscle weakness and wasting seen in this
condition. People with a higher number of CAG repeats tend to
develop signs and symptoms of spinal and bulbar muscular atrophy
at an earlier age.
How do people inherit spinal and bulbar
muscular atrophy?
This condition is inherited in an X-linked recessive pattern. A
condition is considered X-linked if the mutated gene that causes
the disorder is located on the X chromosome, one of the two sex
chromosomes. In males (who have only one X chromosome), one
altered copy of the gene in each cell is sufficient to cause the
condition. In females (who have two X chromosomes), a mutation
must be present in both copies of the gene to cause the disorder.
Males are affected by X-linked recessive disorders much more
frequently than females. A striking characteristic of X-linked
inheritance is that fathers cannot pass X-linked traits to their
sons.
Some females with one altered copy of the AR gene have mild signs
and symptoms related to the condition, including muscle cramps and
occasional tremors.
Where can I find information about diagnosis, management, or
treatment of spinal and
bulbar muscular atrophy?
These resources address the diagnosis or management of spinal and
bulbar muscular atrophy and may include treatment providers.
Growing Stronger, Growing
Better
Global Health
Healthcare Provider
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