How Kennedy Disease Is Different From ALS

Symptoms, Diagnosis, and Treatment for SBMA

Man with muscle cramp
Jan-Otto/Getty Images

Kennedy disease, also known as spinal bulbar muscular atrophy or SBMA, is an inherited neurological disorder. Kennedy disease affects the specialized nerve cells that control muscle movement (specifically, the lower motor neurons), which are responsible for the movement of many muscles of the arms and legs. It also affects the nerves that control bulbar muscles, which control breathing, swallowing, and talking. Kennedy disease can also lead to androgen (male hormones) insensitivity which causes enlarged breasts in men, decreased fertility, and testicular atrophy.

Symptoms of Kennedy Disease

On average, symptoms begin in individuals aged 40-60 years. Symptoms come on slowly, and may include:

  • Weakness and muscle cramps in the arms and legs
  • Weakness of the face, mouth, and tongue muscles. The chin may twitch or quiver, and the voice may become more nasal.
  • Twitching of small muscles that can be seen under the skin.
  • Tremors or trembling with certain positions. The hands may tremble when trying to pick up or hold something.
  • Numbness or loss of sensation over certain areas of the body.

Kennedy disease may have other effects on the body, including:

  • Gynecomastia, meaning enlargement of breast tissue in males
  • Testicular atrophy, where the male reproductive organs diminish in size and lose function.

Causes

Kennedy disease is caused by a genetic defect on the X (female) chromosome. Since males have only one X chromosome, they are most severely affected by the disorder. Females, who have two X chromosomes, may carry the defective gene on one X chromosome, but the other normal X chromosome lessens or hides the symptoms of the disorder. Only a rare case has been noted where a female is symptomatic due to defects in both X chromosomes.

The affected gene codes for androgen receptors, which bind male sex hormones and guide protein production in many areas of the body, including the skeletal muscles and central nervous system motor neurons.

Kennedy disease is rare. Although the Kennedy's Disease Association estimates that it occurs 1 in 40,000 individuals worldwide, other sources list it as far more rare. For example, GeneReviews and the National Organization of Rare Disorders note that it occurs in less than 1 per 300,000 males. As well, it has so far only been found in European and Asian populations and not in African or aboriginal populations.

Diagnosis of the Condition

There are a number of neuromuscular disorders with symptoms similar to Kennedy disease, so misdiagnosis or under-diagnosis may be common.

Often, individuals with Kennedy disease are mistakenly thought to have amyotrophic lateral sclerosis (ALS, or Lou Gehrig's disease). However, ALS, as well as the other similar disorders, does not include endocrine disorders or loss of sensation.

A genetic test can confirm if the Kennedy disease defect is present on the X chromosome. If genetic testing is positive, no other tests need to be done as diagnosis can be made from the genetic test alone.

Treatment for SBMA

Kennedy disease or SBMA does not affect life expectancy, so treatment is focused on maintaining the individual's optimum muscle function throughout his life through some of the following types of therapy:

  • Physical therapy
  • Occupational therapy
  • Speech therapy

These types of therapy are important for maintaining an individual's abilities and for adapting to the progression of the disease. Adaptive equipment such as the use of canes or motorized wheelchairs can help maintain mobility and independence.

Genetic Counseling

Kennedy disease is linked to the X (female) chromosome, so if a woman is a carrier of the defective gene, her sons have a 50% chance having the disorder and her daughters have a 50% chance of being a carrier. Fathers cannot pass Kennedy disease on to their sons. Their daughters, however, will be carriers of the defective gene.

Was this page helpful?

Article Sources

Verywell Health uses only high-quality sources, including peer-reviewed studies, to support the facts within our articles. Read our editorial policy to learn more about how we fact-check and keep our content accurate, reliable, and trustworthy.
  1. La Spada A. Spinal and bulbar muscular atrophy. 1999 Feb 26 [Updated 2017 Jan 26]. In: Adam MP, Ardinger HH, Pagon RA, et al., editors. GeneReviews® [Internet]. Seattle (WA): University of Washington, Seattle; 1993-2019

  2. National Organization of Rare Disorders. Kennedy Disease. Updated 2018.

  3. Genetic and Rare Diseases Information Center. Kennedy disease. Updated September 21, 2015.

  4. Genetics Home Reference. Spinal and bulbar muscular atrophy. Published October 29, 2019.