What Is Kennedy Disease?

Kennedy disease, also known as spinal and bulbar muscular atrophy or Kennedy’s disease, is a slowly progressive neuromuscular condition that affects lower motor neurons (nerve cells located in the brainstem and spinal cord). Lower motor neurons transmit signals to the muscles to produce movement. Kennedy disease involves lower motor neuron degeneration, so this process is disrupted, which causes muscle weakness, wasting, twitching and cramps.

Kennedy disease primarily affects males (occurring in approximately 1 in 150,000). It is very rare in females; however, the mutated gene is located on the X chromosome, so carrier females can pass the disease to their sons or pass the carrier gene to their daughters.

Symptoms

Onset of neurological symptoms usually occurs between the ages of 30 and 60 but can develop as early as the 20s. Symptoms of Kennedy disease include the following:

• Weakness, wasting and cramps of the proximal muscles (those close to the trunk) and bulbar muscles (those of the face, mouth and throat)
• Twitching and tremors
• Dysphagia (difficulty speaking and swallowing)
• Numbness

Male hormone resistance (androgen insensitivity) may start in adolescence and can cause the following symptoms:

• Gynecomastia (enlarged breasts)
• Testicular atrophy (reduction in size or function of the testes)
• Low sperm count, reduced fertility, or infertility
• Erectile dysfunction

Causes

Kennedy disease is caused by a mutation in the androgen receptor (AR) gene which results in androgen receptors that do not function properly. Androgen receptors respond to signals from male hormones (androgens) and are found throughout the body (including the skin, kidney, prostate, skeletal and central nervous system). Kennedy disease is a genetic X-linked condition, which means the mutated gene is carried on the X chromosome (one of the two sex genes).

Because males have only one X chromosome, the condition mostly affects males. Females do not usually display symptoms of Kennedy disease; the exception is if a female has two abnormal X chromosomes. When females are carriers of one abnormal chromosome, the normal X chromosome hides the disease. Females may develop mild symptoms (cramps and twitching) if the mutated gene is on both X chromosomes; however, significant motor neuron disease does not occur in females due to lower levels of circulating androgens.

Risk factors

Factors that increase the risk of developing Kennedy disease include the following:

• Being male
• Having a family history of Kennedy disease
• Being between the ages of 30 and 60

With any X-linked disorder, daughters of an affected male will be carriers of the disease because they inherit the father’s abnormal X chromosome. The sons of an affected male inherit the father’s Y chromosome and, thus, will not have the disease. Sons of female carriers have a 50% chance of inheriting the disease, whereas daughters of female carriers have a 50% chance of being carriers.