Diagnosing Kennedy Disease
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 is caused by a mutation on the X chromosome in the androgen receptor (AR) gene; this mutation results in androgen receptor dysfunction. Androgen receptors respond to signals from male hormones (androgens) and are found throughout the body. Kennedy disease primarily affects males (approximately 1 in 150,000).
Diagnosing Kennedy disease
Diagnosing genetic disorders can be challenging. The diagnostic process for Kennedy disease typically includes a physical exam, medical history, genetic testing, and various other medical tests.
Physical exam and medical history
A physical exam is typically the first step in the diagnostic process. Signs and symptoms of weakness, cramps, and muscle wasting are indicators of Kennedy disease.
A thorough family medical history is also a key component of the diagnostic process. Information regarding any relatives who share similar symptoms or who have a diagnosis of Kennedy disease warrants further testing.
A diagnosis of Kennedy disease is confirmed by genetic testing. If Kennedy disease is present, genetic testing will reveal too many repeats of the nucleotides cytosine, adenine and guanine (CAG) on the androgen receptor (AR) gene. Normally, the CAG nucleotides are repeated up to 36 times. With Kennedy disease, 38 or more repeats of these nucleotides are present.
Because several health conditions share similar symptoms with Kennedy disease, medical tests may be ordered to rule out those conditions before genetic testing for Kennedy disease is done. They may also be ordered if genetic testing is negative for Kennedy disease.
- Blood and lab tests may be ordered to rule out other potential causes of symptoms. Levels of autoantibodies and inflammatory markers may be checked to determine if an autoimmune or autoinflammatory disease is present. A complete blood count and comprehensive metabolic panel may be ordered to review overall health, check the function of the kidneys and liver, and measure blood sugar levels. Thyroid hormone levels may also be checked to ensure the thyroid is functioning properly.
- Imaging studies, such as X-rays, ultrasounds and MRIs, can help determine if structural issues, such as spinal stenosis, are responsible for symptoms. Also, if Kennedy disease is present, imaging may show muscle atrophy.
- Electromyography (EMG) is a test in which a small needle is inserted into a muscle to measure and evaluate electrical activity in the muscle. With Kennedy disease, an EMG is abnormal and may show complex repetitive discharges, fasciculations, and quivering muscles. Muscle involvement is generally observed in three of four regions (face/throat, cervical, thoracic, lumbar).
- Nerve conduction studies involve the placement of two electrodes on the skin to measure the speed of electrical impulses in the muscles. With Kennedy disease, nerve conduction studies may be normal.