Kennedy's disease is a rare genetic neurodegenerative disorder that affects the motor neurons (cells that are important for normal function of the brain and spinal cord). It is a progressive disorder that leads to increasing severity of motor dysfunction and subsequent deterioration of muscle strength, muscle tone, and motor coordination. It was first described by the American physician William R. Kennedy in 1966.
As Kennedy's disease is a progressive neurodegenerative disorder, affected individuals have physical, mental, and emotional impacts. Physically, the neurological degenerative process results in muscle weakness and eventual muscle wasting that can affect the patient's ability to walk or move. Kennedy's disease is also called spinal bulbar muscular atrophy, or SBMA, because both the spinal and bulbar neurons are affected.
Kennedy's Disease is inherited through the X chromosome, and since males only have one X chromosome inherited from their carrier mother, they are usually affected while females are usually carriers. Therefore, sons of carrier mothers will be affected and all her daughters have a 50% chance of being a carrier. Although affected males often have a low sperm count or are infertile, if they are capable of reproducing, all male children will be unaffected and all female children will be unaffected carriers. In some cases, women who are carriers also exhibit clinical symptoms, although they are generally less severe. Kennedy's disease is a rare disease, with only one in 50,000 males affected and no particular pattern among various races or ethnic groups.
Causes and symptoms
Symptoms do not usually develop until between the second and fourth decades of life, although an earlier (and a later) age of onset have been documented. Symptoms initially are mild and include tremors while stretching hands, muscle cramps after exertion, and fasciculations (visible muscle twitches). Muscle weakness often develops in the arms and legs, beginning usually in the shoulder or midsection. It is most noticeable in the legs and the arms. Breathing, swallowing, and talking are functions that require bulbar muscles controlled by motor nerves that communicate with the brain. The effects of bulbar muscle dysfunction can be manifested by slurred speech and dysphagia (swallowing difficulties). In later stages, patients often develop aspiration pneumonia (pneumonia caused by food and fluids traveling down the bronchial tubes instead of the trachea due to poor ability to swallow).
Kennedy's disease is caused by a trinucleotide repeat expansion in the androgen receptor gene. This means that three letters in the DNA alphabet (cytosine-adenine-guanine, or CAG) that are normally repeated 10–36 times expand to produce a larger repeat size of approximately a 40–62 repeated trinucleotide sequence. This sequence is unstable and can change from one generation to the next leading to further expansions. The specific mechanism explaining how this trinucleotide repeat expansion (which leads to an increased length in the protein it encodes) causes the disease is unknown.
Patients with Kennedy's disease usually receive a definitive diagnosis in a clinical molecular genetics laboratory. This requires DNA extraction from blood, followed by testing the gene that causes Kennedy's disease for a mutation. Kennedy's disease can be misdiagnosed as spinal muscular atrophy and Lou Gehrig's disease due to similar symptoms displayed.
The treatment team caring for a patient with Kennedy's disease includes a neurologist , physical therapists, occupational therapists, gastroenterologists, and genetic counselors.
Although research efforts are underway, currently there is no treatment for Kennedy's disease. Medical treatment is based on lessening the symptoms. Physical therapy is useful in reducing the side affects from the progressive muscle weakness.
Recovery and rehabilitation
In the absence of a cure, patients usually do not recover and the symptoms progress during their lifetime. Lifestyle changes may become necessary, especially late in the disease. These changes, in more severe cases, can include (but are not limited to) help eating, wheelchair access at home, and help with using the restroom and changing clothes.
Kennedy's disease is a neurodegenerative disorder that is slow in its progression. It is likely that individuals will become wheelchair bound during the later stages of the disease. Although individuals will have certain difficulties in motor function and may have special needs, the lifespan of affected individuals is not thought to be shortened.
Genetic counseling is important in this disorder since the presence of one affected offspring means that it is likely the disease gene was inherited and that there is a risk that there will be affected offspring in subsequent generations. The possibility of infertility due to low sperm count should also be discussed during the counseling, especially in cases that develop early. Also, gynecomastia (enlarged breasts) in males due to reduced virilization can also have psychosocial consideration and need to be addressed. Erectile dysfunction and/or testicular atrophy may also affect males.
Cooper, D. N., M. Krawczak, and S. E. Antonarakis. "The Nature and Mechanisms of Human Gene Mutation." In The Metabolic and Molecular Basis of Inherited Disease, 7th ed. Edited by C. R. Scriver, A. L. Beaudet, W. S. Sly, and D. Valle. NY: McGraw-Hill, 1995.
Panzarino, Connie. Me in the Mirror. Seal Press, 1994.
"NINDS Kennedy's Disease Information Page." National Institute of Neurological Disorders and Stroke. (April 24, 2004). http://www.ninds.nih.gov/health_and_medical/disorders/kennedy's.htm.
"What Is Kennedy Disease?" Kennedy Disease Association. (April 24, 2004). <http://www.kennedysdisease.org/about.html>.
Kennedy's Disease Association. PO Box 2050, Simi Valley, CA 93062-2050. (805) 577-9591. [email protected] <http://www.kennedysdisease.org/about.html>.
Bryan Richard Cobb, PhD