Kennedy’s disease is named after William R. Kennedy, MD, who first described the condition 1966. Kennedy’s disease (KD), also known as spinal and bulbar muscular atrophy (SBMA), is a rare, slowly progressive X-linked recessively inherited neurodegenerative disorder of lower motor neurons. The estimated incidence is approximately 1 in 40,000 males and is very rare in females. However, KD is the most common adult-onset SBMA, with disease onset ranging from 18 to 64 ages, and unlike other motor neuron diseases, such as amyotrophic lateral sclerosis (ALS), disease progression in KD is relatively slow. KD mainly affects males and is characterised by muscle weakness, atrophy and frequently complicated by other signs and symptoms such as gynecomastia, impotence, testicular atrophy and metabolic changes, suggesting a multisystem involvement. Prominent limb tremor and muscle fasciculations are the most common symptoms in KD patients prior to muscle wasting and weakness in the arms and legs. Leg muscle weakness can also lead to difficulty walking and increased falls risk. Involvement of the bulbar muscles is a frequent finding, accounting for progressive problems with swallowing and speech.
The genetic basis of KD results from an abnormal expansion of a DNA segment called a CAG triplet repeat. The diagnosis is confirmed through molecular genetic testing with affected men having >39 CAG repeats. Although the extended CAG region changes the structure of the androgen receptor (AR), it is unclear how the altered protein disrupts nerve cells in the brain and spinal cord. Studies have shown a fragment of the AR protein containing the CAG segment accumulating within these cells interfering with normal cell function. A recent study reported a correlation between CAG repeat size and electrophysiological abnormalities SBMA. In addition to motor involvement, standard nerve conduction and histopathological studies have disclosed involvement of large myelinated sensory fibres in the spinal nerves of SBMA patients. Neurophysiological examination often reveal reduced or absent sensory responses in spite of minimal, if any sensory symptoms reported by patients. Importantly, there is on average a delay of over 5 years from the onset of weakness to the diagnosis of KD. As such, there is a critical need to develop disease-specific biomarkers in order to provide clues to the underlying pathogenic process in KD patients as well as to facilitate early diagnosis.
Currently, there are no effective treatments or cure for Kennedy’s disease. Some individuals living with KD take medications prescribed by their clinician to help alleviate various symptoms. Others have reported that a smart (light) exercise program coupled with stretching may improve some of the motor symptoms. Researchers, however, continue to strive to find a cure. Recently, a number of MDA researchers have been looking into the possibility of utilising a new class of drugs called the selective androgen receptor modulators (SARMs) to treat SBMA.
Further Information and Support
Kennedy’s Disease Association kennedysdisease.org
National Organisation for Rare Disorders (NORD) rarediseases.org/rare-diseases/kennedy-disease
Reviewed December 2019 by Professor Cindy (Shin-Yi) Lin PhD MEngSci BE, The Kam Ling Barbara Lo Chair in Neurodegenerative Disorder. Central Clinical School, Faculty of Medicine and Health, Brain & Mind Centre. THE UNIVERSITY OF SYDNEY
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