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Charcot Marie Tooth Disease (CMT)
Charcot Marie Tooth – A funny name, but for the estimated 2.6 million people worldwide who have CMT, including myself, it’s no laughing matter. Charcot Marie Tooth actually refers a group of genetic diseases that affect the peripheral nervous system. CMT affects about 1 person in 2500, making it considered to be a rare disease.
Symptoms of CMT are slowly progressive. They include foot bone abnormalities, muscle weakness and degeneration, problems with balance, foot drop, gait abnormalities, and loss of some normal reflexes. Other symptoms of neuropathy include sensory loss and neuropathic sensations and pain. Some people with CMT experience muscle cramping. Scoliosis (curvature of the spine) is often associated with CMT. More rarely, respiratory difficulties occur. These problems will most likely first be detected at the extremities, usually in the feet, then calves, then hands, and then progressing to the forearms. In some cases, use of a wheelchair will become necessary for mobility.
Onset of symptoms is usually before 20 years of age. Generally, the earlier a person begins experiencing symptoms of CMT, the more severe symptoms will become. CMT does not affect life expectancy, but can lead to significant disability.
Currently, the full genetic testing panel performed by Athena Diagnostics tests for 24 genetic subtypes of CMT. The complete CMT panel from GeneDx looks at 51 genetic variants. Research has identified more than 70 genetic subtypes of CMT, however, with new subtypes still being discovered.
While the mechanism behind CMT differs between different subtypes, the pattern of symptoms across forms is similar. At the same time, symptoms vary considerably in severity, age of onset, and presentation across people, even among people with the same type of CMT, and between people from the same family.
Most types of CMT can be inherited through just one dominant gene from one parent. There are also types of CMT that must be transmitted through receiving a recessive gene from both parents or carried on the sex chromosome (X-linked). In less common cases, an individual will be the first in his or family to have CMT, and the disease is caused by a genetic mutation that spontaneously occurred prior to conception.
All types of CMT affect the peripheral nervous system, meaning the nerves that carry signals between the spine and body. If you think about nerves like electrical wires, the myelin sheath functions much like coating on a wire, while the “wire” is called the axon. The defective gene in a person with CMT affects the production of protein in the body that are necessary for proper functioning of either the myelin sheath or the axon.
There are two broad causes behind the symptoms of CMT. In about 70% of cases, the myelin sheath that coats the peripheral nerves starts to break down. Demyelinating forms are usually classified as one of the ‘Type !’ forms of CMT. Myelin (the “coating”) helps to speed up conduction of the nerve signal. As the myelin sheath breaks down, conduction slows down. Over time the axon deteriorates as well.
The most common type of CMT, affecting about half of those diagnosed with CMT, is called CMT Type 1A. This type of CMT affects the myelin sheath and is transmitted through a single gene from a parent.
In about 30% of cases, the axon (the “wire”) becomes directly affected. As the axon deteriorates, the nervous impulse can no longer travel along the axon as well, and symptoms arise. These types are usually classified as Type 2 forms of CMT.
Diagnosis of CMT will be likely to include a neurological exam from a neurologist, medical history, nerve conduction and electromyography (EMG) tests, and possibly genetic testing. Currently, no cure exists for any of the types of CMT, but research has been ongoing. Treatment may include orthotic inserts, braces such as AFOs, physical therapy, occupational therapy, orthopedic surgery, and medication for neuropathic sensations.
Individuals with CMT should avoid certain medications that may increase symptoms. The Charcot Marie Tooth Association (CMTA) publishes a list of these medications.
The CMTA offers many other resources for those with CMT and raises money to provide grants to cutting-edge researchers through their STAR program. The Muscular Dystrophy also provides many resources for those with CMT, including access to comprehensive expert medical care through the MDA Clinics.
Charcot Marie Tooth disease was named for the three physicians who first described CMT in 1886. Sometimes CMT may be referred to as hereditary motor and sensory neuropathy (HMSM) or peroneal muscular atrophy (PMA). While more descriptive, these names are not used as often as that funny name, Charcot Marie Tooth, for this serious disease.
Athena Diagnostics (n.d.). CMT Advanced Evaluation – Comprehensive. Retrieved on 6/26/15 from http://athenadiagnostics.com/view-full-catalog/c/cmt-advanced-evaluation-comprehensive .
Charcot Marie Tooth Association, (n.d.). Homepage. Retrieved on 6/26/15 from http://www.cmtausa.org/ .
Charcot Marie Tooth Association, (2010; updated 3/13/15). An Overview of Charcot Marie Tooth Disorders. Retrieved on 6/26/15 from http://www.cmtausa.org/index.php?option=com_content&view=article&id=70&Itemid=159 .
Charcot Marie Tooth Association, (2015). Medical Alert. Retrieved on 6/26/15 from http://www.cmtausa.org/images/docs/medical_alert.pdf?uuid=6a182c91-4537-6601-e4d2-7f8dd146da5f .
GeneDx, (n.d.). Test Catalog…Charcot Marie Tooth (CMT). Retrieved on 6/26/15 from http://www.genedx.com/test-catalog/disorders/charcot-marie-tooth-cmt/39532/ .
Muscular Dystrophy Association (n.d.). Charcot-Marie-Tooth Disease. Retrieved on 6/26/15 from http://www.mda.org/disease/cmt.html .
Muscular Dystrophy Association (n.d.). Homepage. Retrieved on 6/23/15 from http://www.mda.org/ .
Content copyright © 2015 by Jori Reijonen, Ph.D.. All rights reserved.
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