Congenital myasthenia is an inherited disorder that results in muscle weakness caused by a malfunction at the neuromuscular junction, the area where nerve cells communicate to muscle cells.
Description
Congenital myasthenia is caused by a number of genetic defects that affect the ability of a nerve impulse to move from nerve to nerve, and from the nerve to muscle. The genetic abnormalities can be present in the fetus at the moment of conception or may occur during fetal development. This genetic cause of the disease separates the congenital form of myasthenia from myasthenia gravis and Lambert-Eaton myasthenic syndrome, both of which are caused by the malfunctioning of the immune system.
Demographics
Congenital myasthenia occurs in the young, and occurs with equal frequency in boys and girls. Symptoms tend to appear within the first two years of life. It is common to have siblings who are affected. The disease is extremely rare, occurring in only one to two per million live births.
Causes and symptoms
The root of congenital myasthenia are defects in various genes that play a role in the transmission of nerve impulses. At least a dozen genetic defects have been identified as causes of congenital myasthenic syndromes so far. The defects can affect the manufacture or the release of acetylcholine, a neurotransmitter, or molecule that acts as a communication bridge between adjacent nerves.
As a result of the varying genetic roots of the disease, different congenital myasthenic syndromes exist. These can produce different effects in those who are affected. The symptoms, which usually begin in infancy or toddlerhood, can include a poor sucking response, drooping eyelids (a condition called ptosis), eyes that appear to wander or float (ophthalmoplegia), weakness in facial muscles that is apparent as an abnormal appearance, weakness in the arms and legs, breathing difficulty, delayed development of muscle skills, and a feeling of fatigue. Usually, a parent may notice that the infant is experiencing delays in developmental milestones that require coordinated muscle strength, such as sitting up alone, crawling, or walking. All or just a few of these symptoms can be present in a person with congenital myasthenia. As well, the severity of the symptoms can vary from person to person. Some children may be severely impaired, while others lead near normal lives. Even though children display symptoms, their parents may not be similarly affected.
Diagnosis
The disease is usually diagnosed in the early years of childhood by the abnormal appearance of the face and/or by the noticeable weakening of the arms or legs. A test of muscle strength known as the tensilon test that is considered to be accurate in diagnosis of other forms of myasthenia is usually not specific for congenital myasthenia. Congenital myasthenia is often misdiagnosed as myasthenia gravis or other neuromuscular diseases.
Accurate diagnosis of congenital myasthenia requires specialized testing. These include testing specific nerves to determine if the nerves fatigue more quickly than is normal. While at least a dozen genes that are responsible for the disease are known, genetic testing technology is not currently routinely available. Only a handful of centers in the United States are able to test the anconeus and intercostal muscles to detect the abnormal genes. However, as such technology becomes routine (i.e., gene chips), genetic testing will no doubt become one of the principle means of diagnosis.
Treatment team
Treatment can involve the family physician, a neurologist, family members, and physical therapists. The latter can provide exercises that assist in maximizing muscular strength.
Treatment
Treatment for most types of congenital myasthenia typically involves the use of drugs that help promote the transmission of nerve impulses. Drugs that retard the breakdown of acetylcholine can be used. An example of an acetylcholine sterase is mestinon. Other drugs that show merit in some cases include guanidine, ephedrine sulfate, and albuterol. People may build up a tolerance to ephedrine, which decreases its effectiveness.
As of mid-2004, there were no clinical trials underway or in the planning stages specific for congenital myasthenia. However, agencies such as the National Institute for Neurological Diseases and Stroke continue to fund research that seeks to better understand the underlying genetic bases of congenital myasthenia, and to discover more effective means of increasing nerve signal transmission. Updated information on clinical trials related to congenital myasthenia can be located at the National Institutes of Health website for clinical trials at www.clinicaltrials.org.
Prognosis
With accurate diagnosis, most types of congenital myasthenia can be improved or at least stabilized by the use of drug therapy. More severe forms of the disease may weaken respiratory muscles and result in a reduced lifespan.
BOOKS
Thompson, Charlotte. Raising a Child with a Neuromuscular Disorder: A Guide for Parents, Grandparents, Friends, and Professionals. New York: Oxford Univ. Press, 1999.
National Institute for Neurological Diseases and Stroke (NINDS). P.O. Box 5801, Bethesda, MD 20824. (301) 496-5751. (800) 352-9424. <http://www.ninds/nih.gov>.
National Organization for Rare Disorders (NORD). 55 Kenosia Avenue, Danbury, CT 06813-1968. (203) 744-0100 or (800) 999-6673; Fax: (203) 798-2291. orphan@rarediseases.org. <http://www.rarediseases.org>.
Myasthenia Gravis Foundation of America, Inc. 1821 University Ave. W., Suite S256, St. Paul, MN 55104. (651) 917-6256 or (800) 541-5454; Fax: (651) 917-1835. mgfa@myasthenia.org. <http://www.myasthenia.org>.
