Definition Congenital myasthenia is an inherited condition present at birth that interferes with nerve messages to the muscles. Although some symptoms are similar (muscle weakness worsened by use), congenital myasthenia differs from myasthenia gravis, which usually presents in adulthood and is almost always due to an autoimmune disorder rather than an inherited genetic defect.
Most cases of congenital myasthenia are noticeable at or shortly after birth. In rare cases, symptoms don't present themselves until some time later in childhood or in early adult life.
Normal muscle function requires a chemical messenger called acetylcholine (ACh) to travel from the nerve cell to a receptor on the muscle endplate, in order to stimulate muscle contraction and movement. After the ACh has initiated muscle contraction, it is degraded by an enzyme.
In congenital myasthenia, one of three problems occurs with this system:
- Too little ACh is produced, or its release from the nerve cell is impaired
- The enzyme that should degrade ACh is faulty, resulting in prolonged stimulation of the muscle by excess ACh and ultimately in muscle damage
- The area of the muscle that should be stimulated by the presence of ACh (called the endplate receptor) is defective, and therefore the muscle can not be sufficiently stimulated
Figures regarding the frequency of congenital myasthenia are not available, but it is considered to be a very rare condition.
Causes and symptoms
Most cases of congenital myasthenia are inherited in a recessive fashion, meaning that a baby has to receive a defective gene from each parent to actually manifest the condition.
Babies with congenital myasthenia are often described as "floppy," with weak muscle tone, droopy eyelids, excessive fatigue, compromised eye movements, facial weakness, feeding problems and delayed developmental milestones (such as holding up head, sitting, crawling). In more severe conditions, the muscles that aid breathing are affected, resulting in respiratory difficulties.
The baseline degree of weakness is exacerbated by any activity, including feeding, crying, or moving. Episodes of more severe symptoms may be precipitated by illness, emotional upset, or fever. Some cases of congenital myasthenia progress over time, so that initially mild symptoms can become more severe as the individual ages.
The diagnosis of congenital myasthenia will usually be suspected when a careful physical examination reveals muscle weakness that is worsened by use of a particular muscle. Certainly, a family history of congenital myasthenia heightens such a suspicion.
A test called electromyography measures muscle activity after stimulation. When muscle activity decreases with repeated stimulation, congenital myasthenia is suspected. Testing the blood for the presence of specific antibodies can help distinguish between myasthenia gravis and congenital myasthenia. Very specific microelectrode testing of the muscle endplate receptors can help define whether faulty receptors are responsible for the impairment. Genetic testing and muscle biopsy examination are being researched, but are not currently used for routine diagnosis.
Children with congenital myasthenia will usually be treated by a team consisting of a pediatric neurologist, as well as a physical therapist, occupational therapist, and speech and language therapist. If respiratory problems ensue, a pulmonologist and respiratory therapist may need to be consulted.
There are no treatments available to cure congenital myasthenia. A number of medications may improve symptoms in children with congenital myasthenia. The specific medication that will be most helpful depends on whether the impairment is due to decreased ACh production and release, impaired enzyme degradation of ACh, or faulty ACh receptors in the muscle endplates. Some of the types of medications available include:
- Anticholinesterase medications: Inhibit the degradation of ACh, allowing more to be available to stimulate muscles.
- 3,4, diaminopyridine: Increases the release of ACh from the nerve cells.
- Qunidine or fluoxetine: Prevents overstimulation of ACh receptors on muscle endplates, thus preventing muscles from damage secondary to prolonged stimulation.
The severity of symptoms, responsiveness to medication, and ultimate prognosis varies widely among congenital myasthenia patients.
"Nutritional Disorders of the Neuromuscular Transmission and of Motor Neurons." In Nelson Textbook of Pediatrics, edited by Richard E. Behrman, et al. Philadelphia: W. B. Saunders Company, 2004.
Rose, Michael, and Robert C. Griggs. "Congenital Myasthenias." In Textbook of Clinical Neurology, edited by Christopher G. Goetz. Philadelphia: W. B. Saunders Company, 2003.
Muscular Dystrophy Association. 3300 East Sunrise Drive, Tucson, AZ 85718. (800) 572-1717. firstname.lastname@example.org. <http://www.mdausa.org>.
Rosalyn Carson-DeWitt, MD