Myasthenia Gravis

Myasthenia gravis (MG) is an autoimmune disease in which antibodies against nicotinic acetylcholine receptors (Ach-R) both decrease the number of receptors and change the morphology of the neuromuscular junction (NMJ). The trigger for the autoimmune response is unknown, but a drug-induced form is reversible. Incidence follows a bimodal distribution occurring in women in their second and third decades and in men in their sixth and seventh decades.

Causes

Myasthenia gravis is caused by a defect in the transmission of nerve impulses to muscles. It occurs when normal communication between the nerve and muscle is interrupted at the neuromuscular junction - the place where nerve cells connect with the muscles they control. Normally when impulses travel down the nerve, the nerve endings release a neurotransmitter substance called acetylcholine. Acetylcholine travels through the neuromuscular junction and binds to acetylcholine receptors which are activated and generate a muscle contraction.

In myasthenia gravis, antibodies block, alter, or destroy the receptors for acetylcholine at the neuromuscular junction which prevents the muscle contraction from occurring. These antibodies are produced by the body's own immune system. Thus, myasthenia gravis is an autoimmune disease because the immune system - which normally protects the body from foreign organisms - mistakenly attacks itself.

Symptoms

• Blurry vision and eyelid weakness ++++
• Generalized weakness ++++
• Other bulbar muscles become affected resulting in dysarthria, dysphagia, weakness in the muscles of mastication, and a characteristic nasal-sounding voice. The muscles of the neck, diaphragm, and proximal extremity muscles then become involved in a descending fashion.

Signs

• Weakness of bulbar muscles, particularly eyelid weakness
• Reflexes are usually normal.
• Sensation and mental status are intact.
• Affected muscles are easily fatigable.

Treatment

There are therapies available to help reduce and improve muscle weakness. Medications used to treat the disorder include anticholinesterase agents such as neostigmine and pyridostigmine, which help improve neuromuscular transmission and increase muscle strength. Immunosuppressive drugs such as prednisone, cyclosporine, and azathioprine may also be used. These medications improve muscle strength by suppressing the production of abnormal antibodies. They must be used with careful medical followup because they may cause major side effects.