Myasthenia gravis (MG) is a disorder of neuromuscular transmission characterized by weakness and fatigability of skeletal muscles (ocular, bulbar, limb, and respiratory muscles)1,2. The underlying defect mechanism is the binding of antibodies to the components of the acetylcholine receptors (AChRs) at neuromuscular junctions. MG is the most common disorder of neuromuscular transmission. The incidence occurs in 2 to 7 patients out of a population of 10,000 as far as the prevalence. Women are affected more frequently than men with a ratio of about 3:2. It is estimated that MG affects about 60,000 American and more than 700,000 people worldwide.
MG is not inherited nor contagious. The development of the disorder is spontaneous later in life. There's a temporary form of myasthenia gravis that may develop in newborns when a woman with MG passes the antibodies to her fetus. Neonatal MG usually resolves in 2-3 months. Individuals with MG may experience flare-ups and remission periodically, but remissions are rarely permanent or complete.
There are two clinical forms of MG, ocular and generalized1,2. The ocular myasthenia causes the weakness but is limited to the eyelids and extraocular muscle. In generalized disease, the weakness commonly impacts ocular muscles, but it also affects a variable combination of bulbar, limb, and respiratory muscles. The most common symptoms are visual problems due to ptosis (drooping eyelids) and diplopia (double vision) as two thirds' of all MG patients have ocular related MG. Other common symptoms include sudden, severe muscle weakness and fatigue in the neck or limbs, which change in intensity over days or even hours or worsen as muscles are used. A mask-like appearance or smile may appear grimace, and/or the patients may have difficulty swallowing or pronouncing words.
The diagnosis of MG is established by the history and typical examination findings1,2. A serologic test for autoantibodies and electro-physiologic studies is a reliable laboratory method that can be used to confirm MG. Electro-physiologic studies consist of repetitive nerve stimulation studies and single-fiber electromyography. Anti-AChR antibodies are detectable in about 85% of all MG patients. Only 50% of these patients with weakness are confined to ocular muscles. A negative anti-AChR test does not exclude the disease. In this case, the practitioner should test for Muscle-Specific Kinase (MuSK) antibodies as a diagnostic tool. It is estimated that about 40% of AChR antibody-negative patients with generalized MG present with MuSK. MuSK antibodies are rarely found in AChR antibody-positive patients, or in those with MG limited to ocular muscles. MG patients without anti-AChR or –MuSK may have antibodies to Lipoprotein receptor-related protein-4 (LRP4). Anti-LRP4 antibodies are found in approximately 9.2% (range 2% to 50%) of MG patients who are negative for both anti-AChR and anti-MuSK. LRP4 test may be used in these patients. Bed side diagnosis is another approach. Bed ...