Slight weakness in a limb may be tolerated, but slight weakness in the extraocular muscles would lead to misalignment of the two eyes, even a small degree of which could lead to diplopia. Eyes may also be less able to adapt to variable weakness, because extraocular muscles use visual rather than proprioceptive (body position-sensing) cues for fine-tuning. Immunologic hypotheses proposes that there may be differences in the antibodies in ocular MG versus generalized MG that may favor the muscles responsible for eye movement and eyelid elevation. Physiologic hypotheses propose that it is the unique structure and function of extraocular muscles that predispose them to weakness in MG. Compared to extremity muscles, extraocular muscles are smaller, served by more nerve fibers, and are among the fastest contracting muscles in the body. This higher level of activity may predispose them to fatigue in MG. Additionally, some reports indicate that there may be fewer acetylcholine receptors in extraocular muscles versus limb muscles. Diagnosis
The variable course of MG may make the diagnosis difficult. In brief, the diagnosis of MG relies mostly on the patients history and physical findings, with particular attention to neurologic, eye motility, and eyelid exams. Frequently, patients will describe experiencing alternating ptosis (lid droop in one eye that gets better, then is followed by ptosis in the other eye), as well as diplopia that worsens during the day (with increasing extraocular muscle fatigue). A tensilon (edrophonium chloride) test can be used, which temporarily blocks the breakdown of acetylcholine, and briefly relieves weakness; however, false-negative results are common.
Single-fiber electromyography can be used to electrically stimulate single muscle fibers to determine if there is muscle weakness present. The diagnosis of MG can also be confirmed with blood work that measures the amount of blocking antibody present, but only 70% of ocular MG patients have detectable antibody levels. Additional lab and image tests for commonly associated thyroid, thymus and autoimmune diseases are also advisable. Treatment
The prognosis tends to be good for patients with MG. It is often best not to treat mild cases of MG. Management necessitates avoidance of medications that can worsen neuromuscular transmission, such as aminoglycoside antibiotics, quinolone antibiotics, beta-blockers, chloroquine, anti-arrhythmics, calcium channel blockers, some anticonvulsants and intravenous iodinated contrast should be avoided. MG is characteristically variable in course, with the frequency of diplopia and ptosis affected by environmental, emotional and physical factors such as bright sunlight, stress, viral illness, menstruation, pregnancy, etc. Spontaneous remission can occur in any patient and remain for years. In a study of the natural history of generalized MG among 168 patients (with an average follow-up of 12 years), 14% experienced complete remission. Patients with mild-to-moderate ocular myasthenia are usually treated initially with oral anticholinesterase agents, Mestinon (pyridostigmine) being the most commonly employed. There have not been any randomized clinical trials conducted with these agents, and this treatment is often unsuccessful, particularly in resolving diplopia. Immunosuppressive therapy is then started and the agent of choice is usually prednisone. In a small controlled study this drug demonstrated greater efficacy than pyridostigmine.