leading to a breakdown in communication between the nervous system and the muscle, resulting in muscle fatigue, and sometimes paralysis. Autoantibodies against acetylcholine receptors are detectable in 70–90% of patients with generalized MG, but only 50% in ocular MG. Predilection for eye muscles The precise reasons for the preferential involvement of eye muscles in MG is not well understood, but there are several lines of thought. Functional hypotheses propose that although multiple muscles may be affected, a deficit may be more readily apparent in the eyes for several reasons. 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 patient's 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