Ocular myasthenia

Ocular myasthenia gravis (MG) is a disease of the neuromuscular junction resulting in hallmark variability in muscle weakness and fatigability. MG is an autoimmune disease where anomalous antibodies are produced against the naturally occurring acetylcholine receptors in voluntary muscles. MG may be limited to the muscles of the eye (ocular MG), leading to abrupt onset of weakness/fatigability of the eyelids or eye movement. MG may also involve other muscle groups (generalized MG). Ocular myasthenia gravis (MG) is a disease of the neuromuscular junction resulting in hallmark variability in muscle weakness and fatigability. MG is an autoimmune disease where anomalous antibodies are produced against the naturally occurring acetylcholine receptors in voluntary muscles. MG may be limited to the muscles of the eye (ocular MG), leading to abrupt onset of weakness/fatigability of the eyelids or eye movement. MG may also involve other muscle groups (generalized MG). Although these blocking antibodies may be confined to one of the larger muscles responsible for moving the face or appendages or for breathing, about 90% of MG patients eventually have eye involvement. The most common symptoms are double vision (diplopia) and eyelid drooping (ptosis), whereas the pupil is always spared. Diplopia occurs when MG affects a single extraocular muscle in one eye, limiting eye movement and leading to double vision when the eye is turned toward the affected muscle. Ptosis occurs when the levator palpebrae superioris (the muscle responsible for eyelid elevation) is affected on one or both sides, leading to eyelid drooping. Although these symptoms may not be readily apparent in well-rested patients, weakness can usually be induced with exercise of the commonly affected muscles (e.g. by having the patient look upward for about 60 seconds). In 75% of MG cases, the initial manifestation is in the eye. Within 2 years, 80% of patients with ocular onset of MG will progress to involve other muscle groups, thereby developing generalized MG. If MG is confined to the ocular muscles for more than 3 years, there is a 94% likelihood that the symptoms will not worsen or generalize. Aside from asymmetric ptosis (which becomes worse with fatigue, sustained upgaze, and at the end of the day) and variable limitation of extraocular muscles/diplopia, other clinical signs of ocular MG include gaze-evoked nystagmus (rapid, involuntary, oscillatory motion of the eyeball) and Cogan’s lid twitch (upper lid twitch present when patient looks straight ahead after looking down for 10–15 seconds). Normally, muscle contraction is a result of electrical signals sent from the central nervous system to muscle fibers via nerve impulses. At the neuromuscular junction, this electrical message is converted into a chemical message as acetylcholine is released from nerve fibers and attaches to corresponding receptors on the muscle fiber. In MG, antibodies are produced that block acetylcholine receptors, preventing the molecule from binding to the receptor and 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.