medical condition

Ocular myasthenia
Specialty Ophthalmology

Ocular myasthenia gravis ( MG ) is a disease of the neuromuscular junction resulting in hallmark unevenness in muscle helplessness 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 attack of weakness/fatigability of the eyelids or eye bowel movement. MG may besides involve other muscle groups ( generalized MG ) .

Signs and symptoms [edit ]

Although these blocking antibodies may be confined to one of the larger muscles creditworthy for moving the face or appendages or for breathe, about 90 % of MG patients finally have eye participation. The most common symptoms are bivalent sight ( diplopia ) and eyelid drooping ( ptosis ), whereas the schoolchild is always spared. Diplopia occurs when MG affects a single extraocular muscle in one eye, limiting eye apparent motion and leading to double vision when the eye is turned toward the moved muscle. ptosis occurs when the levator palpebrae superioris ( the muscle creditworthy 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 normally be induced with exercise of the normally affect muscles ( e.g. by having the patient spirit upward for about 60 seconds ). In 75 % of MG cases, the initial materialization is in the eye. Within 2 years, 80 % of patients with ocular onset of MG will progress to involve other brawn groups, thereby developing generalized MG. [ citation needed ] If MG is confined to the eyepiece muscles for more than 3 years, there is a 94 % likelihood that the symptoms will not worsen or generalize. [ citation needed ]

aside from asymmetrical ptosis ( which becomes worse with fatigue, sustained upgaze, and at the end of the day ) and varying restriction of extraocular muscles/diplopia, early clinical signs of eyepiece MG include gaze-evoked nystagmus ( rapid, involuntary, oscillatory gesticulate of the eyeball ) and Cogan ’ s lid twitch ( upper eyelid twitch present when patient looks straight ahead after looking down for 10–15 seconds ) .

Pathophysiology [edit ]

normally, brawn contraction is a resultant role of electrical signals sent from the central nervous system to muscle fibers via steel impulses. At the neuromuscular articulation, this electrical message is converted into a chemical message as acetylcholine is released from nerve fibers and attaches to corresponding receptors on the muscle roughage. In MG, antibodies are produced that block acetylcholine receptors, preventing the atom from binding to the sense organ and leading to a breakdown in communication between the skittish organization and the muscle, resulting in muscle fatigue, and sometimes paralysis. Autoantibodies against acetylcholine receptors are detectable in 70–90 % of patients with generalize MG, but lone 50 % in eyepiece MG .

predilection for eye muscles [edit ]

The precise reasons for the discriminatory interest of eye muscles in MG is not well understand, but there are respective lines of think. functional hypotheses propose that although multiple muscles may be affected, a deficit may be more promptly apparent in the eyes for respective reasons. Slight weakness in a arm may be tolerated, but slight weakness in the extraocular muscles would lead to misalignment of the two eyes, even a belittled degree of which could lead to diplopia. Eyes may besides be less able to adapt to variable weakness, because extraocular muscles use ocular quite than proprioceptive ( body position-sensing ) cues for calibrate. Immunologic hypotheses proposes that there may be differences in the antibodies in ocular MG versus generalized MG that may favor the muscles responsible for center bowel movement and eyelid natural elevation.

Physiologic hypotheses propose that it is the singular 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 shrink muscles in the soundbox. 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 [edit ]

The variable course of MG may make the diagnosis difficult. In brief, the diagnosis of MG relies by and large on the patient ‘s history and physical findings, with especial attention to neurologic, eye motion, and eyelid examination. frequently, patients will describe experiencing alternating ptosis ( hat wilt in one eye that gets better, then is followed by ptosis in the other eye ), a well as diplopia that worsens during the day ( with increasing extraocular muscleman tire ). 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 individual muscle fibers to determine if there is muscle weakness show. The diagnosis of MG can besides be confirmed with blood ferment that measures the amount of blocking antibody present, but alone 70 % of ocular MG patients have detectable antibody levels. Additional lab and effigy tests for normally associated thyroid, thymus gland and autoimmune diseases are besides advisable .

discussion [edit ]

The prognosis tends to be good for patients with MG. It is frequently 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 transmit 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 forcible factors such as bright sunlight, stress, viral illness, menstruation, pregnancy, etc. spontaneous absolution can occur in any affected role and persist 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 dispatch absolution. Patients with mild-to-moderate ocular myasthenia are normally treated initially with oral anticholinesterase agents, Mestinon ( pyridostigmine ) being the most normally employed. There have not been any randomized clinical trials conducted with these agents, and this treatment is frequently unsuccessful, particularly in resolving diplopia. immunosuppressive therapy is then started and the agentive role of option is normally prednisone. In a little controlled study this drug demonstrated greater efficacy than pyridostigmine. [ 1 ] Steroid therapy is controversial, but in another survey the results suggested that prednisone does decrease progress to generalized MG. [ 2 ] There is no single recommended dosing regimen in light of the side effects normally associated with chronic corticosteroid therapy, and the difficulty in weaning patients from steroids without exacerbation of symptoms. Response to prednisone therapy is variable star.

additionally, MG patients should be examined for thymomas, and if found, should undergo surgery to address this condition. A preventive thymectomy is controversial, but has been shown to be helpful in young MG patients with acute disease within 3 years of disease onset, in patients with blow up thymus gland glands and for whom surgery is low-risk, and patients with generalize MG who are unresponsive to medical treatment. The symptoms of ocular MG can besides be addressed by non-medicinal means. ptosis can be corrected with placement of crutches on eyeglasses and with ptosis record to elevate eyelid wilt. Diplopia can be addressed by occluded front with eye patch, frosted lens, occluding contact lens, or by simply placing opaque tape over a fortune of eyeglasses. besides, plastic prism ( Fresnel prism ) can be attached to eyeglasses of a diplopic affected role, allowing for alliance of vision from both eyes in the affect direction, but are much baffling if the degree of muscle weakness, and therefore ocular misalignment, fluctuates frequently .

epidemiology [edit ]

In contrast to generalized MG, strictly ocular MG occurs equally among females and males, has a higher incidence in persons of korean descent, and is likely associated with thyroid gland disease, thymomas ( 20 % incidence ), [ 3 ] and other autoimmune diseases such as scleroderma, systemic lupus erythematosus, arthritic arthritis, Hashimoto ‘s thyroiditis, multiple sclerosis, and thyroid ophthalmopathy .

Notes [edit ]

References [edit ]