Ocular Aspects of Myasthenia Gravis

Jason J. S. Barton, MD, PhD, FRCPC, and Mohammad Fouladvand, MD, Human Vision and Eye movement Laboratory, Departments of Neurology and Ophthalmology, Beth Israel Deaconess Medical Center, Harvard Medical School; and the Department of Biomedical Engineering, Boston University, Boston, Massachusetts.

Semin Neurol. 2000;20(1) 

In This Article

Treatment

Management of myasthenia also includes awareness and avoidance of medications that can worsen neuromuscular transmission ( Table 5 ).[112,121,122] Most frequently reported112 are aminoglycoside antibiotics, beta-blockers,[123,124] and chloroquine. Other clinically important associations include other antibiotics, anti-arrhythmics, calcium channel blockers,[125] some anticonvulsants, and intravenous iodinated contrast.[126] On occasion these drugs may cause weakness in a previously normal subject: in some, this may be the unmasking of underlying myasthenia.[121] Penicillamine does not impair neuromuscular transmission, but may induce auto-antibodies, causing an iatrogenic myasthenic syndrome.[112,121]

Symptomatic treatment can be offered. The mainstays are acetylcholinesterase inhibitors such as pyridostigmine and neostigmine.[45] Ninety percent of patients have some benefit from such drugs, although this may be relatively mild in half.[127] In particular, these may be less effective for diplopia than ptosis.[6] Their main side effects are dose-related and due to muscarinic excess, such as hypotension, bradycardia, excess salivation, abdominal cramps, and diarrhea. Relative contraindications include asthma, arrhythmias, and prostatic hypertrophy. Laxatives may reduce absorption of these drugs.[122]

The major risk of acetylcholinesterase inhibitors in myasthenic patients is cholinergic crisis, in which excessive acetylcholine causes weakness due to depolarization block. Patients unaware that weakness may be caused by an excess as well as a deficiency of medication run this risk if they increase their dose routinely as a response to weakness. Cholinergic crisis may be suspected if weakness is accompanied by salivation, abdominal cramping, and muscle fasciculations; however, it is not always possible to differentiate cholinergic crisis from myasthenic crisis (subacute severe weakness due to cholinergic deficiency), even with an edrophonium test. The safest course is usually to admit the patient to an intensive care unit, consider ventilatory support, stop all medications temporarily, and treat with plasma exchange or possibly intravenous immunoglobulin.

Physical adaptive therapies can be used also as symptomatic treatment, with varying success. While an eye patch is always effective against diplopia, prisms are difficult to use in situations where ocular alignment fluctuates from one minute to the next. Lid crutches are generally more hindrance than help for ptosis. Only in long-standing cases of "burned out" myasthenia, where careful documentation has revealed no further change in severely limited eye muscles, can strabismus surgery be considered, although it is rarely done.

Immunosuppression with drugs is an important goal in the treatment of generalized myasthenia.[122] Low-dose, alternate-day prednisone is helpful in almost 90% of patients with ocular myasthenia also,[127] and improves diplopia in about 75%,[128] with minimal side effects. Initiation of steroid therapy traditionally involves higher dose daily regimens, which can be associated with transient worsening of myasthenic weakness in the first few weeks.[129] Azathioprine is widely used as a steroid-sparing (or steroid-reducing) immunosuppressant in the long-term treatment of myasthenia.[122] There is little data on its use in ocular myasthenia; however, its combination with low-dose prednisone was helpful in 91% of a small sample of 23 patients.[127] It requires monitoring for neutropenia, bone marrow depression, and hepatitis, and is teratogenic.[45] Other immunosuppressant drugs such as cyclosporine[130] are more toxic and there is even less data on their use for ocular myasthenia.

Thymectomy is indicated in those patients with demonstrated thymoma, of course, and is helpful in inducing remission in 10-30% and improvement in up to 80% of patients with generalized myasthenia.[122,131] Whether it should be routinely used in other patients with pure ocular myasthenia, with or without suspected thymic hyperplasia, is more controversial. Most neurologists limit its use in ocular myasthenia to those with severe signs.[132] However, the evidence is mixed. Some claim that it offers no benefit,[127] while others claim significant improvement in 80% of patients who fail to respond or relapse with drug therapy.[133]

Short-term immunomodulation can be achieved with plasmapharesis and intravenous immunoglobulin. These are usually reserved for severe systemic weakness and crisis, and are seldom required for ocular myasthenia.

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