Cases

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Myasthenia gravis (MG): MG was suspected and confirmed by administering a Tensilon test. For this test, 1.0 mL of 1% edrophonium chloride (10 mg/mL, Tensilon), an acetylcholinesterase (AChE) inhibitor, is prepared for intravenous injection. The patient is given an initial dose of 0.1 mL and observed for a response. If the patient's symptoms do not change—as in this case—the remaining 0.9 mL is administered, and the response observed again. With the second dose, this patient's ptosis resolved within seconds but then returned after 5 minutes. MG is an acquired autoimmune disorder clinically characterized by weakness of skeletal muscles and fatigability on exertion. The antibodies in MG are directed toward the acetylcholine receptor (AChR) at the neuromuscular junction (NMJ) of skeletal muscles. The antibodies reduce the number of functioning AChRs and thus interfere with the normal binding of acetylcholine (ACh) to AChRs on the postsynaptic membrane. Because AChE normally hydrolyzes ACh molecules, the administration of an AChE inhibitor, such as edrophonium, causes a transient improvement in symptoms, which strongly suggests MG. The disease occurs in 2 main groups of patients: women in their 20s or 30s and men older than 60 years. The cardinal feature is fatigue and muscle weakness that increases with exercise of the affected muscles but improves with rest. Weakness often begins in the ocular muscles, but it may also begin in the bulbar muscles or in the muscles of the limbs and trunk (Brooke, 1986). Ocular symptoms, such as ptosis or double vision, occur in up to 65% of patients (Oosterhuis, 1982). Bulbar weakness is the presenting symptom in less than 25%. A myasthenic snarl (transverse smile) may be seen when the bulbar muscles are affected. Other bulbar manifestations include nasal speech devoid of consonant sounds (due to weakness of the palate and tongue), difficulty chewing, and (less often) difficulty swallowing and choking on liquids. The problem with chewing may be so severe that patients must support their chin with their hand to finish mastication of solid food. This support may also be necessary if patients have neck-extensor weakness, which can cause their head to fall forward uncontrollably. Limb weakness, mostly in the extensors of the arms and in the flexors of the legs, is the least common initial complaint (14-27% of patients). Symptoms and signs may fluctuate and be absent in the morning. MG is most easily confirmed with an intravenous injection of an AchE inhibitor, such as edrophonium, with which an unequivocal improvement in ptosis or extraocular muscle weakness constitutes a positive response. However, responses can be equivocal or uninterpretable. If so, various electrodiagnostic tests, such as repetitive nerve stimulation or single-fiber electromyography, can demonstrate a defect in neuromuscular transmission. Finally, antibodies against the acetylcholine receptor (AChR-Abs) are present in 80-90% of patients with generalized MG and in about 50% of patients with ocular myasthenia (Phillips, 1990). MG is one of the most treatable neurologic disorders, with more than 90% of patients recovering near-normal function. AChE inhibitors (eg, pyridostigmine, neostigmine) and immunomodulators (eg, steroids, azathioprine, cyclosporine A) are the mainstays of treatment and used in most patients. Other therapeutic options are plasmapheresis, intravenous immunoglobulin (IVIG), and thymectomy. Plasmapheresis (removal of circulating AChR-Abs and immune complexes) is effective. This last option is especially useful in preparation for surgery (because the patient can achieve rapid postoperative recovery) or in short-term management of an exacerbation; improvement is noted within days, though it lasts only 6-8 weeks. The mechanism of action for IVIG in MG is unknown. IVIG often quickly reverses an exacerbation. IVIG is also an alternative to plasmapheresis or use of multiple immunosuppressive agents in patients with refractory myasthenia (Richman, 2003). Thymectomy has been proposed as first-line therapy for patients with generalized MG. The procedure usually induces complete remission, especially in young patients with a hyperplastic thymus, high antibody titer, and short duration of disease (Drachman, 1994).