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Home arrow Environment arrow Inflammatory Disorders of the Nervous System: Pathogenesis, Immunology, and Clinical Management

Clinical Feature

The cardinal feature of MG is fluctuating weakness that is fatigable, worsening with repetitive activities and improving with rest. Weakness is worsened by exposure to heat, infection, and stress [3]. The fluctuating nature of weakness distinguishes MG from other disorders which present with weakness. Typically, the weakness involves specific skeletal muscle groups. The distribution of the weakness is generally ocular, bulbar, proximal extremities, and neck, and in a few patients, it involves the respiratory muscles. In patients with MG, the weakness is mild in 26%, moderate in 36%, and severe in 39%, associated with dysphagia, depressed cough, and reduced vital capacity [27]. Extraocular muscle (EOM) weakness is by far the most common initial symptom of MG, occurring in approximately 85% of patients. Generalized progression will develop in 50% of these patients within 2 years [27]. Early MG usually presents with fluctuating ptosis and diplopia. Diplopia can be elicited by having the patient look laterally for 20-30 s resulting in eye muscle fatigue. The ptosis can be unilateral or bilateral, and sustained up-gaze for 30 or more seconds will usually induce it. The ptosis can be severe enough to totally occlude vision. The most commonly involved EOM is the medial rectus. But, on clinical examination, usually more than one extraocular muscle is weak with pupillary sparing. The weakness does not follow any pattern of specific nerve or muscle involvement, distinguishing it from other disorders such as vertical gaze paresis, distinct cranial nerve palsy, or internuclear ophthalmoplegia (INO).

Bulbar muscle involvement during the course of MG can be seen in approximately 60% of patients. It may present as fatigable chewing, particularly on chewing solid food with jaw closure more involved than jaw opening [38, 39]. Painless dysphagia and dysarthria may be the initial presentation in approximately 15% of patients [39]. The lack of ocular involvement in these patients may result in misdiagnosis as motor neuron disease or primary myopathy. Weakness involving respiratory muscles is rarely the presenting feature of MG, but respiratory insufficiency certainly may occur later as the disease progresses [35]. Respiratory muscle weakness can lead to myasthenic crisis which can be life threatening, requiring mechanical ventilation. It can be precipitated by infections and certain medications such as aminoglycosides, telithromycin, neuromuscular blocking agents, magnesium sulfate, beta-blockers, and fluoroquinolone antibiotics.

Involvement of the limbs in MG produces predominantly proximal muscle weakness. The upper extremities tend to be more often affected than the lower extremities. Occasionally predominant distal muscle weakness occurs [40]. Facial muscles are frequently involved and can make the patient appear expressionless. Neck extensor and flexor muscles are commonly affected. The weight of the head may overcome the extensors, producing a “dropped head syndrome.” Although it has become evident that the natural course of MG with adequate treatment is general improvement in 57% and remission in 13% after the first 2 years, severe weakness can be accompanied by high mortality. Only 20% of patients remain unchanged, and mortality from the disease is 5-9%. Only 4% of the patients who survive the first 2 years become worse. Of those who will develop generalized myasthenia, virtually, all do so by 2-3 years [3].

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