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Key Features

Essentials of Diagnosis

  • Fluctuating weakness of voluntary muscles, producing symptoms such as

    • Diplopia

    • Ptosis

    • Difficulty in swallowing

  • Activity increases weakness of affected muscles

  • Short-acting anticholinesterases transiently improve the weakness

General Considerations

  • Occurs at all ages, sometimes in association with

    • Thymic tumor

    • Thyrotoxicosis

    • Rheumatoid arthritis

    • Lupus erythematosus

  • Onset is usually insidious, but the disorder is sometimes unmasked by a coincidental infection

  • Exacerbations may occur before the menstrual period and during or shortly after pregnancy

  • Symptoms are due to blocks of neuromuscular transmission caused by autoantibodies binding to acetylcholine receptors

  • The external ocular muscles and certain other cranial muscles, including the masticatory, facial, and pharyngeal muscles, are especially likely to be affected

  • The respiratory and limb muscles may also be involved

Demographics

  • Most common in young women with HLA-DR3

  • If thymoma is associated, older men are more commonly affected

Clinical Findings

Symptoms and Signs

  • Initial symptoms

    • Ptosis

    • Diplopia

    • Difficulty in chewing or swallowing

    • Respiratory difficulties

    • Limb weakness

    • Some combination of these problems

  • Weakness

    • May remain localized to a few muscle groups, especially the ocular muscles

    • May become generalized

  • Symptoms often fluctuate in intensity during the day

  • This diurnal variation is superimposed on a tendency to longer-term spontaneous relapses and remissions that may last for weeks

  • Clinical examination confirms the weakness and fatigability of affected muscles

  • Extraocular palsies and ptosis, often asymmetric, are common

  • Pupillary responses are normal

  • The bulbar and limb muscles are often weak, but the pattern of involvement is variable

  • Sustained activity of affected muscles increases the weakness, which improves after a brief rest

  • Sensation is normal

  • Usually no reflex changes

Differential Diagnosis

  • Lambert-Eaton myasthenic syndrome (usually paraneoplastic)

  • Botulism

  • Aminoglycoside-induced neuromuscular weakness

Diagnosis

Laboratory Tests

  • Elevated level of serum acetylcholine receptor antibodies has a sensitivity of 80–90%

  • Certain patients have serum antibodies to muscle-specific tyrosine kinase (MuSK), which should be determined

    • These patients are more likely to have facial, respiratory and proximal muscle weakness than those with antibodies to acetylcholine receptors

  • Other associated antibodies

    • Low-density lipoprotein receptor-related protein 4 (LRP4)

    • Agrin

    • Tests for these antibodies are not widely commercially available

Imaging Studies

  • CT scan of the chest with and without contrast

    • Should be obtained to demonstrate a coexisting thymoma

    • However, normal studies do not exclude this possibility

Diagnostic Procedures

  • Electrophysiology

    • Demonstration of a decrementing muscle response to repetitive 2- or 3-Hz stimulation of motor nerves indicates a disturbance of neuromuscular transmission

    • Such an abnormality may even be detected in clinically strong muscles with certain provocative procedures

  • Needle electromyography

    • Shows a marked variation in configuration and size of individual motor unit potentials in affected muscles

    • Single-fiber electromyography reveals an increased jitter, ...

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