Neuromuscular transmission disorder characterized by fluctuating weakness and fatigability of bulbar and other voluntary muscles without loss of reflexes or impairment of sensation or other neurologic function.

 

Diagnostic criteria:

A. Characteristic signs and symptoms

One or more of the following:

1. Diplopia, ptosis, dysarthria, weakness in chewing, difficulty in swallowing, muscle weakness with preserved deep tendon reflexes, and, less commonly, weakness of neck extension and flexion, and weakness of trunk muscles

2. Increased weakness during exercise and repetitive use with at least partially restored strength after periods of rest

3. Dramatic improvement in strength following administration of anticholinesterase drug (edrophonium (Tensilón®) and neostigmine);

and one or more of the following:

B. EMG and repetitive stimulation of a peripheral nerve: In myasthenia gravis repetitive stimulation at a rate of 2 per second shows characteristic decremental response which is reversed by edrophonium or neostigmine. Single fiber studies show increased jitter.

C. Antibodies to Acetylcholine Receptors

 

Exclusions:

• Congenital myasthenic syndrome, progressive restricted myopathies, steroid and inflammatory myopathies, motor neuron disease

• Multiple sclerosis, variants of Guillain-Barré syndrome (e.g., Miller-Fisher syndrome)

• Organophosphate toxicity, botulism, black widow spider venom

• Eaton-Lambert syndrome

• Stroke

• Medications: neuromuscular blocking agents, aminoglycosides, penicillamine, antimalarial drugs, colistin, streptomycin, polymyxin B, tetracycline

• Hypokalemia; hypophosphatemia

 

Severity: (Osserman classification):

 

• I: Ocular myasthenia
• IIA: Mild generalized myasthenia with slow progression: no crises, responsive to drugs
• IIB. : Moderately severe generalized myasthenia : severe skeletal and bulbar involvement but no crises; drug response less than satisfactory
• III: Acute fulminating myasthenia, rapid progression of severe symptoms, with respiratory crises and poor drug response
• IV: Late severe myasthenia, same as III but progression over 2 years from class I to II

 

 

References: