Disorders of neuromuscular transmission affect the neuromuscular junction; they commonly cause fluctuating muscle weakness with no sensory deficits.
(See also Overview of Peripheral Nervous System Disorders.)
Disorders of neuromuscular transmission may involve
Postsynaptic receptors (eg, in myasthenia gravis)
Presynaptic release of acetylcholine (eg, in botulism)
Breakdown of acetylcholine within the synapse (eg, due to medications or neurotoxic chemicals)
The most common disorder that affects neuromuscular transmission is myasthenia gravis.
Some disorders that affect other areas of the body primarily (eg, stiff-person syndrome, Isaacs syndrome) have neuromuscular manifestations.
Eaton-Lambert syndrome
Eaton-Lambert syndrome is a rare autoimmune neuromuscular junction disorder involving impaired acetylcholine release from presynaptic nerve terminals; it is due to loss of P/Q-type voltage-gated calcium channels on the presynaptic nerve terminals. Repetitive nerve testing at rapid rates (20 to 50 hertz [Hz]) or a single supramaximal stimulation before and after 10 seconds of maximal isometric exercise shows an incremental response of up to 400%. Increases of > 100% are considered diagnostic of a presynaptic disorder of neuromuscular transmission, but an increment of ≥ 60% or greater is highly suggestive.
Botulism
Also due to impaired release of acetylcholine from presynaptic nerve terminals, botulism develops when toxin produced by Clostridium botulinum spores irreversibly binds to a specific receptor (synaptotagmin II) on the presynaptic terminal cholinergic nerve endings. The result is severe weakness, sometimes with respiratory compromise and difficulty swallowing. Other systemic symptoms may include mydriasis, dry mouth, constipation, urinary retention, and tachycardia due to unopposed sympathetic nervous system activity (anticholinergic syndrome). These systemic findings are absent in myasthenia gravis.
In botulism, electromyography (EMG) detects a mild decremental response to low-frequency (2- to 3-Hz) repetitive nerve stimulation but a pronounced incremental response after 10 seconds of exercise or with rapid (50-Hz) repetitive nerve stimulation.
Medications or toxic chemicals
Cholinergic medications, organophosphate insecticides, and most nerve gases (eg, sarin) block neuromuscular transmission by excessive acetylcholine action that depolarizes postsynaptic receptors. Miosis, bronchorrhea, abdominal cramps, diarrhea, and myasthenic-like weakness (cholinergic syndrome) result.
Aminoglycoside and polypeptide antibiotics decrease presynaptic acetylcholine release and sensitivity of the postsynaptic membrane to acetylcholine. At high serum levels, these antibiotics may increase neuromuscular block in patients with latent myasthenia gravis
