(See also Overview of Peripheral Nervous System Disorders Overview of Peripheral Nervous System Disorders The peripheral nervous system refers to parts of the nervous system outside the brain and spinal cord. It includes the cranial nerves and spinal nerves from their origin to their end. The anterior... read more .)
Disorders of neuromuscular transmission may involve
Breakdown of acetylcholine within the synapse (eg, due to drugs 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 Stiff-Person Syndrome Stiff-person syndrome is a CNS disorder that causes progressive muscle stiffness and spasms. (See also Overview of Peripheral Nervous System Disorders.) Stiff-person syndrome (formerly called... read more , Isaacs syndrome Isaacs Syndrome Isaacs syndrome is an autoimmune peripheral nerve disorder that causes neuromuscular manifestations, including continuous muscle twitching (myokymia). (See also Overview of Peripheral Nervous... read more ) have neuromuscular manifestations.
Eaton-Lambert syndrome Neurologic paraneoplastic syndromes Paraneoplastic syndromes are symptoms that occur at sites distant from a tumor or its metastasis. Although the pathogenesis remains unclear, these symptoms may be secondary to substances secreted... read more is due to impaired acetylcholine release from 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.
Also due to impaired release of acetylcholine from presynaptic nerve terminals, botulism Botulism Botulism is poisoning that is due to Clostridium botulinum toxin and that affects the peripheral nerves. Botulism may occur without infection if toxin is ingested, injected, or inhaled. Symptoms... read more 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.
Drugs or toxic chemicals
Cholinergic drugs, organophosphate insecticides Organophosphate Poisoning and Carbamate Poisoning Organophosphates and carbamates are common insecticides that inhibit cholinesterase activity, causing acute muscarinic manifestations (eg, salivation, lacrimation, urination, diarrhea, emesis... read more , and most nerve gases Nerve Chemical-Warfare Agents Nerve agents are chemical-warfare agents that act directly at nerve synapses, typically increasing the activity of acetylcholine. Other chemical agents were used in combat before World War II... read more (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. Long-term penicillamine treatment may cause a reversible syndrome that clinically and electromyographically resembles myasthenia gravis Myasthenia Gravis Myasthenia gravis involves episodic muscle weakness and easy fatigability caused by autoantibody- and cell-mediated destruction of acetylcholine receptors. It is more common among young women... read more . Excessive magnesium orally or IV (with blood levels approaching 8 to 9 mg/dL [4 to 4.5 mmol/L]) can also induce severe weakness resembling a myasthenic syndrome. Immune checkpoint inhibitors (eg, ipilimumab, nivolumab, pembrolizumab), a class of anticancer drugs, have immune-related adverse effects in < 1% of patients; however, these adverse effects (which include myasthenia gravis) continue to be reported.
Treatment consists of eliminating the drug or toxic chemical and providing necessary respiratory support and intensive nursing care. Atropine 0.4 to 0.6 mg orally 3 times a day decreases bronchial secretions in patients with cholinergic excess. Higher doses (eg, 2 to 4 mg IV every 5 minutes) may be necessary for organophosphate insecticide or nerve gas poisoning.