Diagnostic and therapeutic uses of anticholinesterases
THE CONTEXT
This passage continues myasthenia gravis testing and describes several clinical uses and limitations of neostigmine and physostigmine.
Testing for myasthenia gravis
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Short-lasting improvement in the strength of affected muscles is described in myasthenia gravis, rather than other muscular dystrophies.
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If edrophonium is unavailable, the test can use neostigmine 1.5 mg intravenously. Atropine pretreatment may block its muscarinic effects.
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Myasthenic patients are highly sensitive to d-tubocurarine: 0.5 mg intravenously causes marked weakness but is ineffective in non-myasthenic patients. This provocative test is hazardous.
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Demonstration of receptor antibodies in plasma or a muscle biopsy specimen is described as a more reliable test.
Postoperative uses and cobra bite
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Postoperative paralytic ileus or urinary retention may be relieved by neostigmine 0.5–1 mg subcutaneously, provided no organic obstruction is present; this use is uncommon.
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For postoperative decurarization, intravenous neostigmine rapidly reverses paralysis caused by competitive neuromuscular blockers. Atropine or glycopyrrolate is administered beforehand to block muscarinic effects.
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Cobra venom contains a curare-like neurotoxin. Specific antivenom serum is the primary treatment, while neostigmine with atropine can prevent respiratory paralysis.
Physostigmine in poisoning
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Physostigmine is described as a specific antidote for belladonna and other anticholinergic poisoning. It penetrates the blood–brain barrier and antagonizes both central and peripheral actions.
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Physostigmine can itself cause hypotension, arrhythmias and undesirable central effects, so it is reserved as a last resort. Neostigmine does not block central effects but is described as less risky.
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Tricyclic antidepressants, phenothiazines and many antihistamines have additional anticholinergic properties. Physostigmine partly antagonizes their overdose symptoms and coma, but may worsen the fall in blood pressure and arrhythmias.
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Physostigmine has a modest nonspecific arousal effect in central nervous system depression caused by diazepam or general anaesthetics, but this use is rarely warranted.
Alzheimer's disease
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Alzheimer's disease is described as a neurodegenerative disorder with progressive dementia, primarily affecting cholinergic neurons in the brain.