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Anticholinergics, Eye:, Atropine vs hyoscine, Gastrointestinal tract: ,…
Anticholinergics
These drugs act by blocking muscarinic(antimuscarinic) or nicotinic receptors.
Drugs blocking NM receptors are called neuromuscular blocking agents
and those blocking NN receptors are called ganglion blockers.
Eye:
anticholinergic drugs cause mydriasis and cycloplegia.
Atropine, homatropine, cyclopentolate and tropicamide are used as mydriatic and cycloplegic agents.
Mydriatic action is useful in fundus examination(maximum part of retina can be visualized).
Whereas cycloplegic action allows correct assessment of refractive error
due to loss of error resulting from accomadation.
Further pain in iridocyclitis occur due to spasm of ciliary muscle which can be relieved due to cycloplegic action.
Atropine vs hyoscine
Atropine is the longest acting mydriatic which has very long duration of action(3-5 days) in the eye
therefore avoided in adults whereas it has shorter action in other organs.
Hyoscine possess similar cycloplegic action and more potent mydriatic action as compared to atropine.
Its duration of action is also quite long but less than atropine.
Conventional systemic doses of atropine has little effect on eye
whereas equal dose of scopolamine produce definite mydriasis and cycloplegia.
Gastrointestinal tract:
anticholinergic drugs decrease the motility, tone and secretions in the gastrointestinal tract.
Pirenzepine and telenzepine are selective M1 blockers useful in peptic ulcer disease.
Hyoscine, dicyclomine, propantheline, oxyphenonium and clidinium
are useful as antispasmodic agents for the treatment of intestinal colic.
Darifenacin and solefenacin are selective M3 blockers
useful for irritable bowel syndrome and overactive bladder.
Genitourinary tract:
anticholinergic drugs decrease the motility of urinary tract and thus may result in urinary retention
therefore contraindicated in BPH.
Dicyclomine, flavoxate and oxybutynin are useful for the treatment of urinary incontinence(detrusor instability) and renal colic.
Tolterodine, fesoterodine( a prodrug of tolterodine),
darifenacin and solefenacin (selective M3 antagonist) are also useful for urinary incontinence.
Trospium
has minimum CNS penetration because of quarternary amine structure.
It has thus lesser risk of causing impairment of cognition and is safe in elderly also.
It is the only drug from this group that can be used with AcHE inhibitors.
Used for overactive bladder.
Trospium is the only drug in this group that is not metabolized by liver.
Thus it is safe to be used with CYP inhibitors.
Other uses:
Botulinum toxin type A has been approved for the
treatment of strabismus, blepharospasm, cervical dystonia and glabellar lines
whereas botulinum toxin type B has been approved for the treatment of cervical dystonia.
Onabotulinum toxin A has recently been approved
to prevent headaches in adult patients with chronic migraine
(given every 12 weeks as multiple injections)
Atropine
Atropine is the drug of choice for early mushroom poisoning due to inocybe species.
It is contraindicated in poisoning due to amanita muscaria.
Thiotic acid is useful for the late mushroom poisoning due to amanita phalloides.
It is also the drug of choice for organophosphate and carbamate poisoning.
It is used along with neostigmine to decrease its muscarinic side effects for the treatment of myasthenia gravis and cobra bite.
It is also added to diphenoxylate(anti-motility) drug to reduce its addictive potential.
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Adverse effects:
these include dry mouth,
blurred vision due to mydriasis and cycloplegia,
urinary retention, constipation, hyperthermia,
confusion, delirium and restlessness etc.
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CNS:
atropine is a CNS stimulant whereas scopolamine causes CNS depression.
Due to its amnesic and CNS depressant action,
hyoscine induces “twilight sleep”
and has been used as a lie detector or truth serum in suspects.
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Cardiovascular system:
atropine causes bradycardia initially due to inhibition of presynaptic muscarinic receptors M2
but further increase in dose causes tachycardia due to inhibition of post-synaptic M2 receptors.
Useful in the management of arrythmias like AV block and digitalis induced bradycardia.
It has negligible effect on BP and cardiac contractility.
Respiratory system:
anticholinergic drugs reverse the bronchoconstriction caused by stimulation of M3 receptors.
Ipratropium and tiotropium bromide are muscarinic receptor antagonists useful in the treatment of COPD and bronchial asthma.
Ipratropium has non selective action on all muscarinic receptors
whereas tiotropium is somewhat selective blocker of M1 and M3 receptors.
Glycopyrollate is used as pre-anaesthetic medication
to decrease the secretions and reflex bronchospasm during general anaesthesia.
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glands:
anticholinergic drugs decrease the secretion and cause dry mouth, reduced sweating, salivation and lacrimation.
Atropine is contraindicated in children due to risk of hyperthermia due to decreased sweating.
Early mushroom poisoning
Early mushroom poisoning is due to inocybe and clitocybe species.
Symptoms manifest within thirty minutes and are of cholinergic excess like diarrhea, lacrimation and bradycardia.
Therefore DOC is atropine.
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Behavioural therapy(bladder training, pelvic floor exercises, fluid management) is the first line of treatment for overactive bladder whereas antimuscarinic drugs are second line treatment.
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