Pharmacology
Parasympathomimetics
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Get access- Either directly stimulate muscarinic receptors or inhibit acetylcholinesterase
- Prolong the effect of ACh
- Pilocarpine:
- Miotic action on the iris
- Contraction of the longitudinal muscle of the CB
- Pulls on the scleral spur to open up the uveo-trabecular meshwork
- Blocks the uveoscleral drainage route so drainage is confined to the canal of Schlemm
Reduces blood flow to the CB so also reduces aqueous production
Side effects: conjunctival toxicity, iris cysts, cataract, systemic absorption, retinal detachment, punctal stenosis, headaches
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Low dose/diluted pilocarpine can be used to diagnosis Adie’s pupil due to supersensitive iris sphincter caused by loss of postganglionic nerve fibres
Edrophonium: increases concentration of acetylcholine at neuromuscular junction (competitive acetylcholinesterase inhibitor)
Used in Tensilon test to diagnose myasthenia gravis: administered intravenously and any improvement in ptosis or diplopia confirms the diagnosis
Longer acting anticholinesterases (neostigmine, pyridostigmine) can be used to maintain neuromuscular function as treatments
Parasympathetic antagonists (anti-muscarinics)
- Block the site of ACh at muscarinic receptors
- Examples (NB: these are mydriatic AND cycloplegic):
- Atropine: 1% can cause mydriasis for upto 7 days
- Homatropine: 2% causes mydriasis for 6 hours to 4 days
- Cyclopentolate: 1% causes mydriasis for 24 hours
- Tropicamide : 1% causes mydriasis for 4-6 hours
Note
Darker irides generally take longer to attain maximum mydriasis
- Side effects
- Dry mouth
- Facial flushing
- Sweating
- Tachycardia
- Conjunctival hyperaemia
- Blurred near vision
- Photophobia
- Transient rise in IOP (may precipitate angle-closure)
Botulinum toxin: inhibits presynaptic acetylcholine release by irreversible blockage of presynaptic receptors and thus prevents neuromuscular transmission
Cleaves SNARE proteins which are needed for fusion of neurosecretory vesicles with the plasma membrane
- Thus blocks ACh exocytosis
- Long-lasting blockade
- Poisoning is associated with progressive parasympathetic and motor paralysis
- Used to treat blepharospasm and in strabismus management