Cycloplegics and mydriatics Flashcards
How do cycloplegics work?
Cycloplegics are muscarinic antagonist drugs that temporarily paralyze the ciliary muscle and block accommodation.
These effects are mediated through their action on the parasympathetic nervous system (PSNS), specifically by blocking muscarinic acetylcholine receptors (mAChRs); They compete with acetylcholine.
How do mydriatics work?
Mydriatics are muscarinic antagonist drugs. They dilate the pupil (mydriasis) by inhibiting the sphincter pupillae muscle of the iris.
These effects are mediated through their action on the parasympathetic nervous system (PSNS), specifically by blocking muscarinic acetylcholine receptors (mAChRs); They compete with acetylcholine.
Describe the normal action of ciliary muscle and iris sphincter muscle and how cycloplegics and mydriatics then interfere?
Normally Iris sphincter pupillae causes miosis whilst ciliary muscle contracts for accommodation.
- Muscarinic receptors, M3 receptors stimulated.
- These receptors are coupled with G protein GQ.
- This then triggers a cascade of events to increase Ca2+.
- This activates myosin leading to contraction of muscle.
The muscarinic antagonists block these effects. Cycloplegic effects paralyse the ciliary muscle so no accommodation. Mydriatic effect paralyse the iris sphincter pupillae so pupil dilates.
How does phenylephrine work as a mydriatic?
(Has no cycloplegic effect only mydriatic)
- Acts on noradrenaline receptors, alpha 1 receptors stimulated.
- These receptors are coupled with G protein GQ.
- This then triggers a cascade of events to increase Ca2+.
- This activates contraction of iris dilator pupillae for mydriasis.
How does tropicamide work as a mydriatic?
Tropicamide has high affinity for M3 receptors but an even higher affinity for M4. M4 receptors are much more prominent on the iris sphincter muscle, thus tropicamide is better suited for mydriatic effects.
- M4 receptors stimulated.
- These receptors are coupled with G protein GI.
- This then increases Ca2+ intracellular levels
- This activates contraction of iris sphincter muscle, miosis.
- Tropicamide blocks these effects to get mydriasis.
What are the cycloplegic drugs?
Atropine (lasts 1-3h) (recovery 6-12 days) Strongest
Cyclopentolate (lasts 30-60mins) (recovery 24h)
Tropicamide (lasts 30mins) (recovery 6h) Weakest
What are the mydriatic drugs?
Atropine (lasts 45-60mins) (recovery 7days)
Cyclopentolate (lasts 30-60mins) (recovery 24h)
Tropicamide (lasts 15-40mins) (recovery 6h)
Phenylephrine (lasts 10-90mins) (recovery 5-24h)
Why does phenylephrine have such a broad duration?
It is highly protein bound so binds to iris melanin. If px has dark coloured iris then dose will be less effective and will last longer.
How can you prolong the exposure of the drug to the eyes?
Use an ophthalmic insert which is a combined formulation of tropicamide and phenylephrine.
You put the insert in the eye on the lid and leave it to interact with the eye surface.
What are the uses of cycloplegics?
- Examining refractive error
- Diagnosing lazy eye (amblyopia)
- Treating lazy eye (amblyopia)
- Treating ciliary spasms
- Treating accommodative spasms
- Treating pain due to inflammation of anterior uveitis (this works by stopping ciliary spasm)
- Treating atony (ciliary muscle weakness)
- Blood aqueous barrier stabilisation in uveitis
What are the uses of mydriatics?
- Examining in ophthalmoscopy/fundoscopy
- Diagnosing pathologies at the back of the eye
- Treating anterior uveitis to prevent posterior synechiae
- Treating glaucoma caused by anterior displacement of the iris-lens diaphragm
- Dilation of pupil for surgical procedures
Name side effects of cycloplegics
Local:
1. Blurring
2. Stinging
3. Increased IOP
Systemic:
1. Dry mouth
2. Tachycardia
3. Facial flushing
CNS:
1. Ataxia
2. Hallucinations
3. Disorientation
Name side effects of mydriatics
Local:
1. blurring
2. Stinging
3. Increased IOP
Systemic:
1. Cardiovascular effects - due to alpha adrenoreceptor activation from phenyephrine)
2. Tachycardia
3. Headaches
4. Subcranial haemorrhage
What cautions should you take when using cycloplegics/mydriatics?
- Caution with other medications with antimuscarinic effects
- Children under 3 months may develop amblyopia
- Dark pigmented irises
- Small amount present in breastfeeding milk
What contraindication should you keep an eye out for when using cycloplegics/mydriatics?
Narrow angle glaucoma:
if dilation doesnt occur then this is important to take note of as it may be due to papillary block, iris bombe. Could end up completely closing the angle.
What are the muscarinic receptors in the eye?
- Mostly M3 = on iris sphincter, ciliary muscle, ciliary body, trabecular meshwork
- M1, M4 = on iris sphincter, ciliary muscle, ciliary epithelium
- M2 = on longitudinal portion of cililary muscle, prejunctional iris, trabecular meshwork
- M5 = on iris sphincter
