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- Explain the action of mydriatic eye drops and contra-indications to their use
- There are two types of mydriatic drops available and affect the eye iris muscles in different ways. The most commonly used drug is "tropicamide" which is commonly available in 0.5% & 1% minims for single dose usage, and is a muscarinic antagonists (antimuscarinic) which BLOCKS ophthalmic parasympatholytic pathways, preventing the iris sphincter muscle from responding to nerve signals to constrict thus inducing indirectly mydriasis (pupil dilation).
- The second most commonly used mydriatic is "phenylephrine hydrochloride" which is commonly available in 2.5% minims for single dose usage, and acts as a sympathomimetic which MIMICS ophthalmic sympatholytic pathways causing activation of the iris dilator muscles. This causes the dilator muscle to contract, which makes the pupil dilate
- It should be noted that anecdotally in this author opinion, that phenylephrine is rarely used solely for the purpose of pupil mydriasis; lest patients report previous negative reaction to tropicamide. We have had two patients at work who have had an allergic reaction to tropicamide 1% solution resulting them in attending HES (negative drug reaction was NOT due to angle closure). Said patients have and since gone on to insist on either 2.5% phenylephrine or 0.5% tropicamide drops whenever they need dilated examinations
- https://www.drugbank.ca/drugs/DB00809
- https://en.wikipedia.org/wiki/Tropicamide
- https://www.moorfields.nhs.uk/content/pupil-dilating-eye-drops-ointments-mydriatics
- https://www.netdoctor.co.uk/medicines/eye-care/a7116/minims-phenylephrine-hydrochloride/
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