Why are people at more risk of acute angle-closure glaucoma when a tropicamide and phenylephrine combination is used for pupil dilation than when just tropicamide is used?
My experience of using drops to dilate pupils in order to get a better view of the retina is as follows. Eye specialists working in primary care practice-usually community optometrists-who are licensed to use ophthalmic drugs use 1% tropicamide to dilate the pupil. Another name from tropicamide is Mydriacil. Tropicamide is available as a single dose unpreserved preparation and as a multi-use preserved preparation.
Tropicamide is an anticholinergic that blocks the responses of the sphincter muscle of the iris and the ciliary muscle to cholinergic stimulation. In other words, the sphincter muscle is prevented from closing down the pupil and this means that the unopposed dilator muscle pulls the pupil open. Its effect on the ciliary body will not be covered here.
With people who have dark irises, the pigment in the iris soaks up some of the tropicamide which reduces how well it works. A way to get around this is to add another drop to each eye after about 20 minutes of the first drop.
Systemic absorption may be minimised by compressing the lacrimal sac at the medial canthus for one minute during and after the instillation of the drops. This blocks the passage of the drops via the nasolacrimal duct to the wide absorptive area of the nasal and pharyngeal mucosa. In my experience, patients are very rarely asked to do this.
Maximal pupil dilation occurs after 30 to 40 minutes. The effects of the drops normally last between two and four hours but can be up to six hours. Whenever I’ve had tropicamide the effects take about 24 hours to completely go away.
Eye specialists working in secondary care, such as ophthalmologists often use 0.5% and sometimes 1% tropicamide followed by a drop of 2.5% phenylephrine. I cannot find anything about whether the order of drop instillation is important. I can’t think of any reason to think that it would be.
Phenylephrine is a directly acting sympathomimetic agent used topically in the eye as a mydriatic. This means that it acts on the dilator muscle causing it to pull the pupil open. Maximal pupil dilation occurs in 60 to 90 minutes with recovery after 5 – 7 hours. The fact that maximal pupil dilation occurs after an hour or more may explain why patients have to wait so long in eye hospitals. I’ve never experienced phenylephrine in my eyes so can’t comment on how long its effects last.
The combination of tropicamide and phenylephrine means that there is more dilation. Drugs act on the sphincter muscle, preventing it from working, and on the dilator muscle causing it to overwork. This ‘super dilation’ means that more of the iris is folded into the anterior chamber angle and there is a greater chance that effective aqueous drainage is prevented. ‘Super dilation’ is, therefore, more likely to result in acute closed-angle glaucoma in people with narrowed anterior chamber angles.