Use of mydriatic-cycloplegic drugs
As an optometrist who has also worked in eye casualty in hospital I commonly use tropicamide 1 per cent and phenylephrine 2.5 per cent eye-drops for pupil dilation (mydriasis) for indirect ophthalmoscopy.
Tropicamide is an anticholinergic (like atropine) and blocks the cholinergic receptors on the sphincter pupillae muscle in the iris. This muscle is stronger than the other muscle in the iris (the dilatator pupillae muscle) so the tropicamide 1 per cent does usually give a good pupil dilation for indirect ophthalmoscopy.
Phenylephrine is a sympathomimetic like adrenaline and acts on the a-receptors on the dilatator pupillae to cause pupil dilation.
As already stated, the sphincter pupillae muscle is much stronger than the dilatator pupillae muscle so phenylephrine cannot be used on its own; it would not give a sufficiently good pupil dilation. By continuing tropicamide and phenylephrine, this usually gives a slightly better pupil dilation than using tropicamide alone. The pupil will return to its normal undilated state after use of the above combination of drugs within 6 to 8 hours.
So, Mrs Titcomb is quite correct in stating that tropicamide is chosen for routine ophthalmoscopy because it has a rapid onset and brief duration of action.
Unfortunately, the combination of tropicamide and phenylephrine may not properly dilate an eye with a very dark iris because of the mechanical blockade of the drug molecules by the pigment cells and/or the pigment cells binding the drug molecules, as stated by Mrs Titcomb. If this is the case, I tend to use cyclopentolate 1 per cent to try to achieve a better pupil dilation. This is because it seems to be more potent than tropicamide. If one has a very difficult patient, for example, someone with learning difficulties, then the relative or carer could be given atropine eye-drops or ointment to instil for three days prior to the appointment. As discussed by Mrs Titcomb and Dr Wilson, the atropine has a very high affinity for the melanin in the pigment granules in the iris and therefore is released slowly, giving a very prolonged pupil dilation.
The anticholinergics also have a cycloplegic effect: they block the cholinergic receptors on the ciliary muscle, thus paralysing this muscle. This is especially important in young children, particularly those with a squint (lazy eye). Cycloplegia reveals the full refractive error and so the child can be given the correct spectacle strength.
If I may correct Mrs Titcomb in her letter (PJ, January 22, 2000, p133), she stated that optometrists select atropine for cycloplegic refraction. I do not think there is one optician in the country that stocks atropine. Most optometrists’ first-choice cycloplegic is cyclopentolate 1 per cent eye-drops. Indeed the first choice cycloplegic in most hospital eye departments is also cyclopentolate 1 per cent. I have found that cyclopentolate 1 per cent is sufficient in nearly all cases to give adequate cycloplegia.
If cyclopentolate does not give adequate cycloplegia, then atropine 1 per cent eye ointment is used for the three days prior to the child going to the hospital eye department. Unfortunately, on first instilling any of the cycloplegics into the eyes, they cause a stinging sensation for up to about l minute, which is not ideal in young children. Atropine stings less than cyclopentolate.
I hope this furthur informs pharmacists about the use of these drugs.
Citation: The Pharmaceutical Journal URI: 20000304
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