A Clinical Case from the Archives : 26/02/2006

This young labrador is brought to you with persistently dilated pupils but apparently no deficiency with her sight. The owner happens to mention that the dog seems overly interested in a small number of amphibians (she doesn’t know if they are frogs or toads) at the bottom of the garden. Could that have anything to do with this abnormality?

The pupils do seem fixed and dilated – there is no constriction with a bright light. Glaucoma worried me but the dog is normally sighted and the intraocular pressures were normal at 16mmHg. This is then an efferent pupillary defect and might well be related to a pharmacological pupil dilation. Amphibians like this common British toad are known to excrete anti-cholinergic toxins from their skin (Tsuneki et al Alkaloids indolizidine 235B’, quinolizidine 1-epi-207I, and the tricyclic 205B are potent and selective noncompetitive inhibitors of nicotinic acetylcholine receptors.
Mol Pharmacol. 2004;66:1061-9 is a recent report but one I don’t expect you have come across in your ophthalmology reading!) so this is a possible cause of mydriasis but the dog has no other signs of parasympatholytic activity – we await the owner presenting the toad to us as well as the dog!

 

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