The College of Optometrists

I know that for atropine a tonus allowance has to be made and not for cyclopentolate.

Why is this? What is tonus allowance exactly? How much allowance has to be made?

this is due to the way the drugs work. Tonus is the physiological temper the muscle has. When you place anti muscarinic drug in the eye the drug selectively blocks the receptors causing the tonus to change. The more receptors blocked - the greater the tonus change. Also - as soon as the receptors are blocked by the drug - the drug will 'go off' - it will be dispersed by the natural hormones within the receptor which counter it. So the actual tonus is a product of the effectiveness of the drug and how persistant it is.

Atropine is very persistant and also very effective = so it relaxes the muscle the most - and lasts the longest.

Now, as the tonus is pretty constant - get rid of the tonus and the eye becomes artificially long sighted. This you have to account for in your tonus correction.

I hope this helps - steve