Tuesday, May 29, 2018

Facebook: ANS and dilated pupil.

So, this post is regarding answer of our recent pharmacology question posted on facebook medicowesome group. If you still are not following it, please follow for latest updates and interesting questions.

Q) What is the probable diagnosis in a patient with a dilated pupil not responsive in 1% pilocarpine? (AIIMS 2011 Nov)

1) Diabetic 3rd nerve palsy.
2) Adie's tonic pupil.
3) Uncal herniation.
4) Pharmacological block.

Answer is option 4- Pharmacological block.

Let's start with the basics of the ANS to understand the question.

First, imagine an eye with simplest of the structures.
A central area called as pupil. Surrounded by group of muscles called as "constrictor pupillae muscles" which obviously helps in constriction (miosis) of the pupil as their name suggests. They are  further surrounded by "radial muscles" which causes dilation (and mydriasis) of the eye.

Now, each of these muscles will have receptors on them. Receptors need to be stimulated for their respective actions, right? - Yes!
Now, remember - M3 receptors are present on constrictor muscles and Alpha-1 receptors are present on radial muscles.

Now, you must be thinking why I am goofing around with such simple basic concepts?!
Hold on, question may contain confusing options but you already know the answer. Atleast now you do know!
(Read question again and come back!)

Pilocarpine is selective M3 agonist. Stimulation of M3 receptors will lead to miosis.

In pharmacological block, drugs like atropine block the muscarinic receptors present on the pupil. As, the receptors cannot work, pilocarpine cannot produce miosis.

( You really don't need to know all the things to answer MCQs. Sometimes basics are enough!)

Now, let us know more about other options.

1) Diabetic 3rd nerve palsy: Occulomotor motor nerve supplies constrictor puplillae So, palsy of 3rd nerve will cause mydriasis but does it cause any damage to receptors? - No.
So, pilocarpine will respond and thus miosis will occur!

2) Adie's tonic pupil: It manifests as denervation supersenstiviy. Normal pupil responds to 1% pilocarpine but not to dilated solution like 0.05-0.1%. However, in Adie's pupil due to supersenstiviy of receptors, even this diluted solution may results in constriction.

3) Uncal herniation: Pressure on 3rd nerve causes pupil dilation but again it will respond to pilocarpine as receptors are intact.

Some important MCQ points related to above information.
1) Echothiophate is also M3 agonists. It is an anti-glaucomic drug which acts by promoting drainage of fluid via schlemm's canal.
Adverse effect: Causes cataract.

2) Adrenergic drugs causes mydriasis (Stimulation of Alpha-1 receptors) and Anticholinergic drugs (Inhibiting M3 receptors) causes mydriasis and cycloplegia.

1 comment:

  1. Thankyou so much.... Its really healpful in clearing concepts...


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