Basic Anatomy in MRCP(II)

Basic Anatomy in MRCP (II)

I have covered the first six cranial nerves in my previous post, today we are going to talk about another a few interesting points about the first six cranial nerves that are frequently asked in MRCP.


1) Correlation of third cranial nerve with pupil size is an important topic to study. Generally, if you can still remember as a medical student, isolated third nerve palsy is always divided into either a 'medical' or 'surgical' third nerve palsy. As you can remember from my previous post, I told you that third nerve palsy should cause patient to have dilated pupil due to its parasympathetic component.

A COMPLETE third nerve plasy ( Above patient - right third nerve palsy) should have:

i) Dilated pupil (mydriasis)
ii) Ptosis
iii) Deviation of eye laterally and downward ( due to unopposed actions of lateral rectus and superior oblique)

However, you may encounter a patient with third nerve palsy having ptosis and deviation of eye laterally and downward but without dilated pupil. This is what we call as 'medical' third nerve palsy. 'Surgical' third nerve palsy is usually due to compression of the nerve (e.g. by tumour, posterior communicating or posterior cerebral artery aneurysms) results in an acute total (painful) third nerve palsy with a dilated unreactive pupil. Pupillary dilatation occurs early when the nerve is compressed since sympathetic nerve fibres that innervate the iris are carried on the outside of the nerve bundle. Pupillary sparing is characteristic of third nerve lesions caused by infarction in patients with diabetes mellitus and hypertension.

2) Isolated 6th nerve palsy is possible and commonly found in patient with increased intracranial pressure. It is termed as ' False localizing sign' because actually you can’t localize any location at the brain that causing this palsy if you observe only isolated 6th nerve palsy. It occurs only to 6th cranial nerve in increased intra cranial pressure because of its long course intracranially and makes it prone to compression when there is an increased pressure inside the skull.