Cranial Nerve 6: Abducens Nerve

A 6th Nerve Palsy can be quite common in exams, as they can be caused via extra-ocular muscle pathology, rather than any intrinsic lesion in the brain, therefore it is still vitally important.

Anatomy

1. The 6th nucleus lies in the floor of the 4th ventricle at the Pons
2. Axons of the facial nerve loop around the 6th nerve this is called the facial collicus. Note there is a significantly close relationship between the 6th and 7th Nerve at the level of the pons
3. It runs up and anterior to the pons and pierces the dura overlying the basilar part of the occipital bone and runs in Dorello’s canal (piece of bone consisting of the inferior petrosal sinus and 6th nerve) prior to their entrance within the Cavernous Sinus.
4. It makes a sharp turn towards the Cavernous Sinus and runs along the tip of the Petrous temporal bone
5. The 6th nerve has the longest intra-cranial course in comparison to all the cranial nerves.
6. The 6th nerve has a significantly close relationship with the 7th nerve at the petrous temporal bone and greater superficial petrosal nerves. This occurrs at the base of the skull

 

Diagram to show the facial colliculus and the close relationship of the 6th and 7th nerve in the floor of the 4th ventricle

 

 

Basic Representation of the 6th Nerve

 

Anatomy of the Skull Base

 

The 6th Nerve Supplies the lateral rectus muscle and therefore any lesion affecting the 6th nerve can cause diplopia on lateral gaze with the inability of the affected eye to abduct.

Causes of a 6th Nerve Palsy

Like with any exam question it is important to have a structure in your head whilst answering. My guide to answering any question like this is to use an anatomical sieve. This will be more evident in the facial nerve section.

1. Fourth Ventricle- 4th ventricular tumour
2. Pons- tumour, infarct, demyelinating disease
3. Petrous bone- raised intra-cranial pressure, basal skull fracture, skull base tumour
4. Cavernous sinus- thrombosis, laterally expanding pituitary mass
5. Medical- diabetes, vasculitis

It is a very simple but effective approach scoring full marks in the exam!