The sixth cranial nerve (CNVI) is also named the abducens nerve. It only controls eye movement from the lateral rectus muscle of the eye. The primary action is to help the eye move outward, towards the ear - this action is called ABDUCTION - hence, ABducens nerve. CNVI is unique in that it has a long path to its origin that takes sharp turns. The long pathway, location, and anatomical structures that this nerve courses over make it uniquely susceptible to damage from elevated intracranial pressure.
All cranial nerves (all nerves for that matter) have a nucleus (cell body) and axon (that carries nerve impulses away from the nucleus to other structures). The nucleus of CNVI is located in the pons of the brainstem. The axons from CNVI follow the floor of the skull, over the petrous portion of the temporal bone, eventually penetrating through the cavernous sinus and ending in the abducens muscle of the eye.
A quick review of the location and destination of CNIV: -Starts in the PONS -Long course along the base of the skull and over the petrous portion of the temporal bone -Ends in the lateral rectus extraocular muscle
Damage to the trochlear nerve can occur anywhere along its long path. The long portion of this nerve makes it susceptible to small increases in intracranial pressure. The change in pressure can compress or stretch the nerve. An aneurysm, tumor, or abnormal cerebrospinal fluid draining may cause a CNVI palsy. The nerve is also susceptible to poor blood flow from disorders such as high blood pressure or diabetes and is known to be the most likely damaged nerve from viral diseases and demyelination disorders, such as multiple sclerosis.
Patients often have an eye that deviates inwards due to underaction of the lateral rectus muscle. The result is an inability to move the eye outwards, towards the ear.
Root Atlas has a great set of videos that help explain how damage to CNVI affects a patient. Check it out below: