Definition of Trochlear nerve:
Trochlear nerve is the fourth cranial nerve or cranial nerve IV and is entirely motor. It is named Trochlear because it supplies superior oblique muscle of the eye which operates through the pulley like Trochlea.
Among all cranial nerves, the Trochlear nerve has some special features these are.
- It contains the least number of axons so it is the smallest nerve.
- It has a very long course in the cranium so it is the nerve with the greatest length of the cranium.
- All the cranial nerves exit from the skull via ventral aspect but It exits from the skull via dorsal aspect of brain stem.
- It innervates superior oblique muscle on the opposite side from its nucleus.
- It is the only nerve in which all the lower motor neuron fibers decussate.
Embryology of Trochlear nerve :
Trochlear nerve is derived from the basal plate of the embryonic midbrain.
Nucleus of Trochlear nerve:
The Trochlear nucleus is situated in the anterior part of the gray matter that surrounds the cerebral aqueduct of the midbrain . It lies inferior to the oculomotor nucleus at the level of the inferior colliculus . The nerve fibre after leaving the nucleus pass posteriorly around the central Gray matter to reach the posterior surface of the midbrain.
The Trochlear nerve receives corticonuclear fibers from both cerebral hemispheres. It receives the tectobulbar fibers which connect it to visual cortex through the superior colliculus. It also receives fibers from the medial longitudinal fasiculus, by which it is connected to the nuclei of the third, sixth and eighth cranial nerves.
Course of Trochlear Nerve :
As discussed earlier that Trochlear nerve is the most slender of the cranial nerves and it is the only nerve which leaves the posterior surface of the brainstem, emerges from the midbrain and immediately decussates with the nerve of the opposite side. It passes forward through the middle cranial fossa in the lateral wall of the cavernous sinus and enters the orbit through the superior orbital fissure.
Function of Trochlear nerve :
Trochlear nerve innervates the superior oblique muscle of the contralateral eye ball. It is entirely motor and assists in turning the eye downward, laterally, rotate inward and abducts the eye.
Trochlear nerve palsy symptoms :
Damage to the Trochlear nerve result in Trochlear nerve palsy and patient will present with these symptoms.
- Outward rotation of the affected eye called extortion .
- Vertical diplopia.
- Downward gaze weakness and the patient will have difficulty in descending stairs.
- Head tilt opposite to the side of affected eye.
Clinical significance of Trochlear nerve:
1. Vertical diplopia also called double vision :
This is due to the weakness of downward eye movement and the affected eye drift upward relative to the normal eye due to this unopposed movement of both eyes patient will experience diplopia.
2. Torsional Diplopia :
The weakness of the intorsion results in torsional diplopia in which two different visual fields, tilted with respect to each other are seen at the same time.
Examination of Trochlear Nerve :
Trochar nerve is examined by examining the action of the muscle innervated by the Trochlear nerve.
The patient is asked to look down and look in the patient is asked to converge its gaze, read the newspaper and walk downstairs and diplopia associated with these activities is noticed as it is the initial symptom of fourth nerve palsy.