Trochlear Nerve Definition:
The trochlear nerve (the fourth cranial nerve, also called the fourth nerve, IV) is a motor nerve (a “somatic efferent” nerve) that innervates a single muscle: the superior oblique muscle of the eye.
Trochlear Nerve Nuclie:
The nucleus of the trochlear nerve is located in the caudal mesencephalon beneath the cerebral aqueduct. It is immediately below the nucleus of the oculomotor nerve (III) in the rostral mesencephalon.
The trochlear nucleus is unique in that its axons run dorsally and cross the midline before emerging from the brainstem. Thus a lesion of the trochlear nucleus affects the contralateral eye. Lesions of all other cranial nuclei affect theipsilateral side except the optic nerves – cranial nerve II – which innervate both eyes.
Trochlear Nerve Course:
The trochlear nerve emerges from the dorsal aspect of the brainstem at the level of the caudal mesencephalon, just below the inferior colliculus. It circles anteriorly around the brainstem and runs forward toward the eye in the subarachnoid space. It passes between the posterior cerebral artery and the superior cerebellar artery, and then pierces the dura just under free margin of the tentorium cerebelli, close to the crossing of the attached margin of the tentorium and within millimeters of the posterior clinoid process. It runs on the lateral wall of the cavernous sinus, where it is joined by the other two extraocular nerves (III) and the first two branches of the trigeminal nerve (V), Ophthalmic V1 and Maxillary V2. The internal carotid artery and Abducens nerve (VI) run within the cavernous sinus. Finally, it enters the orbit through the superior orbital fissure and innervates the superior oblique muscle.
The superior oblique muscle ends in a tendon that passes through a fibrous loop, the trochlea, located anteriorly on the medial aspect of the orbit. Trochlea means “pulley” in Latin; the fourth nerve is named after this structure.
Trochlear Nerve Palsy Causes and symptoms:
Causes of fourth nerve palsy can be broadly classified as congenital or acquired. Isolated congenital palsies may be heralded by head-tilting to the opposite side of the affected nerve in early childhood. In others a congenital palsy may go unnoticed because of a compensatory mechanism allowing for alignment of the eyes when focusing on an image.
Isolated acquired trochlear nerve palsies can be the result of numerous disorders. Most commonly an underlying cause cannot be found and this is known as an idiopathic palsy. Due to its long course within the brain, the fourth nerve is susceptible to injury following severe head trauma. Depending on the site of nerve compression during trauma one or both nerves may be affected. Aneurysms orbrain tumors may directly compress or result in an increase of intracranial pressure(the pressure within the skull) resulting in nerve palsies.
Disorders such as myasthenia gravis, diabetes, meningitis, microvascular disease (atherosclerotic vascular disease) or any cause of increased intracranial pressure may result in trochlear nerve palsy. A congenital palsy that has gone undetected may manifest itself in adulthood when the compensatory mechanism for ocular alignment is lost. Additionally the removal of a cataract may restore clear vision to both eyes allowing the patient to become aware of their double vision.
A child with a congenital palsy may be found doing a head tilt by his or her parents or relatives. Children will very rarely complain of double vision.
Adults with a new onset fourth nerve palsy will note two images, one on top of the other or angled in position when both eyes are open. Covering of one eye, no matter which one is covered, will resolve their diplopia. Their double vision will worsen when looking down or away from the affected side. If both nerves are affected he or she may experience a horizontal diplopia (two images side by side) when looking downward. If a decompensated palsy is suspected, one should review old photographs to document a pre-existing head tilt to support the diagnosis.
Trochlear Nerve Videos:
Action of Superior Oblique Muscle
Cranial Nerve 4 Palsy
Fourth nerve palsies effect the superior oblique muscle (trochlea).
Trochlear Nerve Nuclie ,Course and Palsy Notes