The trochlear nerve is the fourth cranial nerve and carries motor fibers to the superior oblique muscle. With only approximately 3400 axons, the trochlear nerve is the thinnest and weakest of the twelve cranial nerves1.
Anatomical course
Of all the cranial nerves, the trochlear nerve is the only one that emerges dorsally from the brainstem immediately caudal to the inferior colliculi. It courses in the ambient cistern between the posterior cerebral artery and the SCA, around the cerebral peduncles toward the basal aspect. At the level of the sella turcica, it enters the edge of the tentorial notch. In the lateral wall of the cavernous sinus, the trochlear nerve courses between the oculomotor nerve and the ophthalmic nerve2. Further along, the trochlear nerve passes through the superior orbital fissure and lies outside the annulus tendineus beneath the orbital roof, entering the lateral border of the superior oblique muscle.
Pathologies
The three most common causes of trochlear nerve palsy are3:
- Trauma, due to its long course.
- Ischemia
- Congenital disorders
Symptoms of dysfunction
Damage to the trochlear nerve causes slight inward and upward deviation of the eye with diplopia. Patients have difficulty reading and walking downstairs because downward and nasal gaze cannot be fully performed4.
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