Trochlear Nerve

Dr Hytham Nafady

Introduction

• Trochlear nerve has only somatic motor component.

Somatic motor(general somatic efferent)

Somatic motor innervates the superior oblique muscle of the contralateral orbit.

Anatomy

• Nucleus.

• Intracranial course

• Extracranial course

Segments of the trochlear nerve

• Central,

• Cisternal,

• Cavernous, &

• Orbital

Trochlear nucleus

• Trochlear nucleus is located in tegmentum of midbrain, at the level of inferior colliculus.

• It is located ventral to the periaqueductal grey matter, inferior to the oculomotor nucleus & superior to the medial longitudinal bundle.

Trochlear nucleus

.

* = 4th cranial nerve nucleus

Superior medullary vellum

Central segment

• Course posteriorly & caudally surrounding the aqueduct of sylvius.

• Nerves decussate at anterior medullary velum in the roof of aqueduct before exiting from dorsal aspect of midbrain below the inferior colliculus. 4

Cisternal segment

Course through the ambient cistern between the posterior cerebral & superior cerebellar arteries.

PcoA Cn IV

C n III

Cavernous segment

• The trochlear nerve enters the cavernous sinus superior to the petroclinoid ligament.

• The cavernous segment of the trochlear nerve course through the lateral wall of the cavernous sinus below the oculomotor nerve.

Cn 4 Cn 5 O

Cn 6

Cn 3

Cn 5 mx

Orbital segment

• The trochlear nerve enters the orbit through the superior orbital fissure outside the annular ring of Zinn.

• The orbital segment of the trochlear nerve crosses medially over levator palpebrae superioris and superior rectus muscles before entering the belly of superior oblique muscle

Superior orbital fissure & annular ring of Zinn

Cn 3 i

Cn 3 s

Cn 6

Cn 4

Superior oblique muscleSuperior oblique muscle originates from orbital apex, above annulus, and runs along superonasal aspect of orbit before becoming a tendonous cord. Tendon passes through trochlea and abruptly turns laterally and posteriorly to insert on the globe. Tendon is cordlike as it passes beneath nasal border of superior rectus but fans out to form a broad insertion.

Trochlear nerve palsy

• Nucleus.

• Central segment.

• Cisternal segment.

• Cavernous segment.

• Orbital segment.

Nucleus & central segment

Left superior oblique palsy & right internuclear ophthalmoplegia

Cisternal segment

• Trochlear nerve neuroma.

• Vascular loop compression

Trochlear nerve schwannoma

Trochlear nerve schwannoma along cisternal & cavernous segments

Vascular loop compression of the trochlear nerve by anomalous left PCA

Bilateral trochlear nerve palsies following dorsal midbrain haemorrhage

Isolated trochlear nerve palsy with dorsal midbrain hemorrhage

Atrophy of the superior rectus muscle in trochlear nerve palsy

Superior oblique muscle atrophy

Trochlear nerve palsy

• Diplopia on going downstairs.

Trochlear nerve palsy

Congenital right trochlear nerve palsy with compensatory left head tilt.

Post-traumatic bilateral trochlear nerve palsy with: •right hypertropia on right head tilt & •left hypertropia on left head tilt.