What is the anatomy of the trochlear nucleus relevant to trochlear nerve palsy (fourth nerve palsy) treatment?

Updated: Oct 08, 2019
  • Author: Zafar A Sheik, MD, MD; Chief Editor: Andrew G Lee, MD  more...
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Answer

The trochlear nucleus is located in tegmentum of midbrain, at the level of inferior colliculus. [11, 2] The trochlear nerves decussate at anterior medullary velum in the roof of aqueduct before exiting from dorsal aspect of midbrain. The fourth nerve courses between posterior cerebral and superior cerebellar arteries before entering the cavernous sinus. The fourth nerve then enters the orbit through superior orbital fissure, outside annulus of Zinn. From here, the nerve crosses medially over levator palpebrae superioris and superior rectus muscles before entering the belly of superior oblique muscle.

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

When performing a superior oblique tenotomy, the superior rectus muscle insertion may be used as a landmark. The portion of tendon that is cut during the tenotomy may be isolated by dissecting to a point approximately 8-12 mm posterior to nasal aspect of superior rectus insertion. Broad superior oblique insertion, which is 10-18 mm in length, has great functional importance. Anterior fibers act mainly to intort the globe and do little to abduct or depress the eye. Conversely, more posterior fibers are responsible for abduction and depression but have little torsional action. Surgical procedures designed to alleviate torsional diplopia, such as the Harada-Ito procedure, consist of advancing only anterior fibers of tendon insertion.


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