Block nerve

The trochlear nerve (n. trochlearis) is a motor, thin nerve that emerges from the midbrain behind the plate of the quadrigeminal body, near the frenulum of the superior cerebral velum. The nerve then bends around the cerebral peduncle on the lateral side, passes between it and the temporal lobe of the cerebral hemisphere, runs in the thickness of the lateral wall of the cavernous sinus, and enters the orbit through the superior orbital fissure. In the orbit, it enters the superior oblique muscle of the eye, which it innervates.

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Important features of the IV pair of cranial nerves (trochlear)

• The only cranial nerve emerging from the dorsal surface of the brain.

• Crossed cranial nerve. The nucleus of the trochlear nerve innervates the contralateral superior oblique muscle.
• A very long and thin nerve.

1. The nucleus of the trochlear nerve is located at the level of the inferior colliculi, ventral to the Sylvian aqueduct. It lies caudal to the nuclear complex of the third pair of cranial nerves, merging with it.
2. The bundle consists of axons that curve posteriorly around the aqueduct of Sylvius and completely cross in the superior medullary velum.
3. The nerve leaves the brainstem on the dorsal surface, caudal to the inferior colliculi, bends around it laterally, goes forward under the free edge of the tentorium and passes between the posterior cerebral and superior cerebellar arteries (similar to the III CN). Then it passes through the dura mater and enters the cavernous sinus.
4. The intracovernous part passes in the lateral wall of the sinus, below the third cranial nerve and above the first branch of the trigeminal nerve. In the anterior part of the cavernous sinus, the nerve ascends and passes through the superior orbital fissure above and lateral to the ring of Zinn.
5. The intraorbital portion innervates the superior oblique muscle.

Special tests for the diagnosis of trochlear nerve lesions

The Parks three-step test is very useful in diagnosing trochlear nerve damage:

• First, assess which eye is hypertropic in the primary position. Left hypertropicity may be due to weakness in one of four muscles: the left eye depressor (superior oblique or inferior rectus) or the right eye lifter (superior rectus or inferior oblique);
• second: determine where the hypertropia of the left eye is greater - when looking to the right or to the left. Increase when looking to the left implies either the left lower rectus or the right lower oblique. Increase when looking to the right implies either the left upper oblique or the right upper rectus;
• Third: The Bielschowsky head tilt test identifies the paretic muscle. The patient fixes an object 3 m away straight ahead, and then the head is manually tilted to the right, then to the left. An increase in the left eye hypertropia when tilting to the left implies the left
  superior oblique muscle, and a decrease in the left eye hypertropia when tilting to the right implies the left inferior rectus.

Double Test with Maddox Stick

• Red and green Maddox sticks with vertical cylinders are placed in front of the eyes.
• Each eye will detect a luminous horizontal line.
• In the presence of cyclodeviation, the line seen by the paretic eye will be tilted and therefore different from that seen by the other eye.
• One Maddox stick is then rotated until the lines merge (overlap) and are drawn together.
• The rotation can be measured in degrees and shown as the amount of cyclodeviation.
• Unilateral trochlear nerve lesion is characterized by a cyclodeviation of less than 10.