IV - Trochlear Flashcards
Describe the anatomy of the trochlear nerve
Location: emerges from the dorsal aspect of the brainstem, specifically from the trochlear nucleus located in the dorsal aspect of the midbrain.
Route: follows the longest intracranial course of any cranial nerve, looping around the midbrain before exiting the skull through the superior orbital fissure.
Size: smallest cranial nerve in terms of axonal count; containing the fewest number of axons compared to other cranial nerves.
4th cranial nerve
Only nerve pair exiting from the dorsal side of the brainstem
Decussates within the midbrain, resulting in a contralateral innervation
Outline the pathway of the trochlear nerve
Originate from a pair of symmetrical trochlear nuclei within the medial midbrain at the level of the inferior colliculus.
The left and right nerves then travel dorsally surrounded by the periaqueductal grey matter, decussating before their exit in the dorsal midbrain
The two nerves run on contralateral sides, extend laterally and then anteriorly around the pons, before penetrating the dura above the trigeminal nerve.
In the cavernous sinus, a few sympathetic fibers join the trochlear nerves with possibility of some sensory fibers from the trigeminal nerve.
The nerves then enter the orbits through the superior orbital fissure and continues to extend anteriorly to the superior oblique muscle.
Note: vulnerability to shearing forces in the setting of trauma
What are the functions of the trochlear nerve?
The trochlear nerve primarily innervates the superior oblique muscle of the eye; it also innervates the trochlea which holds the tendon of the superior oblique muscle.
Plays a key role in controlling specific eye movements, including downward and outward eye rotation (extorsion), as well as Intorsion (rotation of eye towards the nose). These movements help to align the eyes properly, maintain visual focus, and coordinate binocular vision.
Proper function of the trochlear nerve is essential for visual coordination and depth perception; enables precise control of eye movements in different directions.
Dysfunction can therefore lead to visual disturbances and challenges in depth perception.
Explain the physiology of the trochlear nerve function
Primarily a motor nerve controlling contraction of the superior oblique muscle of the eye
Nerve impulses travel along trochlear nerve fibers toward the neuromuscular junction (NMJ)
A calcium influx triggers release of acetylcholine (Ach) from nerve terminals; Ach diffuses across the synaptic cleft and binds to nicotinic Ach receptors on the motor end plate of the muscle fiber; the binding causes depolarization of the motor end plate leading to generation of an end plate potential (EPP) – this depolarization activates voltage gated sodium channels resulting in generation of an action potential.
The end result is contraction of the superior oblique muscle contributing to its contribution to precise eye movements
Works in coordination with other cranial nerves, particularly the oculomotor nerve (CN III), abducens nerve (CN VI) and the optic nerve (CN II), to ensure precise eye movements and proper alignment.
This coordination is essential for maintaining visual stability, binocular vision, and depth perception.
Describe the trochlear nerve palsy condition
Trochlear nerve palsy is a neurological condition characterized by dysfunction, weakness or paralysis of the trochlear nerve and, as a result, the superior oblique muscle.
Can either be congenital or acquired; and can be unilateral or bilateral.
The primary symptom is vertical diplopia, which means seeing double images stacked vertically on top of each other; this happens because the superior oblique muscle is unable to properly depress the eye during downward gaze resulting in misalignment of the visual axes.
To compensate for the vertical diplopia, patients often adopt a head tilt away from the affected eye. (in unilateral palsy)
Causes include: trauma, tumors, vascular lesions, congenital, etc.
How do you diagnose a trochlear nerve palsy?
Trochlear nerve palsy can be diagnosed using the Bielschowsky head tilt test; it is used to test for superior oblique muscle weakness or paralysis resulting from dysfunction of the trochlear nerve.
Procedure: the patient is asked to fixate on an object while the head is tilted to one side or the other. The examiner observes the patient’ eye movements and any vertical or torsional deviations of the eyes.
Interpretation: the affected eye will exhibit an over elevation or intorsion when fixating on the object – this compensatory movement occurs because of the weakened superior oblique muscle is unable to depress or extort the eye properly
