Cranial Nerve IV: Trochlear Nerve
What’s unique about the Trochlear Nerve?
The Trochlear nerve is derived from the Latin word, “trochlea,” which means pulley. It supports movement and rotation of the eyes downward and towards the nose. The nerve is thin and delicate and has to travel a long distance before reaching its target, making it vulnerable to compression.
What is the function of the nerve?
Motor: Controls movement of 1 out of 6 extraocular muscles of the eye, the superior oblique muscle, which generally supports movement and rotation of the eyes downward and towards the nose
What are the signs of dysfunction?
Signs of Trochlear nerve palsy include:
• Orientation of the eye upward and outward
• Vertical diplopia, or double vision, often observed when a person looks downwards, such as when walking down the stairs, or when gazing away from the affected side
Trochlear nerve palsy can be present at birth, or it can occur later in life due to:
• Malformation of the nucleus of the nerve, which may be associated with a brainstem stroke
• Damage to the nerve itself, often caused by Traumatic Brain Injury (TBI)
Fibers emerging from the nucleus cross before emerging from the brainstem, resulting in symptoms on the opposite side when the nucleus is affected, whereas damage to the nerve itself results in symptoms on the same side.
How might this nerve be impacted?
Less common conditions may also affect the Trochlear nerve:
• Cavernous sinus syndrome, when an abnormal growth affects the trochlear nerve
• Guillain-Barré syndrome
• Lyme disease
• Meningioma
• Microvascular coronary disease (MCD)
• Shingles (herpes zoster)
• Superior oblique myokymia, episodes of involuntary superior oblique muscle tightening that distort vision
How can you work with this nerve?
•Notice the brainstem, posterior cerebral artery, superior cerebellar artery , tentorium, cavernous sinus, sphenoid, and eye field, and offer decompression.
• Visualize the pathway, from the nucleus at the back of the brainstem, circling the brainstem and passing between arteries and through the tentorium, running across the cavernous sinus, entering the superior orbital fissure of the sphenoid to connect with the superior oblique muscle.
• Notice the quality of potency moving through the nerve
• Hold space for any held patterns to shift
The Trochlear nerve begins at the back of the brainstem. The nucleus of the trochlear nerve is found at the level of the inferior colliculus of the midbrain.
Fibers from each nucleus crossover before the trochlear nerve emerges. This means that the right trochlear nerve originates from the left trochlea nucleus and vice-versa.
The trochlear nerve circles laterally around the brainstem and runs anteriorly towards the eye.
It passes between the posterior cerebral artery and the superior cerebellar artery on the lateral aspect of the pons and then pierces the dura just under the free margin of the tentorium.
It runs on the lateral wall of the cavernous sinus below the oculomotor nerve.
Finally it enters the orbit through the superior orbital fissure.
It then connects to the superior oblique muscle where it provides somatic motor innervation.
The superior oblique muscle originates in the sphenoid bone, proceeds anteriorly where it becomes tendinous, loops through the trochlea, and travels posteriolaterally to attach to the sclera of the eyeball.
The unique path of the superior oblique muscle enables abduction (moving the eyeball laterally), depression (moving the eyeball downwards), and internal rotation (rotating the eyeball medially).