The Anatomy of the Trochlear Nerve

The trochlear nerve is the fourth cranial nerve. It's a motor nerve and provides movement to only one muscle—an eye muscle called the superior oblique, which connects to the top of the eyeball. The tendon of that muscle passes through a structure that's a lot like a pulley. That structure is called the trochlea, which is the Latin word for pulley; this is where the name of the trochlear nerve comes from.

Anatomy

Most of your nerves branch out like trees, with the "limbs" running all through your body, carrying sensory information to and from your brain, and enabling motor function (movement) in your muscles and other moving parts.

It also has the smallest number of axons, which are the protrusions that transmit electrical impulses.

Structure

You have twelve pairs of cranial nerves in your head. They're symmetrical—each one has a right side and left side (but each pair is generally referred to as a single nerve).

The rest of the nerves in your body emerge from the spinal cord but all the cranial nerves, including the trochlear nerve, come from your brain.

Ten of the cranial nerves emerge from the brainstem, which sits at the back of the skull and attaches the brain to the spinal cord. The trochlear nerve is one of these nerves, but it's unique in that it's the only one that comes from the rear of the brainstem. It also runs farther on the inside of the skull than any other cranial nerve and is the thinnest of them.

Location

From the back of your head, the trochlear nerve curves around the brainstem and emerges between two arteries called the posterior cerebral and superior cerebellar arteries. It then goes inside the cavernous sinus and runs along one of its walls.

In the sinus, the trochlear nerve is joined by several other nerves, including the third and sixth cranial nerves (which also serve the eye) and two branches of the trigeminal (fifth cranial) nerve: the ophthalmic and maxillary nerves, which supply sensory innervation to much of the face.

Finally, the trochlear nerve reaches the eye socket and passes above a ring of muscles called the extraocular muscles. It then crosses the roof of the eye socket and connects to the superior oblique muscle.

Anatomical Variations

A section of the trochlear nerve called the cisternal segment does vary in its path through part of the brain, with about 60% of people having the more common route. Either way, this segment of the nerve follows the same path as the superior cerebellar artery, the posterior cerebral artery, and the basal vein of Rosenthal.

This variation is important for neurosurgeons to know so they can avoid causing damage to the nerve.

Function

The trochlear nerve doesn't transmit sensory signals. It functions purely as a motor nerve.

The one muscle it sends signals to—the superior oblique muscle—is one of six muscles that allow the eye to make precise movements for tracking or focusing on an object. This muscle moves the eyeball up and down and left and right.

Associated Conditions

The trochlear nerve can be damaged by injury or as a complication of surgery. It's a fragile nerve made more vulnerable by the path it travels inside the skull, so head trauma is especially likely to damage it.

That damage typically results in impaired function of the superior oblique muscle, meaning the eye can't move inward or down. It's common for trochlear nerve damage and associated eye-movement problems to make it hard for people to see where they're walking, especially when going down the stairs.

This kind of paralysis, either total or partial, is called palsy. Blurry vision or double vision, also called diplopia, can happen as a result of trochlear nerve palsy.

Tilting the head to the side that's not affected can eliminate the doubling. That can help doctors identify the cause of diplopia so that it can be treated properly.

Typically, trochlear nerve palsy is the result of head trauma. Motorcycle accidents are a common cause, but sometimes it can result from even minor head injuries.

Less often, palsy is caused by:

• Diabetes
• Tumor
• Aneurysm (bulging artery in the skull)
• Multiple sclerosis-related nerve damage
• Lyme disease
• Meningioma
• Guillain-Barre syndrome
• Herpes zoster
• Cavernous sinus syndrome

It's more common for just one eye to be impacted by palsy, but it's possible for it to occur in both eyes.

A doctor may suspect trochlear nerve problems when seeing characteristic problems with eye movement, especially if tilting the head helps. The suspicion can be confirmed via computed tomography (CT) scan or magnetic resonance imaging (MRI) of the brain.

Rehabilitation

Treatment of trochlear nerve palsy depends on what's causing the problem. If it's an identifiable cause such as tumor or aneurysm, treating that condition should help resolve the palsy.

If it's due to injury or an unknown cause, eye exercises may help strengthen the muscle and get it working properly again. Additionally, doctors may recommend specialized lenses called prism glasses.

The lenses of prism glasses are thin at the top and thick at the base, which changes the way light moves through them, bending it to compensate for the palsy and eliminate the double image.

In most cases, the palsy will go away eventually.