Trochlear Nerve

What is the trochlear nerve?

Your trochlear nerve, also known as the cranial nerve 4 or CN IV, is a motor (movement) nerve that sends signals from your brain to one of the muscles that control eye movement. You have two trochlear nerves — one for each eye. They’re one of 12 sets of cranial nerves.

Nerves are like cables that carry electrical impulses between your brain and the rest of your body. Many nerves provide sensory information (like touch and temperature sensations) and movement information. Your trochlear nerve is just responsible for movement.

What is the function of the trochlear nerve?

Your trochlear nerve has one job: To supply movement (motor) information to your superior oblique muscle. Together, your trochlear nerve and superior oblique muscle make it possible for you to look down. They also enable you to move your eyes toward your nose or away from it.

The trochlear nerve gets its name from the Latin word “trochleae,” which means “pulley.” A pulley is a device that helps lift and lower an object. Your trochlear nerve controls (innervates) your superior oblique muscle, which is connected to your eye near the top of it. This muscle goes through a sling of connective tissue that acts as a pulley.

Other nerves that control voluntary eye movement include your:

• Oculomotor nerve (CN III).
• Abducens nerve (CN VI).
• Facial nerve (CN VII).

Where is the trochlear nerve?

Even though your trochlear nerve (CN IV) is only responsible for controlling your superior oblique muscle, it has a relatively long course to get to this muscle. Your trochlear nerve starts in your brainstem. It passes through four areas before reaching your superior oblique muscle near the top of your eyeball. These areas include:

1. Trochlear nucleus: This is the part of your trochlear nerve closest to your brain. It’s near the top of your brainstem.
2. Ambient cistern: This area is near your brain’s outer protective tissue (the dura mater, which is part of your meninges).
3. Cavernous sinus: This is a hollow space in the middle of your skull.
4. Orbit: This is the bony socket of your skull that houses your eyeball. Once through the orbit, your trochlear nerve attaches to your superior oblique muscle.

What happens when the trochlear nerve is damaged?

If your trochlear nerve experiences damage, it’ll likely affect your eye movement and vision. Common signs of trochlear nerve damage (often called fourth nerve palsy or trochlear nerve palsy) include:

• Double vision (diplopia).
• Tilting your head to compensate for eye movement or vision challenges. In babies, this may look like torticollis.
• Facial asymmetry (midfacial hypoplasia). This typically affects congenital (from birth) cases.
• Misalignment of your eyes (strabismus).

Causes of trochlear nerve damage

Your trochlear nerve has a very long path through your head. This makes it vulnerable to injury — more so than other cranial nerves. It’s especially vulnerable to damage from head trauma. Common causes include vehicle accidents and boxing. But even mild head injuries can lead to damage.

Other less common causes of trochlear nerve damage include:

• Microvascular disease.
• Pressure from a nearby tumor.
• Increased intracranial pressure.

Some babies are born with trochlear nerve issues. Healthcare providers call this congenital fourth nerve palsy.

How do I keep my trochlear nerves healthy?

Some causes of trochlear nerve damage are unpreventable, like congenital (present from birth) conditions that affect the nerve. But you can take steps to prevent head injuries that could damage your trochlear nerve. These include:

• Making your house safe if you have a baby or toddler (childproofing) to prevent dangerous falls or head injuries.
• Making safe living areas for loved ones with balance issues or an increased fall risk.
• Using a seatbelt when you’re in a vehicle and practicing car seat safety.
• Wearing a helmet when riding a bicycle or motorcycle or playing contact sports.

Other steps that can help keep your nerves healthy in general include:

• Eating nutritious foods, especially those with vitamin D and vitamin B12. These vitamins support nerve health.
• Managing chronic conditions that can damage nerves, like diabetes and high blood pressure.
• Seeking help to quit smoking. Tobacco use slows blood flow to your nerves.
• Reaching and maintaining a weight that’s healthy for you.
• Decreasing stress through meditation, walking or other healthy techniques.

When should I call my healthcare provider?

Contact your healthcare provider if you experience symptoms of fourth nerve (trochlear nerve) palsy. They often include vision changes that worsen when you look down, like blurry or double vision.