The Trochlear Nerve (cranial nerve IV or CN IV) is the fourth cranial nerve of your central nervous system. It is the smallest motor nerve and is responsible for transmitting signals from the brainstem to the superior oblique muscles of your eyes. ¹The superior oblique muscles are extraocular (outside the eyeball) and are attached to the eyeball. They are the longest and thinnest of the six extraocular muscles, and are involved in moving the eye downward, inward and outward.¹

In this article, you will learn about the anatomy and functions of the trochlear nerve. You will also discover how high-impact trauma to the CN IV and superior oblique muscles can reduce your vision and affect your daily activities.

Anatomy of the trochlear nerve

The trochlear nerve pair arises from the trochlear nuclei, located in your midbrain. This nerve travels a unique course, leaving your brainstem from the back side to reach the orbit (eye socket). Both nerves cross over before leaving your midbrain. Hence, each nerve supplies its contralateral side, i.e. the opposite side of the body. The pair then reaches the orbit  through the superior orbital fissure, which also allows oculomotor CN III, abducens CN VI, and trigeminal CN V to pass through.¹

What is the function of the trochlear nerve in eye movement?

The main function of the trochlear nerve is to supply movement signals to your superior oblique muscles, enabling your eyes to move:

• Inward, rotating the top of the eyes towards the nose (intorsion)
• Outward, away from the nose (abduction)
• Downward (depression)¹

This downward and inward gaze helps you with reading and walking downstairs.

The superior oblique muscle is the only eye muscle that lowers your pupil when your eyes move towards the nose (adducted). This function is key when looking for palsy (paralysis) of the superior oblique muscle or the trochlear nerve. During an eye exam, you may be asked to look downwards while your eyes look inward toward your nose. If there is a problem, you may struggle to perform the act. Since the oblique muscles “work in opposite”, your left eye is tested by looking to the right, and your right eye is tested by looking towards the left.¹

What causes trochlear nerve palsy?

The trochlear nerve palsy is a condition in which the nerve is not working correctly or is damaged. This affects the nerve supply to your superior oblique muscles, resulting in reduced or no movement.

Common causes of trochlear nerve palsy are:

• Congenital: present from birth in some individuals. The nerve is absent at birth, or the muscle is underdeveloped
• Head trauma: accidents causing injury to the orbital region and the CN IV nerve. It mainly causes bilateral trochlear nerve injury
• Microvascular ischemia: blocked or restricted blood flow to small blood vessels in diabetics and hypertensive individuals, causing reduced oxygen supply to nerves
• Tumours such as pseudotumour cerebri and meningioma
• Lyme disease: an infection caused by tick bites
• Gullian-Barré syndrome: an autoimmune condition where the immune system attacks the nerves²

What happens if the trochlear nerve is damaged?

When the trochlear nerve is damaged, it usually causes vertical diplopia (double vision when looking down). It also causes images to appear tilted or rotated because your eyes may be misaligned. This can cause you to tilt your head to compensate for diplopia. The diplopia usually affects both eyes.²

In the case of a right eye vertical diplopia, it is most notable when you look downward and inwards toward the nose. This causes difficulty in reading and walking downwards on stairs.

Signs of trochlear nerve palsy

 Symptoms you may experience include:

• Abnormal head posture (called torticollis)
• Facial asymmetry, caused by an underdeveloped midface
• Strabismus, also called eye misalignment, causes the affected eye to appear higher than the normal (hypertropia) or deviated inward towards the nose (esotropia) or outwards (exotropia)²

How to diagnose the trochlear nerve palsy?

You will likely visit an ophthalmologist (eye specialist) for your trochlear nerve palsy. The doctor will ask for your medical history and perform a clinical examination. Your doctor may ask you to cover one of your eyes to note the extent of abnormal eye muscle movement. After assessing your symptoms, your doctor will perform several tests to confirm the diagnosis. These include the following:

Neurological exam

The test involves making an H-shaped pattern using a pen or pencil at a distance of 30 to 40cm in front of you. The fourth nerve palsy causes you to tilt your head away from the affected eye, and the eye moves outwards. This helps distinguish it from sixth nerve palsy, where the head tilt is towards the affected side.³

Parks-bielschowsky 3-step test

This test is usually performed in 3 steps to diagnose an isolated cyclovertical muscle (a muscle that can perform both the vertical and rotational movement) palsy.

• In the first step, your doctor will see which eye is hypertropic in the primary position (one eye is higher than the other)
• Then, your doctor checks for an increase in hypertropia on the right or left gaze
• The third step reveals whether the hypertropia increases in left head tilt or right head tilt

In the case of right trochlear nerve palsy, the right eye will be hypertropic when looking ahead. This becomes more noticeable when looking left, causing you to tilt your head to the right.

Maddox rod test

In the Maddox test, your doctor uses a Maddox rod to determine the direction and amount of deviation (torsion) in the affected eye. This test is used to check for both the horizontal and vertical deviations of the affected eye.

Upright-supine test

The upright-supine test is used to distinguish between the other causes of vertical strabismus from skew deviation (misalignment of eyes where they move in opposite directions due to cerebellar issues).

Imaging tests

A CT scan is performed to determine the trochlear nerve palsy after a head trauma. MRIs are also performed to diagnose brain tumours and haemorrhages that may contribute to CN IV palsy.

How is trochlear nerve palsy treated?

Treatment of trochlear nerve palsy is based on its cause. A person affected by palsy at birth or by trauma can be given prism glasses and a patch. If neither works, your doctor may proceed with surgery. Palsy due to microvascular causes usually resolves on its own in weeks or months. Acquired palsies due to hypertension and diabetes can be relieved by treating the underlying disease.

Summary

The trochlear nerve supplies movement information only to your eyes’ superior oblique muscles. This nerve helps in the upward, downward, and inward movement of the eye. Trochlear nerve palsies may be present at birth, acquired due to head trauma, ischemia caused by diabetes and hypertension, or various other diseases. Symptomatic treatment options involve using prism glasses or a patch. Some palsies resolve on their own, but if the problem persists, then surgery is performed. If you or someone you know suffers from trochlear nerve palsy, they should seek help, get diagnosed for the cause, and start their treatment accordingly.

FAQs

Q1: Which cranial nerve is the smallest?

Ans: The trochlear nerve is the smallest cranial nerve.

Q2: How can you tell if someone has trochlear nerve palsy?

Ans: They will exhibit signs of vertical double vision and a head tilt. They should consult a medical professional if they experience these symptoms.

Q3: Is trochlear nerve palsy permanent?

Ans: Most cases resolve on their own, especially after trauma or treatment of some underlying causes, such as diabetes.

Q4: What is the difference between trochlear (CN IV) and oculomotor (CN III) nerve palsy?

Ans: Trochlear palsy affects only the superior oblique muscle, while oculomotor palsy affects multiple eye muscles and may involve pupil dilation.

Q5: Can trochlear nerve damage affect both eyes?

Ans: Rarely. Most cases are unilateral, but bilateral involvement can occur, especially in severe head trauma or congenital cases.