Trochlear Nerve Palsy

What is Trochlear Nerve Palsy?

The fourth cranial nerve, also known as the trochlear nerve, starts from the middle part of the brain, specifically at a location near the inferior colliculus (an area towards the front of the Sylvian aqueduct). This nerve is responsible for controlling only one muscle, the superior oblique muscle. What’s unique about this nerve is that it’s the only cranial nerve that comes out from the back part of the brainstem and then crosses over to supply the muscle on the opposite side.

This nerve has a long path to follow, which makes it more susceptible to damages. A common issue that doctors often encounter in eye clinics is palsy of the trochlear nerve, which is a condition causing weakness or paralysis of the muscle that this nerve controls.[1]

What Causes Trochlear Nerve Palsy?

There are several different reasons why a person might develop a condition called fourth nerve palsy. This condition affects one of the nerves in your eyes and can change the way your eyes move. In many cases, this condition could be present since birth, but symptoms only show up later in life.

The most common reasons why people develop this condition include it being something they were born with (about 49% of cases), high blood pressure (18% of cases), and experience of head injuries (18% of cases). In other less common cases, it’s caused by high blood pressure combined with diabetes, brain surgery, brain tumors, diabetes alone or after having a specific type of shingles in the eye (each being 1%). Sometimes, the cause remains a mystery (4% of cases).

Other studies have also found similar results, with majority of cases being present from birth and a significant percentage due to injuries. Aside from these common causes, there are also rare conditions that can lead to fourth nerve palsy. These include pseudotumor cerebri (a condition where the pressure inside the skull increases for no apparent reason), meningioma (a type of tumor that grows on the protective layers covering the brain and spinal cord), Lyme disease (an infection caused by tick bites), and Guillain-Barre Syndrome (a rare condition where the body’s immune system attacks the nerves).

Risk Factors and Frequency for Trochlear Nerve Palsy

Trochlear nerve palsy affects about 5.73 out of 100,000 people each year. It is more commonly found in males, possibly because they are more likely to experience head trauma. Most cases of this condition show up in people in their 40s, but those who have had a head injury often show symptoms in their 30s. The delayed onset of symptoms, especially in the 40s, might be due to the body’s diminishing ability to control deviations over time.

• Trochlear nerve palsy affects 5.73 out of every 100,000 people each year.
• This condition is more common in men, likely due to higher instances of head trauma.
• Most people with this condition start showing symptoms in their 40s.
• For people with a history of head injury, symptoms show up in their 30s.
• The delayed onset is likely due to decreased control of deviation over time.

Signs and Symptoms of Trochlear Nerve Palsy

Trochlear nerve palsy might cause people to see double when they look up and down, or see images as being twisted or rotated. This effect might change when the person moves their gaze. Tools like red-green coloured glasses can bring this problem to light accurately.

The main actions of a certain eye muscle (the superior oblique) are to move the eye towards the nose, pointing it downwards, and rolling it inwards. If the fourth cranial nerve has palsy, these actions are affected. Eye examination may show:

• Abnormal head posture: because of the double vision, a patient may turn and tilt their head to the opposite side.
• Facial asymmetry: if the nerve palsy has been there since birth, the middle of the face may not fully develop.
• Strabismus: the affected eye may be misaligned and possibly turned outwards.

A tool known as a Maddox Rod test helps figure out if the eyes are moving abnormally. In the case of fourth cranial nerve palsy, the patient’s perception of a horizontal line will appear tilted downwards because of the abnormal eye movement.

Using the Parks three-step test, a doctor can determine the muscle that is not working properly – causing vertical double vision. This test has three parts:

1. Identifying the eye that is slightly misaligned when looking straight
2. Identifying the misaligned eye when looking right and left
3. Also known as Bielschowsky head tilt test, the head is tilted to each side to determine which side worsens the eye misalignment

In these cases of nerve palsy, the eye misalignment worsens when the person looks in the opposite direction and when they tilt their head to the same side as the affected eye. But, this test is not perfect – it can misdiagnos other conditions as cyclovertical muscle palsy.

In some long-standing cases, the eye misalignment may be the same in different positions of gaze, this is known as the spread of comitance. This condition has features like:

1. Tightening of the muscle on the same side that moves the eye away from the nose
2. Overworking of the muscle on the opposite eye that moves the eye towards the nose
3. Secondary weakness of the muscle on the opposite eye that moves the eye away from the nose

When both eyes have this condition, it can show up as alternating eye misalignment in gazes and tilts, twisting of the eye more than 10 degrees, V- pattern misalignment, two-sided eye fundus twisting, or a chin-down head posture. Sometimes, a case of both eyes being affected might be mistaken as one eye being affected; this condition is known as masked bilateral superior oblique paresis.

Twisting of the eyes can be evaluated using subjective and objective methods. Using the double Maddox rod test for subjecting method, acquired palsy will show a degree of eye twisting, while congenital palsy won’t show one. Eye twisting more than 10 degrees indicates both eyes are affected. Objectively, eye twisting can be measured using an tool that allows doctor to look into the eye (an indirect ophthalmoscope) or retina photography.

Knapp’s classification helps to categorize the condition:

1. Class 1: Eye misalignment is worst when looking up and to the opposite side (due to overworking of the opposite inferior oblique muscle)
2. Class 2: Eye misalignment is worst when looking down and to the opposite side (due to underworking of the affected superior oblique muscle)
3. Class 3: Eye misalignment is worst when looking to the entire field to the opposite side
4. Class 4: Eye misalignment is worst when looking to the entire field to the opposite side and across the lower field
5. Class 5: Eye misalignment is worst across the lower field.
6. Class 6: Both superior oblique muscles are affected
7. Class 7: Due to trauma to the superior oblique muscle, resulting in its weakness and restriction of relaxation (Brown syndrome), also known as ‘Canine tooth syndrome’.

Testing for Trochlear Nerve Palsy

If you’re dealing with fourth nerve palsy, a condition that affects the movement of your eye, extensive neurological testing usually isn’t necessary. Most of the time, this condition is something you’re born with. Your doctor may look for signs such as a tilt in your head that you’ve had for a long time or asymmetry in your face to help rule out any causes that developed later in life.

If the condition isn’t linked to any head trauma and you have other neurological signs, your doctor might suggest neuroimaging, a process that creates pictures of your brain. This is generally done using magnetic resonance imaging, better known as an MRI.

It’s important to note that conditions like myasthenia gravis, a chronic autoimmune disease that causes muscle weakness, can sometimes appear to be fourth nerve palsy. If your doctor suspects this, they might run some tests. These could include looking for a particular antibody in your blood, using computed tomography (also known as a CT scan) to check your chest for an enlarged thymus gland, or conducting nerve conduction studies, which test the speed and strength of signals being sent along your nerves.

Treatment Options for Trochlear Nerve Palsy

Non-surgical management of double vision, also known as diplopia, can involve the use of prism glasses. For cases of sudden diplopia following trauma, botulinum toxin (a type of drug) may be injected into the overactive eye muscle while waiting for natural recovery. Research has shown that this approach can help patients temporarily manage their condition better.

In certain cases, surgical treatment might be required. The type of surgery largely depends on how severe the condition is and how it’s affecting the eye. One common procedure performed by surgeons is the weakening of an eye muscle combined with additional surgery on other eye muscles if necessary. Below are some important principles that guide these surgical interventions:

1. During the operation, the surgeon may perform a ‘forced duction test’ on the eye muscles. This test helps identify loose tendons, often found in cases of a certain type of muscle paralysis. When loose tendons are found, a procedure known as ‘superior oblique tuck’ might be performed.

2. For cases with upward deviation of the eyes (hypertropia) and overactivity in the ‘inferior oblique’ eye muscle, weakening procedures on this muscle are undertaken. This can be achieved by cutting the muscle (myectomy), moving its attachment (recession), or repositioning it to the front (anterior transposition).

When there’s no overactivity in the inferior oblique muscle, other surgical options may include moving the attachment (recession) of either the upper rectus eye muscle on the same side, or the lower rectus eye muscle on the opposite side. In instances where the deviation is more than 15 prism diopters, a combination of various surgical treatments might be used.

Generally, when both eyes are severely affected with rotating outwards, the usual procedures may not be sufficient. A specific procedure called the Harada-Ito procedure could be performed. This approach involves splitting the superior oblique eye muscle and advancing the front half of it. The benefit of this procedure is that the vertical-acting part of the muscle can be left in place while only operating on the front part of the muscle that causes eye rotation. While the Harada-Ito procedure is traditionally performed when the objective torsion is more than 10 degrees, it might be performed for lesser degrees of torsion by some surgeons.

What else can Trochlear Nerve Palsy be?

Skew Deviation is a condition where the body incorrectly assumes that the head is tilted even when it isn’t. This happens due to problems in the brain’s perception of gravity and can result in an abnormal head tilt to compensate. In these cases, one eye might move upward and inward, while the other moves downward and outward. The underlying issue might be a result of blood flow restriction, injury, multiple sclerosis, an abscess, or cancer. Patients might not show symptoms or they could have double vision that doesn’t improve with a head tilt. A specific test, known as the Supine-Upright test, can help diagnose this condition.

Thyroid-related eye disease (TAO) can also mimic a focus nerve issue in the other eye. It presents with symptoms like eye bulging, eyelid retraction, and inability to close the eye fully. X-ray scans often show muscle belly expansion with sparing of the tendon.

In some cases, overactivity of the inferior oblique eye muscle can occur but this doesn’t cause vertical eye misalignment, subjective torsion, and head tilt test remains negative.

Myasthenia gravis too can mimic certain eye muscle issues and needs to be investigated accordingly. The Edrophonium test is commonly used for diagnosis.

A blow to the eye causing a fracture can trap the inferior rectus muscle, causing a misleading three-step test outcome. Signs such as numbness below the eye, the eyeball sinking into the socket, and results from imaging scans can help with the diagnosis.

Finally, patients with early childhood congenital eye muscle issues may show head tilt and face turn which might resemble a disorder known as torticollis. Thus, both eye doctors and bone doctors need to be aware of these conditions for proper treatment.

What to expect with Trochlear Nerve Palsy

In cases of a condition called trochlear nerve palsy, a spontaneous recovery, meaning recovery without any medical treatment, is observed in about 83% of cases, as per a study by Park and colleagues. In fact, over half, or 52.2% of patients, completely recovered on their own. Another study by Phuljhele and team found that either full or partial spontaneous recovery was seen in 45% of patients with a condition that was not present at birth, known as acquired fourth nerve palsy.

If surgery is needed, the success rate is typically high, indicating that most patients recover well after the procedure.

Possible Complications When Diagnosed with Trochlear Nerve Palsy

Surgical correction of trochlear nerve palsy could sometimes end up being excessively adjusted. This overcorrection could change the condition from hypertropia (upward deviation of the eye) to hypotropia (downward deviation of the eye). Such a change could cause diplopia (double vision), which could be difficult for the patient to manage.

In some cases, an issue known as an ‘Iatrogenic Brown Syndrome’ could arise due to an over-tightened superior oblique muscle, resulting in a limitation of the eye’s ability to move up, especially when trying to look towards the nose.

When the fourth nerve palsy is not correctly diagnosed and is thought to be only present in one eye (when it’s actually in both), surgery on the single affected eye could potentially reveal paresis (weakness) in the other, previously undiagnosed eye.

Common Complications:

• Overcorrection leading to changes in eye direction and double vision
• ‘Iatrogenic Brown Syndrome’ due to over-tightened superior oblique muscle
• Undiagnosed bilateral fourth nerve palsy, revealed after surgery

Preventing Trochlear Nerve Palsy

When someone suddenly or gradually starts seeing double vision, with images either tilting or separating vertically, this might be a sign of a condition called trochlear nerve palsy. If you ever face such issues, you should consider consulting with a doctor who specializes in strabismus, a disorder in which the eyes do not align properly. Usually, the symptoms of trochlear nerve palsy can be treated with medication or surgery. However, in some cases, an intensive evaluation of your nervous system might be required.