Fourth cranial nerve palsy (trochlear nerve palsy) is a paralytic strabismus caused by dysfunction of the fourth cranial nerve (trochlear nerve), which innervates the superior oblique muscle. The trochlear nerve is the thinnest and has the longest intracranial course among the cranial nerves. It emerges from the dorsal side of the brainstem, crosses the midline, and innervates the contralateral superior oblique muscle.
The superior oblique muscle has different actions depending on the position of the eye.
Therefore, trochlear nerve palsy results in hypertropia and excyclotorsion on the affected side.
Clinically, it is helpful to classify into the following three types for easier understanding.
The most common cause is trauma. Trauma to the midline of the head or vertex, such as from motorcycle accidents, often results in bilateral palsy. The next most common causes are ischemic and congenital non-decompensated palsy. Population-based studies report that only about 4% are idiopathic. More than 80% of bilateral palsies are unilateral, and more than half of bilateral cases are due to trauma1).
In congenital trochlear nerve palsy, patients may remain asymptomatic for many years by maintaining eye alignment through head tilt. With age-related decrease in fusional amplitude or triggered by illness or stress, decompensation may occur, and patients often notice vertical diplopia in their 20s to 30s. If head tilt can be confirmed in childhood photographs, it provides a clue to a congenital cause.
The main complaint is binocular vertical diplopia or torsional diplopia. The characteristics of symptoms differ depending on the type of palsy.
Since the superior oblique muscle acts to depress the eye in adduction, diplopia worsens in downgaze. Symptoms are often noticed during reading or going up and down stairs. In mild or chronic cases, instead of diplopia, patients may experience blurred vision, difficulty focusing, or dizziness.
Congenital
Head tilt toward the healthy side: Abnormal head posture to compensate for eye position. Accompanied by facial asymmetry.
Large vertical fusion amplitude: Fusion amplitude of 10–15 prism diopters, no diplopia even with hypertropia of 10 PD or more.
Good binocular vision: Stereopsis is maintained as long as eye position is preserved with head tilt.
Decompensated
Severe hypertropia: Large deviation manifesting as exacerbation of congenital condition.
Facial asymmetry: Finding related to long-term ocular torticollis. The line connecting the eye and mouth corner on the healthy side intersects.
Superior oblique muscle atrophy: Atrophy of the superior oblique muscle on the affected side on MRI T1 coronal view is useful for diagnosis.
Acquired
Hypertropia + excyclotorsion on the affected side: Due to reduced intorsion and depression function of the superior oblique muscle.
Inferior oblique overaction: Secondary finding where the affected eye elevates on adduction.
Compensatory head tilt: Head tilt toward the healthy side to reduce diplopia.
Ocular movements often show no obvious abnormalities, and diagnosis is made by eye position testing. In bilateral trochlear nerve palsy, alternating hypertropia (left hypertropia on right gaze, right hypertropia on left gaze) occurs, and if excyclotorsion is 10 degrees or more, bilaterality should be considered.
The superior oblique muscle acts strongly as a depressor during adduction, so diplopia worsens on downward gaze. Symptoms are easily noticed during daily activities such as reading, checking items at hand during meals, or going down stairs. It also worsens on lateral gaze to the side opposite the affected side.
In peripheral trochlear nerve palsy, as with other ocular motor nerve palsies, vascular disorders are often the cause, but aneurysms are rarely the cause. However, in acute-onset cases, the possibility of giant cell arteritis (GCA) should also be considered.
Occurs as part of a broader syndrome related to trauma, tumor, infection, inflammation, etc. Increased intracranial pressure can also be a cause. It is classified based on the affected site (midbrain, subarachnoid space, cavernous sinus, orbit).
In peripheral trochlear nerve palsy, aneurysms are rarely the cause. This is a major difference from oculomotor nerve palsy. However, rare cases have been reported where a subarachnoid aneurysm presents as isolated fourth nerve palsy.
This is the standard method used to diagnose unilateral fourth nerve palsy.
Skew deviation results from damage to fibers from the vestibular nerve to the interstitial nucleus of Cajal in the midbrain and may show a pattern similar to trochlear nerve palsy on the Parks three-step test. Differentiation is important.
| Differentiating Feature | Trochlear Nerve Palsy | Skew Deviation |
|---|---|---|
| Ocular Torsion | Excyclotorsion of the affected eye | Incyclotorsion or bilateral torsion |
| Head Tilt Direction | Tilt toward the healthy side | Tilt toward the side of the hypertropia |
| Effect of body position change | No effect | Positive if change ≥50% |
Treatment of the underlying disease is the highest priority.
Torsional deviation cannot be corrected with prisms, so surgery is necessary when torsional diplopia is the main symptom.
Indications for surgery are as follows.
| Type | Surgical Indication | First-choice Procedure |
|---|---|---|
| Congenital | When head tilt is prominent | Inferior oblique weakening procedure |
| Decompensated | When diplopia is symptomatic | Inferior oblique weakening |
| Acquired | When cyclodiplopia is the main symptom | Inferior rectus recession + nasal transposition |
Inferior oblique weakening (inferior oblique myectomy, recession, or anterior transposition) is performed. If vertical deviation is large, ipsilateral superior rectus recession or contralateral inferior rectus recession is combined.
To eliminate cyclodiplopia, contralateral inferior rectus recession with nasal transposition is the first choice. It can correct both vertical and cyclotorsional deviation simultaneously. Nasal transposition of the inferior rectus by one muscle belly width provides approximately 6–7 degrees of excyclotorsion correction.
Surgical outcomes are good. However, correction of vertical deviation is subtle, and examination on the first postoperative day may require adjustment. The probability of adjustment is about 30%.
Cyclotorsional deviation (excyclotorsion) cannot be corrected with prism glasses. When cyclodiplopia is the main symptom, surgery is indicated. In acquired cases, contralateral inferior rectus recession with nasal transposition is the first choice, correcting both vertical and cyclotorsional deviation simultaneously. See the “Standard treatment” section for details.
The trochlear nucleus is located in the ventral part of the inferior colliculus in the dorsal midbrain, continuous with the caudal end of the oculomotor nucleus. It lies dorsal to the medial longitudinal fasciculus (MLF).
The nerve fibers emerging from the nucleus follow the following pathway:
The trochlear nerve is vulnerable to trauma due to the following anatomical features:
When a strong external force (traffic accident, head trauma) is applied, the dorsal midbrain is pressed against the tentorial edge, and the decussation of the anterior medullary velum is damaged. Because the structure crosses at the midline, bilateral trochlear nerve palsy is likely to occur.
In congenital superior oblique palsy, high-resolution 3T MRI has reported that CN4 is absent in a considerable number of patients. Congenital superior oblique palsy may be classified as congenital cranial dysinnervation disorder (CCDD), but the causative gene has not been identified. Abnormalities of the muscle insertion and hypoplasia of the superior oblique muscle are often more severe than in acquired cases.
In non-isolated fourth nerve palsy, the site of the lesion can be inferred from accompanying symptoms.
Kawamura et al. (2025) reported a 67-year-old man with bilateral trochlear nerve palsy due to a lipoma in the dorsal midbrain. Since his 50s, he had symptoms of the road center line appearing tilted, and MRI revealed a lipoma in the dorsal midbrain. Excyclotorsion was 25 degrees, and stereopsis was absent. Bilateral inferior rectus nasal transposition (with 2 mm additional recession in the right eye) was performed, and postoperatively excyclotorsion improved to 5 degrees, with stereopsis of 60 seconds and disappearance of diplopia1).
Rao et al. (2025) performed selective plication of the anterior one-third fibers of the superior oblique muscle in a 33-year-old man with 20 degrees of excyclotorsion due to bilateral trochlear nerve palsy after a motorcycle accident. Graduated plication of 5 mm in the right eye and 10 mm in the left eye corrected the excyclotorsion, achieving orthophoria and resolution of diplopia in all directions at 3 months postoperatively. This method is non-transecting, reversible, and less invasive than the Harada-Ito procedure or the Fells modification 5).
Kiarudi et al. (2024) reported the first case of fourth cranial nerve palsy in a healthy 10-year-old boy with confirmed asymptomatic COVID-19 infection. He presented with an abnormal head posture and left eye hypertropia; brain MRI was normal. After one year, 10 prism diopters of left hypertropia and inferior oblique overaction persisted, suggesting that post-viral fourth nerve palsy may have a poorer prognosis than sixth nerve palsy 4).
Low et al. (2022) reported a 78-year-old woman with simultaneous oculomotor and trochlear nerve palsy complicating herpes zoster ophthalmicus (HZO). Complete ptosis and ocular movement limitation appeared 12 days after rash onset. After six weeks of oral acyclovir (800 mg five times daily) and steroids, ptosis resolved and ocular movements markedly improved at one year 2).
Subramaniam et al. (2023) reported a 56-year-old woman with isolated fourth nerve palsy due to a posterior fossa arachnoid cyst. She presented with one week of vertical diplopia, and MRI revealed an arachnoid cyst in the left posterior cerebellar region. The cyst was thought to compress the posterior cerebellum and affect the fourth nerve course in the dorsal brainstem. Diplopia showed improvement during follow-up 3).
Iida et al. (2024) reported a 19-year-old man with superior oblique muscle palsy (canine tooth syndrome) due to a dog bite. MRI on day 5 after injury showed inflammation extending from the superior oblique tendon to the trochlea and muscle belly, and early steroid therapy was administered. Although the development of pseudo-Brown syndrome was prevented, diplopia persisted; inferior oblique myectomy was performed at 7 months, improving cyclotorsional and vertical diplopia 6).
