Abducens Nerve Palsy (Sixth Cranial Nerve Palsy)

Key Points at a Glance

Abducens nerve palsy is a condition in which the nerve innervating the lateral rectus muscle is damaged, resulting in limited abduction of the affected eye and incomitant esotropia.
It is the most common ocular motor nerve palsy in adults, with an annual incidence of approximately 11.3 per 100,000 people¹⁾.
In children, it is relatively rare at about 2.5 per 100,000, and since tumors and trauma are the main causes, prompt evaluation is necessary¹⁾.
Horizontal diplopia is the main symptom, worsening with distance vision and gaze toward the affected side.
Most acute cases in adults are due to microvascular ischemia and resolve spontaneously within 3 to 6 months.
In children, benign recurrent abducens nerve palsy is also known, which occurs after viral infection and resolves spontaneously.
Strabismus surgery is considered for persistent cases that have been stable for more than 6 months.

1. What is abducens nerve palsy?

The abducens nerve (cranial nerve VI) is a motor nerve that innervates the lateral rectus muscle. When this nerve is paralyzed, the tension of the lateral rectus muscle decreases, and the action of the antagonistic medial rectus muscle becomes relatively dominant. As a result, impaired abduction of the affected eye and paralytic (incomitant) esotropia occur.

Abducens nerve palsy is the most common isolated ocular motor nerve palsy in adults, with an annual incidence of approximately 11.3 per 100,000 people ¹⁾. In children, it is relatively rare at about 2.5 per 100,000, and is the second most common ocular motor nerve palsy ¹⁾.

The abducens nerve has the longest intracranial course among the cranial nerves. It originates from the nucleus in the dorsal pons, exits the brainstem at the pontomedullary junction, then runs through the subarachnoid space, crosses the petrous apex of the temporal bone, and is fixed in the Dorello’s canal. After that, it passes through the cavernous sinus and superior orbital fissure, and reaches the lateral rectus muscle via the annulus of Zinn in the orbit ¹⁰⁾. Due to this long course, it is susceptible to damage from lesions at various sites.

Q What is the difference between abducens nerve palsy and gaze palsy?

Abducens nerve palsy is a peripheral nerve disorder that causes abduction limitation only in the affected eye. Gaze palsy is a disorder of the brainstem gaze center that impairs conjugate eye movements. Internuclear ophthalmoplegia is a lesion of the medial longitudinal fasciculus that shows adduction limitation. Careful evaluation of eye movements is necessary for differentiation.

2. Main Symptoms and Clinical Findings

Subjective Symptoms

Diplopia (double vision): The most common initial symptom. It presents as horizontal homonymous diplopia, more noticeable at distance than near ¹⁰⁾.
Direction-dependent diplopia: Worsens when looking toward the affected side and improves when looking toward the opposite side.
Headache and eye pain: If caused by increased intracranial pressure, headache may be present. Nausea, vomiting, and pulse-synchronous tinnitus may occur.
Note for children: Infants cannot complain of diplopia, so ocular misalignment (esotropia) or abnormal head posture (face turn) may be the first signs.

Clinical Findings

The main findings of abducens nerve palsy are shown below.

Abduction limitation: The affected eye has limited abduction (outward movement). The degree of limitation ranges from partial to complete palsy.
Incomitant esotropia: The affected eye deviates inward. The deviation increases with distance fixation, which is important for differentiating from comitant esotropia¹⁰⁾.
Abnormal head posture: Turning the face toward the affected side allows the affected eye to fixate in an adducted position, reducing diplopia.
Enlargement of the palpebral fissure: The palpebral fissure may widen during maximal abduction effort.
Nystagmus on abduction: Sensory-motor testing may reveal abducting nystagmus when gazing toward the affected side¹⁰⁾.

Finding	Characteristic
Change in deviation	Distance > near, increases with gaze toward the affected side
Second deviation	Larger than the first deviation
Abduction limitation	Varies from partial to complete

Q In what conditions does bilateral abducens nerve palsy occur?

Bilateral abducens nerve palsy suggests increased intracranial pressure, clivus tumors (e.g., chordoma), meningitis, or meningeal infiltration. In bilateral cases, further evaluation including lumbar puncture is recommended¹⁰⁾.

3. Causes and Risk Factors

The causes of abducens nerve palsy differ significantly between children and adults.

Causes in Children

Congenital

Developmental abnormalities: Congenital palsy due to neuronal migration disorders is rare⁹⁾.

Birth trauma: Trauma during birth can be a cause.

Comorbid conditions: Associated with hydrocephalus and cerebral palsy.

Acquired

Tumors: Pontine glioma, medulloblastoma, and ependymoma are the main causes in children. Peak onset is between 5 and 8 years of age.

Trauma: Occurs after closed head injury via increased intracranial pressure.

Infection/Inflammation: Benign recurrent palsy following viral infection.

Intracranial pressure fluctuations: Secondary to shunt failure or idiopathic intracranial hypertension.

Post-viral abducens nerve palsy in children can recur. A case has been reported in which it occurred after COVID-19 infection at 7 months of age and then recurred twice following gastroenteritis and RS virus infection¹⁾. In a review of 72 cases of recurrent abducens nerve palsy in children, the mean age of onset was 3 years, and 81% recovered completely with observation alone¹⁾.

Abducens nerve palsy in children should be treated as an emergency, unlike in adults. This is because, aside from infections, brain tumors account for most causes. Differentiation from congenital conditions (such as Duane syndrome) is important; if the child is old enough to report double vision, awareness of diplopia or a sudden abnormal head position (compensatory head posture) suggests an acquired cause.

Causes in Adults

Microvascular ischemia: Most common cause, often associated with diabetes and hypertension. Most cases recover spontaneously within 6 months¹⁰⁾.
Trauma: The nerve is vulnerable to injury at the petrous apex due to its fragile course.
Tumors: Skull base tumors (clival meningioma, chordoma, nasopharyngeal carcinoma, metastatic tumors) can be causative³⁾⁶⁾.
Demyelinating diseases: Isolated abducens nerve palsy may present as the initial symptom of multiple sclerosis²⁾.
Inflammatory diseases: Cases of hypophysitis presenting with bilateral abducens nerve palsy have been reported⁷⁾.
Paraneoplastic syndromes: Abducens nerve palsy can be the initial symptom in paraneoplastic syndromes with positive anti-Hu and anti-CV2 antibodies⁸⁾.

The etiology of isolated abducens nerve palsy in patients aged 15–50 years has been reported as vascular disorders 29%, tumors 16%, multiple sclerosis 12%, inflammation 8%, and trauma 6%³⁾.

Classification by lesion site

Nuclear lesions: The abducens nucleus in the pons is affected. Because the ipsilateral facial nerve (cranial nerve VII) pathway is adjacent, facial nerve palsy often accompanies¹⁰⁾.
Fascicular lesion: Causes Foville syndrome (pontine tegmentum lesion) and Millard-Gubler syndrome (pontine ventral lesion).
Subarachnoid space to Dorello’s canal: Traction due to intracranial pressure fluctuations. Presents as non-localizing signs.
Cavernous sinus: Impaired by tumor invasion, internal carotid artery aneurysm, or carotid-cavernous fistula ⁵⁾. Ipsilateral Horner syndrome has high localizing value for posterior cavernous sinus lesions ⁵⁾.

4. Diagnosis and Examination Methods

The diagnosis of abducens nerve palsy is based on clinical examination. It can be clinically diagnosed if abduction deficit and incomitant esotropia of the affected eye are confirmed.

Ophthalmic Evaluation

Visual acuity and refraction test: Essential as baseline data.
Ocular motility evaluation: Assess ductions and versions. Slowing of saccades during lateral gaze aids diagnosis. In children, confirming abduction limitation can be difficult; using the doll’s eye phenomenon can make it easier. It is useful for the examiner to observe from the side of the abduction position.
Strabismus measurement: Measure the deviation at distance and near, and in nine gaze positions. It can also be detected by the cover test.
Fusion range measurement: Used to evaluate binocular vision function.
Fundus examination: Check for papilledema to assess increased intracranial pressure.

Imaging Tests

Gadolinium-enhanced MRI is the most useful imaging test.

MRI is recommended in the following cases.

Patients under 50 years of age¹⁰⁾
When accompanied by pain or other neurological abnormalities
When there is a history of cancer
Bilateral abducens nerve palsy
When papilledema is present
When there is no improvement after 3 months

Even in elderly patients with vascular risk factors, consider imaging if no improvement after 4–6 weeks ¹⁰⁾.

Blood Tests

Complete blood count, blood glucose, HbA1c
Erythrocyte sedimentation rate, CRP
Syphilis serology (FTA-ABS)
Lyme disease antibody titer
If giant cell arteritis is suspected, urgently check ESR and CRP.

If there are signs of increased intracranial pressure, perform lumbar puncture after imaging, and measure opening pressure and analyze CSF (to evaluate infectious, inflammatory, or carcinomatous meningitis)¹⁰⁾.

Differential Diagnosis

It is important to differentiate from other diseases that present with abduction limitation.

Differential Diagnosis	Key Points for Differentiation
Duane syndrome	Narrowing of the palpebral fissure on adduction
Thyroid eye disease	Proptosis, inflammatory signs
Myasthenia gravis	Fatigability, fluctuating diplopia

Duane syndrome is a congenital disorder caused by hypoplasia of the abducens nucleus, characterized by narrowing of the palpebral fissure and retraction of the globe on adduction. These findings are not present in abducens nerve palsy. Fisher syndrome presents with the triad of external ophthalmoplegia, ataxia, and areflexia, and GQ1b antibodies are positive in 90% of cases.

Q What is the most likely cause to suspect first in a child with abducens nerve palsy?

In children, tumor and trauma are the most important causes. In particular, pontine glioma is a representative pediatric brain tumor, and if abducens nerve palsy is accompanied by ataxia or gait disturbance, urgent MRI is necessary. Benign recurrent abducens nerve palsy is a diagnosis of exclusion.

5. Standard Treatment

Treatment of abducens nerve palsy prioritizes treatment of the underlying cause.

Observation

Abducens nerve palsy due to microvascular ischemia often resolves spontaneously within 3 to 6 months¹⁰⁾. About one-third recover within 8 weeks, and if no recovery occurs by 6 months, approximately 40% have a serious underlying disease. Observation is also the basic approach for benign post-viral cases in children.

Conservative Treatment

Occlusion therapy: Use of a Bangerter filter or patch to occlude the affected or healthy eye to eliminate diplopia. In children, alternating occlusion to prevent amblyopia should be considered. However, occluding one eye all day to avoid diplopia or compensatory head posture is contraindicated as it may cause occlusion amblyopia. In children who refuse glasses, occlusion of the healthy (dominant) eye for 1 to 3 hours per day or alternating occlusion is performed to prevent amblyopia due to suppression of the paretic or non-dominant eye.
Prism therapy: Use of Fresnel membrane prisms or prism glasses to correct eye alignment and maintain binocular vision as much as possible. However, due to the incomitance of the deviation, the effect is limited. In mild abducens nerve palsy, prism glasses may be sufficient.

Botulinum Toxin Injection

Botulinum toxin injection into the medial rectus muscle of the affected eye is used to prevent secondary contracture of the medial rectus. It is also used as a temporary treatment before surgery.

Surgical Treatment

Strabismus surgery is performed for persistent cases where orthoptic measurements have been stable for 6 months or more. Prior to surgery, a traction test is performed to assess the presence of restrictions.

Residual Lateral Rectus Function

Recession-Resection (R&R) Procedure: The standard approach is a combination of lateral rectus resection and ipsilateral medial rectus recession in the affected eye.

Contralateral Medial Rectus Recession: A combination of lateral rectus resection in the affected eye and contralateral medial rectus recession is also an option.

Absent Lateral Rectus Function

Transposition surgery: Full tendon transposition, Jensen procedure, Hummelsheim procedure, etc., are considered.

Superior rectus transposition: Combined superior rectus transposition and medial rectus recession is effective in improving esotropia, head posture, and abduction.

6. Pathophysiology and detailed pathogenesis

The abducens nerve originates from the abducens nucleus in the dorsal pons. The nucleus contains motor neurons that innervate the lateral rectus muscle and interneurons that send signals to the contralateral oculomotor nucleus via the medial longitudinal fasciculus. This circuit controls horizontal conjugate movements (saccades)⁹⁾.

Anatomical Course and Vulnerability

The abducens nerve (cranial nerve VI) originates in the hindbrain, runs ventrally, and innervates the lateral rectus muscle⁹⁾. Its course is as follows.

From the nucleus in the dorsal pons, it runs ventrally and exits the brainstem at the pontomedullary junction
It travels a long distance through the subarachnoid space
It is fixed at the petrous apex of the temporal bone in Dorello’s canal (a fibro-osseous canal under the petroclinoid ligament)
It passes through the cavernous sinus (unlike other cranial nerves, it runs within the venous sinus)
It reaches the lateral rectus muscle from the superior orbital fissure through the annulus of Zinn.

Fixation in Dorello’s canal causes vulnerability during intracranial pressure fluctuations. When intracranial pressure increases, the brain shifts downward, and the abducens nerve is stretched in Dorello’s canal. Therefore, abducens nerve palsy appears as a false localizing sign of increased intracranial pressure.

Pathophysiology of microvascular ischemia

Microvascular abducens nerve palsy is self-limiting, and autopsy cases are extremely rare. In the few autopsy findings, extensive demyelination, fragmentation of the nerve sheath, and thickening and hyalinization of the vessel walls in the vasa nervorum have been confirmed.

Association with Congenital Cranial Nerve Motor Abnormalities

Congenital abducens nerve abnormalities are part of a group of congenital cranial dysinnervation disorders (CCDDs) ⁹⁾. CCDDs arise from two mechanisms: impaired specification of motor neurons or abnormal axon growth and guidance. Duane syndrome is caused by hypoplasia of the abducens nucleus and is the most common CCDD, occurring in approximately 1 in 1,000 individuals ⁹⁾. Haploinsufficiency of the SALL4 gene impairs development of the abducens nucleus and causes autosomal dominant Duane-radial ray syndrome ⁹⁾. Moebius syndrome involves the sixth and seventh cranial nerves, presenting with abduction limitation and facial muscle weakness ⁹⁾.

Three mechanisms have been proposed for abducens nerve palsy associated with COVID-19¹⁾.

Baldwin et al. (2024) analyzed 19 cases of isolated abducens nerve palsy related to COVID-19¹⁾. The median latency from onset of systemic symptoms to abducens nerve palsy was 6 days, and the median time to recovery was 30 days. A correlation was found between longer latency to onset and longer recovery time (R²=0.401, p=0.010).

Inflammation hypothesis: autoimmune-mediated nerve damage. Molecular mimicry may be involved.
Coagulation abnormality hypothesis: microvascular ischemia due to endothelial injury.
Direct damage hypothesis: direct viral invasion of the central nervous system. Viral proteins have been detected in cranial nerves in 53% of autopsy cases¹⁾.

7. Latest Research and Future Perspectives (Research Stage Reports)

Abducens Nerve Palsy Due to Multiple Myeloma

Intracranial plasmacytoma is rare, accounting for less than 1% of all intracranial tumors. It occurs in the clivus or petrous part of the temporal bone, compressing the abducens nerve near Dorello’s canal.

Thalambedu et al. (2023) reported two cases of abducens nerve palsy due to central nervous system multiple myeloma and reviewed 28 previous cases from the literature ⁶⁾. Onset occurred in both newly diagnosed and relapsed/refractory cases, with time from symptom onset to diagnosis ranging from days to months. Treatment involved a combination of surgery, radiation, and chemotherapy, leading to symptom improvement.

Nasopharyngeal Carcinoma and Abducens Nerve Palsy

Nasopharyngeal carcinoma is common in Southeast Asia and may present with isolated abducens nerve palsy as the initial symptom.

Lekskul et al. (2021) reported five cases of nasopharyngeal carcinoma presenting with isolated abducens nerve palsy ³⁾. Two patients were under 50 years old, and three were 50 or older; one had vascular risk factors. MRI revealed tumor invasion of the clivus, Dorello’s canal, and cavernous sinus. Chemoradiotherapy improved eye movement in four cases.

Cavernous Sinus Metastasis

Singh et al. (2021) reported a 40-year-old woman with isolated abducens nerve palsy as the initial symptom due to cavernous sinus metastasis from breast cancer ⁴⁾. Contrast-enhanced MRI showed a 20 mm × 10 mm enhancing lesion from the lateral wall of the cavernous sinus to the petrous apex.

Carotid-Cavernous Fistula and Combined Lesions

Yuan et al. (2022) reported three cases of abducens nerve palsy combined with ipsilateral Horner syndrome due to carotid-cavernous fistula ⁵⁾. The two conditions may be discovered simultaneously or sequentially. After repair of the carotid-cavernous fistula, abducens nerve palsy recovers well, but Horner syndrome tends to persist.

Q What happens if abducens nerve palsy does not resolve spontaneously?

If there is no improvement for more than 6 months, imaging studies are necessary to rule out serious underlying disease. Strabismus surgery is considered once eye movements are stable. For details, see the “Standard Treatment” section.

8. References

Baldwin G, Gaier ED, Hennein L. Isolated sixth nerve palsy and COVID-19: a recurrent case in a 7-month-old child and analysis of reported cases. J Neuroophthalmol. 2024;44(3):301-307.
Al-Yousuf N, Aljutaili L, AlHuwais A, et al. Multiple sclerosis presenting with sixth nerve palsy in a child. Int Med Case Rep J. 2021;14:545-550.
Lekskul A, Thanomteeranant S, Tangtammaruk P, Wuthisiri W. Isolated sixth nerve palsy as a first presentation of nasopharyngeal carcinoma: a case series. Int Med Case Rep J. 2021;14:801-808.
Singh A, Sharma P, Pal H, Sharma S, Dixit A. Isolated sixth nerve palsy as the first manifestation of cavernous sinus metastasis from primary breast cancer. Cureus. 2021;13(12):e20094.
Yuan PH, Micieli JA. Simultaneous and sequential development of sixth nerve palsy and Horner’s syndrome from carotid cavernous sinus fistulas. Case Rep Ophthalmol. 2022;13:37-43.
Thalambedu N, Damalcheruvu P, Ogunsesan Y, et al. Sixth nerve palsy from multiple myeloma in central nervous system: case series and review of current literature. Cureus. 2023;15(6):e40998.
Pagoada-Torres JD, Villalobos-Díaz R, Pineda-Centeno LM, et al. Bilateral sixth nerve palsy: a rare presentation of primary hypophysitis. Cureus. 2024;16(4):e58850.
Chandran V, Masnon NA, Nasaruddin RA, et al. A tetrad catastrophe: paraneoplastic syndrome with abducens palsy, intracranial hypertension, and optic neuropathy in primary lung cancer. Cureus. 2024;16(8):e67335.
Whitman MC. Axon guidance molecules and disorders of the cranial motor nerves. Annu Rev Vis Sci. 2021;7:827-850.
American Academy of Ophthalmology. Adult Strabismus Preferred Practice Pattern. 2024.

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