Did you know . . . Abducens nerve palsy, or cranial nerve VI palsy, affects the lateral rectus muscle, leading to esotropia and diplopia.
Understanding its causes, from trauma to systemic diseases, is crucial for proper diagnosis and treatment.
InBrief
by: Evan Mostafa, MD and George Florakis, MD
Cranial nerve VI, the abducens nerve, is solely responsible for motor innervation of the lateral rectus muscle. Abducens nerve palsy refers to dysfunction of the nerve at various points as it traverses from the pons to the lateral rectus muscle. Its location at different aspects of the cranium makes it susceptible to injury from trauma, ischemia, inflammation, infection, and increased intracranial pressure. Patients with abducens nerve palsy experience esotropia of the affected eye, such that the orbit will be turned medially towards the nose.
Signs and Symptoms
Signs and symptoms of abducens nerve palsy include esotropia/strabismus, head turned toward affected eye, diplopia (binocular horizontal), headache, ocular pain, nausea, and vomiting. Strabismus is the general term meaning when the eyes do not align when focusing on an object.
Causes and Risk Factors
In children, abducens nerve palsy may be congenital or acquired, although the latter is more common and typically due to orbital or head trauma. Pediatric patients with Lyme disease, venous sinus thrombosis, or even shunt failure are susceptible to increased intracranial pressure, which can damage the abducens nerve. Meningitis and tumors may also result in damage. In adults, there are similar etiologies (eg, trauma, infection, inflammation, tumors). Multiple sclerosis, microvascular conditions, and diabetes may be risk factors.
Causes and Risk Factors
Injury, especially a skull fracture
Stroke or other vascular conditions (Giant Cell Arteritis)
Infection, such as Lyme disease or a virus
Brain tumor
Inflammation of the nerve, such as from a disease like multiple sclerosis
Increased pressure in the brain, from meningitis or other causes
Hypertension and arteriosclerosis
Diabetes mellitus
Diseases that are present at birth (congenital diseases)
Diagnostic Evaluation and Differential Diagnosis
Patients with abducens nerve palsy should be given a full ophthalmologic examination. Strabismus measurement should be taken. Timing of symptoms may help to determine etiology. Sudden onset refers to microvascular problems, whereas progressive onset can be related to compression. Clinically, the patient will not be able to properly abduct the affected eye. The patient will be have an eye turned medially towards the nose and may compensate by turning their head toward the affected eye. Patients with suspected abducens nerve palsy may be offered magnetic resonance imagining (MRI); however, MRI is not the gold standard and may not be needed. Laboratory testing includes complete blood count, erythrocyte sedimentation rate, C-reactive protein, hemoglobin A1c blood test, Lyme titer, rheumatoid factor test, and antinuclear antibody test. Differential diagnosis includes congenital esotropia, blowout fracture of the orbit, and myasthenia gravis.
Imitators of Cranial Nerve VI Palsy
Thyroid eye disease
Myasthenia gravis
Duane syndrome (congenital strabismus)
Spasm of the near reflex (pseudomyopia, convergent strabismus and miosis)
Old orbital blowout fracture
Congenital esotropia.
Treatment and Recommended Follow-Up
Treatment of abducens nerve palsy differs based on age and etiology. Most adults with microvascular etiology do not require treatment and can see symptoms resolve within months. Patients with increased intracranial pressure can have lumbar punctures or surgeries performed to lessen intracranial pressure, which may resolve the subsequent palsy. (Lumbar puncture in the setting of increased intracranial pressure may be extremely dangerous and must be performed under the direction of an experienced neurosurgeon.) Children with diplopia should be given prism therapy or alternative patching therapy. Use of botulinum toxin and strabismus surgery may be indicated in more severe cases. Systemic diseases such as diabetes should be treated if identified.
Pearl to Know
The abducens nerve is the most frequently injured ocular motor nerve in adults and the second most commonly injured in children. If Lyme disease is suspected and the initial titer is negative, repeat the titer in 1 month. The erythema migrans rash, even without a positive Lyme titer, is pathognomonic for active Lyme disease.
Sources:
Bendszus M, Beck A, Koltzenburg M, et al. MRI in isolated sixth nerve palsies. Neuroradiology. 2001;43:742-745.
Graham C, Mohseni M. Abducens nerve (CN VI), palsy. StatPearls Web site. https://knowledge.statpearls.com/chapter/nurse-anatomy/17031/. April 4, 2019. Accessed October 7, 2019.
Murchison AP, Gilbert ME, Savino PJ. Neuroimaging and acute ocular motor mononeuropathies. Arch Ophthalmol. 2011;129:301-305.
Wysiadecki G, Orkisz S, Gałązkiewicz-Stolarczyk M, Brzeziński P, Polguj M, Topol M. The abducens nerve: its topography and anatomical variations in intracranial course with clinical commentary. Folia Morphol (Warsz). 2015;74:236-244.
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