A 40-year-old woman presents complaining of double vision when both eyes are open, which she noticed when waking that day. She denies a history of trauma, eye pain, ptosis, blurred vision, headache, paresthesias, slurred speech, focal weakness, fevers, or neck stiffness. Her past medical history is significant only for diabetes mellitus since childhood. When asked to fixate on an object in her distant left lateral visual field, the exam reveals what is shown in the photograph.
What is this condition, and how should it be managed? See next page.
Diagnosis: Isolated Sixth Nerve Palsy
The sixth cranial nerve (abducens) innervates the ipsilateral lateral rectus muscle, and is responsible for abduction (lateral gaze) of the eye. The nerve originates from the sixth nerve nucleus located in the brainstem (pons on the floor of the fourth ventricle). It exits into the subarachnoid space at the pontomedullary junction (medial to facial nerve), and then runs superior between the pons and the clivus, pierces the dura, turns sharply anterior at the tip of the petrous temporal bone to enter the cavernous sinus where it runs alongside the internal carotid artery. It then enters the orbit through the superior orbital fissure and innervates the lateral rectus muscle of the eye. This long course makes the sixth nerve susceptible to injury from trauma or impingement by tumors or inflammation.
The sixth cranial nerve is the most commonly injured ocular motor nerve in adults and second most common in children after the fourth nerve. There is no gender predilection for this condition, and it can occur at any age. The etiology of an isolated sixth nerve palsy is surprisingly broad. Nerve ischemia, trauma, or idiopathic causes are the most common etiologies, but other less common but concerning causes include isolated microvascular stroke of the lateral rectus muscle itself, meningitis (including tuberculosis with 15% to 40% having some resulting cranial nerve deficit), increased intracranial pressure, cavernous sinus mass, multiple sclerosis, autoimmune vasculidites, giant cell arteritis, vitamin deficiencies, Lyme disease, and syphilis.
An isolated sixth nerve palsy in a child is assumed to be due to a brain tumor until proven otherwise, but the condition is most commonly related to trauma or a benign, self-limited condition such as viral infection or vaccination. Tumors, conditions causing increased intracranial pressure, Gradenigo syndrome, and type 1 Duane syndrome or Mobius syndrome (rare congenital conditions) also are potential causes.
The differential diagnosis for an isolated sixth nerve palsy in adults includes myasthenia gravis, orbital wall fracture with entrapment, and thyroid-related orbitopathy as well as other more obscure conditions such as orbital inflammatory pseudotumor and convergence spasm.
Patients with an isolated sixth nerve palsy usually present with binocular horizontal diplopia, which is worse for distance than near vision, particularly in the direction of the affected eye. A patient may rotate his head to minimize the diplopia. On examination, the patient is unable to track temporally to the lateral horizontal position, as in the first photograph. Note the ability to track to the contralateral horizontal lateral position is intact in photograph 2.
Although an isolated sixth nerve palsy is typically simple to diagnose clinically, the ED evaluation and management should be focused on determining the possible etiology of the disorder. An attempt should be made to elicit a history of or other symptoms consistent with stroke, cancer, diabetes, thyroid disease, trauma, increased intracranial pressure, or giant cell arteritis. In children, attempt to elicit a recent history of illness or trauma, neurological symptoms, lethargy, hearing loss, or chronic otitis media infections.
On physical exam, it is important to rule out papilledema, note any ptosis, identify any (possibly subtle) associated cranial nerve palsies by performing a comprehensive neurologic evaluation, and evaluate for a tender, enlarged, nonpulsatile temporal artery. Of note, an isolated unilateral sixth nerve palsy should not be considered a “lateralizing” lesion in that there are diffuse and contralateral possible etiologies which can falsely “lateralize” the lesion.
The ED workup should be directed at determining the possible underlying etiology of the isolated sixth nerve palsy, in particular giant cell arteritis because this condition is a vision-threatening ophthalmologic emergency. Magnetic resonance imaging of the brain is indicated for patients under 45, in those 45 to 55 with no history of vascular disease, and those with severe ocular pain, focal neurologic findings, bilateral involvement, multiple cranial nerve palsies, papilledema, or a history of malignancy.
For EDs without emergent MRI capability, a CT scan to rule out stroke or mass lesion, then a lumbar puncture to rule out elevated increased intracranial pressure and meningitis may be reasonable. Also consider sending cerebral spinal fluid to evaluate for syphilis and Lyme disease in patients who are at risk.
Treatment should be focused on addressing any underlying conditions identified during the evaluation. An occlusive patch can be placed temporarily over the affected eye for symptomatic relief of the diplopia. Patients should be sent for an urgent MRI (if not done in the ED) and close follow-up with an ophthalmologist, neuro-ophthalmologist, or neurologist. Prognosis depends on the underlying etiology.