Sixth cranial nerve palsy affects the lateral rectus muscle, impairing eye abduction. The eye may be slightly adducted when the patient looks straight ahead. The palsy may be secondary to nerve infarction, Wernicke encephalopathy, trauma, infection, or increased intracranial pressure, or it may be idiopathic. Determining the cause requires MRI and often lumbar puncture and evaluation for vasculitis.
(See also Overview of Neuro-ophthalmologic and Cranial Nerve Disorders.)
Etiology of Sixth Cranial Nerve Palsy
Sixth cranial (abducens) nerve palsy results from the following:
Typically, small-vessel disease, particularly in diabetics as part of a disorder called mononeuritis multiplex (multiple mononeuropathy)
Microvascular disease secondary to diabetes or hypertension
Sometimes hypertension
Compression of the nerve by lesions in the cavernous sinus (eg, nasopharyngeal tumors), orbit (eg, orbital cellulitis), or base of the skull
Increased intracranial pressure (ICP)
Meningeal carcinomatosis
Aneurysm
Pontine stroke
Tumors affecting the meninges
Rarely, low cerebrospinal fluid (CSF) pressure headache (eg, after lumbar puncture)
Cavernous sinus lesions may be due to thrombosis, infection, a nasopharyngeal tumor, or an aneurysm.
Children with an upper respiratory infection may have recurrent palsy.
However, the cause of an isolated 6th cranial nerve palsy is often not identified.
Symptoms and Signs of Sixth Cranial Nerve Palsy
Symptoms of 6th cranial nerve palsy include binocular horizontal diplopia when looking to the side of the paretic eye. Because the tonic action of the medial rectus muscle is unopposed, the eye is slightly adducted when the patient looks straight ahead. The eye abducts sluggishly, and even when abduction is maximal, the lateral sclera is exposed. With complete paralysis, the eye cannot abduct past midline.
Palsy resulting from a cavernous sinus lesion can cause severe head pain, chemosis (conjunctival edema), anesthesia in the distribution of the 1st and 2nd division of the 5th cranial nerve, and paralysis of the 3rd, 4th, and 6th cranial nerves. Both sides may be affected, although unevenly.
Diagnosis of Sixth Cranial Nerve Palsy
MRI (or CT)
If vasculitis is suspected, erythrocyte sedimentation rate (ESR), antinuclear antibodies, and rheumatoid factor
A 6th nerve palsy is usually obvious, but the cause is not. If retinal venous pulsations are seen during ophthalmoscopy, increased ICP is unlikely.
CT is often done because it is often immediately available. However, MRI is the test of choice; MRI provides greater resolution of the orbits, cavernous sinus, posterior fossa, and cranial nerves. If imaging results are normal but meningitis or benign intracranial hypertension is suspected, lumbar puncture is done.
If vasculitis is suspected clinically, evaluation begins with measurement of ESR, antinuclear antibodies, and rheumatoid factor.
Other tests may be done depending on the suspected cause of 6th cranial nerve palsy.
In children, if increased ICP is excluded, an upper respiratory infection may be the cause of 6th nerve palsy.
Treatment of Sixth Cranial Nerve Palsy
Treatment of the cause
In many patients, 6th cranial nerve palsies resolve once the underlying disorder is treated. Treatment of infection, inflammation, or tumor, when present, may result in improvement.
Idiopathic palsy and ischemic palsy usually abate within 2 months.
Sixth cranial nerve palsy commonly resolves when the cause is nontraumatic and may do so posttrauma.
Key Points
Sixth cranial (abducens) nerve palsy typically results from small-vessel disease, particularly in diabetics, but the cause is often unidentified.
This palsy causes impaired abduction and horizontal diplopia.
To identify the cause, do neuroimaging (preferably MRI), followed by lumbar puncture if imaging results are normal and benign intracranial hypertension is suspected; if vasculitis is suspected, start with erythrocyte sedimentation rate (ESR), antinuclear antibodies, and rheumatoid factor.
If increased intracranial pressure is excluded in children, consider an upper respiratory infection.
Sixth cranial nerve palsy commonly resolves when the cause is nontraumatic and may do so posttrauma; treatment of infection, inflammation, or tumor, when present, may result in improvement.
