Etiology
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)
-
Ischemia
-
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)
-
Benign (idiopathic) intracranial hypertension
-
Meningeal carcinomatosis
-
Aneurysm
-
Pontine stroke
-
Rarely, low cerebrospinal fluid (CSF) pressure headache (eg, after lumbar puncture)
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
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 (eg, due to thrombosis, infection, tumor, or an aneurysm) 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
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.
In children, if increased ICP is excluded, an upper respiratory infection may be the cause of 6th nerve palsy.
Treatment
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 usually resolves whether a cause is identified or not.
