What is a Sixth Nerve Palsy?
The sixth cranial nerve (abducens nerve) is responsible for innervating the lateral rectus muscle which pulls the eye laterally (abducts the eye). A sixth nerve palsy is a weakness or decrease in innervation to the lateral rectus muscle. The result is the loss or decrease in the ability of the eye to turn outward and the eyes will typically turn in towards the nose.
A sixth nerve palsy is unilateral in most cases and rarely bilateral.
What are the Symptoms of a Sixth Nerve Palsy?
Horizontal double vision (images are doubled side by side) is the main symptom of a sixth nerve palsy. When unilateral, the double vision will be worse or the images farther apart when the patient looks laterally to the side of the sixth nerve palsy. Many people turn their head toward the side of the sixth nerve palsy to decrease the double vision. Children under age six or seven will not have double vision as their brains can suppress an eye and not see double vision causing amblyopia.
What Causes a Sixth Nerve Palsy?
The sixth nerve has a long course from the brainstem to the lateral rectus muscle. Therefore, there are many causes of a sixth nerve palsy occurring along its pathway.
Diabetes is the most common cause
High blood pressure
Increased intracranial pressure
Cavernous sinus mass
Other Causes of Double Vision That Mimic Sixth Nerve Palsy
Thyroid eye diseases (Graves’ disease)
Diagnosis of a Sixth Nerve Palsy
The workup for a sixth nerve palsy includes blood test, MRI, and workup by a neuro-ophthalmologist or neurologist to determine the cause. This will lead to the proper treatment of the sixth nerve palsy. Many cases of sixth nerve palsy are temporary as with diabetes and will resolve in a few weeks or months.
Temporary Treatment of Sixth Nerve Palsy
Fresnel prisms are thin flexible prisms that can be attached to a patient’s glasses or glasses can be made without a prescription to hold the prisms. Prisms bend light so that the double vision can be decreased or treated while the patient is recovering from the sixth nerve palsy. The Fresnel prisms are easily attached to the glasses.
Long Term Treatment
If after a year and the palsy has not resolved, other forms of treatment may be considered.
If the deviation is not too large, permanent prisms can be placed in regular eyeglasses.
The surgical procedure will be determined by the level of function of the sixth nerve. Complete loss of function will require a transposition surgery such as Jensen’s or whole muscle transposition.