Cranial Nerve Palsies
Strabismus or misalignment of the eyes, can be caused by palsies or weakness of certain cranial nerves (CN). There are 12 specialized cranial nerves that course through the brain and control various functions and sensations of the head and neck. Four of these nerves are involved with eye and eyelid movements: CN III (3rd), CN IV (4th), CN VI (6th), and CN VII (7th).
Third Cranial Nerve Palsies
Third cranial nerve palsies can result in drooping of the eyelid (ptosis) and an outward drifting of the eye (exotropia). ). The affected eye is unable to look in towards the nose, up, or down. Sometimes the pupil is also abnormally enlarged on that side. Third nerve palsies can be acute or chronic. Any sudden third nerve palsy can be a neurologic emergency, caused by an abnormality in the brain like bleeding, a tumor, or an aneurysm. In children, third nerve palsies can be congenital, secondary to serious head trauma, vascular abnormality, tumor, infection, demyelination, inflammation, or even a migraine.
Third nerve palsies require a neurologic work-up, typically including neuroimaging with a brain MRI or CAT scan. Depending on the causative factors, the misalignment and ptosis caused by third nerve palsies can be addressed with eye muscle and eyelid surgery, once the problem has stabilized.
Fourth Cranial Nerve Palsies
Fourth cranial nerve palsies are more common in the pediatric population. The fourth cranial nerve controls the superior oblique eye muscle, responsible for moving the eye downwards when looking towards the nose. A partial or complete palsy of the fourth cranial nerve may result in a head tilt. Infants noted to have torticollis, or a sustained head tilt, may be referred to a pediatric ophthalmologist to be evaluated for a congenital fourth nerve palsy. Fourth nerve palsies can also be caused by head trauma, infection, or a brain mass. In young children, however, they are most likely to be benign and congenital, not requiring a neurologic work-up. A fourth nerve palsy can be treated effectively with eye muscle surgery to one or both eyes.
Sixth Cranial Nerve Palsies
The sixth cranial nerve innervates the lateral rectus muscle, which helps keep the eye straight and allows it to move outwards towards the temple. Sixth nerve palsies of one or both eyes result in crossing of the eye(s) and an inability to move the eye out to the side. This may cause double vision, with two images being side by side. Sixth nerve palsies can occur due to an infection or inflammation of the brain or the brain’s coating (meninges); high intracranial pressure due to bleeding, swelling, or mass of the brain; head trauma; post viral infection; stroke; congenital causes; idiopathic (not known). Depending on the cause for the sixth nerve palsy, the resulting cross and double vision may be managed with patching, prism glasses, Botox injections, or eye muscle surgery.
Seventh Cranial Nerve Palsies
The seventh cranial nerve is also called the facial nerve as it innervates the muscles of the eyelids and mouth. A seventh nerve palsy, also called a Bell’s palsy can result in a sag of the lower eyelid as well as poor ability to close the eyes properly on the affected side. This can result in the tear film not staying firmly against the eye’s surface to keep it lubricated. In addition, the weak eyelids may not be able to protect the front of the eye from drying out, especially during sleep. This may result in chronic redness and irritation of the eye, and can even damage the vision due to corneal compromise. Seventh nerve palsies can be idiopathic (cause unknown); due to infections like Lyme disease; or secondary to trauma, tumor, stroke, bleeding, or inflammation. They also require a neurologic work-up. Chronic seventh nerve palsies may require eyelid surgery to improve the ability for proper lid closure.