Paralysis of the 3rd cranial nerve or common oculomotor nerve
The third cranial nerve controls the movements of four muscles moving the eye. This nerve also controls the movements of the pupil, the upper eyelid and the accommodation.
The paralysis of the 3rd cranial nerve may be complete or partial, involving all innervated muscles or only a few of them. In the case of complete paralysis, there is a total ptosis, a completely closed eyelid and deviation of the eye outward and downward, a dilated pupil and having no reaction to light.
The congenital form is usually rare, with the pupil spared and associated with amblyopia. The cause may not be clear. Many cases have no obvious cause and are probably secondary to an abnormal nerve development.
Acquired paralysis can be caused by brain trauma, viral infections, tumors, aneurysms, diabetes, and hypertension. Congenital abnormalities of the 3rd cranial nerve include restrictive syndromes such as Congenital Fibrosis of Extraocular Muscles (CFEOMs).
Unfortunately, total paralysis has a bad prognosis for binocular vision. Through surgery, it is possible to obtain a rough realignment in the primary position using eyeglasses, often without eliminating the diplopia. In the case of partial paralysis, surgery can be of great help to re-align the eyes and to regain binocular vision.
The muscle deficit that is often associated with the eyelid hemorrhage deficiency will result to hypotrophy of the eye and ptosis. The isolated muscle deficit of the superior rectus muscle of the eye is generally congenital. The surgical correction involves a recession of the inferior rectus muscle of the eye that is associated with the resection of the superior rectus muscle. Strabismus is corrected by modulating the strength of the contralateral synergist muscle of the eye, the muscle that contracts to make a determined movement. For the total isolated deficit, Knapp’s intervention will be used that involves the transposition of the two straight horizontal muscles to the sides of the upper ridge. Only then will ptosis be corrected.
The deficit in the inferior oblique muscle of the eye is very rare. The condition that simulates this picture is Brown’s Syndrome. A differential diagnosis is made with the evaluation of passive motility in such kind of case.
Paralysis of the 4th cranial nerve or Trochlear nerve palsy
The fourth cranial nerve innervates the superior oblique muscle. This disease causes weakness or paralysis to the Superior Oblique Muscle and can be mono or bilateral.
The muscular actions of the superior oblique muscle are abduction, depression, and internal rotation of the eye. So muscle paralysis causes a reduction in the eye depression, hence a relatively higher eye position than the contralateral eye. The affected eye is also rotated outward and slightly inside that is with hypertrophy and the head will be tilted to the shoulder of the healthy eye. Eye elevation is the most visible aspect of the paralysis. There are two major causes, the first is being congenital and the second is cranial trauma. Rare cases are due to stroke, tumors, aneurysms.
The treatment of choice in congenital forms is surgery. This minimizes diplopia, reduces hypertropia, and corrects the tilt of the head to the shoulder. Surgery is performed on one or both eyes according to the severity of the paralysis. The correct approach involves the recession of the inferior oblique muscle and the possible resection or duplication of the superior oblique muscle, or the recession of the inferior rectus muscle of the contralateral eye. (See also the video: https://www.mariaelisascarale.it/deficit-del-4-nervo-cranico)
A similar picture is found in plagiocephaly or flat head syndrome, where the deficit of the superior oblique muscle is due to a cranial malformation.
Paralysis of the 6th cranial nerve or abducens nerve palsy
The sixth cranial nerve innervates the lateral rectus muscle that rotates the eye outward and its weakness results to the eyes crossing inward toward the nose, with an abnormal head position turned to the problem area. The most frequent causes are stroke, trauma, viral diseases, brain tumors, inflammation, infections, migraine, and increased intraocular pressure. Palsy may be present at birth but the most common cause in children is trauma. In adults, it is stroke. Sometimes the cause may remain unknown despite in-depth investigations.
The 6th cranial nerve has a very long course between the brain and lateral rectus muscle. Depending on the location of the damage, other neurological structures may be affected, so the disorder may be associated with hearing loss, facial problems, ptosis or other concerns with eye movements. Viral paralysis usually regresses completely. Post-traumatic ones partially regress and complete improvement usually occurs in the first six months.
Prism use can eliminate diplopia in the primary position, head straight, but not in all directions. The prismatic power can be reduced as the paralysis improves. The injection of botulinum toxin into the medial rectus muscle may be a possibility within the first month of the onset of the paralysis.
Occlusion on an eye in children should be carefully monitored because there is a risk of an onset of amblyopia. After the observation period, at least 6 months, it is possible to perform an extraocular muscle surgery to achieve correct eye alignment and expand the binocular vision field.