Ocular motor cranial nerve palsies
Oculomotor nerve palsy is an eye condition resulting from damage to the third cranial nerve or a branch thereof. As the name suggests, the oculomotor nerve supplies the majority of the muscles controlling eye movements (4 out of the 6 extracocular muscles. All except the Lateral Rectus and Superior Oblique). Thus, damage to this nerve will result in the affected individual being unable to move their eye normally.
In addition, the nerve also supplies the upper eyelid muscle (levator palpebrae superioris) and It is accompanied by parasympathetic fibers innervating the muscles responsible for pupil constriction (sphincter pupillae) . The limitations of eye movements resulting from the condition are generally so severe that the affected individual is unable to maintain normal alignment of their eyes when looking straight ahead, leading to strabismus and, as a consequence, double vision (diplopia).
In complete third nerve palsy, there is ptosis with a divergent and slightly depressed eye. Extraocular movements are restricted in all directions except laterally (preserved lateral rectus function). Intact fourth nerve (superior oblique) function is detected by inward rotation on attempted depression of the eye.
Pupillary involvement, manifesting as a relatively dilated pupil that does not constrict normally to light, usually means compression, which may be due to aneurysm of the posterior communicating artery or uncal herniation due to a supratentorial mass lesion.
In acute painful isolated third nerve palsy with pupillary involvement, posterior communicating artery aneurysm must be excluded. Pituitary apoplexy is a rarer cause. Causes of isolated third nerve palsy without pupillary involvement include diabetes mellitus, hypertension, giant cell arteritis, and herpes zoster.
Fourth nerve palsy causes upward deviation of the eye with failure of depression on adduction. In acquired cases, there is vertical and torsional diplopia that is most apparent on looking down. Trauma is a major cause of acquired— particularly bilateral—fourth nerve palsy, but posterior fossa tumor and medical causes, such as in third nerve palsy, should also be considered. Similar clinical features are seen in congenital cases due to developmental anomaly of the nerve, muscle, or tendon.
Sixth nerve palsy causes convergent squint in the primary position with failure of abduction of the affected eye, producing horizontal diplopia that increases on gaze to the affected side and on looking into the distance. It is an important sign of raised intracranial pressure and may also be due to trauma, neoplasms, brainstem lesions, petrous apex lesions, or medical causes (such as diabetes mellitus, hypertension, giant cell arteritis, and herpes zoster).
In isolated ocular motor nerve palsy presumed to be due a medical cause, brain MRI is not always required initially, but it is necessary if recovery has not begun within 3 months.
Ocular motor nerve palsy accompanied by other neurologic signs may be due to lesions in the brainstem, cavernous sinus, or orbit. Lesions around the cavernous sinus involve the first and second divisions of the trigeminal nerve, the ocular motor nerves, and occasionally the optic chiasm. Orbital apex lesions involve the optic nerve and the ocular motor nerves.
Myasthenia gravis and thyroid eye disease (Graves ophthalmopathy) should be considered in the differential diagnosis of disordered extraocular movements
Prismatic Measurement Exam- When needed, your doctor will conduct a thorough measurement of the ocular misalignment using prisms while covering one eye or the other.
Magnetic Resonance Imaging (MRI) Scan- Your doctor may order an MRI to rule out tumors and other structural abnormalities as these can cause similar symptoms.
Therapy- Treatment of cranial nerve palsies and ocular motor disorders depends on the cause. In some cases, you may need to wear prism glasses (specialized prescription).
Eye muscle surgery- In some cases, you may need to undergo eye muscle surgery.