Oculomotor nerve palsy

Presentation

A complete oculomotor nerve palsy produces an outward and downward displacement of the eye, known as exotropia and hypotropia. The outward deviation results from unopposed tone of the lateral rectus muscle, while the downward deviation arises from unopposed action of the superior oblique muscle. Affected individuals also exhibit ptosis (drooping eyelid) and mydriasis (pupil dilation).

Causes

Oculomotor palsy can stem from various conditions. Non-traumatic pupil-sparing palsies are termed "medical third," while those affecting the pupil are called "surgical third." Most congenital cases are idiopathic, though some show familial tendency with autosomal recessive inheritance. Causes include aplasia or hypoplasia of supplied muscles, severe birth trauma, vascular disorders (diabetes, heart disease, atherosclerosis, aneurysm, especially of the posterior communicating artery), space-occupying lesions, inflammation, infection, trauma, demyelinating disease, autoimmune disorders like myasthenia gravis, postoperative complications, and cavernous sinus thrombosis.

Mechanism

The oculomotor nerves arise from different subnuclei in the midbrain, course through various brain structures, and branch into superior and inferior divisions after exiting the cavernous sinuses. Lesions along this path produce different pathological features. The parasympathetic fibers lie on the nerve surface, supplied by pial blood vessels, while the core contains the main trunk supplied by vasa vasorum. Pathologies affecting the core without involving the surface spare the pupillary reflex ("medical" palsy), whereas external compression or trauma affects the entire nerve, causing "surgical" palsy with pupillary involvement.

Ischemic stroke selectively damages somatic fibers over parasympathetic fibers because ischemia affects the vasoneurium from outside inward, and somatic fibers are located in the inner part of the nerve. A similar mechanism applies in diabetes. Thus, ptosis and impaired movement occur in almost all forms, but pupillary abnormalities are more common with trauma and "surgical third" palsies. A posterior communicating artery aneurysm compresses the entire third nerve, blocking both somatic and autonomic conduction.

Diabetic oculomotor neuropathy typically spares the pupil, thought to be due to the outermost location of pupillomotor fibers, which are less prone to ischemic damage than the innermost fibers.