Oculomotor nerve palsy
Oculomotor nerve palsy | |
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Other names | Third nerve palsy |
Eye nerves diagram | |
Specialty | Ophthalmology, neurology |
Oculomotor nerve palsy or oculomotor neuropathy
Presentation
A complete oculomotor nerve palsy will result in a characteristic displacement outward (exotropia) and downward (hypotropia). The outward displacement occurs because the lateral rectus muscle (innervated by the sixth cranial nerve) maintains muscle tone in comparison to the paralyzed medial rectus. The downward displacement occurs because the superior oblique muscle (innervated by the fourth cranial or trochlear nerve) is unantagonized by the paralyzed superior rectus, inferior rectus and inferior oblique. The affected individual will also have a ptosis, or drooping of the eyelid, and mydriasis (pupil dilation).
Causes
Oculomotor palsy can arise as a result of a number of different conditions. Non traumatic pupil-sparing oculomotor nerve palsies are often referred to as a "medical third", with those affecting the pupil being known as a "surgical third".
Congenital oculomotor palsy
The origins of the vast majority of congenital oculomotor palsies are idiopathic. There is some evidence of a familial tendency to the condition, particularly to a partial palsy involving the superior division of the nerve with an autosomal recessive inheritance. The condition can also result from aplasia or hypoplasia of one or more of the muscles supplied by the oculomotor nerve. It can also occur as a consequence of severe birth trauma.
Acquired oculomotor palsy
- Space occupying lesions or tumours, both malignant and non-malignant
- Inflammation and infection
- Trauma
- Demyelinating disease (multiple sclerosis)
- Autoimmune disorders such as myasthenia gravis
- Post-operatively as a complication of neurosurgery
- Cavernous sinus thrombosis
Mechanism
As the pair of oculomotor nerves arises from different subnuclei in the midbrain, courses through different structures in the brain and branches into superior and inferior divisions after exiting the cavernous sinuses, any lesions along its path will produce different pathological features of the third nerve palsy. The parasympathetic aspect of the nerve (which constricts pupils and thicken the lens) is located on the nerve surface, supplied by pial blood vessels. The nerve's core contains the main trunk of the oculomotor nerve, supplied by vasa vasorum. Thus pathologies affecting the nerve's core without affecting the superficial part of the nerve (thus sparing the pupillary reflex) are known as "medical" oculomotor nerve palsy. The "surgical" type of oculomotor nerve palsy is caused by external structures compressing on the nerve or trauma, which affects the entire nerve, thus affecting pupillary reflex.[2]
Oculomotor palsy can be of acute onset over hours with symptoms of headache when associated with diabetes mellitus. Diabetic neuropathy of the oculomotor nerve in a majority of cases does not affect the pupil.[3] The sparing of the pupil is thought to be associated with the microfasciculation of the fibers that control the pupillomotor function located on the outmost aspect of the occulomotor nerve fibres; these fibres are spared because they are outermost and therefore less prone to ischemic damage than are the innermost fibres.[4]