A complete oculomotor nerve palsy will result in a characteristic displacement outward (exotropia) and downward (hypotropia).

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".

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.

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.

Therefore, while almost all forms cause ptosis and impaired movement of the eye, pupillary abnormalities are more commonly associated with trauma and the "surgical third" rather than with ischemia (the "medical third").

The compression of the external autonomic fibres renders the pupil nonreactive and leads to the "surgical third" nerve palsy.