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.Ischemic stroke selectively affects somatic fibers over parasympathetic fibers, while traumatic stroke affects both types more equally. Ischemic stroke affects the vasoneurium, which starts to supply the nerve from outside to inside. As the somatic fibers are located in the inner part of the nerve, these fibres are affected more in the setting of ischemia. A similar mechanism is also accurate for diabetes. 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"). A posterior communicating artery aneurysm will generally cause compression of the entire third nerve and will this prevent any nerve signal conduction, affecting the somatic system as well as the autonomic. The compression of the external autonomic fibres renders the pupil nonreactive and leads to the "surgical third" nerve palsy. 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. 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.
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