Cranial Nerve 3 Palsy - Surgical CN3

The picture below shows a young girl with complete ptosis of the left eye. She presented with progressive and worsening ptosis affecting her left eyelid. At the same time, she reported having diplopia as well. 

Upon elevating her left upper eyelid, her left eyeball is displaced inferolaterally in neutral position. This finding is consistent with a left CN 3 (occulomotor) palsy. The eyeball is deviated in such a manner due to unopposed action of the left lateral rectus and left superior oblique muscles which are innervated by the left CN 6 (abducens) and left CN 4 (trochlear) nerves respectively. 

It is also important to take the opportunity to assess the left pupil size to determine the possible cause of her left CN 3 palsy. In this picture, it can be seen that her left pupil is dilated compared to the right pupil. This suggests that she may have a space occupying lesion compressing the CN3. In most cases, a cerebral aneurysm (particularly from the posterior communicating (PCOM) artery) need to be looked for. The pathogenesis of a dilated pupil in such a scenario is that an aneurysm arising from the PCOM artery compresses on the CN 3 putting pressure on, and damaging the parasympathetic nerve fibres that innervate the constrictor pupillae muscle that is responsible for pupillary constriction. These parasympathetic nerve fibres run on the surface of the CN 3, therefore when there is an external compression onto the CN3, the parasympathetic nerve fibres would be affected as well.

Once a surgical cause i.e. an aneurysm has been excluded, then a medical cause can still be possible. Bear in mind, the parasympathetic nerve fibres that innervate the constrictor pupillae muscle that is responsible for pupillary constriction is also supplied by tiny blood vessels called vasa nervorum. The vasa nervorum are equally susceptible to damage from medical diseases e.g. connective tissue disorders, diabetes mellitus, atherosclerosis etc like other arteries elsewhere in the body. Once the blood supply to the parasympathetic nerve fibres are disrupted, there will be ischemic damage followed by impaired parasympathetic nerve function resulting in a dilated pupil.

CCE.