 The ptosis caused by the oculomotor palsy can be unilateral or bilateral, as the subnucleus to the levator muscle is a shared, midline structure in the brainstem.In cases in which the palsy is caused by the compression of the nerve by a tumor or aneurysm, it is highly likely to result in an abnormal ipsilateral papillary response and a larger pupil.Surgical third nerve palsy is characterized by a sudden onset of unilateral ptosis and an enlarged or sluggish pupil to the light.