A patient presented with sudden onset of weakness involving his left upper and lower limb. On examination, he was conscious, with a dilated non-reactive right pupil, normal power in the right upper and lower limbs, and a left hemiparesis. What is the likely site of lesion? Outline your reasoning.
Right side of the midbrain. It is a crossed hemiplegia suggesting a brainstem stroke and the 3rd nerve nucleus is located in the midbrain.
This question interrogates one's knowledge of the highly regarded Talley and O'Connor manual of physical examination.
Let us reason through this. A left hemiparesis suggests right sided cerebral pathology.
A right sided unreactive pupil suggests that the third nerve is somehow involved, and the nucleus for it is in the midbrain. Ergo, the right midbrain has infarcted. The hemiparesis reflects damage to the right-sided motor tracts above the level of the decussation (which is at the medulla).
The Internet Stroke Centre has an excellent summary of stroke syndromes.
Clinical Examination of the Critically Ill Patient, 3rd edition by L.I.G. Worthley - which can be ordered from our college here.
Clinical Examination: whatever edition, by Talley and O'Connor. Can be acquired wherever good books are sold or stolen.