A previously fit and well 8-year-old boy, was brought into the emergency department by ambulance following a road traffic collision ....

A previously fit and well 8-year-old boy, was brought into the emergency department by ambulance following a road traffic collision in which he was a pedestrian hit by a car driving at 40 mph. There was dried blood over his hair. He was intubated and ventilated and made no spontaneous respiratory effort. Oxygen saturations were 97% in FiO2 100%. His heart rate was 70, capillary refill time 2 seconds and blood pressure 185/90. His Glasgow Coma Score was 3.
On examination of his eyes, the right eye was deviated down and outwards with a fixed and dilated pupil; the left eye was in the midline position with a pupil that constricted to light.

Which ONE of the following cranial nerve(s) is involved in the localizing sign identified in the
clinical examination?
A. Cranial nerve I
B. Cranial nerve I and III
C. Cranial nerve III
D. Cranial nerve IV
E. Cranial nerve IV and VI

Answer:
C. Cranial nerve III

Discussion: Uncal herniation commonly causes compression of cranial nerve III, which originates in the anterior aspect of the midbrain.
Cranial nerve III compression results in:
• Parasympathetic paralysis and pupillary dilation of the eye on the affected side.
• Loss of innervation of all ocular motility muscles except for lateral rectus (cranial VI) and the
superior oblique (cranial nerve IV) resulting in the deviation of the eye to a ‘down and out’ position.

Pupillary dilation often precedes the somatic motor effects of cranial nerve III compression because the parasympathetic fibres surround the motor fibres of the nerve and so are compressed first. Initially pupillary dilation is ipsilateral to the side of the lesion (localizing sign). However, as intracranial pressure continues to rise, the contralateral cranial nerve III is also compressed and bilateral ‘fixed and dilated’ pupils are seen.