Hutchinson's pupil is a clinical sign in which the pupil on the side of an intracranial mass lesion is dilated and unreactive to light,[1] due to compression of the oculomotor nerve on that side.
These can be due to concussion injury to the brain and is associated with subdural haemorrhage and unconsciousness.
The parasympathetic fibers to the pupil are responsible for pupillary constriction.
In stage 2, the parasympathetic fibers on the side of injury are paralysed, leading to dilatation of pupil.
In stage 3, the parasympathetic fibers on both sides are paralysed - leading to bilateral pupillary dilatation.