The most widely recognised cranial nerve deficit following head injury is that of CN III.

It may be injured following any head injury in which there is raised intracranial pressure (ICP), for example extradural haemorrhage (Fig 1).

The medial part of the temporal lobe (uncus) herniates down, compressing the third nerve against the fixed edge of the tentorium (Fig 2). Symptoms are an ipsilateral fixed dilated pupil (the parasympathetic nerves are injured first as they are most superficial) associated with a reduced conscious level.

An ocular palsy may develop later as the more central part of the nerve becomes injured. The trochlear (CN IV) and abducens nerves (CN VI) can also be involved.

Isolated third nerve palsy secondary to minor head injury has also been reported [4].

Fig 1: Extradural haemorrhage with midline shift Fig 2: Herniation of the uncus (blue arrow) and third nerve palsy