The dilated pupil and brain herniation

This paper recalls the descriptions and early ideas about the dilated pupil accompanying raised intracranial pressure resulting from head injuries and space-occupying lesions. The observation of the ominous fixed, dilated pupils in those with expanding brain lesions dates to Richard Bright and Jonathan Hutchinson in the 19th century, but its significance and mechanisms were only debated in the early years of the 20th century. Compression or stretching of the oculomotor nerve were considered possible causes, but the related mechanisms of coning and the importance of lateral shift were only more recently realised.