An association between sub-arachnoid haemorrhage and influenza A infection.

The sera of 25 patients with confirmed sub-arachnoid haemorrhage were assayed by complement fixation for 14 viruses. Twenty (80%) showed elevated titres to influenza A virus compared with 3 out of 25 (12%) of control subjects matched for age, sex and district and 3 out of 17 (17%) of a group of patients with other neurological disorders. A seasonal variation in the notification of sub-arachnoid haemorrhage appeared to be related to changes in the incidence of influenza A infection.