Background Acute subdural haematoma (aSDH) is a rare complication of aneurysmal subarachnoid haemorrhage (SAH) and is associated with poor clinical condition on admission and poor outcome.

Objective The aim of this study was to assess whether aneurysmal aSDH is an independent risk factor for poor outcome.

Methods In a series of 1632 patients retrieved from our prospectively collected single centre SAH database and fulfilling prespecified inclusion criteria, we found 53 patients with aSDH on the initial CT scan. From the same series, we collected 660 patients in whom aSDH was ruled out by reviewing the initial CT scan. We compared the risk of poor outcome at discharge and at 3 months between patients with and without aSDH by calculating crude risk ratios (RRs) with corresponding 95% CIs, and adjusting for age, sex, location and treatment modality of the aneurysm that bled, clinical condition on admission, intracerebral haemorrhage, intraventricular haemorrhage and hydrocephalus, with Poisson regression.

Results Patients with aSDH had a higher risk of poor outcome at discharge (crude RR 1.59; 95% CI 1.35 to 1.86) and at 3 months (crude RR: 2.17, 95% CI 1.79 to 2.62) than patients without aSDH. After simultaneous adjustment for five characteristics that affected the crude RR, the RR for poor outcome for patients with aSDH at discharge was 1.15 (95% CI 0.97 to 1.37) and at 3 months 1.30 (95% CI 1.04 to 1.62).

Conclusions The presence of aSDH in patients with aneurysmal SAH is an independent risk factor for poor outcome at 3 months.