The diagnosis of subarachnoid haemorrhage.

Lumbar puncture (LP) has for a long time been the mainstay of diagnosis in patients who presented with symptoms or signs of subarachnoid haemorrhage (SAH). At present, computed tomography (CT) has replaced LP for this indication. In this review we shall outline the reasons for this change in diagnostic approach. In the first place, there are drawbacks in starting with an LP. One of these is that patients with SAH may harbour an intracerebral haematoma, even if they are fully conscious, and that withdrawal of cerebrospinal fluid (CSF) may occasionally precipitate brain shift and herniation. Another disadvantage of LP is the difficulty in distinguishing between a traumatic tap and true subarachnoid haemorrhage. Secondly, the use of CT within the first two or three days offers a wealth of information about the origin and extent of the haemorrhage, and about the presence of early complications requiring urgent treatment. An early brain scan also serves as a baseline against which future changes can be measured. The information about the location of the haemorrhage is especially valuable in patients with SAH and a negative angiogram. This is a heterogeneous group of patients which will be discussed separately. The review continues with guidelines about the interpretation of the CSF in patients with a negative CT scan, which is the only remaining indication for LP in the diagnosis of SAH. The final paragraph points out the many pitfalls in the diagnosis of rebleeding by analysis of CSF samples, which makes serial CT scanning by far the preferred method.