A non-contrast CT scan of the head is the
first-line investigation. CT scans have high
sensitivity (close to 100%) for diagnosing
aSAH within the first three days of the bleed
(Ferguson 2009). Negative CT scans should be
followed 12 hours later by a lumbar puncture
to look for yellowing of the cerebrospinal fluid
(xanthochromia), which is present following
aSAH (Ferguson 2009, Connolly et al 2012,
RCP 2012). If aSAH is diagnosed, the next
step is to locate the aneurysm and visualise
its shape. This will help guide decisions about
how to treat the aneurysm. Cerebral digital
subtraction angiography is commonly used
for this purpose. CT angiography (CT head
with contrast medium to visualise the cerebral
arteries) is another option, but it is not clear
whether this scan is sensitive enough to identify
very small aneurysms of less than 3mm
(Connolly et al 2012).