Central nervous system disorders

IV./3.2.: Computed tomography – CT

IV./3.2.1.: Role of CT in SAH diagnostics

If a SAH is suspected, the first imaging method of choice is the CT. On CT images, the strong hyperdensity of the freshly effused blood can be easily differentiated in the basal cisterns and, if the blood extends superiorly, in the subarachnoid space along the fissures and sulci, demonstrating a very characteristic pattern (detects over 90% of cases which rate decreases later). Thus, unenhanced CT scan performed in the first day is effective in almost all cases. A peripheral hyperdensity is visible which fills the basal cisterns, and extends along the sulci cranially, even until the convexity – in contrast to the anyway hypodense liquor.

Hematoma is located in the external liquor space, between the pia mater and the arachnoidea. Beyond the filling of basal cisterns hyperdensity extends into the Sylvius fossa and the interhemisphaeric fissure as well. The localisation of the ruptured aneurysm can be judged by the localisation of the sooner appearing/larger amount of bleeding (i.e. e.g. frontal, interhemispheric or posterior fossal predominance of the larger mass of hematoma).

Several days/one week later the hyperdensity of the bleeding – due to the attenuative effect of the existing and continously replenishing liquor, and the degradation of the blood in the extravasal space – decreases, which reduces the diagnostic hit safety, since the density difference between the hematoma and the liquor will be less and less sufficient to detect the hemorrhagic borderline zone. In extensive cerebral edema, venous outflow of the subarachnoid space is hampered due to compression, stasis appears as hyperdensity in the cerebral superficial subarachnoid space, which can lead to a positive misconception (so-called "pseudosubarachnoid hemorrhage").

On the other hand, very small amount of filmlike subarachnoid hematoma can be easily „overlooked”. In this case, CT scan is recommended to be repeated several hours later, if no MR is available and SAH is very seriously suspicious clinically. Lumbal punction is a very sensitive method, but not specific and besides an invasive method. Lumbal punction is prohibited to be performed in case of increased intracerebral pressure due to risk of herniation! Pay attention, since CT can demonstrate the initial brain stem herniation which is a contraindication.

IV./3.2.2.: Bleeding severity scale of Fischer

IV./3.2.3.: Role of CT in the prognosis of SAH

CT has not only a good diagnostic reliability but also plays a role e.g. in the prediction of vasospasm as well (its development correlates with the amount of bleeding). CT has also a role in follow-up examinations, in the recognition of hydrocephalus.

IV./3.2.4.: CT-based differential diagnostics

Differentiation of primary (resultant from aneurysm) and traumatic SAH is not always easy in absence of anamnestic data (unconscious or embarrassed patient), therefore, localisation can be directional. However, sometimes it is not easy to differentiate a traumatic, frontal, basal or superficial hematoma from a bleeding resultant from the anterior communicating artery.

IV./3.2.5.: CT angiography (CTA)

This method can give the size of the aneurysm, spaciousness of the origin, ratio of the neck and maximal diameter (for planning of subsequent intervention) accurately.