CT of Subarachnoid Hemorrhage

      The CT scan is important, first of all, to document the subarachnoid hemorrhage and to assess the amount of blood in the cisterns. Detection of subarachnoid blood is very dependent on how early the scan is obtained. Data in the literature vary from 60-90%. If the scan is obtained within four to five days, the detection rate is very high. Secondly, the CT helps localize the site of the aneurysm. This can be done by the distribution of blood within the cisterns and also with dynamic scanning following an IV bolus of contrast. Thirdly, the CT is important to evaluate complicating factors such as cerebral hematoma, ventricular rupture, hydrocephalus, cerebral infarction, impending uncal herniation and re-bleed. Endnote

      Regarding CT patterns of ruptured aneurysm, an anterior communicating aneurysm is suggested by blood in the cisterna lamina terminalis, anterior pericallosal cistern, and interhemispheric fissure. Identification of clot within a cistern makes this sign more specific. There may be extension of blood into the septum pellucidum and lateral ventricle, and hematoma in the inferomedial frontal lobe. Localizing posterior communicating artery aneurysms is more difficult because the blood is usually diffuse within the cisterns. Intracerebral hematoma or ventricular rupture is unusual with posterior communicating aneurysms. Rupture of a middle cerebral aneurysm is characterized by blood in the sylvian fissure and a hematoma in the temporal lobe, which may also rupture into the adjacent temporal horn. Posterior fossa aneurysms often do not have good localizing findings on the CT scan.

      It is not uncommon to find a small amount of blood in the ventricles in patients with subarachnoid hemorrhage. That does not necessarily mean that direct ventricular rupture has occurred because subarachnoid blood can enter the ventricular system in a retrograde manner. Ventricular rupture from a bleeding aneurysm is usually more dramatic, often showing a cast of blood or clot in a lateral ventricle. A subarachnoid hemorrhage with blood in the lateral ventricle is usually due to an anterior communicating aneurysm. Middle cerebral aneurysm is another possibility, but that should be associated with a temporal hematoma. Similarly, pericallosal aneurysms can rupture into the ventricle but then there should be hematoma in the corpus callosum as well.

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      What is the role of a contrast scan in subarachnoid hemorrhage? The combination of clinical and plain scan findings is often fairly conclusive that a subarachnoid hemorrhage has occurred. If emergency arteriography is considered, contrast limitations need to be considered. We obtain the contrast scan if the diagnosis is in doubt, or if the plain scan shows a large intracerebral hematoma that needs emergency evacuation and there is no time for the angiogram. The detection rate of aneurysms with contrast scanning ranges from 40% for posterior communicating to 80% for anterior communicating, middle cerebral and basilar aneurysms. A common problem is that the subarachnoid blood obscures the enhancing aneurysm.

      Conventional MR sequences are very insensitive for detecting subarachnoid hemorrhage. Clots within cisterns can be detected, but in general, MR is not the procedure of choice in the work-up of patients with subarachnoid hemorrhage. Due to the flow void phenomenon, aneurysms about the circle of Willis can be identified on spin-echo MR images. Endnote With fluid-attenuated inversion recovery (FLAIR) sequences, the CSF is dark, so that subarachnoid hemorrhage can be seen more easily. These sequences may be helpful for detecting subarachnoid blood in the posterior fossa where CT has difficulty and in the sulci over the cerebral convexities. Endnote  

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