Diagnosis Of Subarachnoid Hemorrhage And Aneurysms

The first diagnostic test for a suspected aSAH is a noncontrast head CT (Fig. 1). The sensitivity of detecting an aSAH within the first 24 hr of hemorrhage is 92% and decreases by about 7% each 24 hr thereafter (8). A false positive may occur in the rare case of generalized brain edema that causes venous congestion in the subarachnoid space, mimicking an aSAH (9). The Fisher Scale (Table 1) assigns a numeric rating of the hemorrhage and facilitates prediction of the risk for vasospasm by grading the amount of blood on initial presentation.

In suspected aSAH cases, if the head CT is not diagnostic, a lumbar puncture is mandatory. The lumbar puncture remains the most sensitive test for aSAH. Once a hemorrhage occurs in the subarachnoid space and blood becomes mixed in the cerebrospinal fluid (CSF), sufficient lysis of the red blood cells and formation of bilirubin and oxyhemoglobin form within 6 to 12 hr

Figure 1 Noncontrast head computed tomography showing aneu-rysmal subarachnoid hemorrhage. Acute blood in the subarachnoid space appears as diffuse hyperintensities in the chiasmatic, Sylvian, and interhemispheric cisterns. Also note intraventricular hemorrhage in the fourth ventricle associated with hydrocephalus (enlarged temporal horns).

(9), giving the CSF a yellow tinge, or xanthochromia, after centrifugation. Therefore, in addition to routine CSF labs, CSF bilirubin should be checked.

The gold standard in the diagnosis of aneurysms is intra-arterial (IA) digital subtraction angiography. This enables visualization of the aneurysm in relation to its parent vessel, definition of the collateral circulation, and assessment for vasospasm. To assess all of these characteristics thoroughly, it is imperative that the angiogram includes contrast injection of both carotid arteries and both vertebral arteries a 4-vessel angiogram), with multiple views (anteroposterior, lateral, and oblique) of each injection. The risk of such a study in qualified centers is very low. One meta-analysis reported a transient or permanent neurologic complication risk of 1.8% in patients with aSAH and 0.3% in patients without aSAH (10). The risk of permanent neurologic damage is as low as 0.09% (11 ).

Procedural risks are eliminated in MRA or CTA, but their detection rate is lower. They are especially useful in planning for surgery when definition of the surrounding anatomy is necessary, as three-dimensional reconstruction with interactive manipulation of the views is possible. However, they remain inadequate replacements for IA angiography in the diagnosis of aneurysms at this time. Direct comparisons of CTA and MRA with IA angiograms showed that the accuracy of CTA/MRA is approximately 90% and is improving (12). CTA sensitivity and specificity are 91% and 95%, respectively (13). In the period prior to 1995, CTA accuracy was 84%, and in the period subsequent to that, it was 93% (12). MRA sensitivity and specificity have been reported at 83% and 97%, respectively (14). However, the detection rate decreases dramatically with smaller aneurysms and becomes negligible for sizes less than 3 mm. MRA accuracy is reported at 90% and has not changed significantly. Although further improvements are expected for these noninvasive tests, IA angiograms remain the gold standard for detection of intracranial aneurysms. Therefore, it is crucial that patients with suspected aSAH have the following tests in this order: 1) noncontrast head CT, 2) lumbar puncture if head CT is nondiagnostic, and 3) IA angiography in cases with confirmed aSAH.

Table 1 The Fisher Scale Grades the Amount of Hemorrhage on a Diagnostic Head Computed Tomography, Which Then Can Be Used to Assess for Risk of Vasospasm
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