Imaging has taken a more prominent role in the diagnosis of subarachnoid hemorrhage than in the past when lumbar puncture, still very useful, predominated. CT will detect blood locally or diffusely in the subarachnoid spaces or within the brain or ventricular system in more than 90 percent of cases and in practically all cases in which the hemorrhage has been severe enough to cause momentary or persistent loss of consciousness (Fig. 33-26). MRI also detects blood in the proton-density sequence; after a day has passed, this is also appreciated with the FLAIR sequence and some groups perform MRI in preference to CT. The blood may appear as a subtle shadow along the tentorium or in the sylvian or adjacent fissures, it is more easily appreciated in the noncontrast study. A large localized collection of subarachnoid blood or a hematoma in brain tissue or within the sylvian fissure indicates the adjacent location of the aneurysm and the likely region of subsequent vasospasm, as already noted. When two or more aneurysms are visualized, the CT can identify the one that had ruptured by the clot that surrounds it. Also, coexistent hydrocephalus will be demonstrable. If the CT documents subarachnoid blood with certainty, a spinal tap is not necessary.
In where subarachnoid hemorrhage is suspected but not apparent on imaging studies, a lumbar puncture should be undertaken. Usually the CSF becomes grossly bloody within 30 min or sooner of the hemorrhage, with RBC counts up to 1 million/mm3 or even higher, however, blood may not be easily apparent in a lumbar puncture minutes after the hemorrhage. The CSF in the first days is under increased pressure, as high as 500 mm H2O—but usually closer to 250 mm H2O—an important finding in differentiating spontaneous subarachnoid hemorrhage from a traumatic tap. With a relatively mild hemorrhage, there may be only a few thousand cells but a severe headache syndrome from subarachnoid hemorrhage is usually associated with at least several hundred cells. It is also probably not possible for an aneurysm to rupture entirely into brain tissue without some leakage of blood into the subarachnoid fluid. In other words, the diagnosis of ruptured saccular aneurysm is essentially excluded if blood is not present in the CSF, provided the spinal fluid is examined more than 30 min after the event. Xanthochromia is found after centrifugation of the CSF if several hours or more have elapsed from the moment of the ictus. In a patient who reports a headache that was consistent with subarachnoid hemorrhage but that had occurred several days earlier, the CT may be normal and xanthochromia is the only diagnostic finding. To determine whether xanthochromia is present, fresh CSF must be centrifuged in a tube with a conical bottom and the supernatant compared to clear water in good light or preferably examined by spectrophotometric techniques. It has been our experience that most hospital laboratories cannot be depended on to give accurate results for this test from visual inspection alone. Also helpful after several days is the MRI taken with the FLAIR sequence as mentioned, which will demonstrate blood (the proton-density sequence is more sensitive to blood in the first day).