The problem of a “traumatic tap” often clouds the early diagnosis, and several aids to detecting this misleading laboratory result are discussed in Chap. 2. Here it is reiterated that in addition to the absence of xanthochromia, the most important features indicating that blood has been spuriously introduced by entering small veins in the epidural space with the lumbar puncture needle are the clearing of blood as one continues to collect fluid and a marked reduction in the number of RBCs in serial tubes of spinal fluid. A normal opening pressure also suggests puncture of a local vessel rather than a ruptured aneurysm. The combination of subarachnoid hemorrhage and a traumatic tap generally requires that vascular imaging procedures be performed to resolve the issue of the presence of an aneurysm that could potentially rerupture. In both a traumatic puncture and early in subarachnoid hemorrhage, the proportion of WBCs to RBCs in the CSF is usually the same as in the circulating blood (approximately 1:700), but in some patients with genuine hemorrhage a brisk CSF leukocytosis appears within 48 h, sometimes reaching more than 1,000 cells/mm3. The protein is slightly or moderately elevated and in some instances glucose is reduced sometimes dramatically so. Elevated CSF pressure has already been mentioned as favoring a genuine hemorrhage rather than traumatic tap.
Digital subtraction angiography with bilateral carotid and vertebral contrast injections is the most sensitive means of demonstrating an aneurysm. In addition to other causes of subarachnoid hemorrhage, approximately 5 to 10 percent of patients with apparent aneurysmal rupture will not have an aneurysm evident. Some of these instances may be the result of the obliteration of the lesion in the process of rupture. Patients with the typical clinical picture of spontaneous subarachnoid hemorrhage in whom an aneurysm or arteriovenous malformation cannot be demonstrated angiographically have a better prognosis than those in whom the lesion is visualized (Nishioka et al). In a series of 323 angiographically negative cases followed for an average of 10 years, there was rebleeding in only 12 (Hawkins et al). After 22 years, 69 percent of these patients had survived. If the first angiogram does not reveal an aneurysm, it is customary to repeat the study in several weeks, in part because it has been considered that vascular spasm may have earlier obscured the aneurysm. Even when no vasospasm visualized, it is occasionally the case that a second study shows the lesion. If the first study involves all cerebral vessels and uses several views of the basal circulation, it has been our experience that the second arteriogram is infrequently revealing, but we follow general practice and repeat it nonetheless. Imaging of the spinal vasculature rarely reveals a source in these circumstances, 3 of 75 patients in a series reported by Germans and colleagues.