It has not been firmly established if alternative forms of imaging, namely CT and MRI angiography are as dependable as conventional angiography in excluding and aneurysm but several series suggest they may not be entirely adequate. Even when MR angiography demonstrates the aneurysm, if surgery is contemplated, the surgeon usually requires the kind of anatomic definition that can be better obtained by conventional digital subtraction angiography. CT and MRI angiographic have the potential advantages of showing the lesion in relation to the adjacent brain, soft tissue and bone in multiple views (Fig. 33-27). Digital subtraction angiography with bilateral carotid and vertebral artery 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 subarachnoid hemorrhage due to aneurysmal rupture will not have an aneurysm evident. Some of these instances may be the result of 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 an angiogram in several weeks in part because it has been considered that arterial spasm may have earlier obscured the aneurysm. Even when no vasospasm is visualized, it is occasionally the case that a second study shows the aneurysm. It has not been firmly established if alternative forms of arterial imaging, namely CT and MRI angiography, are as dependable as digital subtraction angiography in identifying an aneurysm but several series suggest they may not be entirely adequate. Even when CT or MR angiography demonstrates the aneurysm, if surgery is contemplated, the surgeon usually requires the kind of anatomic definition that is better obtained by conventional digital subtraction angiography. CT and MR angiographic techniques do have the potential advantage of showing the vascular lesion in relation to adjacent brain parenchyma, soft tissue, or bone (Fig. 33-26).
Another clinical circumstance with a favorable outcome is a limited perimesencephalic hemorrhage as described by van Gijn and colleagues. The cisterns surrounding the midbrain and upper pons are symmetrically filled with blood, the headache is mild, and signs of vasospasm do not develop. No aneurysm is found at the expected site for blood in this region, that is, at the top of the basilar artery. The patient usually does well and a second arteriogram is probably not required. It has been speculated that the bleeding has a venous rather than an arterial-aneurysmal source.
Rebleeding The outstanding characteristic of this condition, mentioned earlier, is the tendency for the hemorrhage to recur from the same site in more than one-third of patients, often catastrophically. This threat colors all prognostications and dominates modern treatment strategies including early obliteration of the aneurysm by surgery or endovascular techniques. There does not appear to be a way of determining reliably which patients will bleed again. The cause of recurrent bleeding is not understood but is related to naturally occurring mechanisms of clot lysis, perhaps due to factors in the CSF, at the site of initial rupture, usually at the dome of the aneurysm. Rates and timing of rebleeding are discussed further on under “Prognosis.”