The same causes (anticoagulation, blood dyscrasia with coagulopathy, and arteriovenous malformation [AVM]) may underlie bleeding into the epidural or subdural space and give rise to a rapidly evolving compressive myelopathy. In some cases, as in those of Leech and coworkers, one cannot ascertain the source of the bleeding, even at autopsy. Epidural or subdural bleeding, like epidural abscess, represents a neurologic emergency and calls for immediate localization by imaging and, in some cases, surgical evacuation.
Advances in the techniques of selective spinal angiography and microsurgery have permitted the visualization and treatment of vascular lesions that cause bleeding with a precision not imaginable a few decades ago. These procedures make it possible to distinguish among the several types of vascular malformations, arteriovenous fistulas, and vascular tumors, such as hemangioblastomas, and to localize them accurately to the spinal cord, epidural or subdural space, or vertebral bodies. This subject is discussed further on.
Vascular Malformations and Dural Fistulas of the Spinal Cord
These lesions cause both ischemic and hemorrhagic lesions. Some are true arteriovenous malformations (AVMs), implying a congenital connection between the two sides of the circulation and others are more limited fistulas in the dura, probably mostly acquired for various reasons. The distinction is in the size of the nidus of communication between an artery and a vein and the size and location of feeding and draining vessels. The classification of spinal AVMs is confusing, in part because the enlarged draining veins by which the lesions were formerly identified are probably secondary features. A more useful categorization reflects the appearance and location of the malformation: (1) arteriovenous malformations that are strictly intramedullary or that also involve the meninges and surrounding structures, such as the vertebral bodies, to a limited extent; (2) a variety of intradural perimedullary fistulas that lie on the pial and subpial surface of the cord (these probably conform most closely to the lesion described by Foix and Alajouanine discussed in the earlier section “Foix-Alajouanine Myelopathy”); and (3) dural fistulas. There is insufficient pathologic material to determine whether these represent distinct pathologic entities or simply differing degrees and configurations of a common developmental process but, as mentioned, the last of these types may be acquired from local venous occlusions and the other types do not originate in this way. Once recognized, treatment of a spinal cord malformation of any type may be an urgent matter, especially in cases with rapid clinical deterioration and impending paralysis.
Dural arteriovenous fistula The entity is addressed first because it has emerged as the most common type, at least in our practices. Fistulas within the dura that overlies the spinal cord are capable of causing a myelopathy, sometimes several segments distant from the vascular lesion. Most are situated in the region of the low thoracic cord or the conus and have a limited venous draining system. Some are in a dural root sleeve and drain into the normal perimedullary coronal venous plexus. Men seem to be affected disproportionately.
The presenting clinical features in our patients have included slowly progressive bilateral but asymmetric leg weakness with variable sensory loss. According to Jellema and colleagues, who studied 80 patients with spinal dural fistulas, the most common initial symptoms were gait imbalance, numbness, and paresthesias. As the process progressed, the majority developed urinary problems, leg weakness, and numbness in the legs and buttocks. The degree of leg weakness varied greatly and back pain in their series was infrequent and has not been a consistent feature in the patients under our care.