Pathophysiology of Type I Spinal Dural Arteriovenous Malformations
Joshua B. Bederson, MD*
Robert F. Spetzler, MD
Division of Neurological Surgery, Barrow Neurological Institute, Mercy Healthcare Arizona, Phoenix, Arizona
*Current Address: Department of Neurological Surgery, Mount Sinai Medical Center, New York, NY
Classification of spinal arteriovenous malformations (AVMs) into distinct groups is based on angiographic criteria, but few histological data support the most popular nomenclature. Questions exist regarding the pathogenesis and precise fistula location in spinal dural AVMs. We reviewed the clinical course, intraoperative hemodynamic data, and histological specimens from 12 patients whose lesions were treated surgically. Longitudinal histological studies of two surgical specimens in patients with the classical clinical and angiographic criteria for “dural” AVMs demonstrated that the AV fistula was located in the subarachnoid space. Postoperatively, seven patients improved neurologically, four stabilized, and one was worse (mean follow-up, 18.7 ± 5.7 months). Spinal cord perfusion pressure, defined as mean arterial minus spinal intradural venous pressure, increased from 36.1 ± 15.7 to 57.1 ± 9.1 mm Hg after closure of the AV fistula (p < 0.05). After fistula resection, draining vein pressure decreased (44.1 ± 17.7 to 22.7 ± 8.9 mm Hg; p < 0.05) but was still more than two times greater than central venous pressure (6.7 ± 2.0 mm Hg) or epidural venous pressure (12.5 ± 4.4 mm Hg; p < 0.05), demonstrating restriction of spinal cord venous outflow. These findings suggest that clinical and angiographic distinctions between so-called “dural AVMs” and “intradural perimedullary AV fistulae” may be unreliable. The hemodynamic profile of these lesions appears to change over time and may reflect progressive restriction of venous outflow.
Key Words : intrathecal perimedullary arteriovenous fistula, spinal dural arteriovenous malformation, type
Clinical and angiographic distinctions between dural (Type I) and intrathecal (Type IV) spinal arteriovenous malformations (AVMs) are based on the presumed location of the AV nidus.20,23,27,56,63,71 These differences have recently been summarized in an authoritative report by Mourier et al.64 Type I AVMs are fed by dural arteries only; the AV shunt is located within the thickness of the dura; and they become symptomatic with a progressive myelopathy or cauda equina syndrome in middle-aged patients. In contrast, Type IV malformations occur in younger patients and may present catastrophically with subarachnoid hemorrhage. The AV fistula is located in the subarachnoid space. Merland and colleagues have classified Type IV spinal AVMs into three subgroups.56,64
Despite recent advances in understanding Type I and Type IV spinal AVMs, questions remain concerning the location of the fistula, their pathogenesis and natural history, and the cause of neurological symptoms. Although extensive angiographic documentation of these lesions exists, histopathological data have rarely been published. We present two patients with clinical and angiographic features of Type I AVMs whose fistula was located intrathecally. These findings raise questions about the current nomenclature. Furthermore, intraoperative pressure measurements from the overall series of 12 patients provide further information about the pathogenesis of the AV fistula and the mechanisms that might produce venous hypertension and neurological symptoms.