Intradural perimedullary and subpial AVM The pial fistulous arteriovenous communication that involves the superficial aspect of the cord to a variable extent is the least frequent in this category but probably of a similar nature to the dural type; it may be related (or identical) to the vascular lesion in the earlier discussed Foix-Alajouanine process. In contrast to dorsal arteriovenous malformations, these fistulas tend to involve the lower thoracic and upper lumbar segments or the anterior parts of the cervical enlargement. The patients are often younger and the sexes are equally affected. The clinical syndrome may take the form of slow spinal cord compression, sometimes with a sudden exacerbation, or the initial symptoms may be apoplectic in nature, either because of thrombosis of a vessel or of a hemorrhage from an associated draining vein that dilates to aneurysmal size and bleeds into the subarachnoid space or cord (hematomyelia and subarachnoid hemorrhage); the latter complication occurred in 7 of 30 cases reported by Wyburn-Mason.
Diagnosis These lesions—dural or parenchymal—may be apparent on MRI or CT myelography by the presence of one or more enlarged and serpiginous draining vessels in the subarachnoid space; just as often, they are not visualized by these methods (Jones et al). For this reason, the possibility should come to mind that an otherwise unexplained myelopathy with signs of congestion of the cord on MRI may be the result of a vascular malformation. However, several studies, such as the one by Toossi and colleagues, have suggested that the absence of both T2 hyperintensity and flow voids on MRI makes the presence of a dural fistula unlikely and spares the patient from the need for angiography.
Imaging features that have been emphasized with dural fistulas include enlargement of the spinal cord at the level of the lesion and T2 bright signal of the swollen cord over several segments, but these are not invariable. Infrequently, the draining surface vessels are evident on MRI (Fig. 42-8A). Because of the slow blood flow within the vascular lesion, the affected region may have a hypointense T1 signal. Hurst and Grossman have commented on the presence of peripherally located regions of T2 hypointense signal changes. Many of these changes are reversed by surgical or endovascular interventions that ablate the malformation. There is variable enhancement although, with increasingly improved MRI techniques, more fistulas are becoming apparent. Some remarkable ones appear as a multiple small enhancing areas that are like hairs standing on end, coating the cord over several levels.
The diagnosis is usually established through selective angiography, which shows the fistula in the dura overlying the cord or on the surface of the cord itself but the most conspicuous finding is often the associated early draining vein (Fig. 42-8B). Demonstration of the fistula requires the injection of feeding vessels at numerous levels above and below the suspected lesion, because the main artery of origin is often some distance away from the malformation. The small angiodysplastic vessels of the Foix-Alajouanine lesion may not be opacified with angiography. In rare instances, the fistula or high-flow arteriovenous malformation lies well outside the cord, for example, in the kidney, and gives rise to a myelopathy, presumably by raising venous pressures within the cord.