Dural arteriovenous fistula (SDAVF) is an intravertebral vascular malformation that has only been gradually recognized in the last 20 years. Since 1977, when Kendall and Logue first reported 10 cases of SDAVF, more than 260 cases have been reported in China and abroad. Due to the development of interventional neuroradiology and microneurosurgery, the current understanding of this disease has been greatly improved. Here is an introduction to the research progress of SDAVF at home and abroad in recent years.
1, SDAVF etiology
Recent research findings increasingly support the view that acquired factors play a decisive role. The main supporting points are: (1) Histological studies of the dural vasculature show the existence of potential arteriovenous traffic in the normal dura. (2) Cases of occlusion of intracranial venous sinuses followed by dural arteriovenous traffic have been reported. (3) Some cases after intracranial surgery have presented with dural arteriovenous traffic in a lesion that was not present on preoperative angiography. (4) Occlusion or stenosis of the venous sinuses has been reported in the literature to precede the development of post-traumatic fistulae. (5) Most patients do not present with symptoms until after the age of 40 years, and the fistula is characterized by slow blood flow and insignificant enlargement of the blood supply vessels, in contrast to the presentation of fistulas in congenital lesions. (6) The lesion is located in the dura, but venous drainage is limited to the coronary venous system. (7) Since the dura at the spinal nerve root is close to the intervertebral disc, vertebral bone or ligament, the dura is extremely vulnerable to damage from trauma at this location.
2.SDAVF anatomy
The blood circulation in the spinal canal is a relatively independent system, especially the intravertebral venous system is quite different from other venous systems in the body, so Batson proposed the concept of the 4th venous system in 1940, in which there is no valve in the venous vessels, thus providing anatomical conditions for the occurrence of SDAVF. The disease is characterized by an arteriovenous traffic lesion in and around the proximal dura that surrounds the spinal nerve roots, usually with one or several blood supplying arteries and a drainage vessel, with the fistula located on the lateral side of the dura near the intervertebral foramen or below the “axilla” that corresponds to the cuff of the nerve root, and rarely on the anterior side.
3.Pathophysiology of SDAVF
It is currently believed that the cause of the patient’s spinal cord injury symptoms is a slow increase in venous pressure in the coronary plexus leading to a decrease in the intramedullary arteriovenous pressure gradient. The process is: (1) arteriovenous traffic at the intervertebral foramen causes arterial blood to back up into the root medullary vein. (2) The direction of blood flow in the root medullary vein is opposite to normal, so blood flows backward into the coronary plexus, which increases venous pressure and stagnates blood. (3) Similar changes occur in the intramedullary vessels, resulting in an increase in tissue pressure, a progressive decrease in autoregulation, and local edema and ischemic changes. For example, in “Foix-Alajouanine syndrome”, the venous pressure is increased due to inadequate venous drainage rather than spontaneous thrombosis of the lesion. The time of blood flow through the spinal cord is also prolonged from the normal 15-20 seconds to 40-60 seconds.
4. Clinical manifestations of SDAVF
Merland et al. concluded that most of the “posterior medullary hemangiomas” are root medullary arteriovenous fistulas, which is what we call SDAVF, so its incidence is higher than that of spinal hemangiomas.
4.2 Age and gender According to the 97 patients reported by Symon, Rosenblum, Hassler, and Ling Feng, the age of onset ranged from 22 to 76 years, but middle-aged and elderly patients were more common. There were more males than females, and male patients accounted for 86,6%.
4.3 Lesion sites From the examination results and surgical findings of the above 97 patients, it can be seen that the lesions were mainly concentrated in the lower thoracic and lumbar spine, and other sites were rare.
4.4 Symptoms and signs The disease is a non-self-limiting disease, and once the disease is developed, the symptoms will be progressively aggravated and eventually lead to irreversible damage to the nervous system. Although the symptoms can be reduced or disappear after treatment, they can be aggravated or relapse. Symon and Keonig found that the initial symptoms of the disease were atypical, similar to the initial manifestations of any spinal cord compression, mainly abnormal cone function. Changes in sensory, motor, and sphincter function occur later, and by the time of presentation the most common symptom is defecation and urination dysfunction, which is significantly earlier than in other extramedullary benign tumors and intramedullary gliomas. Symptoms may be exacerbated by increased venous pressure due to exercise, specific positions, pregnancy, and Valsalva maneuvers. The usual sign is damage to upper and/or lower motor neurons in the lower extremities, and Keonig found in a study of 20 patients with this disease that 95% of patients had signs of lower motor neuron damage, with sensory abnormalities due to damage to the posterior spinal cord and spinal thalamic tract being the most frequent. decreased and a few (13%) developed muscle atrophy. In contrast, subarachnoid hemorrhage is rare in patients with this disease and was not seen in 1 of Symon’s patients, but 6 cases presented with arachnoiditis, an early sign of subarachnoid hemorrhage as described by Pia.
5. Diagnosis of SDAVF
The disease is difficult to diagnose early because of its sporadic onset, insidious onset, and long and progressive course, and many patients have severely lost the ability to move on their own before being diagnosed and operated on. The only way to diagnose the disease is selective spinal arteriography, which clearly shows the abnormal vessels at the lesion. Because the plane of clinical signs and the actual plane of the lesion can be completely inconsistent, and because Chaloupka and Gobin et al. first found two SDAVFs in one patient by imaging in 1995, it is necessary to perform a complete spinal angiogram, i.e., including all arteries supplying the spinal cord. However, it has also been noted that individual cases showed worsening of symptoms after performing angiography. Other methods, such as spinal canal imaging, can show dilated, tortuous vessels, but cannot be distinguished from lesions such as spinal cord hemangiomas; cerebrospinal fluid examination often shows “protein-cell separation”, i.e., elevated protein content with normal cell counts, which is common in intraspinal tumors and therefore not specific for diagnosis.
The widely accepted criteria for diagnosis are: (1) age >40 years, especially in male patients; (2) abnormal sensory, motor and sphincter function in both lower extremities, with progressive worsening of symptoms and development of signs; (3) selective myelography reveals abnormal traffic in the dural fistula and arterioles.
6.SDAVF treatment
6,1 Treatment principle Since the natural course of the disease is progressive and the neurological damage is irreversible in the late stage, SDAVF should be treated surgically at an early stage. symon, Oldfield et al. believe that the surgical treatment of this disease should be achieved by blocking this traffic between arterioles. And the premise of surgical resection of the lesion is that (1) the blood supplying artery of the lesion does not supply the spinal cord at the same time and (2) the nerve root involved is not functionally important. If embolization is used, all the blood supplying arteries should be embolized by long-acting emboli to prevent the occurrence of recanalization, and these arteries cannot be the supplying vessels of the spinal cord at the same time.
6,2 Surgical approach
6,2,1 Fistulotomy (blocking) The dural cuff around the intervertebral foramen is exposed by removing part of the lamina and the fistula is removed or clamped closed under direct vision. This procedure has significant efficacy without risk because it is adequately exposed, simple, and completely resected. The disadvantage is that the portion of the nerve root involved that crosses the dura is removed, and in some patients, a dural repair is performed.
6,2,2 Drainage vein resection Because the artery supplying the SDAVF, such as the spinal branch of the intercostal artery, may be the vessel from which the root medullary artery, including the Adamkiewicz artery, originates, resection or blockage of the fistula can cause ischemia and necrosis of the corresponding spinal cord segment. Therefore, the procedure to be performed in this case is to preserve the fistula and directly remove part of the subdural drainage vein.
6,2,3 Embolization Cahan et al. stated that endovascular embolization is effective and that the aim of treatment is to embolize the fistula, not the draining vein, because the draining vein itself also has the function of draining blood from normal spinal veins, and if it is occluded, clinical symptoms will be aggravated. niimi and Berenstein et al. applied acrylic material as an embolus in 49 patients with SDAVF, with Merland also achieved a 66% cure rate by applying a super-selective cannula to the spinal artery and injecting a small amount of pure IBCA or NBCA to occlude the fistula. Therefore, Niimi states that the application of embolization is the treatment of choice for SDAVF.
6,2,4 Other methods such as laminectomy decompression and partial coronary plexus resection have been proven to be ineffective and even aggravate the disease, and have now been abandoned.
6,3 The efficacy of treatment depends not only on the location of the lesion, but also on the timing of surgery and the surgical approach chosen. Keonig pointed out that early diagnosis and early treatment are the only way to achieve satisfactory treatment results. In addition, if the fistula is blocked by surgery or embolization, the patient’s symptoms will mostly improve, while if the fistula is not completely blocked, the clinical symptoms will recur.
7. Outlook
SDAVF research has made great progress, but many questions remain to be elucidated, such as the epidemiology of the disease, which is more prevalent in middle-aged and elderly men, but the exact mechanism is still unclear; what is the mechanism by which the latent arteriovenous traffic under normal conditions reopens under pathological conditions? Experimental animal models of the disease have still not been successfully established, etc. Therefore, we should strengthen the basic research of SDAVF to guide clinical treatment and prevention.