Dural arteriovenous shunts (DAVSs, or Dural arteriovenous fistulas (DAVFs)) are abnormal arteriovenous fistulas within the dura mater. Theoretically, they can occur anywhere in the dura mater, but are most commonly found near the venous sinuses. Veins may drain into the venous sinus, the dural vein, a soft meningeal vein that drains in the reverse direction to the surface of the brain or cerebellum, a perimedullary vein, or any combination of these forms. The supplying artery is usually the adjacent dural artery, less commonly a branch of the periosteal artery, and rarely the soft meningeal artery.
The concept and nomenclature of the disease are not uniform, and it was formerly referred to as dural arteriovenous malformations (DAVMs), which include congenital arteriovenous fistulas of juvenile origin, often associated with other complex congenital malformations such as arteriovenous malformations of the Galen vein or arteriovenous malformations in the brain parenchyma. DAVSs or DAVFs specifically refer to acquired dural arteriovenous fistulas that originate in adulthood. Dural arteriovenous fistulas discussed here do not include congenital cases.
DAVFs account for approximately 5-20% of intracranial vascular malformations, with the transverse sigmoid sinus area being the most common. With the development of imaging techniques in recent years, the detection rate has increased. The exact etiology is unclear, and most scholars believe that DAVF is an acquired disease with common triggers such as head trauma, cranial surgery, and clinical conditions that can cause hypercoagulable states such as pregnancy, infection, and oral contraceptives. It is now believed that these triggers trigger the localized dural angiogenesis process. Vascular endothelial growth factor (VEGF) and basic fibroblast growth factor, which are associated with angiogenesis, are highly expressed in DAVS. The formation of new venous channels allows localized direct communication between the periosteal, dural, and even soft membrane arteries and veins. If vascular neovascularization is accompanied by loss of small venous surface features, it tends to form venous or venous sinus thrombosis. The consequences of the above vascular neovascularization and venous thrombosis processes depend on the location and other remaining venous access. The clinical presentation and course of DAVS are closely related to these two factors.
Venous sinuses and plexus
The morphological and physiological characteristics of the dural venous sinuses change with age. At birth, all the veins of the brain return to the posterior septal sinuses (PSS). The spongy venous plexus (customarily called the cavernous sinus, not a true venous sinus) primarily drains venous blood from bony structures in the nasal vestibule, orbit, and midline of the skull base. A few months after birth, the veins of the lateral fissure begin to merge into the cavernous plexus, allowing venous blood from the infant’s brain to enter the external jugular venous system through the orbital or pterygoid plexus. The jugular venous bulb matures after birth and gradually degenerates its adjacent Medial occipital sinus and Marginal sinus. However, the degeneration of specific venous sinuses during embryonic or infantile life may be incomplete or may still have underlying lacunae. The changes in the direction of drainage of the above mentioned venous sinuses at different stages of development and the changes in hemodynamics caused by the closure of fontanelle have led to specific characteristics of the connections between the individual venous plexuses or sinuses.
The venous sinuses or plexuses at the base of the skull are of cartilaginous origin, whereas the venous sinuses on the convex side of the brain are of membranous origin. These distinctions are morphologically indistinguishable but have different biological properties. The differences in origin may explain the different relationships between bone, periosteum, dura mater, and veins in different types of lesions.
The differences in venous structures and dural anatomy at different sites determine the specific vascular constitutive features of the vascular nests of dural arteriovenous fistulas and the different natural course of the disease.
The transverse sinus extends from the sinus sink to the confluence of the supratentorial sinus on both sides, followed by the sigmoid sinus, which merges into the jugular venous bulb, and the transverse and sigmoid sinuses together are called the lateral sinus. In the classification “transverse sinus area DAVF”, the area involving the sinus sink up to the adjacent jugular venous bulb, this area actually includes the transverse sinus, which receives cortical reflux, and the sigmoid sinus, which does not receive cortical reflux. Thus, there is a distinction between these two parts in terms of cortical venous reflux.
Within the venous sinuses sometimes a separation may exist and form parallel venous channels. One of them may be used for cortical drainage and the other may be used only for DAVF drainage and may be a therapeutic target.
There are no dural venous sinuses in the midline of the skull base and spine. DAVSs can also occur in these areas, but do not first drain into the dural venous sinuses.
The veins that flow through the dura mater vary in length and either extend from the subdural cortical veins or converge on the epidural venous flow before they merge into the venous sinuses. They are commonly found in the pars sinus of the azygosity, in the lateral sinus adjacent to the canopy, or in the middle skull base adjacent to the sieve plate. DAVSs occurring at these sites were once called extradural dural arteriovenous fistulas.
The root medullary veins of the spinal cord cross the dura mater and converge in a venous lake between the periosteum and the dura mater, sometimes between two adjacent arterial basins, so that sometimes the blood supply for a spinal arteriovenous fistula can come from the root artery of an adjacent segment.
The veins accompanying the convex dural artery (often located in the cranial sulcus) form their own network. These arteries are never involved in the formation of DAVSs unless direct trauma or arterial pathology causes the artery to rupture and communicate with the adjacent venous network.
There is a rare type of DAVS in which the supplying artery is from the dural artery and the fistula and draining vein are located within the skull, the so-called osteodural arteriovenous fistula. Those with convex sites are easier to identify. Those occurring at the base of the skull are easily confused with dural arteriovenous fistulas, which are often located in the pterygoid or occipital bone near the dura mater. It is more common in females in the cranial region and in males on the spinal side. In addition, it is often associated with reconstruction of bony structures.
In addition to the above subtypes, there are subtype A, which describes the type of arterial supply: simple arteriovenous fistula with a single arterial supply; and subtype B, multiple arteriovenous fistula with multiple arterial supplies. Type IIB, for example, refers to multiple arteriovenous fistulas with multiple arterial supplies and venous return with both venous sinus, dural and soft meningeal venous return, and so on.
Headache and intracranial murmur are the common symptoms, other symptoms include vision loss, proptosis, conjunctival congestion, eyelid varices, head varices, thickened and pulsating scalp arteries, mental status changes, neurological dysfunction, intracranial hemorrhage and cranial hypertension manifestations, etc.
1, cranial nerve symptoms: cranial nerve blood supply from the dural artery, dural arteriovenous fistula can cause blood theft and cause cranial nerve dysfunction. cranial nerve symptoms often improve after DAVF is cured. There is no evidence that DAVF can cause blood theft in the brain or spinal cord. Therefore, symptoms in the brain or spinal cord are always related to the venous side of the fistula. Sometimes mechanical factors such as epiglottis or retrobulbar edema can also cause ptosis and limited eye movement, which must be differentiated from true neurological dysfunction.
2. Tinnitus and audible intracranial murmur: Pulsatile tinnitus is caused by eddy currents in the paraventricular sinus or adjacent draining veins. 40% of patients with tinnitus can hear a pulsatile murmur with a stethoscope. DSA should be performed in case of sudden change or disappearance of the murmur.
3. Focal neurological dysfunction: Focal neurological dysfunction in adults is seen in patients with soft membrane, cortical or perimedullary venous reflux. The symptoms depend on the area of venous hypertension caused by venous reflux and are essentially symptoms of ischemia. Before treatment it is seen that cortical venous flow does not enter the DAVF drainage vein, whereas after successful treatment it is seen that cortical venous flow enters this vein and the symptoms improve. Compression of the local venous lake can cause focal symptoms in pediatric patients, but does not cause clinical symptoms in adult patients.
4.Whole brain neurological dysfunction: Commonly, DAVF in the transverse sinus or sinus confluence area or high flow can cause whole brain neurological dysfunction such as dementia in adult patients. Especially when the main venous sinus is narrowed or occluded, it can cause reversal of the direction of blood flow in the superior sagittal sinus and the straight sinus, and the whole brain venous hypertension can cause cerebrospinal fluid absorption disorder. It also causes optic papillary edema, intracranial hypertension and ventricular enlargement.
5.Intracranial hemorrhage: It is a common complication of type II and type III DAVF, which is caused by the rupture of a high pressure regurgitant vein with reverse drainage. The site of bleeding can be located in the subdural, subarachnoid space or intracerebral. The overall hemorrhage rate is in the range of 35% to 42%. In type III, the hemorrhage rate is up to 48%, which is higher than type II (11%). Intracortical hematomas are more common than those in the subdural or subarachnoid space. Any part of DAVF with soft membrane or cortical venous drainage can cause intracranial hemorrhage, but due to the specificity of local venous anatomy, those with canopy or anterior skull base are more prone to hemorrhage, which can be as high as 75%~95%.
6, the influence of hormones: pregnant women and premenstrual female DAVF patients can have increased clinical symptoms, these patients can often benefit from estrogen therapy. The exact target of estrogen action is not clear, and the vascular architecture of these patients is not specific. DAVF in the pterygoid and cavernous sinus regions has a strong female predisposition and is sensitive to hormonal changes.
7. Other associated conditions: DAVF is an acquired disease of adulthood and is not associated with congenital or developmental disorders. However, we also note that patients with DAVF may occasionally be associated with intracranial or multiple AVMs, cavernous hemangiomas, congenital hemorrhagic capillary dilatation (HHT) or Rendu-Osler-Weber syndrome (ROW), and intracranial or extracranial aneurysms. These co-morbidities should also be considered in the management of DAVF.
Multifocal DAVF occurs in 8.1% of the Toronto series, and the incidence of cortical reflux is three times higher in these cases than in other patients.
8. Progression and changes in clinical presentation
Thrombosis of the draining veins of DAVSs may lead to the diversion of venous drainage or prompt a shift to transmural or cortical drainage, which can cause local neurological dysfunction or bleeding. But thrombosis can also cause so-called spontaneous healing. For type I DAVSs in the cavernous sinus area with slow blood flow and no cortical reflux, in the experience of Huashan Hospital, compression of the carotid artery can be practiced under specialist supervision, which can sometimes promote this spontaneous healing. Secondary glaucoma or exacerbation of orbital symptoms should be closely noted and may be due to supraorbital or posterior venous thrombosis. Ocular venous thrombosis can cause ischemia of the optic nerve and may require thrombolytic or hormonal therapy.
Changes in clinical presentation (e.g., loss of pulsatile flow sounds) are usually seen in associated venous thrombosis and less frequently in spontaneous changes in blood flow in DAVSs. High-flow multifocal juvenile DAVSs distributed along the transverse sinus that persist into adulthood often cause cerebral venous return disturbances, and clinical symptoms may progressively worsen depending on the difference in the potential to form compensatory venous return pathways.
Angiography should be re-examined after changes in clinical presentation.
The natural course of the disease is more unpredictable. It was not until 1997 that a prospective study of 102 intracranial DAVSs was reported by Davies et al. of Toronto, with a mean follow-up of 33 months and 91% completing follow-up. 55 patients did not have soft membrane and cortical venous reflux, 81% of the 32 who did not receive any treatment had spontaneous remission of symptoms, and 86% of the 23 who received treatment had remission. 46 patients had soft membrane and/or cortical venous reflux (14 of them refused treatment). This group of patients presented with new onset non-hemorrhagic neurological dysfunction in 11% per year and a 20% annual bleeding rate. At follow-up, none of the venous drainage types appeared to change, and all those with unsuccessful treatment or persistent floppy vein reflux developed intracranial hemorrhage or neurologic dysfunction at follow-up. By 2002, the number of follow-up cases with chondral venous reflux in this group increased to 118, which showed an annual rate of new neurological deficits of 6.9%, an annual bleeding rate of 8.1%, and an annual mortality rate of 10.4% (van Dijk et al. Duffau et al. (1999) reported that 35% of bleeding cases rebleed within 2 weeks of onset.
The above data suggest that patients with type II and type III DAVSs with soft and/or cortical venous reflux should be treated aggressively, while conservative treatment is advocated for type I patients without reflux. In the literature, conversion from type I to type II and III has been reported, but it is rare (<1%) and most of them are after surgical or interventional treatment, with few spontaneous transformations.
Treatment indications and strategies are based on the type of venous drainage of DAVF. Patients with type I, type II and type III often cause neurological dysfunction and intracranial hemorrhage.
Surgical treatment.
Surgical treatment of DAVF is extremely challenging and requires adequate preoperative preparation for blood transfusion. Surgery must be performed by two skilled neurosurgeons working together to minimize intraoperative bleeding. With the help of a neurointerventional surgeon, embolization of the major blood supply arteries can be performed preoperatively, but embolization of large vessels such as the main trunk of the external carotid artery should not be performed as it may affect postoperative incision healing.
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