11 Critical Care and Neuroanesthetic Considerations for Arteriovenous Malformation and Arteriovenous Fistula Surgery
Critical care and anesthetic management are essential components of a multidisciplinary approach to the treatment of arteriovenous malformations (AVMs). Presenting symptoms of AVMs include hemorrhage, seizure, headache, and focal neurological deficits and will predict the most important first steps. Hemorrhage is the most common presenting event and requires cautious and rapid blood pressure control. Timing of surgical correction is debatable, with risk and benefits to both ultra-early resection and delayed treatment after rupture. Seizures are the most common presenting symptoms in unruptured AVMs and are treated with standard antiepileptic medications. While AVMs are a rare cause of headaches, most patients with AVMs have headaches. The least common presenting symptom is focal neurologic deficit. During open or endovascular repair, optimal control of blood pressure, intracranial pressure, ventilation, and oxygenation and monitoring are critical for the prevention of secondary brain injury, and assist the surgeon to optimize outcomes for these patients. Because of abnormal vasculature, cerebral tissues around the AVM are often underperfused and are at risk of further ischemia in the setting of hypotension. Many anesthetics including inhaled medications can increase intracranial pressure and should be avoided. Additional brain relaxation can be accomplished with hyperosmotic therapy. Intraoperative monitoring with neurophysiologists can be complementary to anesthetic and surgical planning; short-acting paralytic agents are preferred if neuromonitoring will be utilized. Occasionally, awake craniotomies can be performed to preserve eloquent areas in appropriately selected patients. With careful alignment of the management of medical and surgical care, superior patient care can be achieved.
Keywords: critical care, anesthesia, hemorrhage, ischemia, intracranial hypertension, seizure, neuromonitoring, ruptured arteriovenous malformation
- Common clinical presentations of arteriovenous malformations include hemorrhage, seizures, headaches, and focal neurological deficits.
- Acute medical therapy of arteriovenous malformations and arteriovenous fistulas is critical in multidisciplinary care of patients harboring these lesions in the neurointensive care unit, particularly for patients presenting with hemorrhage but also for patients undergoing surgical and/or endovascular interventions.
- Anesthesia is crucial for successful execution of surgical and endovascular procedures for arteriovenous malformations and arteriovenous fistulas.
While the goals of surgical and endovascular techniques are to maximize neurological function and minimize disability and death caused by arteriovenous malformations (AVMs), the medical and anesthetic management of these patients is critical to the concurrent care while in the operating room or endovascular suite and throughout their hospital course. Cerebral AVMs can present with several distinct clinical scenarios dictating the most urgent concern, often prior to surgical or endovascular procedures, and influencing the next steps of care. The most common initial clinical presentation is hemorrhage. Over 50% of the presentations involve rupture and hemorrhage. Another 30% present with seizures.1 Less commonly, patients can present with headaches or focal neurologic deficits. Due to more frequent imaging, currently approximately 1.2% of AVMs are asymptomatic incidental radiographic findings.2 Patients will most commonly present between the ages of 20 and 39 years, but it is not uncommon to present in the second or fifth decades of life.3 The location of the lesion influences clinical presentation. Most AVMs are located in lobar areas of the cerebrum, with an even distribution among AVMs found in the frontal, parietal, occipital, and temporal lobes.4 Less commonly, AVMs are located in deep brain or infratentorially. During procedures, caution must be used to prevent rapid changes in blood flow or intracranial pressure (ICP) from medication administered from anesthesia colleagues.
11.2 Materials and Methods
Materials used in this chapter were based on literature searches from PubMed with the focus on cerebral AVMs with supplementation from literature reviews for anesthetic and medial management. Images are taken from the clinical experiences of the authors.
11.3 Presenting Symptom
The overall reported risk for rupture of a cerebral AVM is 2 to 4% per year. While the natural history of AVMs can vary greatly, the overarching goal of treatment is to prevent rupture and hemorrhage while minimizing risks of other complications. Lesions that present with hemorrhage have the highest risk of future hemorrhage, especially in the first year. Additionally, the risk for second hemorrhage in the first year after a rupture is between 6 and 15%, and is suspected to be higher than quoted in previous studies.5 AVMs with a larger nidus, greater than 5 cm, are most likely to rupture, but AVMs with a smaller nidus (< 3 cm) are more likely to present initially as a hemorrhage. Smaller AVMs are less likely to cause other symptoms such as seizures, headaches, and neurological deficits before hemorrhage.1 Other significant risk factors for rupture include deep or infratentorial location, exclusively deep venous drainage, and associated aneurysms.5 ► Fig. 11.1 is an example of an AVM presenting as a cerebellar hemorrhage on noncontrasted head computed tomography (CT) in a 14-year-old who presented with acute loss of consciousness. ► Fig. 11.2 represents his cerebral angiogram.
Fig. 11.1 Noncontrasted head CT. 14-year-old male who presented with coma from cerebellar AVM hemorrhage.
Fig. 11.2 Angiography. 14-year-old male who presented with coma from cerebellar AVM hemorrhage.
Intracerebral hemorrhages (ICHs) caused by AVMs have better prognosis for recovery than spontaneous ICHs, with over a 20-fold decrease in mortality for hemorrhages caused by AVM rupture.6 This highlights the prognostic importance of appropriate imaging used to identify the underlying cause of the hemorrhage before prognosticating for neurologic outcome. Bleeding patterns and patient demographics can also be important in outcome and should be considered in discussions with patients and families. Some studies have shown that parenchymal bleeding appears to be associated with a lower Glasgow Coma Score (GCS) on presentation; however, nonparenchymal hemorrhages, especially basal cistern subarachnoid hemorrhage (SAH), are associated with worse overall outcomes in patients.7 Other studies have found parenchymal bleeding to be associated with unfavorable outcomes, along with higher patient age and patients who undergo hematoma evacuation.8 The ICH score is used to predict outcomes in patient functionality and mortality for nontraumatic ICH. Components include GCS on presentation, ICH volume, presence of intraventricular hemorrhage (IVH), supratentorial or infratentorial location, and age of the patient. It is also accurate in predicting functional outcomes for risk stratification in patients with ICH due to a ruptured AVM, but is less reliable predicting mortality because ICHs due to AVM rupture have a much lower risk of death than other types of spontaneous ICHs.9
The timing of interventional treatments after hemorrhage can play an important role in critical care decisions. The most common neurosurgical interventions include microsurgical resection, embolization, and stereotactic radiosurgery. In conservatively treated ruptured AVMs, there are a significant number of patients who suffer an early rerupture within the first few weeks. Perinidal and perihematomal edema on follow-up imaging 2 weeks after rupture indicates a higher risk for early rebleed and should be treated more aggressively.10 Periprocedural rupture rates after endovascular occlusion of the AVM have been reported to be between 4 and 16%.11 It is important to closely monitor the neurological exam post embolization and obtain prompt imaging with significant changes. Periprocedural bleeding after endovascular occlusion should especially be considered if the patient has an intranidal aneurysm or draining vein occlusion or if any complications were reported during the procedure.11
About 25% of patients with cerebral AVMs also have a cerebral arterial aneurysm, with the majority occurring in the feeding artery of the AVM, and the rest either intranidal or not closely associated with the AVM.12 There is a significantly elevated risk of hemorrhage with associated aneurysms, especially in aneurysms greater than 5 mm in diameter.13 While most patients present with ICH or IVH, patients with feeding artery aneurysms are also at risk of SAH and subsequent complications associated with it.12
Medical management of ruptured AVMs is best implemented with a multidisciplinary team including neurosurgical, neurointerventional, and neurocritical care teams. Initial diagnostic testing should include basic medical work-up with complete blood counts, prothrombin time, partial thromboplastin time, imaging, and accurate GCS and ICH scores.14 Some patients may require intubation for failure to protect the airway during coma or seizure. Diagnostic evaluations begin with a noncontrast CT to initially evaluate the location and extent of any potential hemorrhage. Noninvasive vascular imaging such as CT angiography can be helpful to look for an underlying vascular malformation including AVMs, cavernous malformations, and aneurysms, to plan the next phase of treatment, or to identify active bleeding.14 Magnetic resonance imaging (MRI) is also often obtained to help delineate lesion and hemorrhage location within the brain more accurately. Catheter cerebral angiography is the final diagnostic step and can be performed if the patient has stabilized and does not require emergent decompressive surgery. Digital subtraction angiography is the gold standard for identifying and classifying underlying vascular malformation.
There are limited data for the medical management for AVMs, as surgeries aimed at prevention of rebleeding, both open and endovascular, are treatments of choice; however, pathophysiology and treatment are based on studies in all ICH patients and expert opinion for management of acutely neurologically injured patients. Blood pressure management is crucial in patients with ICH from ruptured AVMs, as it is with ICH without underlying structural lesions. Evidence shows that when systolic blood pressure of less than 140mm Hg is achieved and consistently controlled, patients have better functional outcomes and are safe, especially when kept between 130 and 139 mm Hg.15 The titration of intravenous short-acting antihypertensives is recommended to reduce the risk of hypotension and blood pressure variability.14 All anticoagulation and antiplatelet medications should be stopped and reversed if taken recently. Blood glucose should be kept below 185 g/dL, as continuously elevated blood glucose levels have been associated with worse outcomes.16 Acute antiepileptic treatment is often started in the intensive care unit (ICU), especially in lobar and cortical hemorrhages, but has not been proven to change outcomes.16 Continuous electroencephalography can help detect seizures including nonconvulsive status epilepticus in unresponsive patients.17
Unlike with a ruptured aneurysm, where most of the complications are attributable to hydrocephalus and less likely due to local mass effect from intraparenchymal hemorrhage, both are equally important to consider in the pathology of AVM rupture.18 Perinidal edema and hematoma volume expansion can occur with ICH caused by ruptured AVMs. The surrounding edema is caused by a complex web including vasogenic edema from plasma protein extravasation and clot retraction, inflammatory cascades, and other factors.19 Intracranial hypertension is managed with standard practices including osmotic therapy (hypertonic saline or mannitol), sedation, or decompressive surgery for more severe cases or for failed medical management.20 Forty-four percent of patients with ruptured AVMs require early external ventricular drain (EVD) placement, with 18% becoming shunt dependent, requiring ventriculoperitoneal shunting.18 Placement of external ventriculostomy is based on specific patient factors. Criteria often used to perform ventriculostomy include GCS< 9, evidence of herniation and/or increased ICP, significant IVH, or hydrocephalus.21 IVH, low GCS scores, and the presence of an associated aneurysm are risk factors for both EVD placement and shunt dependence.18
While acute neurological decline due to mass effect is an indication for emergent surgery, current practice, often, is to perform AVM resection surgery electively, after a period of 1 to 4 weeks, to allow for patient recovery from the initial ictus and to allow swelling to subside and associated clot to liquefy. Recommendations for emergent clot evacuation include a decreased level of consciousness due to ICH, a hematoma volume greater than 30 mL in the temporal lobe or posterior fossa, or a hematoma volume greater than 60 mL in the cerebral hemispheres.17 Recently, some neurosurgeons performed early resection of lower grade AVMs over delayed treatment. One study showed a mortality of 7.4% for low-grade AVMs microsurgically resected within the first day compared to 23 to 29% from natural history data with delayed resection.22 This technique may prevent “ultra-early’ rebleeds, which occur within the first 24 hours of hemorrhage, the riskiest time in the first year after event.22 The outcomes for early resection are best predicted by severity upon admission, and are less related to location, ICH size, and grade of the AVM.23 This is discussed further in other chapters.
Postoperatively, patients are at risk of neurological changes because of edema and postoperative bleeding. The first and oldest of the two theories to explain such events is called normal perfusion pressure breakthrough. Theoretically, the parenchyma around the AVM is in a chronically hypoperfused state and, therefore, has decreased autoregulation of local blood flow compared to normal vessels.24 When there is more blood flow, the vessels without the ability to autoregulate will rupture. A newer theory called occlusive hyperemia postulates that stagnant blood flow in former feeding arteries leads to hypoperfusion and ischemia in surrounding tissue; simultaneously, venous outflow of surrounding parenchyma is obstructed, leading to passive hyperemia.24 Progressive ischemia and hyperemia combine for edema and postoperative bleeding. The true answer could lie on a spectrum between the two theories, with currently unknown factors involved. Another factor, discussed elsewhere, to explain postoperative hemorrhage is potential residual nidus. Most surgeons perform intraoperative angiography or immediate postoperative angiography to reduce any potential chance of residual nidus. Regardless, progressive edema and bleeding should be closely monitored after open resection and treated swiftly.
Vasospasm after AVM rupture is extremely rare, even with blood in the subarachnoid space. Multiple case reports have been published detailing delayed cerebral ischemia from angiographically confirmed vasospasm, especially in the presence of intraventricular blood.25,26 Of the 20 cases of delayed ischemia from vasospasm in the literature, only 56% had SAH, but interestingly, all cases involved IVH.27 Only 6.3% of patients with SAH from AVMs developed vasospasm detected on angiography compared to approximately 70% of aneurysmal SAH patients developing angiographically visible vasospasm.27 Vasospasm should be considered in patients with delayed neurologic deficits after a ruptured AVM, especially in the setting of associated SAH or IVH.25 Empiric delayed angiography to assess for vasospasm is probably unnecessary,27 although treatment principles for delayed cerebral ischemia secondary to vasospasm in the setting of completely resected AVM (or secured AVM-associated aneurysm) would be analogous to those followed in the setting of aneurysmal SAH including induced hypertension, hypervolemia, and possibly endovascular therapy.
Seizures are the most common presentation for unruptured AVMs. Most of these patients presenting with seizures harbor cortical AVMs. The locations have equal distribution with the exception of occipital AVMs, which have lower seizure rates. Larger nidus is a strong predictor of seizure presentation.28 The exact etiology for the association of AVMs and seizures has not been completely determined, but several mechanisms have been proposed. These include neuronal destruction and degeneration, changes in glial structure and physiology, and formation of reactive oxygen species.29 There has been no distinction of seizure semiology specifically associated with AVMs. ► Fig. 11.3 demonstrates a cortical AVM that presented with seizures on fluid-attenuated inversion recovery (FLAIR) sequence MRI. ► Fig. 11.4 represents the angiogram demonstrating the AVM.
Fig. 11.3 T2 FLAIR MRI. 17-year-old female who presented with seizure and headache due to cortical AVM and intraventricular rupture.