Arteriovenous Malformations of the Basal Ganglia and Thalamus

Patient Selection

The overall risk of hemorrhage for cerebral arteriovenous malformations (AVMs) is about 2 to 4% per year. ¹ Annual rupture rates are lower for unruptured AVMs compared to patients with prior AVM bleed at 2.2 versus 4.5%, respectively. ² Hemorrhage rates for AVMs in the thalamus and basal ganglia may be higher. In a review of untreated, patients with >500 patient-years of follow-up who were ultimately referred to Stanford for evaluation, the pretreatment annual rupture rate was 9.8% per year, ³ and deep AVM location has also been identified as an independent risk factor for hemorrhage in large natural history studies. ¹^, ² Periventricular location and deep venous drainage have been shown to increase the risk of bleeding of cerebral AVMs, and a history of previous hemorrhages correlates with hemorrhage recurrence for AVMs of the thalamus and basal ganglia. Hemorrhage from a basal ganglia or thalamic AVM also carries a risk of serious morbidity, with up to 85% of such cases developing hemiparesis or hemiplegia. The higher risk of hemorrhage and greater morbidity from hemorrhage should be factored into the decision as to whether to treat basal ganglia and thalamic AVMs. However, safe surgical access to the thalamus and basal ganglia is limited, and morbidity from treatment complications in these eloquent locations can be similarly devastating, making the assessment of risk–benefit challenging.

An important factor in patient selection is whether the AVM and/or any hemorrhage from it reaches a pial or ventricular surface. In such cases, if one can expose the AVM from that surface, then resection is at least theoretically possible. If no pial or ventricular surface is reached, then the approach has to transgress brain tissue, the function of which has to be potentially expendable without causing morbidity. There are several surgical corridors that can be used to access these deep-seated lesions. Small AVMs of the medial thalamus that are accessible through the lateral ventricle can be resected with minimal morbidity. AVMs limited to the pulvinar can often be removed safely. AVMs in the anterior thalamus or basal ganglia presenting to the insular region can be accessed through a transsylvian exposure with care taken to spare the motor fibers traversing the posterior limb of the internal capsule. Oftentimes, a multimodal approach employing embolization and radiation followed by subsequent delayed microsurgery can be used. If the lesion is large and does not present to a ventricular or pial surface, one may consider either or both stereotactic radiosurgery and embolization.

Poor surgical candidates include patients with severe comorbidities, elderly patients, and patients with devastating neurological deficits. Patients with lesions located in the posterior limb of the internal capsule are also excluded because of the high risk of permanent deficits. In patients with asymptomatic basal ganglia and thalamic AVMs, the risk of surgical morbidity and other patient-specific characteristics should be carefully weighed against the natural history of the lesion and patient-specific factors, such as lesion location, patient age, comorbidities, angiographic features, and history of hemorrhage. Patients presenting with hemorrhage are known to have a worse natural history and poorer outcomes after rehemorrhage; thus, carefully tailored low-morbidity treatments with a goal of AVM obliteration can decrease future hemorrhage risk and the associated morbidity.

24.2 Preoperative Preparation

24.2.1 Imaging

Preoperative magnetic resonance imaging and catheter angiograms are essential to understanding AVM features, anatomy, and location. In some cases, preoperative diffusion tensor imaging can be useful to localize nearby traversing motor tracts and understand their position relative to the nidus, aid in surgical approach selection, and enhance the intraoperative neuronavigation. We highly recommend the use of neuronavigation systems to guide the approach and resection of deep AVMs.

24.2.2 Embolization

On traditional catheter angiography, these AVMs are generally fed from the medial and lateral lenticulostriate arteries, recurrent artery of Heubner, thalamogeniculate arteries, thalamoperforating arteries, and anterior and posterior choroidal arteries. Almost all AVMs in the basal ganglia and thalamus have deep venous drainage. In patients with AVMs amenable to embolization, we recommend staged embolizations spaced at least 1 week apart to reduce the volume of AVM and potential bleeding during surgery. We never attempt to embolize more than 30% of the AVM at any session because more aggressive embolization can cause swelling and hemorrhage.

24.2.3 Anesthetic Technique

Surgery is performed under general endotracheal anesthesia. The anesthesiologist should control the patient’s mean arterial pressure (MAP) between 70 and 80 mm Hg throughout induction and surgical opening, and at 60 to 70 mm Hg during resection of the AVM through emergence from anesthesia. We also recommend mild hypothermia with a target core temperature of 33°C to 34°C achieved via a cooling blanket or femoral venous catheters.

24.2.4 Monitoring

Electrophysiological monitoring and mapping are important tools in resection of AVMs. The use of continuous bilateral upper and lower somatosensory evoked potentials and motor evoked potentials along with cortical mapping can be invaluable in decreasing the risks associated with the surgical and endovascular management of these lesions.

24.2.5 Additional Preparation

Prior to surgery, we sometimes use a ventriculostomy or lumbar drain for brain relaxation prior to positioning. However, this is not necessary if an intraventricular approach is used because cerebrospinal fluid (CSF) can be drained directly with opening of the ventricle. Additional steps to induce brain relaxation include hyperventilation or diuresis or both. The groin should also be prepared for possible intraoperative angiography.

24.3 Operative Procedure

24.3.1 Operative Positioning and Exposure

There are five main surgical approaches used individually or in combination: frontal, interhemispheric transcallosal, parietal interhemispheric transcallosal, occipital transtentorial infrasplenial, supracerebellar infratentorial, and transsylvian and transcortical (frontal or parietal) ( ▶ Fig. 24.1 and ▶ Fig. 24.2). The approach is determined by the location of the lesion. AVMs of the caudate are approached through a frontal interhemispheric transcallosal exposure ( ▶ Fig. 24.3), whereas AVMs in the thalamus are exposed through a parietal interhemispheric transcallosal approach ( ▶ Fig. 24.4 and ▶ Fig. 24.5).

Fig. 24.1 (a-c) Approaches to basal ganglia and thalamic arteriovenous malformations. The translucent oval areas indicate the anatomical location of the lesion and the arrows show the corresponding approach. (1) Frontal interhemispheric transcallosal. (2) Parietal interhemispheric transcallosal. (3) Occipital transtentorial infrasplenial. (4) Supracerebellar infratentorial. (5) Transsylvian. (6) Transcortical (Video 24.1).

Fig. 24.2 Patient positioning, incision, and bone flap for the different approaches. (a) For the frontal and parietal interhemispheric transcallosal approach, the patient is positioned supine, and the head is flexed 20 to 30 degrees and elevated above the heart. Alternatively, the patient can be positioned in a lateral or three-quarter prone position with the right side down, which allows the right hemisphere to fall away from the falx. (b) A trapdoor skin incision over the midline is performed, and the craniotomy is placed one-third behind the coronal suture and two-thirds in front for the frontal approach or with the anterior craniotomy border behind the postcentral gyrus for the parietal approach. (c) Frontotemporal craniotomy for the transsylvian approach is done with the patient supine and the head rotated 20 degrees and extended. (d) The supracerebellar infratentorial approach is done in the sitting position, allowing the cerebellar hemispheres to fall down with gravity. Alternatively, the modified prone Concorde position can be used. Slight flexion of the head opens the access to the posterior fossa, but too much flexion has to be avoided so as not to impair venous outflow. (e) Three-quarter prone position is chosen for an occipital transtentorial infrasplenial approach. (f) This position allows the occipital lobe to fall away from the falx, with gravity limiting the need for retraction and opening the view to the ambient and quadrigeminal cisterns.

Fig. 24.3 (a) A 28-year-old male presented with intracerebral hemorrhage from a 6-cm large, right basal ganglia/thalamic arteriovenous malformation (AVM) after unsuccessful proton beam radiation therapy 8 years prior. (b) Typical flow voids are seen in the T2 axial and (c) sagittal magnetic resonance imaging (MRI). After four embolization sessions, the AVM flow was reduced by 75%. The remaining AVM was fed by branches of the right middle cerebral artery (MCA) (d) (arrow, right ICA injection), right lateral lenticulostriates (arrow), and anterior (e) and posterior (f) thalamoperforators (arrow, vertebral artery injection) and had both superficial and deep drainage. A 1-cm intranidal aneurysm was also found. Following the embolizations, staged surgery was performed using three separate approaches. (g) First, a frontal interhemispheric transcallosal approach was used to resect the anterior and medial portion of the AVM. Special care was taken to leave draining veins intact. The interhemispheric fissure with the pericallosal arteries is seen (arrow). (h) The AVM surface was exposed, and this portion of the AVM was subsequently resected. (i) View into the lateral and third ventricle (asterisk) after resection of the AVM. (j) In a second stage, a parietal transcortical approach was used to resect the posterior and lateral aspect of the AVM. A third operative stage used a frontal transcortical exposure. (k) Postoperative anteroposterior and (l) lateral right internal carotid and anteroposterior (m) and lateral (n) vertebral injections show complete resection of the AVM (Video 24.1).