Surgical Management of Brainstem and Cerebellar Arteriovenous Malformations

Introduction

Brainstem and cerebellar arteriovenous malformations (AVMs) are rare vascular lesions that represent a minority of all intracranial AVMs. Although they are often grouped together, their specific anatomic location within the posterior fossa has important clinical implications for prognosis and treatment. For example, while cerebellar and pial-based brainstem AVMs are frequently amenable to surgical resection, AVMs involving the deep cerebellar nuclei or located within the brainstem parenchyma are typically treated with embolization and/or radiosurgery. Treatment strategies for infratentorial AVMs have evolved dramatically since Olivecrona first resected a cerebellar AVM in 1932, particularly with the development of selective endovascular embolization techniques and stereotactic radiosurgery. ^, Current treatment algorithms involve a multidisciplinary approach, where surgical resection, stereotactic radiosurgery, and endovascular embolization are used alone or in combination to manage these complex lesions.

Anatomy and Classification

Infratentorial AVMs are located in the brainstem and the cerebellum. Based on their location in the posterior fossa, cerebellar and brainstem infratentorial AVMs are frequently discussed together. Increased risk of hemorrhage is a hallmark of infratentorial AVMs and further distinguishes these lesions from their supratentorial counterparts. ^{, ,} Nevertheless, there are marked differences in the prognosis and treatment of infratentorial AVMs based on location, ^, which warrant in-depth consideration and classification based on the AVM’s relationship to surrounding structures.

Brainstem AVMs are subdivided into lesions of the midbrain, pons, and medulla. Han and colleagues further categorized brainstem AVMs into six categories based on surface presentation: (1) anterior midbrain, (2) posterior midbrain, (3) anterior pontine, (4) lateral pontine, (5) anterior medullary, and (6) lateral medullary. Depth as well as relationships to surrounding brainstem nuclei and white matter tracts determine resectability of brainstem AVMs. Many brainstem AVMs involve the pial surface rather than the brainstem parenchyma. These lesions are more frequently amenable to safe resection, or even “occlusion in situ” by interrupting the feeding arteries circumferentially, followed by the draining veins, while leaving the nidus on the brainstem surface in order to minimize damage to the surrounding tissue during resection. Conversely, parenchymal brainstem AVMs have an arterial supply primarily consisting of perforating arteries and often require an approach through critical nuclei and fiber tracts, precluding safe surgical resection.

According to Robert and colleagues, cerebellar AVMs can be anatomically classified based on involvement of the cerebellar hemisphere (42 patients, 71%), deep cerebellar nuclei and/or cerebellar peduncles (20, 35%), and vermis (21, 36%). In the same study, eloquent location was defined as involving the cerebellar nuclei or peduncles, allowing for extrapolation of the Spetzler and Martin classification system to AVMs of the posterior fossa. ^{, ,} There is considerable variability in the anatomy of arterial feeding vessels to cerebellar AVMs. In general, cerebellar hemispheric AVMs are supplied by SCA, AICA, and PICA branches, and rarely involve the fourth ventricular ependyma. Vermian AVMs are most often supplied by the SCA and PICA and can extend into the fourth ventricular roof. Tonsillar AVMs are supplied by PICA branches and often project dorsally, away from the fourth ventricle ( Fig. 56.1 ).

Epidemiology and Natural History

The annual incidence of brain AVMs has been estimated to be 0.82 to 1.8/100,000. ^, Infratentorial AVMs account for 5 to 15% of all brain AVMs. Of these, about 15% occur in the brainstem or CPA and 85% occur in the cerebellum. Infratentorial AVMs present at a mean age of 36 years while brainstem AVMs present at a mean age of 48 years. ^, Reports are inconsistent regarding gender predominance.

Infratentorial AVMs hemorrhage at a higher rate than their supratentorial counterparts and hemorrhage is the initial presentation in 72% to 92% of cases. ^, Several risk factors for bleeding have been described, including deep venous drainage, presence of associated aneurysms, and outflow venous stenosis. ^, Unlike supratentorial AVMs, patients rarely present with seizures. The absence of seizures as a presenting symptom prior to hemorrhage likely contributes to the increased frequency of hemorrhage as the initial presenting symptom. The annual hemorrhage rate for cerebellar AVMs was 8.6% during a 4-year follow-up in one report, but may be up to 15% in the first year. Annual repeat hemorrhage rate is substantially higher in brainstem AVMs with reports ranging from 15 to 17.5%. Mortality from the initial presenting hemorrhage has been reported from 27% up to 67%. ^, Five years after diagnosis, annual hemorrhage rate drops to 1.5%. Each subsequent hemorrhage is associated with 30% neurologic morbidity and 10% mortality. These high hemorrhage rates and associated morbidity and mortality justify more aggressive treatment of infratentorial AVMs as opposed to continued observation.

Clinical Presentation

As already mentioned, 72% to 92% of patients with infratentorial AVMs initially present with hemorrhage. ^{, ,} Presenting symptoms are severe headache (45%), sudden loss of consciousness (37%), nausea and vomiting (5%), and focal neurologic deficits (3%). Patients must undergo an initial thorough neurologic evaluation and close observation for any early neurologic deterioration. Imaging studies and early decisions regarding life-saving procedures, such as placement of a ventricular drain, decompressive craniectomy, and/or hematoma evacuation must be made. Treatment of the AVM can be deferred in most cases. In one published series, acute suboccipital decompressive craniectomy was performed in 9% of patients and 4% underwent hematoma evacuation. In this same report external ventricular drainage was required in 23%. Notably, 9% of the patients in this series eventually underwent ventriculoperitoneal (VP) shunt placement. In a different report, 79% presented with intracerebral hematoma, 79% with SAH, 53% with IVH, and 82% presented with hydrocephalus. In this cohort, 42% underwent cerebellar hematoma evacuation in the acute setting.