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Callosal and Periventricular AVMs


Pearls

  • Callosal and periventricular AVMs are quite rare.

  • Their deep location and association with critical neurovascular structures indicate patients should seek help from experienced centers.

  • Indications for treatment are similar to those for all other iAVM locations.

  • Radiosurgery, although less effective at obliteration, should be considered.

  • A thorough understanding of the surgical anatomy is required for the treatment of these complex lesions.

Introduction

Intracranial arteriovenous malformations (iAVMs) are among the most challenging pathologies neurosurgeons face. Long-term studies have allowed for a better understanding of the natural history of iAVMs and the significant morbidity and mortality associated with their rupture. It is widely accepted that ruptured iAVMs should be treated, whether with microsurgical resection or stereotactic radiosurgery (SRS). Treatment of unruptured iAVMs, however, remains a controversial topic since the results of the ARUBA study (A Randomised Trial of Unruptured Brain AVMs).

Callosal and periventricular AVMs (CPV AVMs) represent a particularly rare subtype of AVM accounting for roughly 5% of all iAVMs. Given their deep location and involvement of critical neurovascular and subcortical structures, treatment of these lesions carries a high risk. It should be noted that in this chapter, we have considered periventricular AVMs to be those with a nidus primarily within the lateral and third ventricles. This does not include AVMs within the thalamus or basal ganglia, which are discussed separately.

The treatment algorithm for these unique AVMs closely resembles that for iAVMs in general. Patients with ruptured CPV AVMs almost always present with intraventricular hemorrhage, requiring urgent placement of a ventriculostomy for management of the ensuing hydrocephalus. Once clinically stable, these patients should undergo definitive treatment of the lesion, given the risk of rerupture and its associated morbidity or mortality. Unruptured CPV AVMs should be treated in cases in which the patients suffer intractable and debilitating headaches, and treatment should be strongly considered in patients under the age of 40 years in light of their high lifetime risk of rupture.

For CPV AVMs, our practice is to favor microsurgical resection with or without endovascular embolization if the AVM nidus measures less than 4 cm. This corresponds to a Spetzler-Martin grade of III or less, given that this location is largely considered noneloquent and deep venous drainage is almost always present. For the pediatric population, microsurgical resection is particularly favored due to the negative side effects of radiation and the high lifetime risk of rupture. For CPV AVMs larger than 4 cm, SRS delivered as a single dose or as volume-staged treatment is preferred.

This chapter reviews the relevant anatomy, subtypes of CPV AVMs, useful surgical approaches, and application of SRS.

Anatomy

Understanding ventricular anatomy is critical to the microsurgical management of CPV AVMs. The ependymal walls serve as important landmarks for navigating this deep brain region where there are no cortical surfaces. The lateral ventricles are composed of the frontal horn, body, atrium, occipital horn, and temporal horn. Choroid plexus can be found in the body, atrium, and temporal horns, and this is where periventricular AVMs are typically located, as the choroid plexus is a highly vascular structure. The critical neural structures in the deep brain form a C shape centered around the thalamus and consist of the corpus callosum, the caudate nucleus, and the fornix. The lateral ventricles also form a C shape and are wedged between the corpus callosum and caudate nucleus ( Fig. 33.1 ).

Fig. 33.1, Neuroanatomy of the periventricular region. The major neural structures are shaped in an inward and anteriorly tilted “C” centered around the thalamus. Although these deep structures serve a range of high-level functions, they are considered noneloquent.

Corpus callosum

The corpus callosum is the largest transverse white matter tract that connects the two cerebral hemispheres. It is divided into the rostrum and genu anteriorly, the body, and the splenium posteriorly. The rostrum makes up the floor of the frontal horns of the lateral ventricles, and the genu makes up the anterior wall. The body drapes over the roof of the body of the lateral ventricles, and the splenium comprises the medial wall of the atrium, connecting the occipital lobes.

Fornix

The fornix contains hippocampomammillary fibers and begins as the fimbria in the temporal horn of the lateral ventricle. It wraps around posterosuperiorly as the crus around the pulvinar of the thalamus and joins together as a single tract overlying the thalamus and forming the inferior lining of the body of the lateral ventricles. The fornix then splits into the columns and ends in the mammillary bodies at the anterior margin of the foramen of Monro.

Caudate

The caudate forms the lateral walls of the lateral ventricles, with its head bulging into the frontal horns and its tail wrapping around the atrium and ending adjacent to the temporal horn.

Choroidal fissure

The choroidal fissure is the attachment of the choroid plexus to the ventricular wall at the junction of the thalamus and the fornix. It begins at the foramen of Monro, extends along the superior surface of the thalamus, and ends in the temporal horn at the inferior choroidal point behind the head of the hippocampus. The choroid plexus arises from a clear membrane called the tela choroidea, which attaches to the choroidal fissure via the tenia choroidea.

Velum interpositum

An important potential space to understand with respect to AVMs in this region is the velum interpositum. The velum interpositum makes up the roof of the third ventricle and comprises five layers: the fornix superiorly, two layers of teniae surrounding a layer of critical blood vessels that includes the internal cerebral vein and the medial posterior choroidal artery, and a layer of choroid plexus that is in continuity with that in the body of the lateral ventricles. The third ventricle is a narrow, funnel-shaped chamber located between the two thalami; it communicates with the lateral ventricles through the foramina of Monro. The roof is described earlier and bordered by the foramina of Monro anteriorly and the pineal recess posteriorly. The floor of the third ventricle spans from the optic chiasm and chiasmatic recess anteriorly to the aqueduct of Sylvius posteriorly.

Arteries

A schematic illustration of the arterial anatomy of the periventricular region is shown in Fig. 33.2 .

Fig. 33.2, Arterial anatomy of the periventricular region. The lateral ventricles are supplied by three choroidal arteries (the anterior choroidal artery [aChA] , medial posterior choroidal artery [mpChA] , and lateral posterior choroidal artery [lpChA] ). The aChA arises from the supraclinoid internal carotid artery (ICA) and courses underneath the optic apparatus into the temporal horn. The mpChA arises from the crural segment of P 2 (P 2 A) and wraps posteriorly around the tectum into the velum interpositum, ending in the floor of the body of the lateral ventricle through the foramen of Monro. Lastly, the lpChA arises from the ambient segment of P 2 (P 2 P) and takes a direct course into the atrium. ACA , Anterior cerebral artery; BA , basilar artery; CalA , calcarine artery; CMA , callosomarginal artery; MCA , middle cerebral artery; PCA , posterior cerebral artery; PcaA , pericallosal artery; PCoA , posterior communicating artery; POA , parieto-occipital artery.

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