Brainstem and pineal region

Fourth ventricle

The fourth ventricle is a rhomboid structure, located within the posterior fossa and bounded anteriorly by the brainstem, posteriorly by the cerebellum and laterally by the cerebellar peduncles ( Ch. 24 ). It communicates with the third ventricle through the cerebral aqueduct at its rostral apex; it also opens to the cisterns of the posterior fossa caudally through the foramen of Magendie and laterally, at its broadest point, to the cerebellopontine angles through the foramina of Luschka ( Fig. 28.1 ).

The floor of the fourth ventricle is constituted by the dorsum of the brainstem and is highly eloquent. The rostral two-thirds are posterior to the pons and the caudal third is posterior to the medulla. A median sulcus runs from the aqueduct to the obex at its caudal tip, anterior to the foramen of Magendie, which divides the floor into two equal halves. The level of the foramina of Luschka corresponds to the junction between the pons and medulla. The sulcus limitans runs parallel to the median sulcus; it delineates a medial section adjacent to the midline at the median sulcus from a lateral section overlying the vestibular nuclei known as the vestibular area. The medial section is elevated above the rest of the floor and is called the median eminence. This is wider rostrally than caudally. In its rostral region it is highly prominent; it overlies the ascending part of the facial nerve as it crosses over the nucleus of the abducens nerve. More caudally, in turn, it overlies the hypoglossal nucleus, the vagal nucleus and the area postrema; the latter lies just rostral to the obex. The vestibular nuclei lie deep to the vestibular area and extend into the lateral recesses. The cochlear nuclei form a prominence within the most lateral part of the vestibular area. The striae medullaris form a useful landmark, extending medially from the region of the foramina of Luschka to disappear into the median eminence.

The roof of the fourth ventricle is pyramidal, with its apex at the fastigium. In its upper half, the roof consists of the superior medullary velum, a thin layer of white matter across the midline, connecting the deep surface of the superior cerebellar peduncle on each side. The top of the cerebellar vermis, the lingula, sits on the dorsal surface of this part of the roof. The inferior medullary velum contributes to the roof of the lower fourth ventricle in the midline and is continuous with the superior medullary velum at the fastigium. At its inferior margin it is attached to the tela choroidea at the telovelar junction. The tela carries the choroid plexus on its ventricular surface; it extends from the nodule of the vermis laterally into the lower half of the lateral recess. However, in its inferior extent, the tela is attached to the inferior cerebellar peduncles as they border the lower fourth ventricle towards the obex. Gaps in the tela form the foramen of Magendie and the foramina of Luschka.

The foramina of Luschka connect the fourth ventricle with the cerebellopontine angles. The paired foramina arise caudal to the cerebellar peduncles, with the inferior peduncle forming their rostral wall. The flocculus extends from the lateral part of the tela choroidea, emerging at the superior aspect of the recess. It is a useful landmark at surgery in this region. The foramen has important relationships to cranial nerves in the region. The vestibulocochlear nerve crosses its floor, whereas the facial nerve arises superior to it and the glossopharyngeal and the vagus nerves arise ventral to it.

The cerebellar tonsils are the lowest part of the cerebellar hemispheric surface and their anatomy is relevant to the telovelar approach. The tonsillar peduncles are white matter bundles that connect the cerebellar hemispheres to the tonsils along their superolateral borders. Below the peduncles, the tonsils, on their lateral surfaces, are adjacent to the biventral lobules of the cerebellum. The cerebellomedullary fissure separates the tonsils dorsally from the medulla oblongata ventrally. The ventral surface of the superior poles of the tonsils overlies the lower roof of the fourth ventricle, which at this level consists of the inferior medullary velum and the tela choroidea. The tonsils form a barrier to the opening of the lower fourth ventricular cavity; separating the two tonsils surgically allows access to the ventricle.

The dentate nucleus, the largest of the deep cerebellar nuclei, is located just rostral to the superior pole of the tonsil, within the cerebellar white matter in the superior part of the lateral recess, near the inferior medullary velum. It receives afferents from the premotor and supplementary motor areas, as well as the spinocerebellar tract. The most caudal component of the vermis, seen between the tonsils, is the uvula. The component just cranial to this is the pyramid, which is attached to the medial cerebellar hemisphere on both sides. This point of attachment is the medial part of the dentate nucleus. It is also useful to remember that the facial colliculus and the nucleus of the abducens nerve within the floor of the fourth ventricle are at the same level as the dentate nucleus. The dentatothalamic tract, running within the superior cerebellar peduncle, is the main efferent tract of the dentate nucleus. The fibres of the superior cerebellar peduncle ascend medial to the middle cerebellar peduncle, forming the lateral walls of the upper fourth ventricle.

Tumours in or arising from the brainstem often distort the nuclei within the floor of the fourth ventricle, underlining the importance of intraoperative monitoring and mapping during surgical resection. Cranial nerve nuclei may be displaced and compressed ventrally by a tumour and become evident only on mapping once the resection is under way. This is typical for the lower cranial nerve nuclei with medullary brainstem tumours; pontine tumours displace the facial nerve nucleus around the edge of the tumour.

The posterior fossa syndrome (PFS), characterized by mutism, ataxia and emotional lability, occurs in up to 25% of surgical resections of tumours within the fourth ventricle. Risk factors include age under 5 years, large tumours and medulloblastoma. Although initially described as transient, deficits in speech and mobility often persist. Studies correlating clinical PFS with tractography have identi­fied injury to the dentatorubrothalamic tract as central to the aeti­ology of the condition. Hypertrophy of the inferior olivary nucleus is an associated change on long-term surveillance imaging. Although the precise aetiology remains unclear, a diaschisis syndrome involving acute reduction of input from the dentate nucleus to the thalamo­cortical motor system has been postulated.

Traditionally, the transvermian approach has been used to access lesions within the fourth ventricle. Injury to the vermis, particularly the lower vermis, has been associated with PFS and ataxia. Arguably, by preserving the vermis, the telovelar approach is less likely to lead to PFS; however, this is unclear from the current literature. This approach involves dissection of the tela choroidea and the inferior medullary velum, hence exposing the entire length of the fourth ventricle up to the aqueduct through the cerebellomedullary fissure. Dissection begins in the plane between the medial tonsil and the lateral surface of the uvula ( Fig. 28.2 ). Upward and lateral retraction of the tonsil exposes the tela choroidea and the inferior medullary velum. The posterior inferior cerebellar artery is protected and the tela is incised. This incision is extended superiorly through the inferior medullary velum. Large fourth ventricular tumours stretch both the tela and the velum, and facilitate this step. This approach also allows early definition, and protection, of the fourth ventricular floor and cerebellar peduncles, as well as early identification of arterial supply to the tumour from the posterior inferior cerebellar artery, with the potential to minimize blood loss in small children. It also facilitates access to the lateral recesses of the fourth ventricle. The telovelar approach may reduce the need for permanent cerebrospinal fluid (CSF) diversion after posterior fossa surgery; this is probably related to the wide opening between the cisterna magna and the foramen of Magendie.

Although the telovelar approach is appropriate for most lesions within the fourth ventricular cavity, tumour location within the rostral third of the fourth ventricle has been associated with a lower rate of complete surgical resection. A study using image guidance to compare access using transvermian and telovelar approaches demonstrated that exposure of the lateral recesses and foramina of Luschka was more complete with the telovelar approach, whereas the rostral half of the fourth ventricle was more easily accessed through a transvermian approach.