Brain Stem

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The brain stem, which consists of the midbrain, pons, and medulla oblongata, connects the cerebrum above to the spinal cord below. It is a highly organized structure that, in addition to conveying ascending and descending tracts, contains the nuclei of the cranial nerves III to XII and is responsible for a number of complex functions, including control of respiratory and cardiovascular activity and regulation of the level of consciousness.

The midbrain is the shortest segment of the brain stem. It consists of a ventral and a dorsal portion separated by the ventricular space. The dorsal or posterior portion is called the tectum and consists of four rounded swellings, the paired superior and inferior colliculi. The ventral or anterior portion is called the tegmentum and contains the reticular formation, the nuclei of cranial nerves III and IV, and ascending and descending pathways.

The pons is readily identified from the midbrain and from the medulla oblongata (whose description follows) as a large bulge on the ventral surface of the brain stem lying on the dorsum sellae of the sphenoid bone. The dorsal or posterior surface of the pons is taken up superiorly by the superior cerebellar peduncles, while inferiorly it forms the upper part of the floor of the fourth ventricle. The pons contains ascending and descending tracts and connections with the cerebellum, as well as the nuclei of cranial nerves V to VIII.

The medulla oblongata extends from the lower limit of the pons to a level just above the first pair of cervical nerves, where it is continuous with the spinal cord. It is somewhat pyriform in shape, larger superiorly than inferiorly. The dorsal surface of the medulla oblongata forms the lower part of the floor of the fourth ventricle in the cephalad half. The ventral surface is made up of the pyramids containing the corticospinal tracts. The medulla oblongata contains the nuclei of cranial nerves IX through XII, which exit along its lateral aspects. The caudal half of the medulla oblon-gata represents a transition to the spinal cord, with cross-sectional organization and a central canal similar to the cord.

Given the complexity of the structure of the brain stem and the proximity of the motor and sensory tracts and cranial nerve nuclei, it is not surprising that disturbances in brain-stem function can be seen with very small lesions. Until the advent of computerized tomography (CT) in the late 1970s, the brain stem could be imaged only by indirect means. Even CT is not ideal, primarily because of distortions of the image caused by the surrounding bone. Magnetic resonance imaging (MRI) is now the imaging modality of choice for patients in whom a lesion in the brain stem is suspected.

Lesions that may affect the brain stem include vascular malformations such as angiomas, which may be a cause of spontaneous hemorrhage; infectious diseases such as tuberculosis, although this is rare in developed countries; de-myelinating diseases such as multiple sclerosis; and tumors.

Classically occurring in children at a median age of between 5 and 10 years, brain-stem tumors account for as many as 15% of all brain tumors in this age group. Evidence suggests that the incidence of brain-stem tumors has been increasing, although this is probably due in large part to better detection using MRI of lesions that in the past may have gone undiagnosed.

There are several types of brain-stem tumors, each characterized by a distinct clinical presentation and MRI appearance. These include focal tumors, most often seen in the midbrain; dorsal exophytic tumors that grow from the dorsal aspect of the medulla into the fourth ventricle; and cervicomedullary tumors that originate in the upper cervical cord or medulla and grow posteriorly to project into the fourth ventricle. These types account for approximately 20% of all brain-stem tumors. Some, notably focal tumors arising in the tectum, may do well without any therapeutic intervention. Others may do well after surgery alone or, if this is not possible, after treatment with radiotherapy. In contrast, approximately 80% of all brain-stem tumors are of the so-called "diffuse intrinsic kind." These are highgrade astrocytomas that grow very rapidly and cause multiple neurological deficits. Surgery is not indicated, and these patients are treated with radiotherapy alone. In spite of a satisfactory early response to treatment, outcome is very poor. The median time to progression after treatment with radiotherapy is only of the order of 6 months, and the median survival time is less than 1 year. Less than 10% of patients will be alive 2 years or more after treatment. Several groups are actively undertaking research studies using various types and combinations of chemotherapy and biological agents given along with radiotherapy in the hope of achieving an improved outcome for children with these tumors, as yet without success.

Carolyn R. Freeman McGill University Health Center/ Montreal General Hospital, Montreal, Quebec, Canada

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