What is the brain stem responsible for

Publisher Summary

This chapter discusses the brainstem. The study of the brainstem helps to examine the way grey and white matter of the cervical spinal cord are rearranged in the medulla. On each side of the medulla, corticospinal fibers are located ventrally in the pyramids. Two tracts in the spinal ventral funiculus are displaced dorsally in the medulla by the pyramid and medial lemniscus; these are the tectospinal tract and the medial longitudinal fasciculus. The spinal pattern of grey matter is altered in the medulla. Motor nerve roots emerge from the upper cervical cord in two separate rows for the muscles of dissimilar developmental origin. The ventral nerve roots supply myotomal muscles. Other fibers emerge dorsally from the lateral surface of the cord to form the spinal accessory (XI) nerve, ascending through the foramen magnum to supply branchial arch musculature. Visceral components of cranial nerves are represented in the medulla by vagal and glossopharyngeal nuclei. The vagus provides a parasympathetic supply to thoracic and abdominal viscera, the latter supplemented by the sacral parasympathetic.

Organ Function and Clinical Significance

The brainstem serves as a conduit from the brain to the cranial nerves and spinal cord. As a result, the brainstem is involved with motor, sensory, and special sensory function, as well as regulation of temperature, cardiac function, respiratory function, and consciousness. It is well accepted that the entire brainstem may be treated to 54 Gy using conventional fractionation with minimal risk of late brainstem toxicity. Small volumes of brainstem may tolerate higher doses. Similar to the brain, the brainstem is heterogeneous, and it is not well known which regions are most susceptible to radiation-induced damage.

2.1.3 Brainstem

The brainstem serves as the physical structure that connects the brain to the spinal cord, containing portions of the mid and hindbrain regions. This structure plays several significant roles in maintaining proper function of the nervous system and its control over our bodies. The brainstem regulates and maintains the cardiac and respiratory systems. Additionally, it plays an important role in controlling consciousness and the process of sleeping. Injuries to the brainstem can be life threatening, as they can result in an inability to breathe involuntary, which often will lead to death. In addition to controlling these major bodily functions, the major nerve conduction pathways (often referred to as tracts) that regulate our ability to sense start in the brain, then move through the brainstem and into the spinal cord before branching out all over the body. The nerves responsible for sensing and movement in the face branch out from the brainstem, enabling us to respond to stimuli. These nerves are known as cranial nerves because they exit the skull. The medulla oblongata, a cone-shaped region of the brainstem, serves as the continuation of the spinal cord into the skull. Accordingly, it forms the lowest region of the brainstem. It also controls the autonomic activity of the heart and lungs, including involuntary activity. Thus, it is critical to have a functioning medulla oblongata for survival. Injuries to this specific region of the brainstem are often critical and life threatening as mentioned previously, an inability to breathe can lead to death. While the pons region of the brainstem is relatively small, its function is critical in several ways. It relays messages between the cerebrum and the cerebellum to ensure proper brain activity. It regulates the processes of sleeping and dreaming as it is the location in the brain where rapid eye movement (REM) sleep associated with dreaming occurs. Overall, the brainstem performs highly critical functions for maintaining proper bodily function and survival.

Reticular Formation

The brainstem reticular formation is not a separate anatomic structure but is instead distributed throughout the core of the brainstem from the medulla into the midbrain. It plays a fundamental role in arousal and consciousness, control of movement and sensation, and in regulation of visceral functions. A subset of the reticular formation neurons sends fibers to the intralaminar thalamic nuclei, which in turn project widely throughout the cerebral cortex. Damage to these ascending brainstem fibers, collectively called the ascending reticular activating system, results in loss of consciousness and coma, even in the absence of any damage to the cerebral hemispheres. Thus the cerebral cortex requires input from the brainstem to maintain awareness and arousal. In addition, the reticular formation contains neuronal circuits responsible for regulation of respiration, for cardiovascular responses to blood pressure and oxygen level modulations, as well as for coordination of swallowing and other oromotor functions. Finally, complex reflexes such as walking and maintenance of body orientation with respect to gravity are coordinated by the brainstem reticular formation and may occur in the absence of input from higher brain regions. Immaturity of the brainstem reticular formation, in particular of its serotonergic component, has been implicated in the etiology of SIDS.14

Brainstem Stroke

Brainstem stroke can result in “bulbar palsy,” affecting the lower motor neurons of cranial nerves IX through XII in side or outside of the brainstem. Patients with brainstem stroke have greater occurrence of persistent dysphagia than those with hemispheric stroke.7,59 With lateral medullary infarction (Wallenberg syndrome), oral control may be near intact, but the ability to trigger and achieve an effective swallow is weak bilaterally, despite a unilateral lesion.7 Reduced laryngeal elevation, unilateral pharyngeal weakness, and reduced adduction of the vocal cords may be seen, resulting in aspiration.95 A delayed or absent swallow response may be seen.55,60 Recovery does occur in 88% of patients; however, it takes longer than in those with hemispheric stroke.60

Brainstem

The brainstem, also spelled brainstem, is divided into three regions: the superior midbrain, the middle pons, and the inferior medulla oblongata. Each of these is about 1 in in length, and together, they make up only 2.5% of the total brain mass. The brainstem tissues are similar to those of the spinal cord. There is a deep gray matter surrounded by white matter fiber tracts. Differently, there is a nucleus of gray matter inside the white matter, which does not occur in the spinal cord. The brainstem is discussed in detail in Chapter 9.

Distal Field Ischemia in the Brainstem

Brainstem lesions due to hypotension have been hypothesized but seldom documented. Romanul637 identified the medial zone at the tegmentobasal junction of the mid-pons (the area damaged in central pontine myelinolysis) as a possible zone of vulnerability between medial penetrating branches and tegmental supply from the circumferential cerebellar vessels supplying the tegmentum, but did not refer to necropsy specimens supporting this idea. Jurgensen and colleagues645 described a 44-year-old epileptic woman who was found to be hypotensive, hypothermic, and comatose after presumed multiple-drug injection. Necropsy revealed bilateral, symmetric, round, hemorrhagic infarcts distributed in a columnar fashion in the lateral brainstem tegmentum extending the length of the lower pons and medulla. They were between the short lateral circumferential penetrators and the lateral edge. There were also bilateral hemorrhagic lesions in the lateral putamen. Gilles646 described isolated necrosis of brainstem nuclei in children after hypotension.

The authors have seen two adult patients with a clinical picture after hypotension that closely mimicked pontine hemorrhage: small pupils, absence of horizontal gaze, and deep coma. In one of these patients, no gross lesion was visible at postmortem, but extensive necrosis of brainstem nuclei was seen microscopically, especially in the pons. Postmortem examination was not performed in the other patient. Lance and Adams647 described patients with hypotension who made fine or coarse muscular jerks, especially on conscious attempts at precise movement. These researchers called this phenomenon “intention and action myoclonus.” Myoclonus was related to probable cerebellum system damage but was not precisely localized. Keane648 wondered whether the sustained upward gaze found in 15 patients who had suffered a cardiorespiratory arrest could be due to a "symmetrical cerebellar hypoxic change" that was found at postmortem in the 6 autopsied patients. Downward nystagmus and severe truncal and extremity cerebellar dysfunction were also noted in several patients, leading Keane648 to suggest a posterior circulation locus for the pathogenic mechanism.

Introduction

Brainstem gliomas account for 10%–20% of pediatric brain tumors [1]. The median age at diagnosis ranges from 5 to 9 years of age. Notably, brainstem gliomas represent a range of diseases rather than a single entity, ranging from low-grade tumors which may require only limited treatment and carry a favorable prognosis to high-grade tumors that are rapidly progressive despite aggressive therapy.

About 80% of brainstem gliomas are located in the pons, whereas the remaining 20% can be found in the medulla, midbrain, or cervicomedullary junction. Historically, pediatric brainstem gliomas have been divided into three main categories with distinct imaging and clinical features: diffuse intrinsic pontine gliomas (DIPG), focal tectal gliomas, and posterior exophytic/cervicomedullary gliomas [2]. DIPG account for 80% of brainstem gliomas. The typical duration of symptoms in DIPG patients is short (< 2 months), and they classically present with ataxia, long tract signs, and/or cranial nerve deficits. DIPG are recognized by characteristic imaging findings of a diffuse pontine lesion, encasing the basilar artery. At the time of diagnosis, contrast enhancement on magnetic resonance imaging (MRI) may or may not be present, and is not predictive of outcome [3]. The diagnosis can be made based on clinical and imaging features alone, and when biopsied the histologic appearance may range from diffuse astrocytoma or anaplastic astrocytoma to glioblastoma. Regardless of initial histological grade, however, progression-free survival (PFS) in DIPG patients is generally 5–6 months and overall survival (OS) < 1 year [1,2].

Posterior exophytic/cervicomedullary gliomas account for 10%–15% of brainstem gliomas. These patients usually have a longer duration of symptoms at diagnosis (> 2 months) and often present with headache and emesis, as well as dysphagia and weakness. The tumors usually arise from the floor of the fourth ventricle or from the cervicomedullary junction. On imaging, they tend to be focal with a posterior exophytic component, and typically show enhancement on postcontrast MRI. When biopsied, these tumors have pilocytic astrocytoma histology [2]. The third type of brainstem glioma is composed of focal tectal gliomas, and these account for about 5% of brainstem gliomas. These tumors often have > 2 months of symptoms at diagnosis and usually have signs of elevated intracranial pressure such as headaches and emesis. They are located in the region of the tectal plate and commonly have hydrocephalus on imaging. Histology is usually that of low-grade glioma [2]. Of note, brainstem gliomas associated with NF1 are considered as a separate entity. Patients with NF1 more commonly have optic pathway gliomas, but a small percentage can develop gliomas in the brainstem. These tumors tend to behave less aggressively, can sometimes be asymptomatic, and regress on occasion.

Alternative diagnoses should be considered when evaluating focal brainstem gliomas, namely primitive neuroectodermal tumors (PNET) of the brainstem. These are often characterized by younger age of onset, a focal nonenhancing tumor, and aggressive tumor behavior and growth [4]. Of note, recent advances in molecular classification have allowed the demonstration that a significant proportion of tumors previously diagnosed as PNETs, in fact display molecular profiles indistinguishable from other CNS entities, namely ATRTS, ETMRs, and HGG [5]. Other possibilities to consider in the differential diagnosis include histiocytic lesions, tuberculoma, or abscess. In these cases, pathological diagnosis with a biopsy is indicated.

Factors that seem to affect prognosis of brainstem gliomas include presentation with cranial nerve palsies, evidence of hypodense tumor involving the entire brainstem, and malignant tumor histology such as mitoses. These are typically associated with shorter survival [6]. Favorable prognostic factors include the diagnosis of NF1, symptom duration > 12 months at diagnosis, calcification noted on the tumor, exophytic location of the glioma, and low-grade pathology including the presence of Rosenthal fibers [7].

Brainstem

Throughout the brainstem, anatomic regions are broadly subdivided in the ventral to dorsal direction as base, tegmentum, and tectum. The base is located ventrally and consists mostly of long white matter tracts (cerebral peduncles, basis pontis, and medullary pyramids). The tegmentum lies dorsal to the base and ventral to the cerebral aqueduct or fourth ventricle. Among other structures, it contains the reticular formation, a centrally located, relatively uniform gray matter that lacks strict organization and boundaries, but is critical to the control of basal bodily activities, including cardiovascular tone, respiration, and consciousness. The tectum is the area located dorsal to the brainstem ventricular compartments, serving as their roof. Together, the tectum and tegmentum house most of the cranial nerve nuclei and also provide much of the integrative function of the brainstem.

The locations of cranial nerve nuclei display the same general pattern throughout the brainstem. Nuclei are located in the dorsal tegmentum in the vicinity of the fourth ventricle. Motor nuclei are located medially, sensory nuclei are located laterally, and the autonomic nuclei are found between them.

Functional considerations

The brainstem links the cerebrum, spinal cord, and cerebellum. Nerve tracts traveling through the brainstem relay signals from the cerebellum to parts of the cerebral cortex that are involved in motor control. This allows coordination of fine motor movements needed for many different activities. Functionally, the brainstem controls several important body functions. These include alertness, arousal, breathing, blood pressure regulation, digestion, heart rate, other autonomic functions, and relay of information between peripheral nerves and spinal cord to the upper parts of the brain. Therefore, when the brainstem is injured, there can be difficulties with mobility and coordination of movements. Walking, writing, eating, and many other activities may become very difficult. A brainstem stroke can destroy tissue required for the direction of respiration, heart rhythm, and swallowing. It can also affect hearing, speech, limb movement, and normal sensations throughout the body.

Section review

What are the three tracts of the corticospinal pathway?

In the corticospinal tracts, where do the axons synapse?

Which body functions are regulated by the brainstem?