The brain, cushioned by watery fluid on all sides, resides inside the skull. It resembles a fleshy, shelled walnut in shape. Like a walnut, it has two distinct but connected halves. These are called the left and right cerebral hemispheres. Although the hemispheres appear to be mirror images of each other, the similar-looking parts of each do different things.

Like a walnut’s exterior, the brain’s surface folds in and out. These wrinkles increase the surface area of the brain. Its surface is divided by especially deep grooves, called fissures or sulci. The brain contains major nerves—such as the optic nerves, which connect it to the eyes. There are also nerves that control movement of arms and legs, the ability to maintain a heart rate, and level of awareness. Blood vessels carry oxygen and nutrients to all parts of the brain and spinal cord, and channels called aqueducts allow the watery fluid (called cerebrospinal fluid [CSF]) to move throughout the brain and spinal cord (central nervous system or CNS).

The brain has several protective coverings that prevent injury. The bones of the skull and the spine provide the outermost protection. Inside the skull are three thin membranes called the meninges, which surround the brain and spinal cord. The outer meninge is called the dura mater, the inner meninge is the pia mater, and the middle meninge, which carries the blood vessels, is called the arachnoid (see Figure 2–1). The brain and spinal cord are also protected by the CSF that flows between the layers of meninges.

Figure 2–1: The meninges

Cerebrum

The largest region in the brain is the cerebrum (also called the supratentorial region). It is made up of two cerebral hemispheres (left and right). The two hemispheres are separated by a large groove, called the cerebral fissure. Deep within the cerebral fissure is a bundle of nerve fibers called the corpus callosum, which transmits information between the two sides of the cerebrum.

The cerebrum interprets sensory input from all parts of the body and also controls body movements. It is the part of the brain responsible for thinking, reasoning, learning, controlling movement, and processing emotions and memories.

The cerebrum is divided into four areas (called lobes) on each side of the brain: the frontal, temporal, parietal, and occipital lobes. The corpus callosum connects the two parts of each lobe on both sides of the brain. Structures on one side of the brain control the opposite side of the body. For example, any movement of the arms and legs on the right side of the body is controlled by the left cerebral hemisphere.

Your child’s doctors will attempt to determine which side of your child’s brain is dominant. Dominance is important for tumors that are near the hearing or speech and language processing areas. It may be difficult for the surgeon to remove all of the tumor on a dominant side in these areas without damaging speech or hearing. Pre-operative testing (functional MRI scans) and intraoperative monitoring (mapping) are vital when dealing with tumors in the dominant hemisphere. These tools, discussed in Chapter 6, Coping with Procedures, and Chapter 10, Surgery, allow the surgeon to remove as much tumor as possible while preserving function. The parts of the cerebrum are shown in Figure 2–2.

Frontal lobes

The frontal lobes, located directly behind your forehead, are your brain’s main planning and personality centers. They process and store information that helps you think ahead, use strategy, and respond to events based on past experiences and other knowledge. A small part of the frontal lobe is involved in articulating speech. Another small strip of the frontal lobe helps control movement. Malfunctions in the frontal lobe may lead to poor planning, impulsiveness, and certain types of speech problems. Symptoms are more pronounced if the tumor crosses the corpus callosum and affects both frontal lobes. Symptoms of frontal lobe tumors include:

• Seizures
• Changes in ability to concentrate
• Poor school performance
• Changes in social behavior and personality

Figure 2–2: Basic brain anatomy

Toward the back section of the frontal lobe is the motor cortex, the part of the brain that controls movement of the head and body parts on the opposite side of the body. Because the location of motor nerves varies a little in each person, mapping of the motor cortex using a functional MRI, and using this information while monitoring during surgery, helps surgeons know the exact location of these nerves so as not to injure them during tumor removal. For more information, see the “MRI” section of Chapter 6, Coping with Procedures, and the “Intraoperative monitoring” section of Chapter 10, Surgery.

Temporal lobes

The temporal lobes are located at the sides of the brain. Hearing, memory, and speech and language processing are managed by the temporal lobes. They are the places where information taken in from the various senses is integrated, permitting complex thoughts, movements, and sensations to be formulated and acted upon. Within the temporal lobes is the amygdala, which controls social behavior, aggression, and excitement. The hippo-campus, located in the temporal lobes, is involved in storing memories of recent events. Depth perception and sense of time are also controlled by the temporal lobes.

Tumors in the temporal lobes can cause seizures and changes in behavior. When a tumor grows in the temporal lobes, the brain has a hard time filtering out extra information, and sensory information and memories may start to blend together in unfamiliar ways. Sounds may be perceived as having colors, for example, or your child may have an unsettling feeling of déjà vu.

Figure 2–3: MRI showing temporal lobe tumor

Parietal lobes

Directly behind the frontal lobes and above the temporal lobes are the parietal lobes. In some people, the motor cortex, which controls arm and leg movement, extends into the parietal lobes. The parietal lobes process all sorts of sensory information coming in from the body, including data about temperature, pain, and taste. The parietal lobes also control language and the ability to do arithmetic. When the parietal lobes are not functioning properly, sensory information is not processed correctly, and your child may have a hard time making sense of her environment.

The rear of each parietal lobe, next to the temporal lobe, has an area important in processing the auditory and visual information needed for language. When children have tumors in these lobes, they do not understand what is being said when someone speaks to them.

Abnormal movements or weakness in the arms and legs, memory problems, language problems, and seizures are associated with tumors in the parietal lobes. During a seizure affecting these areas, strange physical sensations may be felt, such as a crushing pressure or a tingling feeling in part of the body.

Occipital lobes

The occipital lobes serve as the visual centers of the brain. They are responsible for making sense of the information that comes to the brain from the eyes through the optic nerves. The left occipital lobe deals with input from the right eye, and the right lobe deals with input from the left eye. Tumors in the occipital lobes are associated with visual field cut (loss of central or peripheral vision) on one side or the other.

Posterior fossa

The posterior fossa (also called the infratentorium) is located at the very back of the brain, on top of the spinal cord. It includes the cerebellum, the brainstem, and the fourth ventricle (see the “Ventricular system” section and Figure 2–6 later in the chapter).

Figure 2–4: Side view of the brain

Sixty percent of all childhood brain tumors originate in the posterior fossa. Symptoms of tumors in this area of the brain include:

• Signs of increased intracranial pressure (headache, vomiting, unsteady gait, double vision, sleepiness, or lethargy)
• Weakness of cranial nerves (visual or hearing problems, weakness or drooping on one side of the face, eye movement problems, difficulty swallowing, difficulty breathing)
• Unsteady gait (ataxia)

Cerebellum

The cerebellum is about one-eighth the size of the cerebrum. Tumors growing in the cerebellum can cause coordination and balance problems. The child may have an uneven walking pattern or may continually fall to one side. Difficulty judging distances or reaching for and grabbing an object are symptoms of a tumor in this area.

Brainstem

The brainstem is the relay center for transmitting messages between the brain and other parts of the body. All sensory information from the body goes through the brainstem on its way to the rest of the brain, and all motor messages from the brain to the rest of the body travel through the brainstem. The three parts of the brainstem are:

• Midbrain, which processes vision and hearing and coordinates sleep and wake cycles
• Pons, which controls eye and facial movements and links the cerebellum to the cerebrum
• Medulla, which controls breathing, swallowing, heart rate, and blood pressure

The 12 cranial nerves originate in the brainstem, primarily in the pons and medulla. They control eye and facial movements, vision, taste, hearing, swallowing, and movement of the neck and shoulders.

Symptoms of tumors in the brainstem include:

• Abnormal eye movements or a droopy eyelid
• Drooping of the face or facial asymmetry
• Difficulty swallowing or breathing
• An uneven walking pattern
• Weakness on one or both sides of the body, often seen first in an arm and/or a leg

Diencephalon

Near the center of the brain is the diencephalon, which is made up of several tiny but extraordinarily important structures: the thalamus, the hypothalamus, the pituitary gland, and the pineal gland.

All sensory information passes through the thalamus before being sent to the forebrain for more advanced interpretation—except for information gathered via the sense of smell, which takes a different route. The hypothalamus is small, but it has a big role in managing everything from hunger to digestion to muscle contractions. It has direct control over the pituitary gland, which produces hormones and similar chemicals. The pineal gland helps to govern the body’s sense of time and rhythm, including regulation of the reproductive cycle.

Tumors that grow in the diencephalon cause:

• Disruption of hormone production
• Abnormal growth (usually delayed growth)
• Excessive thirst and frequent urination
• Drowsiness or changes in level of consciousness
• Difficulty with vision
• Memory problems
• Weakness of one or both sides of the body

Optic nerves

The optic nerves carry information from the eyes to the occipital lobes. The nerve from the right eye goes to the left occipital lobe, and the nerve from the left eye goes to the right occipital lobe. These two nerves cross at a place called the optic chiasm, near the hypothalamus. Tumors that affect the optic nerves cause changes in visual acuity (the ability to see) or visual fields (peripheral vision).