At the base of the brain (at the top of the spinal cord) lies the brain stem. In evolutionary terms, this is the most ancient part of the central nervous system. The brain stem maintains basic life support functions: breathing, heart rate, blood pressure and digestion. An extensive stroke affecting the brain stem is usually fatal; when patients do survive, some sort of artificial life support is often necessary. Since the brain stem also helps maintain consciousness, many patients with a major stroke in this area fall into a coma. A coma can also result when a stroke in the cerebrum, which surrounds the brain stem, causes swelling that puts pressure on the brain stem. (Many small brain-stem strokes are now detectable on MRI scans, however, and these patients do well.) Above the brain stem is the cerebellum, which controls coordination, balance and posture. Early symptoms of a stroke in the cerebellum include dizziness (vertigo), nausea and vomiting. Later symptoms are clumsiness, shaking or difficulty controlling certain muscles. Above this part of the brain is a group of structures known as the limbic system, which we share in common with all other mammals. The limbic system is responsible for the primal urges and powerful emotions that ensure self-preservation: rage, terror, hunger, and sexual desire. Growth and reproductive cycles also are governed by the limbic system. Strokes in this area are rare, but when they do occur, basic animal drives may be severely limited or, conversely, patients may lose their natural inhibitions. Surrounding these more primitive brain structures is the cerebrum, the largest portion of the brain in humans and a common site of strokes. Its convoluted outer layer of gray matter, known as the cortex, is the seat of conscious thought, perception, voluntary movement and integration of all sensory inputs. A unique feature of mammalian brains is the division of the cerebral cortex into two halves, or hemispheres, each responsible for a different set of duties. In most people, the right hemisphere specializes in matters of spatial relationships, color perception, visual interpretation and musical aptitude. The left half of the brain typically oversees analytical tasks such as mathematical computation and logical reasoning; in right-handed people, the left hemisphere possesses particular regions dedicated to linguistic tasks, such as comprehending words and formulating speech. The two hemispheres constantly communicate with one another via a thick neural connecting cable known as the corpus callosum. Each hemisphere governs movement and sensory perception on the opposite side of the body. Therefore, a stroke in the left hemisphere can result in paralysis on the right side of the body. Each hemisphere of the cerebral cortex is further subdivided into four distinct sections, referred to as lobes. The frontal lobe, as its name suggests, is situated at the front of the brain, behind the brow. Among other things, this area deals with motor function; that is, its neurons send signals that initiate muscle activity throughout the body. Damage to one side of the brain in a specific part of the frontal lobe called the motor cortex results in weakness or paralysis somewhere on the opposite side of the body. In addition to paralysis of the limbs and torso, muscles on one side of the face or mouth may be affected, altering the person's appearance or ability to speak clearly (a condition known as dysarthria). Difficulty in speaking, writing or gesturing is known as expressive aphasia. It can result when a stroke affects the frontal lobe on the dominant side (for example, the left side of someone who is right-handed). The frontal lobe manages more abstract types of movement as well, including activities that require sequential steps. Consequently, a stroke may make it difficult or impossible to carry out a complex task, such as preparing a meal. Finally, the foremost portion of the frontal lobe governs highly abstract processes such as insight, initiative and social inhibitions. A stroke here could result in uncharacteristically impulsive or uninhibited behavior. On the other hand, profound apathy, lethargy and a lack of intentional behavior may result; this condition is known as abulia. Behind the frontal lobe, the parietal lobe is dedicated to receiving and interpreting sensory input from all parts of the body. Common problems resulting from parietal strokes are sensory loss, numbness and visual loss on the side of the body opposite the damaged hemisphere. Damage to the highly specialized sensory cortex may result in agnosia, leaving the stroke survivor unable to interpret incoming visual, auditory or tactile stimuli, even though the senses of vision, hearing and touch are mechanically intact and function normally. Another common consequence of a stroke involving the parietal lobe is a phenomenon known as neglect. Patients exhibiting neglect will typically stop perceiving or acknowledging events, and even sensations, on the side of their body opposite the affected cerebral hemisphere. (Patients with a stroke in the right side of the brain are particularly prone to neglect their left side.) The temporal lobe is situated at ear level, underneath both the parietal and frontal lobes. It is dedicated to hearing, auditory perception and the storage of memories. Strokes in the temporal lobe only rarely cause hearing loss; however, they commonly result in language deficits known as aphasia, a term referring collectively to problems understanding speech, verbalizing thoughts, reading or writing. Memory loss is also a common consequence of stroke in this brain region. However, memory deficits may be only temporary, since the temporal lobe on the other side of the brain can eventually compensate (unless, of course, both sides of the brain have been affected). The occipital lobe lies at the rear of the cerebral cortex, in the back of the skull. It is dedicated entirely to the perception and interpretation of visual data delivered from the eyes via the optic nerves. A stroke in the right side of the occipital lobe does not cause blindness in the left eye; instead, it causes hemianopia, which leaves the victim blind to the left field of vision, or “half blind” in both eyes, while the right field of vision remains normal in both eyes. (The patient may interpret this as a loss of vision in one eye only.) An occipital lobe stroke can also result in loss of the ability to recognize and interpret visual stimuli, such as faces. In addition to the deficits covered above, a stroke may produce other long-term complications, including seizures, impaired concentration, poor judgment, erratic sleep cycles, loss of libido, emotional instability and depression. Immobility following a stroke may lead to aspiration pneumonia (inhalation of food and other particles into the lungs due to an inability to swallow and cough properly), bedsores, deep vein thrombosis (formation of painful blood clots in the legs, also known as thrombophlebitis), limb contractures (tightening of the muscles in the limbs), incontinence and urinary tract infection. Last Updated: 4/14/2003
The Johns Hopkins University 1996-2003. All rights reserved. This information is not intended to provide advice on personal medical matters, nor is it intended to be a substitute for consultation. |
