sections include: mechanisms, diagnosis and treatment
“What a piece of work is a man,” Shakespeare wrote, “in form and moving how express and admirable.” Our brains and bodies can perform an almost infinite variety of movements, from pounding a heavy hammer on a narrow spike, to graceful high dives through the air, to picking up and turning one thin page of a book without tearing it. Through our motor systems, we interact with and change our environment. But the corticospinal, or pyramidal, motor system, from the brain to the nerves to the muscles, needs instructions to work well.
The directions for the pyramidal system reside in the brain in what we call praxis programs. You can also think of these as your “how to” system. They provide such instructions to your motor system as:
- How to select the tools and actions you need to solve a mechanical problem.
- How to position your hands to hold and use the tools.
- How to move your limbs and body in or through space; these movements require both knowing where you are (egocentric information) and where your target is (allocentric information).
- How rapidly to move.
- How much force to apply.
- How to order certain movements to achieve a goal.
We develop these programs largely through practice on small tasks, especially in the early months and years of life, until we no longer have to think about them except when faced with a very new challenge. But when areas of the brain suffer damage, the programs may be lost. Disorders of this “how to” system are called apraxias.
We use our arms and legs most often to carry out tasks, so apraxias are most apparent when they interfere with limb movement. There are two major groupings of limb apraxias: general problems, and those interfering with specific tasks. Of the five major forms of general limb apraxia, each is defined by the nature of errors that a person makes, and each has a different neuropsychological mechanism. They are:
- limb-kinetic: the person suffers a loss of deftness
- ideomotor: the person makes errors involving space and time
- dissociative: the person can correctly imitate actions, but cannot correctly perform them as described verbally
- ideational: the person cannot perform a series of acts leading to a goal
- conceptual: the person makes content errors and loses mechanical knowledge such that he or she may be unable to use alternative tools or solve novel problems
Three other forms of limb apraxia are task-specific. People with apraxic agraphia have problems printing or writing letters, but as long as they have no other language disturbances they may still be able to spell aloud correctly and type. People with constructional apraxia have problems with drawing or copying pictures. Those with dressing apraxia have trouble dressing themselves. The specificity of these problems indicates that each task—writing, drawing pictures, dressing—is coordinated by a particular area of the brain.
In general, performing any skilled action requires at least four levels of processing:
- conceptual-semantic knowledge—basically, knowing what you want to accomplish, and how it might be done
- spatial-temporal information—knowing how to hold tools or objects you need to work with and move these tools to accomplish their desired action
- development of motor or innervatory programs— being able to coordinate your bodily movements through nervous system signals
- motor activation—being able to start and control those movements
Each of the apraxic disorders discussed above is related to dysfunction at one or more of these levels of action programming.
The most common disorders that cause apraxia are strokes (ischemic, hemorrhagic) and such degenerative diseases as Alzheimer’s or corticobasal degeneration. However, any disease that produces cortical dysfunction, including brain tumors and trauma, may cause apraxia.
Different apraxias are associated with damage to different parts of the brain. Limb-kinetic apraxia, for example, is usually associated with dysfunction of the motor or premotor cortex.
Ideomotor apraxia may be associated with injury to either the parietal lobe or the premotor cortex. People with parietal lesions may be impaired not only at gesturing, pantomiming, and working with tools but also at discriminating and comprehending other people’s gestures.
Individuals with dissociative apraxia have dysfunctions in the posterior hemisphere. Those with ideational apraxia often have widespread dysfunction, but when the problems are localized they are most likely to be in the frontal lobes. The locus of lesions that cause conceptual apraxia has not been fully determined.
Dressing apraxia is associated with right parietal lesions, apraxic agraphia with left frontal or parietal lesions, and constructional apraxia with left or right parietal lesions.
For right-handed people, most of these apraxias are likely to be associated with dysfunctions in the left hemisphere of the brain. However, dressing apraxia is more likely to arise from problems in the right hemisphere, and constructional apraxia may stem from dysfunction in either half.
Diagnosis and Treatment
Apraxia is not a disease in itself but rather a sign of some other, underlying disease causing damage to the brain. Therefore, when people develop the symptoms of apraxia, neurologists must evaluate them, diagnose the underlying diseases, and, when possible, start treatment. Those treatments are discussed in the relevant sections of this book.
People with apraxia from static deficits—meaning problems that are not getting worse—can often be retrained to perform the same tasks using undamaged areas of their brains. Unfortunately, this type of rehabilitation program is not widely available.
Many activities of daily living, such as cooking, building, and fixing things, require people to perform skilled acts. An apraxic disorder can therefore be terribly disabling. Furthermore, in order to compensate for a disability or avoid problems caused by it, a person must know that he or she is disabled, and often people with apraxia do not recognize their limitations. Caregivers must therefore be certain that patients with apraxia do not attempt tasks that may cause them to injure themselves or others.
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