Motor apraxia is condition that interferes with the sufferer's ability to plan and perform voluntary movements with his or her body. This inability remains even when the sufferer understands how to perform the movement, wishes to move, and has no physical injury or disability in the impaired body part that would prevent him or her from moving normally. It is caused by damage to the cerebrum, a part of the brain involved in voluntary motor control.
There are several types of motor apraxia, with symptoms depending on the precise nature of the damage. It can affect areas of the body including the limbs, mouth, and eye sockets. It impairs the ability to properly perform motor tasks and in serious cases can interfere with a person's ability to move to such an extent that he or she is not able to live independently. Common causes include head trauma, stroke, and degenerative diseases in the nervous system.
Motor apraxia is often distinguished from verbal apraxia, or apraxia of speech, which affects the sufferer's ability to speak by interfering with the movement of his or her mouth while trying to form words. While verbal and motor apraxia are both caused by damage to the motor centers of the brain and often occur together, the latter term is commonly used to refer specifically to non-speech impairments of voluntary motor control. Bruns ataxia, a disorder that interferes with walking due to lack of muscle coordination, is sometimes called gait apraxia but is actually an unrelated condition.
Several types of apraxia are defined according to the part of the body they affect. Limb-kinetic apraxia interferes with a person's ability to make precise movements with her arms, legs, and fingers. This can affect both gross motor skills, such as walking, and fine movements, such as the ability to button a shirt or tie knots.
Buccofacial apraxia affects voluntary facial movements, such as winking. Ocular motor apraxia interferes with eye movements and the ability to rapidly move the eyes, making it difficult for the sufferer to follow moving objects or control the direction he looks in without turning his entire head. Motor-oral apraxia interferes with non-verbal oral movements, such as chewing. This is a distinct condition from verbal apraxia, and although they often occur together, it is possible to have one and not the other.
Ideomotor apraxia is a form of motor apraxia that interferes with hand gestures and tool use and especially with the ability to mimic or pantomime these actions when instructed to do so. For example, if a motor apraxia sufferer is given a toothbrush and told to pretend to use it or shown a gesture and told to imitate it, his or her ability to do so accurately will be impaired. In many people with ideomotor apraxia, their ability to spontaneously perform these tasks on their own initiative is intact.
For example, a sufferer may be able to raise her arm to get the attention of a waiter in a restaurant or hold and use a toothbrush normally while actually brushing her teeth, but lose the ability to do so when asked to raise her arm or pretend to brush her teeth by someone else. The disorder can take forms such as clumsy or inaccurate movement, slowness, or the inability to hold an object correctly. Someone suffering from ideomotor apraxia may even attempt to perform a different, inappropriate task. For example, a person may respond to instructions to mime brushing his teeth by attempting to use the toothbrush to comb his hair or write as if it were a pen. Despite what this may seem to suggest, the problem is not caused by an inability to understand the instructions, but by the nervous system's inability to turn conscious intent into specific muscle movements.
Many sufferers are seriously impaired in their ability to use their hands to operate tools or make gestures, even when acting spontaneously, and people who suffer significant impairment only when acting on instructions may still suffer minor deficits in some aspects of spontaneous motor control. Ideomotor apraxia is usually the result of a brain lesion caused by an interruption of blood supply or ischemia, most commonly due to a stroke. Lesions in many areas have been seen in different ideomotor apraxia patients, with the premotor and parietal areas of the left hemisphere being the most common. Ideomotor apraxia can also have other causes, such as Alzheimer's disease, Parkinson's disease, and other degenerative disorders of the nervous system.