The term apraxia will be used here to mean inability to follow a motor command that is not due to a primary motor deficit or a language impairment. It is apparently caused by a deficit in higher-order planning or conceptualization of the motor task. You can test for apraxia by asking the patient to do complex tasks, using commands such as "Pretend to comb you hair" or "Pretend to strike a match and blow it out" and so on. Patients with apraxia perform awkward movements that only minimally resemble those requested, despite having intact comprehension and an otherwise normal motor exam. This kind of apraxia is sometimes called ideomotor apraxia. In some patients, rather than affecting the distal extremities, apraxia can involve primarily the mouth and face, or movements of the whole body, such as walking or turning around.
Unfortunately, the term "apraxia" has also been attached to a variety of other abnormalities—for example, "constructional apraxia" in patients who have visuospatial difficulty drawing complex figures, "ocular apraxia" in patients who have difficulty directing their gaze, "dressing apraxia" in patients who have difficulty getting dressed, and so on. It is unclear at present whether these various types of "apraxia" are related in some way, or are caused by completely different mechanisms.
Although apraxia indicates brain dysfunction, it can be caused by lesions in many different regions, so exact localization is often difficult. Apraxia is commonly present in lesions affecting the language areas and adjacent structures of the dominant hemisphere. This can make it challenging to prove that the deficit is apraxia rather than impaired language comprehension. Often we can make the distinction by asking the patient to perform a task and, if he fails, demonstrating several tasks and asking him to choose the correct one.