Pathophysiology of Apraxia

Apraxia is a syndrome reflecting motor system dysfunction at the cortical level, exclusive of primary motor cortex. In planning movements, previously learned, stored complex representations of skilled movements are used. These 3-dimensional, supramodal codes, also called representations or movement formulae, are stored in the inferior parietal lobule of the left hemisphere. Diseases that involve this part of the brain, including strokes, dementias, and tumors, can cause loss of knowledge about how to perform skilled movements.

Apraxia can occur with lesions in other locations as well. Information contained in praxis representations is transcoded into innervatory patterns by the premotor cortices, including the supplementary motor area (SMA) and possibly the convexity of the premotor cortex; the information is then transmitted to the primary motor cortex and a movement is performed. Lesions of the SMA or other premotor cortices also can cause apraxia; in this case, knowledge about movement is still present, but the ability to perform movement is absent.

Apraxia also occurs with lesions of the corpus callosum, such as tumors or anterior cerebral artery strokes. Although the corpus callosum is not known to be involved directly in the performance of skilled movements, it contains crossing fibers from the right hemisphere to the premotor cortex. This type of apraxia represents a classic disconnection syndrome; patients with callosal apraxia typically are apractic only with the left hand.