Unravelling the role of the insula in apraxia: A systematic review of VLSM studies in stroke patients

Apraxia refers to a variety of higher-order motor disorders not primarily caused by motor, sensory or cognitive impairment. Evaluation of apraxia typically involves gestural imitation, pantomime, and actual tool use. Lesion studies correlate the ability of movement control to cortical areas such as the inferior frontal gyrus, inferior parietal, and temporal lobes. Apraxia-related lesions often extend to subcortical regions, white matter, and the insula. Despite its involvement in several cognitive and sensorimotor-related processes, the insula's role in gesture execution remains unclear. Functional imaging revealed a role of the left anterior insula in tool recognition and object use. Individual experimental studies fail to explain the involvement of the insula in praxic abilities, however involvement of the right insula during gesture imitation was documented. Stroke affecting the middle cerebral artery frequently damages the insula, suggesting its secondary role in apraxia. We conducted a systematic review of studies in apraxic stroke patients which used VLSM. 31 of the papers identified through keyword searches met inclusion criteria. Consistent with the literature, most of the selected studies identify apraxia as a typical syndrome of LBD. The left insula has been found to be one of the most frequently impaired areas in patients who manifested apraxia after a stroke, particularly when assessed with pantomime and imitation tasks. Although the role of the insula is not always investigated and is sometimes indicated only in relation to language abilities, its structural and functional integrity seem closely related to the quality of praxic abilities.