American Board of Psychiatry and Neurology (ABPN) Practice Exam 2026 - Free ABPN Practice Questions and Study Guide

Pure motor hemiparesis typically arises from a lesion in the internal capsule, which is a critical pathway for motor fibers traveling between the cortex and the brainstem/spinal cord. This pathway contains descending corticospinal fibers that are essential for voluntary motor control. In cases where there is an infarction or injury in this area, it can disrupt the transmission of motor signals, resulting in weakness on one side of the body, which is characteristic of pure motor hemiparesis.

The internal capsule's organization is such that it carries motor information from the entire contralateral side of the body in a compact manner. Thus, damage here can lead to a more pronounced motor deficit without involvement of sensory pathways, as the sensory fibers travel in a different location and may not be affected by the same stroke.

In contrast, lesions in the midbrain, cerebellum, or thalamus may produce associated deficits, such as ataxia, sensory loss, or other complications that distinguish the hemiparesis as mixed or include sensory involvement. Thus, the specificity of pure motor hemiparesis as a result of strokes localized in the internal capsule makes it the most common association.