Lesions of the Brainstem

Abstract

The effects of a brain stem lesion depend upon the anatomic and physiologic roles of the neurons and tracts damaged. Tracts passing through or commencing within the brain stem generally are oriented in a plane parallel to the longitudinal axis of the brain stem. Examples are the spinothalamic tract, medial lemniscus, and corticospinal tract. In contrast, motor cranial nerve nuclei are restricted to a single subdivision of the brain stem and their intramedullary roots course in a plane perpendicular to the longitudinal axis. Tract signs alone are of limited value in determining the level of injury. For example, in the case of a lesion involving the corticospinal tract, the spinal cord cannot be excluded. Likewise, cranial nerve signs by themselves may be insufficient to determine whether the lesion is in the brain stem or peripheral nerve. However, combining the signs of long tract involvement and cranial nerve impairment provides the framework to determine the precise site of a lesion. Thus, for this purpose, it is important to know the longitudinal and transverse organization of the brain stem.