What is the approach to localizing a brain stem lesion?
As a consequence of the unique anatomic arrangements in the brain stem, a unilateral lesion within this structure often causes “crossed syndromes,” in which ipsilateral dysfunction of one or more cranial nerves is accompanied by hemiplegia and/or hemisensory loss on the contralateral body.
Exquisite localization of a brain stem lesion depends on signs of long-tract (corticospinal and spinothalamic pathways) dysfunction to identify the lesion in the longitudinal (or sagittal) plane and on signs of cranial nerve dysfunction to establish its position in the cross-sectional (or axial) plane.
Localization of disorders of the brain stem can be simplified by summarizing the patient’s neurologic deficits to answer the following questions: Is the lesion affecting unilateral or bilateral structures of the brain stem? What is the level of the lesion? If the lesion is unilateral, is it medial or lateral in the brain stem?