Figure 53: Vascular Distribution and Anatomy Relevant to the Lateral Medullary (Wallenberg) Syndrome

This axial section of the medulla highlights those structures that, when damaged, are responsible for the vestibular and ocular motor features of the Wallenberg syndrome. The nucleus prepositus hypoglossi (NPH) and medial vestibular nucleus (MVN) complex is important for horizontal gaze-holding (neural integration). Fibers from the horizontal semicircular canals project to the MVN, thus providing an explanation for an abnormal ipsilateral head impulse test (HIT), which is occasionally seen with a lesion involving the vestibular nucleus. Gaze-evoked nystagmus and loss of the horizontal vestibulo-ocular reflex in Wernicke's encephalopathy can also be explained by NPH-MVN dysfunction. Fibers coming from the (left) peripheral utricle also synapse in the (left) vestibular nucleus, explaining why a skew deviation is common in a Wallenberg syndrome. With a left Wallenberg skew, the left hypotropic eye is predictably ipsilateral to the injury because the lesion is caudal to the pontomedullary decussation of the utricle ocular motor fibers (and the ocular tilt reaction [OTR] will be ipsiversive - e.g., head tilt, ocular counterroll are toward the left). If the same pathway (that originated in the left peripheral utricle) is affected rostral to the utricle fiber decussation (e.g., right medial longitudinal fasciculus or right interstitial nucleus of Cajal lesion), the right hypertropic eye is predictably ipsilateral to the injury ( causing a skew deviation with a right hypertropiawith a , and is also responsible for the hypertropic eye being ipsilateral to the injury rostral to its decussation (e.g., and the OTR will contraversive - e.g., head tilt, ocular counterroll are toward the left). Although various pathways exist from the utricle and semicircular canals to their respective ocular motor nuclei, many of these pathways travel through the medial longitudinal fasciculus (MLF, which is not yet carrying interneurons from the 6th nucleus). Involvement of the (left) inferior cerebellar peduncle (ICP) may result in asymmetry of the saccade facilitation/inhibition pathways, resulting in ipsilateral (left) ocular lateropulsion, ipsilateral (leftward) saccadic hypermetria and contralateral (rightward) saccadic hypometria in a (left) Wallenberg syndrome.