Walsh & Hoyt: Involvement of the Trigeminal Pathways in the Brainstem

The location of a brainstem lesion is usually indicated by distinctive combinations of cranial nerve palsies and long tract signs. We are concerned in this section with the diagnostic features of facial hypalgesia and hypesthesia that accompany accompany some of these brainstem syndromes. In most instances, facial sensation is impaired on the side of the lesion, but occasionally, sensory loss is present on the side opposite the lesion. In contrast to peripheral involvement of the trigeminal nerve, sensory loss from brainstem lesions is commonly dissociated. Thus, perception of pain and temperature may be profoundly affected whereas modalities such as light touch and pressure are spared. Sensory defects with peripheral lesions correspond precisely with the distribution of various divisions and branches of the trigeminal nerve, whereas sensory defects from trigeminal lesions in the brainstem may not adhere strictly to these classic subdivisions. Thus, in brainstem lesions, the sensory loss might involve one small area of the mandibular region or extend into an adjacent region subserved by the maxillary nerve. Correlation of these ""unorthodox"" sensory defects with the size and location of the lesion is far from exact, but certain guidelines have emerged from pathologic studies of acquired disease as well as from tractotomies performed for the relief of facial pain.