MR appearance of trigeminal and hypoglossal motor denervation

PURPOSE: To illustrate and describe the appearance of both long-standing and relatively recently occurring motor denervation of the hypoglossal nerve and of the third (mandibular) division of the trigeminal nerve (V3), with emphasis on findings particular to MR imaging. METHODS: Findings from 11 patients with V3 denervation and from seven patients with hypoglossal denervation resulting from a variety of abnormalities were reviewed retrospectively. The motor denervation appearance and functional compromise of the affected musculature are described in terms of the chronicity of the denervation process. RESULTS: The appearance of V3 and hypoglossal motor denervation varies with the chronicity of the process. Long-standing denervation results in extensive fatty replacement and a decrease in the size of the affected musculature. Relatively recently occurring denervation results in abnormal contrast enhancement and edemalike signal changes in the denervated musculature. Fatty replacement was observed acutely in hypoglossal denervation but did not manifest until the subacute stage in V3 denervation. Increased volume of the denervated musculature may also accompany acute denervation signal changes. CONCLUSION: V3 and hypoglossal denervation have a variable appearance depending on the chronicity of the process. Recognition of MR imaging patterns of denervation may allow earlier diagnosis of a denervating lesion and may help to distinguish denervation from similar- appearing processes, such as infection or neoplasia.