Introduction

Although clinical neurophysiological techniques can be applied to nearly all cranial nerves, this chapter will focus on those readily studied in standard EMG laboratories. Evoked potential studies of the optic and auditory nerves are discussed in Chapters 25 and 26, and only brief mention will be made of specialized techniques applied to extraocular and laryngeal muscles. Standard nerve conduction studies and EMG can assess the facial, trigeminal, spinal accessory, and hypoglossal nerves and the muscles they innervate. However, a few general caveats apply to the study of these cranial nerves. First, normal values for conduction studies are not as well-established as those for limb nerves and, therefore, side-to-side comparisons become particularly important. Second, the short or inaccessible extracranial course of some nerves prohibits direct assessment of conduction velocity. In such cases, reflex studies and latency measurements provide the next best option. Finally, EMG of cranial muscles requires a delicate approach and familiarity with specific motor unit potential (MUP) characteristics.

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