J Neurosurg 88:718–725, 1998 718 J. Neurosurg. / Volume 88 / April, 1998 ARLY trigeminal evoked potentials (TEPs) can be recorded from the scalp after selective stimulation of the three trigeminal branches with electrodes inserted into the infraorbital, supraorbital, and mental foramina. 7,8,11 The TEPs consist of a series of short-dura- tion components, of which the first three are the most important because of their stability and consistency in both appearance and timing. 12 When obtained after stimu- lation of the infraorbital nerve, these components have been named W1 (0.9 msec), W2 (1.84 msec), and W3 (2.54 msec); 7,10 similarly, those obtained from the supraor- bital nerve have been named SW1 (0.95 msec), SW2 (2.2 msec), and SW3 (2.89 msec) 8 and those from the mental nerve have been named MW1 (1.9 msec), MW2 (2.75 msec), and MW3 (3.5 msec). 11 The origin of the respons- es obtained by stimulation of the infraorbital nerve has been established by simultaneous scalp and direct record- ings during percutaneous surgeries performed on the tri- geminal nerve 4 and by correlating changes in the TEPs’ morphology and latency with the presence of discrete lesions at various locations. 5,9 The sensitivity and localiza- tion power of TEPs have also been shown, and these char- acteristics have been used in the monitoring of thermo- lesioning procedures for trigeminal neuralgia. 6 Responses after stimulation of the infraorbital nerve are the easiest to obtain, because of their comparatively high amplitude and because the infraorbital foramen is larger than the others. However, the need for a comprehensive functional explo- ration of the trigeminal pathway demands that the re- sponses from supraorbital and mental nerves should also be thoroughly studied and their origins established. This is the main aim of this study. In addition, further evidence for the precise locations of generator sites of infraorbital TEPs has been obtained, conduction times and velocities have been precisely calculated, sources of possible errors in computing conduction velocities and in estimating the electrode position relative to the nerve have been identi- fied, and the basis for very accurate monitoring of scalp and direct responses during operations has been estab- lished. Materials and Methods The data presented here were collected during a 15-year period from a total of 64 patients who underwent thermorhizotomy and posterior fossa exploration for decompression of the trigeminal root. The recordings were made by the author at the Walton Centre for Neurology and Neurosurgery, Liverpool, United Kingdom, and at the Pain Relief Center of the National Institute for Cancer, Genoa, Italy. Approval of the procedures had been granted by the ethical committees of both institutions and informed consent was obtained from each patient. A portion of the data was collected retrospec- tively from recordings not expressly obtained for this study. Be- cause more than one nerve (infraorbital, supraorbital, and mental) could be stimulated in one patient, the total number of recordings on which this work is based is actually higher than the number of patients. The detailed number of recordings for each procedure is given in each Results section. Generator sites of early scalp potentials evoked from the three trigeminal branches MASSIMO LEANDRI, M.D. Interuniversity Center for Pain Neurophysiology, University of Genoa, Genoa, Italy Object. The aim of this study was to seek evidence about the generators of the first three components of the scalp’s early trigeminal evoked potentials (TEPs) obtained by stimulation of the supraorbital (SW1, SW2, and SW3), infraor- bital (W1, W2, and W3) and mental (MW1, MW2, and MW3) nerves. Methods. Simultaneous scalp and depth recordings were measured during surgical procedures in which thermorhi- zotomy and microvascular decompression were performed. Conclusions. Direct evidence was found that the origin of MW1 lies in the mandibular nerve at the foramen ovale, whereas the origin of W1 in the maxillary nerve at the foramen rotundum and the origin of SW1 in the ophthalmic nerve at the superior orbital fissure could only be inferred. The generators of SW2, W2, and MW2 were found to be on the nerve root at a distance of 10 mm from the pons. Calculations based on conduction velocity suggested that the generators of SW3, W3, and MW3 were inside the brainstem, at distances between 16 mm and 20 mm from the root entry zone. Recordings obtained in eight patients with discrete surgical lesions of the trigeminal pathway confirmed the sites of origin of the early components and further proved that only the fastest group of fibers is responsible for scalp re- sponses. KEY WORDS • generator site • trigeminal nerve • evoked potential E