298

J. Dent. 1992;

Trigeminal potentials:

somatosensoty evoked a normal value study

M. A. Pogrel, D. Mouhabaty, Departments

20: 298-301

T. Dodson, I. Rampil and M. Grecco

of Oral and Maxillofacial

Surgery and Anesthesia,

University

of California, San Francisco,

USA

ABSTRACT Normal somatosensory evoked potentials

were obtained from the lower lip of40 volunteers. Efforts were made to exclude artefact. Aconsistent triphasic wave form ofthree peaks and troughs was defined. There were greater variations in amplitude than latency between subjects. Statistically, one side of the lower lip can be used as a control for the contralateral side, but it may not be possible to have reliable normal values between subjects. Somatosensory evoked potentials may however represent an objective method of evaluating trigeminal sensory nerve function. KEY WORDS:

Somatosensory

J. Dent. 1992; April 1992)

20: 298-301

evoked potentials, Trigeminal (Received

12 December

nerve, Inferior alveolar nerve

199 1; reviewed

13 February 1992;

accepted 27

Correspondence should be addressed to: Dr M. A. Pogrel, Department of Oral and Maxillofacial Surgery, Box 0440,

University of California, San Francisco, CA 94 143-0440,

INTRODUCTION Neuropraxia, axonotmesis and neurotmesis of the inferior alveolar and infraorbital nerves are responsible for considerable morbidity in dentistry and oral and maxillofacial surgery, following trauma (Schwartz, 1973; Girard, 1979), wisdom tooth removal (VanGool et al., 1977; Merrill, 1979; Kipp et al., 1980; Goldberg et al., 1985; Osborn et al., 1985; Alling, 1986; Wofford and Miller, 1987), and orthognathic surgery (Walter and Gregg, 1979; Zaytoun et al., 1986). Damage to the lingual nerve also occurs following lower third molar removal (Schwartz, 1973; VanGool et al., 1977; Rud, 1984; Mason, 1988). Attempts have been made to carry out microneurosurgical repair and nerve grafting procedures on the sensory branches of the trigeminal nerve in order to restore continuity and thereby restore nerve function (Mozsary and Syers, 1985), but disagreement exists on the timing and success of these procedures (Donoff and Guralnick, 1982; Wessberg et al., 1982; Rood, 1983; Mozsary and Middleton, 1984). The methods presently used to evaluate trigeminal sensory nerve function rely almost totally on subjective testing (Walter and Gregg, 1979; Campbell et al., 1987; Robinson, 1988). In order to accurately evaluate trigeminal nerve damage and the response to various treatment @ 1992 Butterworth-Heinemann 0300-5712/92/050298-04

Ltd.

USA

modalities, a reproducible objective means of testing trigeminal nerve function is desirable. This study investigates the use of somatosensory evoked potentials (SEP) as an objective monitor of nerve function. Since their first description in 1875 (Caton, 1875), there has been a vast literature regarding SEPs (Chiappa, 1983; Nuwer, 1986; Aminoff, 1987,1988). However, the response elicited from the trigeminal nerve has only rarely been studied. Evoked potentials of the dental pulp have been monitored using intradentinal or intrapulpal electrodes (Chatrian et al., 1982; Femandes de Lima et al., 1982), and similarly evoked potentials from the gingiva appear to be obtainable (Bennett and Jannetta, 1980). The tooth pulp has been utilized in research protocols since it displays ‘an all or none’ response (i.e. the dental pulp can only respond with pain and it has a well-defined threshold). There are a small number of papers describing SEPs obtained from the upper lip (Drechsler, 1980; Chapmanet al., 1985), the lower lip (Findler and Feinsod, 1982; Barker et al., 1987b; Godfrey and Mitchell, 1987; LeBanc et al., 1987) and the tongue (Ishiko et al., 1980). The blocking effect of lidocaine injection on the SEP obtained from direct stimulation of teeth has also been described (Gehrig et al., 1981). In addition, the normal trigeminal evoked potential (Stohr and Petruch, 1979; Barker et al.,

Pogrel et al. : Trigeminal somatosensory evoked potentials

1987a; Seki, 1987; Fagade and Wastell, 1990) has been described. There is little consistency in the results obtained from these studies and in many cases a number of possible variables involved are not discussed, making comparisons between different studies difficult.

MATERIALS

AND

METHODS

Forty volunteers, ranging in age from 22 to 43 years (mean age 27 years), underwent testing. Subjects with positive medical histories or taking medications known to alter nerve conduction velocity or EEGs were disqualified, as were subjects with existing dental pathology, or a history of mandibular nerve block within the previous 6 weeks. Stimuli consisted of bipolar pulses of 100 us duration applied at a rate of 3.1 stimuli per second to the vermillion border of the lower lip. Nicolet (Nicolet Biomedical, Madison, WI, USA) paediatric somatosensory stimulating electrodes with an interelectrode distance of 14 mm were used. The stimulus intensity was set at two times sensory threshold. Intensities ranged from 6 to 18 mA (mean = 10.8 mA). Intensities were below motor threshold in all cases. Because latencies are short, a near-field configuration of receiving electrodes is appropriate since the electrode can be placed immediately over the appropriate sensory cortex. The stimuli were generated and the data collected and averaged using a Nicolet Pathfinder II (Nicolet). The bandpass was set to 5-1000 Hz. Sensitivity was 200 mV. A notch filter and a high frequency digital smoothing filter were used. The recording period was set to 50 ms to encompass the primary overload potential response. Six sets of average data were collected. Each set consisted of 256 accepted stimulations of the lower lip. Three sets of data were collected from the right side of the lip and three sets from the left. The resultant cerebral activity was recorded via 30 gauge, 12 mm subdermal platinum recording electrodes placed in the scalp at C3’ and C4’ (i.e. over the corresponding primary sensory cortex) with a reference electrode at CZ on the lo-20 electrode system (Jasper, 1958). Recordings were made from the contralateral recording electrodes only. Subjects were grounded over the left brachioradialis muscle. Electrode contact impedances were maintained at or below 5 ka. Since the stimulus electrodes are so close to the recording electrodes, amplifier saturation artefact was felt to be a significant potential problem and may have been responsible for some of the inconsistences in previously reported results. To that end, we used a lowered filter at 5 Hz and did the following tests: 1. The ipsilateral lower gingiva was stimulated instead of the lip. This should rule out any muscle artefact since the gingiva does not contain muscle. The evoked potential recorded was identical to that from the lip. 2. The possibility of 60 Hz artefact being recorded was investigated by using the recording electrodes as an

299

antenna near to power points and light sources, etc. This did not produce any data. 3. The possibility of conduction being via skin or muscle and not via nerve was investigated by placing the stimulating and recording electrodes in the same contiguration as on the skull but on a large piece of beef steak. All stimuli were rejected as artefact. 4. Ipsilateral recordings were attempted and most responses were rejected as artefact and those which were accepted had long latency periods with low amplitude and no fixed pattern. The database was constructed and managed using an IBM-PC clone and Lotus l-2-3 (Cambridge, MA, USA). Subsequent data analysis used Statistical Analysis Systems version 6.06 (Cat-y, NC, USA). The mean latency and amplitude with associated standard deviations and 95% confidence intervals were computed for each of the three peaks and troughs of the triphasic wave generated by stimulating the mental nerve for each study patient. In addition, the mean difference between the right and left mental nerve with the associated standard deviations was computed for each peak and trough of the generated triphasic wave. A paired t test was used to determine if the mean difference between the right and left sides was equal to zero.

RESULTS Forty volunteers with a mean age of 27 years (range 22-43 years) enrolled in the study. There were 46% males and 54% females. In all cases, a triphasic waveform was produced. The waveform consisted of three peaks and troughs (negative and positive deflections) preceded by a high amplitude spike representing the applied stimulus (Fig I). The total time-course (75 ms) is the interval between stimulus to the lip and response at the primary sensory cortex. Data are summarized in Tables I and II. The mean latency (ms) for each peak (Nl, N2, N3) and each trough (P2, P3) with their associated standard deviations and 95%

I

I

I

I

I

I

I

15

30 Time

I

45

1

I

,

60

(ms)

Fig. 1. The mean of three SEPs (three runs of 256 accepted potentials) from the lower lip. The high amplitude spike at time zero is the stimulus artefact.

300

J. Dent. 1992; 20: No. 5

Tab/e II. Amplitude

Table 1. Latency (milliseconds) Variable

Mean

N, PI N, P, N,

8.39 k 0.65 15.24 + 0.7 1 24.8 1 + 1.02 32.06 + 1.09 41.30 -t 1.26

*Sample

+ s.d. *

Mean differ (R vs L) -0.01 0.08 0.30 0.09 0.53

+ + f f f

0.99 1.39 2.1 1 2.36 1.97

P-value t 0.93 0.69 0.38 0.81 0.09

size = 40.

t Paired t test.

confidence intervals are summarized in Table I. The mean latencies for each peak and trough varied significantly among individual volunteers. The mean differences in the latencies between the right and left sides are also presented in Table I. The mean differences for each peak and trough did not differ significantly from zero. Similar descriptive statistics were computed for the amplitude for each peak and trough, and they are presented in Table II. The mean amplitudes for each peak and trough varied significantly among the volunteers. The mean differences between right and left sides did not differ significantly from zero.

DISCUSSION The consistent and reproducible morphology of the waveform and the high degree of concordance, with regard to latency and amplitude, of the two sides of a given individual are strong evidence that the data being recorded is the evoked potential of the inferior alveolar nerve. Overall, latencies showed considerable variation among individuals. However, since latencies between the right and left sides of an individual were not significantly different, it appears that one side of the lower lip can be compared with the contralateral side, and each patient may serve as their own control in cases of unilateral nerve injury. Mean amplitude was variable among individuals, and within the limits of this study it varied proportionally with stimulus intensity, with individual subjects showing high concordance between the right and left sides. As with other nerve studies, latency may be expected to represent a more meaningful basis for comparison than the amplitude. Overal latencies and amplitudes vary significantly among individuals possibly due to differing sizes and weights. When other published studies are examined, there are a small number of triphasic patterns from the trigeminal nerve with similar latency periods and, although other studies show somewhat different patterns, many of them do show events at the same latency periods (Findler and Feinsod, 1982; Seki, 1987; Stohr and Petruch, 1987). With the accumulation of further data, SEP testing may prove to be an objective means of assessing sensory nerve damage and serve as a valuable diagnostic and prognostic

Variable N, PI N* P, N,

Mean 0.14 -2.51 0.94 - 1.27 1.77

(microvolts) + s.d. * + + Ik f k

0.53 0.79 0.44 0.69 0.82

Mean differ (R vs L) 0.15 0.03 0.1 1 0.009 -0.20

k + kf I!I

P-value t

0.67 1.08 0.76 0.092 1.48

0.17 0.84 0.92 0.95 0.39

*Sample size = 40. t Paired t test.

tool in oral and maxillofacial surgery, if one side of the subject can act as control for the affected side.

CONCLUSIONS 1. A consistent pattern of brain electrical activity can be recorded at the surface of the scalp following stimulation of the lower lip using SEP testing. 2. One side of the lower lip can act as a normal control for the contralateral side with regard to both latency and amplitude of the SEP waveform. 3. SEPs may represent an objective, non-patientdependent means of evaluating sensory nerve function in the maxillofacial region.

Acknowledgements This study was supported by a University Academic Senate Research Grant.

of California,

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