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The blink reflex and the corneal reflex are followed by cortical activity resembling the nociceptive potentials induced by trigeminal laser stimulation in man
Neuroscience Letters 310 (2001) 37±40
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The blink re¯ex and the corneal re¯ex are followed by cortical activity resembling the nociceptive potentials induced by trigeminal laser stimulation in man Marina de Tommaso*, Giuseppe Libro, Marco Guido, Vittorio Sciruicchio, Francomichele Puca Interuniversity Center for the Study of Headache and Neurotransmitter Disorders of the Central Nervous System, Perugia, Roma, Sassari, Bari, Napoli, Firenze, Italy Received 15 May 2001; received in revised form 21 June 2001; accepted 6 July 2001
Abstract Laser stimulation of the supraorbital regions evokes brain potentials (LEPs) related to trigeminal nociception. The aim of this study was to record the R2 component of the blink re¯ex and the corneal re¯ex in 20 normal subjects, comparing the scalp activity following these re¯exes with the nociceptive potentials evoked by CO2 laser stimulation of supraorbital regions. Cortical and muscular re¯exes evoked by stimulation of the ®rst trigeminal branch were recorded simultaneously. The R2 component of the blink re¯ex and the corneal re¯ex were followed by two cortical peaks, which resembled morphologically N-P waves of LEPs. The two peaks demonstrated a difference in latency of approximately 40 ms, which is consistent with activation time of nociception. This ®nding suggests that these re¯exes are induced by activation of small pain-related ®bers. q 2001 Elsevier Science Ireland Ltd. All rights reserved. Keywords: Blink re¯ex; Corneal re¯ex; Laser stimulation; Pain-related potentials
Recently, it was demonstrated that CO2 laser stimulation of mechano-thermal nociceptors in the perioral and supraorbital regions evokes brain potentials (LEPs) useful for detecting selective trigeminal nociception dysfunction [4,7]. The condition used for examination of cortical responses to small ®ber stimulation is subjective pain sensation. In the face, particularly in the supraorbital zone, pain produced by an electric or CO2 laser, or any type of stimulator, induces a blink re¯ex. Although this muscular activity may compromise the recording of genuine cortical activity, the amplitude and latency of the trigeminal nociceptive potentials generally enable the muscular re¯ex responses to be well recognized and seldom mistaken for cortical activity [4]. The R2 component of the blink re¯ex is assumed to be partly elicited by nociceptive Ad ®ber input [5,6]. However, some authors maintain that it is mainly a mechanoceptive * Corresponding author. Clinica Neurologica I Policlinico, Piazza Giulio Cesare 11, 70124 Bari, Italy. Tel.: 139-080-5478565; fax: 139-080-547-8532. E-mail address: [email protected] (M. de Tommaso).
re¯ex [1], and further studies attribute the nociceptive quality to a later response named R3 [8]. Nevertheless, electric stimulation of the cornea also evokes blinking responses, which should be purely nociceptive, as the cornea is innervated by small myelinated afferent ®bers [2]. Therefore, the aim of this study was to record the R2 blink re¯ex and corneal re¯ex in normal subjects, comparing the cortical activity induced by electric stimulation of the supraorbital nerve and cornea with the nociceptive potentials evoked by CO2 laser stimulation of the supraorbital regions. Twenty healthy subjects, ten women and ten men, 25±45 years old (mean age 32.2 ^ 5.5), were selected based on the absence of any current or previous internistic, neurologic or psychiatric illness, and recent intake of psychoactive and analgesic drugs. The subjects gave their consent to the study, which was approved by the local Ethics Committee. They were instructed to keep their eyes open throughout the study. Cortical and muscular re¯ex responses evoked by stimulation of the ®rst trigeminal branch were simultaneously recorded through disc electrodes (impedance less than 5 kV) placed at the vertex (Cz) and referred to linked ear lobes (A1A2). Electro-oculographic (supraorbital electrode) and electromyographic (EMG) recordings from the
0304-3940/01/$ - see front matter q 2001 Elsevier Science Ireland Ltd. All rights reserved. PII: S03 04 - 394 0( 0 1) 02 08 6- 9
38
M. de Tommaso et al. / Neuroscience Letters 310 (2001) 37±40
orbicularis oculi muscle were measured according to method of Cruccu et al. [4]. Each subject received electrical stimulation of the supraorbital nerve and cornea as well as CO2 laser stimulation of the skin above the eyebrow. The supraorbital nerve was stimulated by means of surface electrodes placed longitudinally on the nerve, 2 cm from emergence of the supraorbital nerve. Stimuli were square wave negative single pulses of 0.1 ms duration, delivered unilaterally by a constant current isolated unit; the stimulus intensity was gradually increased in 1 mA steps until a clear R2 component was detectable in at least three out of ®ve repetitions. The cornea was stimulated by a saline-soaked thread resting on the cornea [1], increasing electrical intensity in 0.1 mA steps, to obtain a clearly recognizable re¯ex from the orbicularis oculi muscle in at least three out of ®ve repetitions. For LEPs, a CO2 laser stimulator (Neurolas, Electronic engineering, Florence, Italy) was used. Brief radiant heat pulses were delivered at 4.5 W intensity, increasing the stimulus duration in 5 ms steps to reach a pinprick sensation in at least three out of ®ve stimuli (beam diameter, 2.5 mm, 5 mm 2; duration 5±30 ms). The skin above the eyebrow was stimulated, and laser beam was moved slightly for each stimulus to minimize the effects of skin burns, nociceptor fatigue, and central habituation [4]. Subjects were asked to count the laser and electric stimuli to avoid distraction; they received two series of about 50 electric stimuli for the cornea and the supraorbital nerve and 40 laser stimuli on both sides. EEG and EMG were sampled at a rate of 2048 cps for 1 s following stimulus onset. Signals were ampli®ed with cortical potentials and re¯ex responses ®ltered by 0.5±80 Hz and 50±3000 Hz bandpass, respectively. An interstimuli interval of at least 1 min was used for all types of stimulation, to avoid habituation of both
re¯ex and cortical responses. The responses to single trials were individually stored and subsequently reviewed for artifacts before averaging. Only responses in which cortical activity was well separated from blinking responses were selected, with each average consisting of 25 samples, repeated at least once to ensure reproducibility of the potentials. The blinking responses obtained by the orbicularis oculi electrode were superimposed rather than averaged. The differences in latencies and amplitudes between cortical responses evoked by the three kinds of stimulation were subjected to a repeated ± measures ANOVA with contrast analysis. The Spearman test was applied to evaluate the correlation between the latency of laser and electrical cortical potentials. Electrical stimulation provoked a well recognizable corneal re¯ex (mean intensity 0.5 ^ 0.25 mA on the right side and 0.55 ^ 0.18 mA on the left side) and blink re¯ex R2 component (mean intensity 12 ^ 4.5 mA on the right side and 13.5 ^ 6.2 mA on the left) in all subjects. In 15 subjects, the corneal stimulation evoked a large vertex response, consisting of a negative component with a latency of about 150 ms, followed by a positive peak at approximately 230 ms (Table 1; Fig. 1). A positive peak with a latency of around 85 ms was recognizable in 14 subjects. The mean duration of corneal re¯ex was 47 ^ 8.5 ms on the right and 46 ^ 7.6 ms on the left. Thus, it occurred at the beginning of the scalp response in a few cases, making the ®rst positive peak unclear. In ®ve subjects, a sustained closing of the eyes followed corneal stimulation on both sides, causing the cortical potential to be undetectable. The R2 component obtained by electrical stimulation of the supraorbital nerve was followed by a large scalp potential in 16 subjects. This consisted of a negative peak occurring at a
Table 1 Features of re¯ex responses and cortical waves by electric stimulation of cornea and supraorbital nerve and laser stimulation of the skin above the eyebrow in a group of normal subjects
Electric stimulation Cornea M SD
Blinking response N-wave latency (ms) latency (ms)
P-wave latency (ms)
Peak-to-peak amplitude (mV)
Laser/Electric N-wave latency difference (ms)
Right
Left
Left
Right
Left
Right
Right
Left
39.9 3.6 35±45 (n 20)
38.8 156.1 3.1 26.1 34±44 119±177 (n 15)
155.1 15.9 127±175
226.2 34.4 186±255 (n 15)
221.7 27.4 185±260
11.8 13.8 8.8 12.3 9±26 8.8±41 (n 15)
39.44 18.5 35±58 (n 15)
43.3 24.4 36±60
33.9 155.4 3.1 23 27±38 126±180 (n 16)
168.8 32 115±178
223.4 39.3 180±270 (n 16)
239.4 39.3 189±284
14.4 18.3 8.9 12.3 13±30 12±31 (n 16)
38.7 12.2 34±57 (n 16)
42.6 11.5 36±58
70.4 4.5 68±77 (n 5)
202.2 24.2 150±226 (n 19)
289.4 27.5 240±331 (n 19)
291.9 31.2 240±338 (n 19)
12.9 4.2 9±21 (n 19)
Supraorbital nerve M 35.1 R2 SD 3 30±38 (n 20) Laser stimulation Supraorbital area M 70.2 SD 2.8 66±74 (n 7)
Right
210.2 20.1 150±230 (n 19)
Left
13.6 4.2 7.5±21 (n 19)
M. de Tommaso et al. / Neuroscience Letters 310 (2001) 37±40
Fig. 1. Recordings from the right orbicularis oculi muscle (1) and Cz (2) in a 30-year-old man after electric stimulation of the cornea (A) and supraorbital nerve (B), and laser stimulation of the skin above the eyebrow (C). The corneal re¯ex and the blink re¯ex are followed by a potential on Cz resembling the morphology of the N-P wave induced by laser stimulation, with a difference in latency of about 40 ms. The laser stimulus induced a blinking response at a latency of about 70 ms. At bottom the grand average (G.A.) resulting from stimulation of the right side in 13 subjects: cortical potentials induced by laser stimulation (thick line) and by cornea (thin line) and supraorbital nerve electric stimulation (gray line) are shown.
39
mean latency of approximately 155 ms, and a positive one at a mean latency of about 223 ms (Table 1; Fig. 1). The mean R2 duration was 31.5 ^ 8.7 on the right and 30.9 ^ 1.1 on the left side. However, a R3 component appeared subsequently in four subjects on the right side and in three on the left side, lasting for 20.3 ^ 4.5 ms on the right and 19.6 ^ 3 ms on the left side; in these cases the scalp response was contaminated. An early positive peak at about 86 ms was detectable on both sides in 12 subjects. In all subjects, laser stimulation with a mean duration of 20 ^ 5.59 ms on the right and 19.44 ^ 5.27 ms on the left evoked a scalp response on at least one side, with a large Nwave occurring about 210 ms after the stimulus. The P-wave occurred after a mean latency of about 289 ms (Table 1; Fig. 1). Two subjects did not attend to the stimulus, as they failed to count the stimuli, and the vertex response was detectable only for stimulation of one side. A blink-like response with a latency around 70 ms preceded the onset of the N scalp component in ®ve subjects on both sides, and in two subjects on one side; in two cases it was large and contaminated the earlier scalp response. However, a positive peak at roughly 110 ms was detectable bilaterally in 18 subjects. No signi®cant difference was observed between the peak latencies evoked by corneal and supraorbital nerve stimulation. However, the latency of LEPs was signi®cantly increased (P , 0:05), with the difference from the latency of the cortical potentials evoked by electrical corneal and supraorbital nerve stimulation computed in each case (Table 1). The peak-to-peak amplitude of the main cortical potential did not differ signi®cantly between the three kinds of stimulation. Thirteen subjects showed a clear cortical potential under all methods of stimulation. The latency of the negative cortical potential evoked by electrical stimulation of the cornea and supraorbital nerve showed a signi®cant correlation with N-wave LEPs latency. The results of Spearman test was 0.8675 on the right and 0.8786 on the left for the corneal evoked potential (P , 0:001), and 0.7210 on the right and 0.7111 on the left for the supraorbital nerve evoked cortical wave (P , 0:01). Laser stimulation of the supraorbital zone evoked a scalp response in our series. This response should be considered a speci®c trigeminal nociceptive potential, exploring conduction along the small trigeminal ®bers and the central nociceptive pathways [4]. Stimulation of the cornea and supraorbital nerve at intensities adequate for the corneal re¯ex and the blink re¯ex R2 evoked a cortical potential at the vertex. This resembled the trigeminal LEPs in both amplitude and morphology. The difference in latency was approximately 40 ms, consistent with nociceptor activation time [9]. This cortical potential could be pain-related, resulting from impulses ascending through small myelinated or unmyelinated ®bers. The cornea is innervated by Ad and C ®bers only, and the corneal re¯ex is mediated by small myelinated ®bers: our ®ndings con®rm its nociceptive properties [2,3].
40
M. de Tommaso et al. / Neuroscience Letters 310 (2001) 37±40
The nociceptive nature of the R2 component is debated. The longer latency of the corneal re¯ex compared with the electrically evoked R2 has been interpreted in terms of different afferents: Ab ®bers being responsible for R2, and Ad producing the corneal re¯ex [1]. Modulation of re¯exes by a potent narcotic-analgesic drug, fentanyl, and by a sedative-hypnotic and muscle relaxant drug, diazepam, was tested in a pharmacological trial. This study showed that the R2 appears to be inhibited mostly by diazepam, and the corneal re¯ex is suppressed by fentanyl. The authors concluded that the corneal re¯ex is purely nociceptive whereas R2 is mediated by tactile afferents [3]. Alternatively, there is some evidence that R2 is mediated by nociceptive afferents: laser stimulation of the skin above the supraorbital foramen induces a re¯ex blinking response, resembling the R2 component in both latency and duration [6]. Induction of a cortical potential such as nociceptive LEPs in relation to occurrence of the R2 could con®rm that this blink re¯ex component is partly induced by small ®ber activation. The R3 component occurred rarely in our series, so the systematic study of the latency and duration of cortical potentials following the R3 was not possible for this later re¯ex, which strongly prevents the recording of genuine neural activity. The recording of corneal and supraorbital nerve nociceptive potentials could be dif®cult in clinical practice, because averaging must be preceded by selection of responses in which cortical activity is well separated from blinking responses. With respect to nociceptive potentials evoked by trigeminal electric stimulation, the LEPs induced by ®rst trigeminal branch stimulation appeared less compromised by the occurrence of blinking artifacts; moreover, in previous reports the skin bordering the lower and upper lips was preferred to the zone above the eyebrow, in order to avoid an excess of blinking re¯exes [4]. The application of trigeminal LEPs following supraorbital zone stimulation may be preferred when a selective involvement of the ®rst
trigeminal branch is supposed. In this case, the trigeminal nociceptive potentials following corneal and supraorbital nerve electrical stimulation could also support the clinical evaluation. Further studies on larger series randomly assigned to the three kinds of stimulation are needed to con®rm the af®nity of the trigeminal potentials following corneal and supraorbital nerve electrical stimulation with nociceptive LEPs.
[1] Berardelli, A., Cruccu, G., Manfredi, M., Rothwell, B.L. and Marsden, C.D., The corneal re¯ex and the R2 component of the blink re¯ex, Neurology, 35 (1985) 797±801. [2] Cruccu, G., Berardelli, A. and Manfredi, M., Afferents for the human corneal re¯ex, Neurology, 234 (1987) 64. [3] Cruccu, G., Ferracuti, S., Leardi, M.G., Fabbri, A. and Manfredi, M., Nociceptive quality of the orbicularis oculi re¯exes as evaluated by distinct opiate- and benzodiazepine-induced changes in man, Brain Res., 556 (1991) 209± 217. [4] Cruccu, G., Romaniello, A., Amantini, A., Lombardi, P., Innocenti, P. and Manfredi, M., Assessment of trigeminal small®bers function: brain and re¯ex responses evoked by CO2laser stimulation, Muscle Nerve, 22 (1999) 508±516. [5] Ellrich, J., Anderson, O.K., Treede, R.D. and Arendt-Nelsen, L., Convergence of nociceptive and non nociceptive input onto the medullary dorsal horn in man, NeuroReport, 9 (1998) 3213±3217. [6] Ellrich, J., Bromm, B. and Hopf, H.C., Pain evoked blink re¯ex, Muscle Nerve, 20 (1997) 265±270. [7] Kazarians, H., Scharein, E. and Bromm, B., Laser evoked potentials in response to painful trigeminal nerve activation, Int. J. Neurosci., 81 (1995) 111±122. [8] Rossi, B., Risaliti, R. and Rossi, A., The R3 component of the blink re¯ex response induced by activation of high threshold cutaneous afferents, Electroenceph. clin. Neurophysiol., 73 (1989) 334±340. [9] Treede, R.D., Jahnke, M.T. and Bromm, B., Functional properties of CO2 laser activated nociceptive ®bers in an intact human skin nerve, In Bromm, B. (Ed.), Pain Measurement in Man: Neurophysiological Correlates of Pain, Elsevier, Amsterdam, 1984, pp. 65±78.
www.elsevier.com/locate/neulet
The blink re¯ex and the corneal re¯ex are followed by cortical activity resembling the nociceptive potentials induced by trigeminal laser stimulation in man Marina de Tommaso*, Giuseppe Libro, Marco Guido, Vittorio Sciruicchio, Francomichele Puca Interuniversity Center for the Study of Headache and Neurotransmitter Disorders of the Central Nervous System, Perugia, Roma, Sassari, Bari, Napoli, Firenze, Italy Received 15 May 2001; received in revised form 21 June 2001; accepted 6 July 2001
Abstract Laser stimulation of the supraorbital regions evokes brain potentials (LEPs) related to trigeminal nociception. The aim of this study was to record the R2 component of the blink re¯ex and the corneal re¯ex in 20 normal subjects, comparing the scalp activity following these re¯exes with the nociceptive potentials evoked by CO2 laser stimulation of supraorbital regions. Cortical and muscular re¯exes evoked by stimulation of the ®rst trigeminal branch were recorded simultaneously. The R2 component of the blink re¯ex and the corneal re¯ex were followed by two cortical peaks, which resembled morphologically N-P waves of LEPs. The two peaks demonstrated a difference in latency of approximately 40 ms, which is consistent with activation time of nociception. This ®nding suggests that these re¯exes are induced by activation of small pain-related ®bers. q 2001 Elsevier Science Ireland Ltd. All rights reserved. Keywords: Blink re¯ex; Corneal re¯ex; Laser stimulation; Pain-related potentials
Recently, it was demonstrated that CO2 laser stimulation of mechano-thermal nociceptors in the perioral and supraorbital regions evokes brain potentials (LEPs) useful for detecting selective trigeminal nociception dysfunction [4,7]. The condition used for examination of cortical responses to small ®ber stimulation is subjective pain sensation. In the face, particularly in the supraorbital zone, pain produced by an electric or CO2 laser, or any type of stimulator, induces a blink re¯ex. Although this muscular activity may compromise the recording of genuine cortical activity, the amplitude and latency of the trigeminal nociceptive potentials generally enable the muscular re¯ex responses to be well recognized and seldom mistaken for cortical activity [4]. The R2 component of the blink re¯ex is assumed to be partly elicited by nociceptive Ad ®ber input [5,6]. However, some authors maintain that it is mainly a mechanoceptive * Corresponding author. Clinica Neurologica I Policlinico, Piazza Giulio Cesare 11, 70124 Bari, Italy. Tel.: 139-080-5478565; fax: 139-080-547-8532. E-mail address: [email protected] (M. de Tommaso).
re¯ex [1], and further studies attribute the nociceptive quality to a later response named R3 [8]. Nevertheless, electric stimulation of the cornea also evokes blinking responses, which should be purely nociceptive, as the cornea is innervated by small myelinated afferent ®bers [2]. Therefore, the aim of this study was to record the R2 blink re¯ex and corneal re¯ex in normal subjects, comparing the cortical activity induced by electric stimulation of the supraorbital nerve and cornea with the nociceptive potentials evoked by CO2 laser stimulation of the supraorbital regions. Twenty healthy subjects, ten women and ten men, 25±45 years old (mean age 32.2 ^ 5.5), were selected based on the absence of any current or previous internistic, neurologic or psychiatric illness, and recent intake of psychoactive and analgesic drugs. The subjects gave their consent to the study, which was approved by the local Ethics Committee. They were instructed to keep their eyes open throughout the study. Cortical and muscular re¯ex responses evoked by stimulation of the ®rst trigeminal branch were simultaneously recorded through disc electrodes (impedance less than 5 kV) placed at the vertex (Cz) and referred to linked ear lobes (A1A2). Electro-oculographic (supraorbital electrode) and electromyographic (EMG) recordings from the
0304-3940/01/$ - see front matter q 2001 Elsevier Science Ireland Ltd. All rights reserved. PII: S03 04 - 394 0( 0 1) 02 08 6- 9
38
M. de Tommaso et al. / Neuroscience Letters 310 (2001) 37±40
orbicularis oculi muscle were measured according to method of Cruccu et al. [4]. Each subject received electrical stimulation of the supraorbital nerve and cornea as well as CO2 laser stimulation of the skin above the eyebrow. The supraorbital nerve was stimulated by means of surface electrodes placed longitudinally on the nerve, 2 cm from emergence of the supraorbital nerve. Stimuli were square wave negative single pulses of 0.1 ms duration, delivered unilaterally by a constant current isolated unit; the stimulus intensity was gradually increased in 1 mA steps until a clear R2 component was detectable in at least three out of ®ve repetitions. The cornea was stimulated by a saline-soaked thread resting on the cornea [1], increasing electrical intensity in 0.1 mA steps, to obtain a clearly recognizable re¯ex from the orbicularis oculi muscle in at least three out of ®ve repetitions. For LEPs, a CO2 laser stimulator (Neurolas, Electronic engineering, Florence, Italy) was used. Brief radiant heat pulses were delivered at 4.5 W intensity, increasing the stimulus duration in 5 ms steps to reach a pinprick sensation in at least three out of ®ve stimuli (beam diameter, 2.5 mm, 5 mm 2; duration 5±30 ms). The skin above the eyebrow was stimulated, and laser beam was moved slightly for each stimulus to minimize the effects of skin burns, nociceptor fatigue, and central habituation [4]. Subjects were asked to count the laser and electric stimuli to avoid distraction; they received two series of about 50 electric stimuli for the cornea and the supraorbital nerve and 40 laser stimuli on both sides. EEG and EMG were sampled at a rate of 2048 cps for 1 s following stimulus onset. Signals were ampli®ed with cortical potentials and re¯ex responses ®ltered by 0.5±80 Hz and 50±3000 Hz bandpass, respectively. An interstimuli interval of at least 1 min was used for all types of stimulation, to avoid habituation of both
re¯ex and cortical responses. The responses to single trials were individually stored and subsequently reviewed for artifacts before averaging. Only responses in which cortical activity was well separated from blinking responses were selected, with each average consisting of 25 samples, repeated at least once to ensure reproducibility of the potentials. The blinking responses obtained by the orbicularis oculi electrode were superimposed rather than averaged. The differences in latencies and amplitudes between cortical responses evoked by the three kinds of stimulation were subjected to a repeated ± measures ANOVA with contrast analysis. The Spearman test was applied to evaluate the correlation between the latency of laser and electrical cortical potentials. Electrical stimulation provoked a well recognizable corneal re¯ex (mean intensity 0.5 ^ 0.25 mA on the right side and 0.55 ^ 0.18 mA on the left side) and blink re¯ex R2 component (mean intensity 12 ^ 4.5 mA on the right side and 13.5 ^ 6.2 mA on the left) in all subjects. In 15 subjects, the corneal stimulation evoked a large vertex response, consisting of a negative component with a latency of about 150 ms, followed by a positive peak at approximately 230 ms (Table 1; Fig. 1). A positive peak with a latency of around 85 ms was recognizable in 14 subjects. The mean duration of corneal re¯ex was 47 ^ 8.5 ms on the right and 46 ^ 7.6 ms on the left. Thus, it occurred at the beginning of the scalp response in a few cases, making the ®rst positive peak unclear. In ®ve subjects, a sustained closing of the eyes followed corneal stimulation on both sides, causing the cortical potential to be undetectable. The R2 component obtained by electrical stimulation of the supraorbital nerve was followed by a large scalp potential in 16 subjects. This consisted of a negative peak occurring at a
Table 1 Features of re¯ex responses and cortical waves by electric stimulation of cornea and supraorbital nerve and laser stimulation of the skin above the eyebrow in a group of normal subjects
Electric stimulation Cornea M SD
Blinking response N-wave latency (ms) latency (ms)
P-wave latency (ms)
Peak-to-peak amplitude (mV)
Laser/Electric N-wave latency difference (ms)
Right
Left
Left
Right
Left
Right
Right
Left
39.9 3.6 35±45 (n 20)
38.8 156.1 3.1 26.1 34±44 119±177 (n 15)
155.1 15.9 127±175
226.2 34.4 186±255 (n 15)
221.7 27.4 185±260
11.8 13.8 8.8 12.3 9±26 8.8±41 (n 15)
39.44 18.5 35±58 (n 15)
43.3 24.4 36±60
33.9 155.4 3.1 23 27±38 126±180 (n 16)
168.8 32 115±178
223.4 39.3 180±270 (n 16)
239.4 39.3 189±284
14.4 18.3 8.9 12.3 13±30 12±31 (n 16)
38.7 12.2 34±57 (n 16)
42.6 11.5 36±58
70.4 4.5 68±77 (n 5)
202.2 24.2 150±226 (n 19)
289.4 27.5 240±331 (n 19)
291.9 31.2 240±338 (n 19)
12.9 4.2 9±21 (n 19)
Supraorbital nerve M 35.1 R2 SD 3 30±38 (n 20) Laser stimulation Supraorbital area M 70.2 SD 2.8 66±74 (n 7)
Right
210.2 20.1 150±230 (n 19)
Left
13.6 4.2 7.5±21 (n 19)
M. de Tommaso et al. / Neuroscience Letters 310 (2001) 37±40
Fig. 1. Recordings from the right orbicularis oculi muscle (1) and Cz (2) in a 30-year-old man after electric stimulation of the cornea (A) and supraorbital nerve (B), and laser stimulation of the skin above the eyebrow (C). The corneal re¯ex and the blink re¯ex are followed by a potential on Cz resembling the morphology of the N-P wave induced by laser stimulation, with a difference in latency of about 40 ms. The laser stimulus induced a blinking response at a latency of about 70 ms. At bottom the grand average (G.A.) resulting from stimulation of the right side in 13 subjects: cortical potentials induced by laser stimulation (thick line) and by cornea (thin line) and supraorbital nerve electric stimulation (gray line) are shown.
39
mean latency of approximately 155 ms, and a positive one at a mean latency of about 223 ms (Table 1; Fig. 1). The mean R2 duration was 31.5 ^ 8.7 on the right and 30.9 ^ 1.1 on the left side. However, a R3 component appeared subsequently in four subjects on the right side and in three on the left side, lasting for 20.3 ^ 4.5 ms on the right and 19.6 ^ 3 ms on the left side; in these cases the scalp response was contaminated. An early positive peak at about 86 ms was detectable on both sides in 12 subjects. In all subjects, laser stimulation with a mean duration of 20 ^ 5.59 ms on the right and 19.44 ^ 5.27 ms on the left evoked a scalp response on at least one side, with a large Nwave occurring about 210 ms after the stimulus. The P-wave occurred after a mean latency of about 289 ms (Table 1; Fig. 1). Two subjects did not attend to the stimulus, as they failed to count the stimuli, and the vertex response was detectable only for stimulation of one side. A blink-like response with a latency around 70 ms preceded the onset of the N scalp component in ®ve subjects on both sides, and in two subjects on one side; in two cases it was large and contaminated the earlier scalp response. However, a positive peak at roughly 110 ms was detectable bilaterally in 18 subjects. No signi®cant difference was observed between the peak latencies evoked by corneal and supraorbital nerve stimulation. However, the latency of LEPs was signi®cantly increased (P , 0:05), with the difference from the latency of the cortical potentials evoked by electrical corneal and supraorbital nerve stimulation computed in each case (Table 1). The peak-to-peak amplitude of the main cortical potential did not differ signi®cantly between the three kinds of stimulation. Thirteen subjects showed a clear cortical potential under all methods of stimulation. The latency of the negative cortical potential evoked by electrical stimulation of the cornea and supraorbital nerve showed a signi®cant correlation with N-wave LEPs latency. The results of Spearman test was 0.8675 on the right and 0.8786 on the left for the corneal evoked potential (P , 0:001), and 0.7210 on the right and 0.7111 on the left for the supraorbital nerve evoked cortical wave (P , 0:01). Laser stimulation of the supraorbital zone evoked a scalp response in our series. This response should be considered a speci®c trigeminal nociceptive potential, exploring conduction along the small trigeminal ®bers and the central nociceptive pathways [4]. Stimulation of the cornea and supraorbital nerve at intensities adequate for the corneal re¯ex and the blink re¯ex R2 evoked a cortical potential at the vertex. This resembled the trigeminal LEPs in both amplitude and morphology. The difference in latency was approximately 40 ms, consistent with nociceptor activation time [9]. This cortical potential could be pain-related, resulting from impulses ascending through small myelinated or unmyelinated ®bers. The cornea is innervated by Ad and C ®bers only, and the corneal re¯ex is mediated by small myelinated ®bers: our ®ndings con®rm its nociceptive properties [2,3].
40
M. de Tommaso et al. / Neuroscience Letters 310 (2001) 37±40
The nociceptive nature of the R2 component is debated. The longer latency of the corneal re¯ex compared with the electrically evoked R2 has been interpreted in terms of different afferents: Ab ®bers being responsible for R2, and Ad producing the corneal re¯ex [1]. Modulation of re¯exes by a potent narcotic-analgesic drug, fentanyl, and by a sedative-hypnotic and muscle relaxant drug, diazepam, was tested in a pharmacological trial. This study showed that the R2 appears to be inhibited mostly by diazepam, and the corneal re¯ex is suppressed by fentanyl. The authors concluded that the corneal re¯ex is purely nociceptive whereas R2 is mediated by tactile afferents [3]. Alternatively, there is some evidence that R2 is mediated by nociceptive afferents: laser stimulation of the skin above the supraorbital foramen induces a re¯ex blinking response, resembling the R2 component in both latency and duration [6]. Induction of a cortical potential such as nociceptive LEPs in relation to occurrence of the R2 could con®rm that this blink re¯ex component is partly induced by small ®ber activation. The R3 component occurred rarely in our series, so the systematic study of the latency and duration of cortical potentials following the R3 was not possible for this later re¯ex, which strongly prevents the recording of genuine neural activity. The recording of corneal and supraorbital nerve nociceptive potentials could be dif®cult in clinical practice, because averaging must be preceded by selection of responses in which cortical activity is well separated from blinking responses. With respect to nociceptive potentials evoked by trigeminal electric stimulation, the LEPs induced by ®rst trigeminal branch stimulation appeared less compromised by the occurrence of blinking artifacts; moreover, in previous reports the skin bordering the lower and upper lips was preferred to the zone above the eyebrow, in order to avoid an excess of blinking re¯exes [4]. The application of trigeminal LEPs following supraorbital zone stimulation may be preferred when a selective involvement of the ®rst
trigeminal branch is supposed. In this case, the trigeminal nociceptive potentials following corneal and supraorbital nerve electrical stimulation could also support the clinical evaluation. Further studies on larger series randomly assigned to the three kinds of stimulation are needed to con®rm the af®nity of the trigeminal potentials following corneal and supraorbital nerve electrical stimulation with nociceptive LEPs.
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