New techniques in female pudendal somatosensory evoked potential testing

Objective—The primary nerves innervating the female genitalia are the dorsal nerve of the clitoris (DNC) and the perineal nerve, which innervate the clitoris and the external genitalia/distal vagina, respectively. We describe two novel electrodiagnostic techniques for evaluating the integrity of these female genital somatosensory pathways.

Subjects and methods—Seventy-seven healthy women (mean age 29.3 years) were enrolled for this study. We performed DNC somatosensory evoked potentials (SEPs), stimulating through self-adhesive disk electrodes on either side of the clitoris. Perineal nerve SEPs were evoked through a vaginal probe. Cortical responses were measured through cup electrodes affixed to the scalp at Cpz and Fpz. Stimulus parameters were duration 0.1?ms, frequency 4.1?Hz, filters 5–5,000?Hz, at three times sensory threshold.

Results—DNC and perineal nerve SEPs from both the right and left sides were reproducible and easily discerned. The mean P1 latencies were: right DNC 39.4?ms (SD 2.8?ms), left DNC 39.3?ms (SD 3.3?ms), right perineal nerve 37.8?ms (SD 3.4?ms), left perineal nerve 37.6?ms (SD 3.1?ms). We recorded SEP responses from 90 to 92% of subjects for the DNC, and 69% for the perineal nerve.

Conclusions—We are able to evoke somatosensory potentials from the four primary somatic nerves that mediate female genital cutaneous sensation. In healthy subjects, the DNC responses are robust and maintain laterality. The perineal nerve responses are less consistently obtained, but when recorded, are easily discerned. These preliminary data provide a foundation from which to study female genital innervation, particularly as it applies to sexual function.     

Clinical value of the pudendal somatosensory evoked potential

The pudendal evoked potential was recorded in 126 patients who had presented with various uro-genital complaints. The patients were divided into two groups depending on whether or not there were clinical signs of neurological disease. Group I consisted of 83 patients (66%) who on clinical examination were neurologically normal. In group 11 there were 43 patients (34%) who had physical signs suggesting underlying neurological pathology. The pudendal evoked potential was abnormal in 10 patients from the group with neurological signs (group 11) but only 1 patient from group I (a man who had made an excellent recovery from previous transverse myelitis). The conclusion of this study is that the pudendal evoked potential is of no greater value than the clinical examination in the assessment of patients with uro-genital dysfunction. The recommendation that the potential should be recorded as part of the routine assessment of patients suspected of having a neurogenic disorder of the bladder and sexual function should be reconsidered.     

The development of the somatosensory evoked potential in the unanaesthetized fetal sheep

Somatosensory evoked potentials were recorded in utero from 13 chronically instrumented fetal lambs (97 to 148 days of gestation) following electrical stimulation of the upper lip or upper limb. Several clear and reproducible peaks were observed. Following upper lip stimulation, peaks were seen with mean peak latencies of 9, 13.2, 17.8, 21.3, 33.8 and 206 ms at a gestational age of 125 days. Similar peaks, but of slightly later mean latencies, were seen following limb stimulation. These peaks demonstrated significant gestational age related falls in peak latencies (P less than 0.05). Several of the mid to late latency peaks, notably those occurring at 21.3, 33.8 and 206 ms, demonstrated changes (P less than 0.05) in both latency (longer in low voltage) and amplitude (reduced in low voltage) dependent on electrocorticographic state. Rate of stimulus presentation also had a significant effect on both amplitude and latency of several peaks (P less than 0.05) with this effect lessening with advancing gestational age. Evoked potentials can thus be successfully obtained from chronically instrumented fetal lambs and provide a useful indice for studies of neural maturation.     

Origin of frontal N15 component of somatosensory evoked potential in man

Origin of the frontal somatosensory evoked potential (SEP) by median nerve stimulation was investigated in normal volunteers and in patients with localized cerebrovascular diseases, and the following results were obtained.

• 1.(1) In normal subjects, SEPs recorded at F3 (or F4) contralateral to the stimulating median nerve were composed of P12, N15, P18.5 and N26. Similar components were recognized in SEP recorded at Fz.
• 2.(2) In patients in whom putaminal or thalamic hemorrhages had destroyed the posterior limbs of the internal capsules, frontal N15 and parietal N18 (N20) disappeared. These components were also absent in patients with cortical (parietal) infarctions. Among these patients, the thalamus was not affected in cases with putaminal hemorrhages and cortical infarctions.

These facts indicate that the generator of the frontal N15 does not exist in the thalamus but that it originates from the neural structure central to the internal capsule, which suggests a similarity to the generator of the parietal N18.Because N15 was recorded in the midline of the frontal region with shorter latency than parietal N18, the frontal N15 might represent a response to the sensory input of the frontal lobe via the non-specific sensory system.     

Detection of neurological injury using time-frequency analysis of the somatosensory evoked potential

Somatosensory evoked potentials (SEPs) can be monitored during critical surgery to help detect or possibly prevent post-operative injury to the brain. This paper presents the application of time-frequency analysis to detect both temporal and spectral changes in the SEP waveform that may occur due to injury. Time-frequency distributions, which provide a measure of signal energy at both a specific time and frequency, were computed for averaged SEPs acquired from anesthetized cats during various stages of hypoxic injury and then recovery. Wigner distributions of SEP waveforms were found to contain a peak of signal energy at a specific time and frequency, a peak that is altered during injury. Four characteristics of the distribution peak that demonstrate changes due to injury were computed: peak time, peak frequency, peak power, and peak sharpness. Peak time was found to increase while peak frequency, peak power, and peak sharpness were found to decrease during injury. Furthermore, the total signal power in a time-frequency space around the normal peak location was monitored by developing a time-frequency window filter (TFWF) method. For all cases, onset of hypoxia was detected an average of 2.75 min earlier by the TFWF method than by the conventional amplitude measurement method. Time-frequency analysis of EP signals may therefore be useful as a monitoring tool for early detection of brain injury.     

Neurometric evaluation of the cortical somatosensory evoked potential in acute incomplete spinal cord injuries

The clinical application of evoked potentials has at times been criticized for its failure to provide an objective quantifiable assessment of the processing of sensory information and a reflection of nervous system integrity at least comparable in accuracy to other methods of assessment. The challenge is to quantify the SEP in a manner that accurately reflects the function of the somatosensory system.Therefore, neurometrics, an approach which emphasizes the transformation of the neurophysiological data to a common metric of relative probability by reference to normative standards and the classification of critical features of data by multivariate statistics, was employed.The 3-part study involved the evaluation of the diagnostic, prognostic and localization functions of the cortical tibial SEP in patients with incomplete spinal cord injuries. Two neurometric indices which correlated well with concurrent and future neurological status were developed. The distributions of influence of dorsal columns and spino-thalamic tracts on the SEP were established.     

Three-dimensional human somatosensory evoked potentials

Median nerve somatosensory evoked potentials were recorded from 30 normal adults using conventional scalp derivations and an orthogonal bipolar surface electrode montage. This allowed the determination of the spatial orientation of the hypothetical centrally located equivalent dipole derived from the evoked response recorded in 3-dimensional voltage space. The 3-dimensional voltage trajectory describing changes in equivalent dipole orientation and magnitude revealed 4 major apices between 5 and 25 msec, 3 of which corresponded to the traditional P14, N20 and P25 peaks. A fourth apex at 17 msec was not as evident in the conventional recordings and signaled a transition from a vertical P14–N18 generator process to a horizontal N20 generator process. The normal within- and between-subject variability of trajectory apices, segments and planes are described, along with the theoretical and practical implications of this recording technique.     

Short-latency somatosensory evoked potentials in brain-dead patients

Ten adult brain-dead patients were evaluated for the presence of clearly defined median nerve short-latency somatosensory evoked potentials (SSEPs). All met clinical criteria recommended by the President's Commission report (1981), had positive apnea tests, and had electrocerebral silent EEGs. P13-P14 and N20 were absent in all scalp-scalp channels, although 3 patients showed P13-P14 in scalp-non-cephalic channels. Of 6 patients showing N13, 3 lacked P13-P14. Our data suggest a characteristic destruction of N20 and rostral P13-P14 generators, with variable rostral-caudal loss of lower generators, SSEPs can provide valuable information about brain-stem activity in the evaluation of suspected brain-dead patients.     

Ipsilateral median somatosensory evoked potentials recorded from human somatosensory cortex

Somatosensory evoked potentials (SEP) to ipsilateral and contralateral median nerve stimulations were recorded from subdural electrode grids over the perirolandic areas in 41 patients with medically refractory focal epilepsies who underwent evaluation for epilepsy surgery. All patients showed clearly defined, high-amplitude contralateral median SEPs. In addition, four patients showed ipsilateral SEPs. Compared with the contralateral SEPs, ipsilateral SEPs were very localized, had a different spatial distribution, were of considerably lower amplitude, had a longer latency (1.2–17.8 ms), did not show an initial negativity, and were markedly attenuated during sleep. Stimulation of the subdural electrodes overlying the sensory hand area was associated with contralateral hand paresthesias, but no ipsilateral hand paresthesias occurred. It was concluded that subdurally recorded cortical SEPs to ipsilateral stimulation of the median nerve (M) reflect unconscious sensory input from the hand possibly serving fast bimanual hand control. The anatomical pathway of these ipsilateral short-latency MSEPs is not yet known. Transcallosal transmission seems unlikely because of the short delay between the ipsilateral and contralateral responses in selected cases. The infrequent occurrence of ipsilateral subdurally recorded SEPs and their low amplitude and limited distribution suggest that they contribute very little to the short-latency ipsilateral median SEPs recorded on the scalp.     

Steady-state analysis of somatosensory evoked potentials

We report the development of a new method for frequency domain analysis of steady-state somatosensory evoked potentials (SEPs) to amplitude-modulated electrical stimulation, which can be recorded in significantly less time than traditional SEPs. Resampling techniques were used to compare the steady-state SEP to traditional SEP recordings, which are based on signal averaging in the time domain of cortical responses to repetitive transient stimulation and take 1–2 min or more to obtain a satisfactory signal/noise ratio. Median nerves of 3 subjects were stimulated continuously with electrical alternating current at several modulation frequencies from 7 to 41 Hz. Amplitude modulation was used to concentrate the power in higher frequencies, away from the modulation frequency, to reduce the amount of stimulus artifact recorded. Data were tested for signal detectability in the frequency domain using the T_circ² statistic. A reliable steady-state response can be recorded from scalp electrodes overlying somatosensory cortex in only a few seconds. In contrast, no signal was statistically discriminable from noise in the transient SEP from as much as 20 s of data. This dramatic time savings accompanying steady-state somatosensory stimulation may prove useful for monitoring in the operating room or intensive care unit.     

Presurgical functional localization of primary somatosensory cortex by dipole tracing method of scalp-skull-brain head model applied to somatosensory evoked potential

The aim of the present study was to explore the utility of dipole tracing (DT) of a scalp-skull-brain (SSB) head model in preoperative functional localization of the human brain. Nine patients who underwent surgery of mass lesions around the central sulcus (CS) were employed. By using SSB/DT, dipole source location of early cortical components of the somatosensory evoked potential (SEP) was estimated before surgery. Motor cortex, CS and primary somatosensory cortex were determined by cortical SEP during surgery. After surgery precise functional mapping was reproduced in MRI, and the accuracy of DT was evaluated by measuring the distance between estimated dipole source and the posterior bank of the CS. We defined this distance as localization error of DT. In 4 cases without structural change around the sensorimotor cortex, localization error ranged from 1 to 4 mm with an average of 2 mm. In 5 cases with structural alteration of sensorimotor cortex, localization error ranged from 6 to 10 mm with an average of 8 mm. The difference in localization error between the two groups was statistically significant, and may have been caused by changes of conductance near sensorimotor cortex in the latter group. Functional localization by DT was accurate and useful. But localization error could not be ignored in cases with structural alteration in the sensorimotor cortex.     

Long sensory tracts (cuneate fascicle) in cervical somatosensory evoked potential after median nerve stimulation

Low amplitude high frequency waves (LHW) were investigated in normal and patient cervical somatosensory evoked potentials after median nerve stimulation (CSEP) in parallel to normal and patient conducted somatosensory evoked potentials (SEP) after tibial nerve stimulation. Normal recordings were obtained in five subjects undergoing dorsal root entry zone (DREZ) coagulation for pain relief. Patient recordings were obtained in 11 subjects suffering from either syringomyelia, spinal cord tumour, or both. All recordings were made intraoperatively from the dorsal spinal cord surface using the subpial recording technique. Normal CSEP showed typical triphasic potential starting with an initial P9, followed by N13 and a final positivity, P1. Numerous LHW were superimposed on slow triphasic potential. To improve the visibility of LHW, slow triphasic potential was removed from the original CSEP. Potentials thus obtained contained only high frequency components of CSEP, i.e. LHW. They were compared with conducted SEP after tibial nerve stimulation. Comparison revealed similarities in high frequency, low amplitude and general wave form, LHW thus showing characteristics of conducted potential. Duration was found to be significantly shorter than normal duration in both patient LHW (Student's t-test, P<0.0005) and patient conducted SEP (Student's t-test, P=0.064). A shorter duration was associated with worsening of configuration in patient LHW and patient conducted SEP. These changes of LHW could not be connected with distortion of N13 seen in patient CSEP. A shorter duration and worsening of configuration in patient LHW were most prominent in cases with a loss of vibration and posture senses, but were also observed in cases where only pain and temperature senses were affected. We therefore concluded that cuneate fascicle is the most likely generator of LHW, although the participation of other cervical long sensory tracts, e.g. spinothalamic tract, cannot be ruled out.     

慢性不可预见性应激对大鼠恐惧条件反射和躯体感觉诱发电位的影响

目的:探讨慢性不可预见性应激对大鼠恐惧条件反射以及体感诱发电位的影响并分析可能的神经电生理机制。方法:26只雄性SD大鼠(190~200 g)随机分成两组(n=13):对照组和模型组。用慢性不可预见性应激刺激模型组大鼠,用恐惧条件反射实验检测两组大鼠的恐惧反应,用躯体感觉诱发电位检测大鼠脑电活动。结果:与对照组相比,模型组大鼠在恐惧记忆阶段不动时间百分比减小(56.64%±13.78%vs69.72%±18.10%,P<0.05),躯体感觉诱发电位的第二个正向波(P2)潜伏期也明显缩短(70.54±10.13 msvs78.46±7.80 ms,P<0.05)。相关性分析显示大鼠恐惧条件反射的不动时间与躯体感觉诱发电位潜伏期存在正相关(r=0.507,P<0.05)。结论:慢性不可预见性应激抑制大鼠恐惧反应,并缩短体表感觉诱发电位的潜伏期,恐惧反应行为与体感诱发电位潜伏期存在正相关,提示大鼠恐惧反应与体表感觉诱发电位可能有共同的神经递质机制。     

Temperature-dependent hysteresis in somatosensory and auditory evoked potentials

Fourteen adult patients undergoing open heart surgery under induced hypothermia had median nerve, short-latency somatosensory evoked potentials (SSEPs) recorded during cooling (from 36°C to 19°C) and subsequent rewarming. Similar data on another group of patients who had brain-stem auditory evoked potentials (BAEPs) were also analyzed. Hypothermia produced increased latencies of the major SSEP and BAEP components and the latencies returned to normal with subsequent warming. The temperature-latency relationship during the cooling phase was significantly different from that during the warming phase. For SSEP components the temperature-latency relationship was linear during cooling and curvilinear during warming, whereas for BAEP it was curvilinear both during cooling and warming. Furthermore, the regression curves were different during the two phases of temperature manipulation, particularly for temperatures below 30°C both for SSEP and BAEP components. At the onset of warming there was an initial exaggerated warming response on the evoked potential (EP) latencies and amplitude of the EP components. The temperature-latency regression curves were uniformly less steep during the warming phase compared to those during cooling. These findings suggest the existence of hysteresis in the relationship between temperature and EP latencies. The latencies at a given temperature below 30°C depend on whether that temperature is reached during cooling or during warming.     

The relationship between median nerve somatosensory evoked potential latencies and age and growth parameters in young children

Median nerve somatosensory evoked potentials (SEPs) were recorded in a group of 35 normal children aged 6–36 months and related to body length, body weight and head circumference. Recordings were made while the child was awake using the following derivations: ipsilateral clavicle - Fpz; seventh cervical vertebra (CV7) - Fpz; contralateral C′ - Fpz. the clavicular-Fpz response was bilobed in 77% of the recordings, the initial being designated CL1 and the following negativity, CL2. Early negativities, coincident in latency with CL1 and CL2, were often present inin CV7-Fpz recordings, however, a negativity longer in latency than CL2 was always present, was designated CVN, and preceded a prominent positivity. A scalp-recorded early negativity (N1) and a following positivity (P1) were well defined in all recordings.The latency of CL1, CL2 and CVN increased with an increase in age and stature. In contrast, significant negative correlations were found between the latency of N1, P1, age and statur. The CL1-CVN interpeak latency did not correlate significantly with age, while the central conduction time (CVN-N1) declined with an increase in age and stature. These findings confirmed the complexity of SEP absolute and interpeak latency changes with an increase in age and stature in young children.