Measuring pain in non-verbal patients is a challenge. In this study we combine EEG recording with stimulation using a flat-tip probe to detect noxious-evoked brain activity in an objective manner.
Pain is an unpleasant sensory and emotional experience. In non-verbal patients, it is very difficult to measure pain, even with pain assessment tools. Those tools are subjective or determine secondary physiological indicators which also have certain limitations particularly when exploring the effectiveness of analgesia. As cortical processing is essential for pain perception, brain activity measures may provide a useful approach to assess pain in infants. Here we present a method to assess nociception with electrophysiological brain activity recordings optimized for the use in newborn infants. To produce highly standardized and reproducible noxious stimuli we applied mechanical stimulation with a flat-tip probe, e.g., PinPrick, which is not skin-breaking and does not cause behavioral distress. The noxious-evoked potential allows the objective measurement of nociception in non-verbal patients. This method can be used in newborn infants as early as 34 weeks of gestational age. Moreover, it could be applied in different situations such as measuring the efficacy of analgesic or anesthetic drugs.
Pain is an unpleasant sensory and emotional experience associated with actual or potential tissue damage, or described in terms of such damage1. The inability to communicate verbally does not negate the possibility that an individual is experiencing pain but makes it very challenging to assess pain-relieving treatment, for example in newborn infants2. Several behavioral and physiological indicators are used to assess pain in non-verbal patients. Different scales have been developed over the years, the choice depends on the type of stimulus, gestational age and the environment in which the neonates are embeded3,4,5. These pain assessment tools either rely on the rater's interpretation or they demand secondary physiological indicators.
In this video, we present a method to assess nociception with electrophysiological recordings optimized for use in newborn infants. Nociception is defined as the neural process of encoding noxious stimuli. Thus, quantitating nociception is an elegant and objective method to determine the neural input in a non-verbal person. Moreover, cortical activity detected by electroencephalography (EEG) is correlated with the intensity of noxious events5,6.
The method presented here combines EEG recording with noxious stimuli produced by a mechanical stimulation with a flat-tip probe, also called a pinprick7, which is not skin-breaking and does not cause behavioral distress6. It has been shown, that nociception following punctuate stimulation is predominantly mediated by Aδ-fibers, does not need a skin breaking lesion8 and the magnitude of the nociceptive-specific potential is not dependent on sleep state9. The probe is well accepted also by parents when applying this method in a study setting with neonates. The probe is electronically linked to the EEG recording system enabling the EEG recording to be precisely tagged when the probe contacts the skin. This greatly simplifies the process of time locking and is the premise for all subsequent EEG analyses. In order to minimize preparation time of the infant EEG recording we used a modified international 10/20 electrode placement system where we reduced the number of electrodes to the minimum requirement of three electrodes (Figure 1). The central vertex Cz electrode, where the noxious-evoked brain activity is maximal9,10,11, was used together with one reference and one ground electrode.
The study was approved by the Competent Ethics Committee of Northwestern Switzerland (EKNZ 2015-079) and informed written parental consent for the participant was obtained before the measurement.
1. Preparation
2. Measurement
3. Data Analysis
Figure 1 is the diagrammatic representation of the electrode positioning using the modified international 10/20 electrode placement system. Figure 2 shows the EEG activity recorded shortly before and after application of one single noxious stimulus using a flat-tip probe with 32 mN force, stimulation occurred as described in the protocol to the neonate's right hand (Figure 2A). The noxious-evoked response is visible at approximately 300 ms post stimulus onset (time 0). Figure 2B demonstrates the average response of 50 stimuli applied with a force of 32 mN to the same patient. Note that the noxious-evoked potential is more clearly if more stimuli are applied. Woody filtering, as shown in Figure 2C, can be used to adjust for slight variability in the response latency. The predefined template11, shown in red, is projected onto the EEG to calculate the magnitude of the noxious-evoked response.
Figure 1: Diagrammatic representation of the electrode positioning using the modified international 10/20 electrode placement system.
Figure 2: EEG activity recorded in response to the experimental noxious stimulation. A flat-tip probe stimulus (32 mN) was applied to the right hand of one patient, the time window of interest 200 – 500 ms after the stimulus is shaded. Panel A: EEG activity after one single stimulus, the principal peak at 250 ms is marked with an arrow. Panel B: Averaged response following 50 stimuli. Panel C: Woody filtered data with projected template (red). Please click here to view a larger version of this figure.
The approach presented here shows how noxious-evoked brain activity in neonates can be measured in an objective way using EEG recording and flat-tip probe stimulators to apply experimental noxious stimuli. This technique can be used in various clinical settings to detect nociception, e.g. in non-verbal persons such as neonates. The complete study can be done within 15 min, including placing the baby, identifying, preparing and mounting the electrodes, and finally applying and recording the 50 noxious stimuli. The stimuli do not cause the infant behavioral distress, and this approach provides an objective method with which to assess nociception in infants.
In this study, we used a flat-tip probe stimulus with a mild force of 32 mN. Higher forces of 64 mN or 128 mN can also be applied in neonates but may lead to increased movements artefacts due to limb withdrawal. We noticed that in our study neonates accepted a force of 32 mN very well and did not withdraw their limbs, whereas a stimulus with 128 mN force leads to bilateral reflex withdrawal6. It has been shown that the noxious-evoked potential is greater depending on the force of the flat-tip probe stimulus (32 vs. 64 mN)6,14. Verriotis et al. showed that children at the age of 1 year have higher amplitude in their event-related vertex potential and higher pain scores than newborn infants15. Therefore, the force of the flat-tip probe stimulus can be adapted depending on the age of the patient16.
Previous studies recorded nociception from neonates being nursed supine or prone or even on their side. To keep experimental settings constant, we recommend keeping one position especially when performing a study with several participants. We found that performing the measurement in a supine position worked well. We performed the flat-tip probe stimuli on the neonate's hand. Positioning and handling of the neonate may be easier if applying the stimuli to the hand compared to the foot. The foot in fact is more commonly used in studies, however, we found that stimulating the back of the hand can be easier than accessing the foot. Reflex withdrawal is a useful additional measure of nociceptive activity, which can be incorporated into a multidimensional assessment of pain in this challenging non-verbal population17.
Despite avoiding any discomfort and movement artefacts there is always spontaneous neuronal activity. Thus filtering and averaging is important in order to attenuate the background noise and visualize the event related potential. Also, repetition of flat-tip probe stimuli, e.g. 50, strongly improves the signal-to-noise ratio. To analyze the data, time-locking of all the measurements is crucial.
A further technical challenge in neonates shortly after birth is the presence of vernix caseosa coating the skin of newborn infants including the head, which is why their skin needs to be cleaned carefully with an extra prepping paste even when active electrodes are used. In our opinion, active electrodes are better suited for this method than passive electrodes because they are less sensitive to external movement on the wires. Also, they are easier to use since they accept higher impedances due to the inbuilt pre-amplifier. However, passive electrodes can be used as well.
For data analysis, we used the method described by Hartley et al.6,11. Another method is the time-frequency analysis demonstrated by Hu et al.18.
Taken together, the technical methods, setup, and interpretation of the results require a trained team with preferably two people doing the measurement, one applying the stimuli while the other is checking the EEG. If the neonate is unsettled, the EEG will be contaminated with movement artifacts and it is not possible to perform the measurements. Also, because of validity, it is mandatory to have a certain number of stimuli, which can be challenging.
Nociception matures in early life and as early as 34 weeks' gestation the neonates' brain can distinguish between touch and nociception17. Under the age of 34 weeks' gestation infants are more likely to generate nonspecific neuronal bursts19 whereas in late-preterm infants starting at 34 weeks' gestation the here described method can be used11. This method opens the door for a variety of research investigations. For example, it could be used to test the impact of birth-related stress on nociception20 or to test the specific impact of various analgesic drugs in neonates and infants6,21. For example, the protocol presented here has been recently used in a study demonstrating the efficacy of topical local anesthetics in significantly reducing the noxious-evoked potential when a noxious stimulus was applied to a treated foot as compared to background activity or to the untreated foot11.
In conclusion, noxious-evoked brain potentials in EEG recordings make it possible to objectively investigate surrogate measures of pain perception in non-verbal patients. This method is applicable for use in the clinical setting in research investigations. By time-locking the flat-tip probe stimuli to the EEG recording it is possible to reliably evaluate the electrophysiological nociceptive response.
The authors have nothing to disclose.
The authors would like to acknowledge Caroline Hartley and Rebeccah Slater (Department of Paediatrics, University of Oxford, UK) for critical reviewing our paper and Walter Magerl (Department of Neurophysiology, Center of Biomedicine and Medical Technology Mannheim (CBTM), University of Heidelberg, Germany) for supporting us with technical equipment and knowledge.
Easycap | EASYCAP GmbH | AC-32-C | EEG caps for infants sizes 34 and 36 |
actiCAP | Brain Products GmbH | BP-04243-SIG | active electrodes |
ImpBox | Brain Products GmbH | impedance measurement | |
V-Amp | Brain Products GmbH | EEG recording device | |
Contact trigger for pinprick stimulation | MRC Systems GmbH | ||
PinPrick stimulator set | MRC Systems GmbH | ||
EEG prepping paste | USB Pharmacy | contains sodium chloride, pumice stone, propylene glycol | |
SuperVisc | EASYCAP GmbH | Electrolyte-Gel for active electrodes | |
Brain Vision Recorder | Brain Products GmbH | ||
Brain Vision Analyzer | Brain Products GmbH | ||
MATLAB using EEGLAB | Swartz Center for Computational Neuroscience, University of California San Diego | For EEG processing, including averaging of all EEG epochs |