Introduction The human spinothalamic tract (STT) conveys information about noxious mechanical as well as noxious and non-noxious thermosensory stimuli. In the clinical and laboratory setting, pinpricks as well as heat stimuli, facultatively with the recording of evoked potentials, are commonly used in order to assess the functional integrity of the STT. Quantitative sensory testing methods have also been employed, however, these approaches rely on a psychophysical correlate and have to be considered as surrogates, as they do not directly and fully objectively probe the functionality of the STT. Although applying these methods is already comprehensive in the clinical routine, from a neurophysiological perspective, they fall short of an objective electrophysiological outcome parameter for the non-nociceptive pathway for cold detection, which is also transmitted through the STT. Furthermore, cold allodynia, a positive symptom frequently encountered in patients with spinal cord injury pain, has so far escaped proper appreciation, owing to the absence of an objective method in order to functionally dissect the underlying spinal pathways. We have demonstrated the feasibility of the recording of cold-evoked potentials in healthy human volunteers (Baumgartner et al., 2012), and present preliminary data of patients with circumscribed lesions of the spinal cord. Methods CEPs were elicited using a contact-thermode stimulator (CHEPS, Medoc Ltd.) and recorded from scalp electrodes. Starting at a baseline temperature of 35 °C, cool ramps of minus 3 °C were applied bilaterally in different dermatomes in healthy volunteers and patients with defined spinal cord injuries in blocks of 20 stimuli each (ISI 8–10 s). Results The evoked potential for cold stimuli consisted of a vertex negativity (N2) followed by a positivity (P2) with amplitudes corresponding to the stimulus intensity. Conductance velocities were in the A-fiber range. Ratings for the cool intensity showed little habituation, and were never painful. Peripheral adaptation to the innocuous cold stimuli, applied under our experimental paradigm, was very little. Local application of menthol (40% in ethanol) for 30 min had a sensitizing effect, resulting in a shorter latency of the CEPs. In patients with spinal cord injuries a loss or reduction of CEPs was observed. Conclusion Inoccuous cold stimuli applied to the skin reproducibly evoke cortical potentials. Adding cold evoked potentials as a further outcome parameter to well-established methods such as contact heat or laser evoked potentials may increase the overall sensitivity of these functional STT measurement techniques. The combination may enable the detection of pathological changes within the thermal and nociceptive pathways that might otherwise escape observation. Considering the fact that CEPs can reliably be elicited by non-painful cold stimuli, movement artefacts due to nociceptive reflexes are less often seen. As intensity ratings in healthy volunteers were never painful, the acquisition of evoked potentials in patients with various pain conditions, possibly even in areas of hyperalgesia, may be facilitated. Futhermore, smaller regions such as the face or certain dermatomes are better accessible since changing the location of the thermode within the regions of interest in order to reduce adaptation is not necessary. Overall, cold evoked potentials provide an additional and complimentary approach towards the functional examination of the spinothalamic tract in health and disease.
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Apr 8, 2017
Kozo Nakai + 5
Open Access