4. Neurophysiological methods in evaluation of pain in multiple sclerosis

Abstract

Pain of various origin is a common symptom in patients with multiple sclerosis (MS) with the estimated prevalence between 50% and 85%. Central neuropathic pain (which is defined as a pain arising as a direct consequence of a lesion or disease affecting the somatosensory nervous system) affects about 30% of this group of patients. According to current EFNS (European Federation of Neuropathic Societies) guidelines on neuropathic pain assessment, neurophysiological and psychophysiological methods play an important role in the diagnostic process of neuropathic pain, together with clinical examination and validated screening tools and questionnaires, and functional neuroimaging. Psychophysiological measures are mainly based on quantitative sensory testing (QST) and its dynamic applications. Common QST methods may be defined as a measurement of perception in response to external stimuli of controlled intensity, which allows the evaluation of detection and pain thresholds for various modalities. A detailed QST protocol has recently been developed and validated by German Research Network on Neuropathic Pain. This protocol consists of the assessment of 13 parameters reflecting the sensitivity and pain perception for thermal, touch, pressure, vibration and pinprick stimuli, and allows the detailed clinical description of particular patient with the evaluation of complex sensory profile. In neuropathic pain patients, it may help to characterize particular painful neuropathic syndromes, and predict or monitor treatment effects including effect of treatments upon different pain components. So called “dynamic QST” (dQST) is a group of methods, where the pain-perceiving system is stimulated in a way that exposes a certain mechanism of pain processing, particularly its central modulation These methods include the tests of central integration, such as temporal summation (TS, also called wind-up) and spatial summation, and tests of descending control, e.g., the conditioned pain modulation (CPM). These methods can’t be used for diagnosis or confirmation of the presence of pain. However, their setting is probably one of the factors, predetermining the development of chronic pain. They thus may reflect the ,,pro-nociceptive“ disposition of the particular patient. Among neurophysiological methods, pain-related evoked potentials and some of the pain-related reflexes are the most relevant tests for the assessment of pain in MS patients. Pain-related evoked potentials are the easiest and most reliable neurophysiological methods for assessing the function of nociceptive sensory pathways. Various types of stimuli can be used to evoke the response of pain-related neuronal structures: laser-evoked potentials (LEPs), contact-heat evoked potentials (CHEPs) and potentials elicited by a surface concentric electrode that provides a preferential activation of superficial terminals (i.e. small-diameter afferents) (PREPs). These methods show a clear correlation with pain and are highly specific in its confirmation, while their sensitivity depends on the definition of abnormality (being quite low if only the absence of the response is considered to be abnormal, with a rapid increase if also a reduction of amplitude is taken into consideration). Pain-related reflexes appear to be diagnostically useful particularly for facial pain (e.g. trigeminal neuralgia, which represents one of the most frequent types of pain in MS patients). Two brainstem reflexes (early (R1) blink reflex and early (SP1) masseter inhibitory reflex) are efficient tools to reveal symptomatic forms of trigeminal neuralgia with a very satisfactory sensitivity and specificity. The cutaneous silent period (CSP) is a spinal inhibitory reflex with cortical modulation. The response is recorded from the small hand muscles after noxious stimulation of the fingers, which reflects the suppression of activity in spinal motor nuclei. The method has been tested in various pain conditions and is clearly related to clinical symptoms of thermal and pain perception disturbance. CSP was not systematically studied multiple sclerosis, but has repeatedly been shown to display clear abnormalities in spinal cord lesions (e.g. cervical myelopathy). However, its correlation with presence of pain seems to be limited. Supported by MH CZ-DRO (FNBr, 65269705) and IGA CR NT13523-4.