Electrophysiology in diagnosis and management of neuropathic pain

Electrophysiological techniques demonstrate abnormalities in somatosensory transmission, hence providing objective evidence of 'somatosensory lesion or disease' which is crucial to the diagnosis of neuropathic pain (NP). Since most instances of NP result from damage to thermo-nociceptive pathways (thin fibres and spino-thalamo-cortical systems), specific activation of these is critical to ensure diagnostic accuracy. This is currently achieved using laser pulses or contact heat stimuli, and in a near future probably also with contact cold and intra-epidermal low-intensity currents. Standard electrical stimuli, although of lesser diagnostic yield, are useful when large and small fibres are affected together. Nociceptive evoked potentials to laser (LEPs) and contact heat (CHEPs) have shown adequate sensitivity and specificity to be of clinical use in the differential diagnosis of NP, in conditions involving Aδ of C-fibres and spino-thalamo-cortical pathways. LEPs have also a role in the detection of patients at risk of developing central post-stroke pain after brainstem, thalamic or cortical injury. Cognitive cortical responses and autonomic reactions (sympathetic skin responses) reflect pain-related arousal and can document objectively positive symptoms such as allodynia and hyperalgesia. They are of help in the differential diagnosis of somatisation disorders, by discriminating conscious simulation (malingering) from conversive sensory loss. The electrophysiological approach to patients suspected, or at risk, of NP is a cost-effective procedure that should never be absent in the diagnostic armamentarium of pain clinics.