Pain Neurophysiology

Nerve conduction studies are used in the neurological routine diagnostics to assess functional loss of the motoric and sensory nerve fibres. The sensory nervous system may be tested with somatosensory-evoked potentials (SEP), therefore, electrical stimuli are synchronised with the EEG and latencies and amplitudes of the evoked potentials are analysed, respectively.

There is major diagnostic gap when using SEP, since only 20% of the sensory system, i.e. only thickly-myelinated A-beta fibres are assessed. The remaining 80% of different nerve fibre sub-classes (small fibres) require a different electrophysiological approach.

The central nervous system receives signals of different modalities by different sub-classes of nerve fibres. Cold sensations are mediated by cold-mediating A-delta fibres, painful heat is mediated by heat-sensitive A-delta fibres (AMH II), painful mechanical stimuli are mediated by mechano-sensitive A-delta-fibres (AMH I), warm sensation by unmyelinated C-fibres.

Due to the poor selectivity of SEPs, almost all nerve fibre sub-classes are elicited by the electrical stimulus. Nevertheless, only the strongest and fastest signal is visible in the EEG (gate-control-theory, first-come-first-serve hypothesis), i.e. an A-beta-fibre mediated potential.

New selective diagnostic tools are required in order to perform a complete assessment of the somatosensory nervous systems. With these, sensory deficits may be detected in the early stage of disease, which enables early treatment decisions. Typical examples of disorders with sensory deficits are diabetic polyneuropathy, radiculopathy and inflammatory demyelinating disorders.