Assessment of a New Measurement Technique for Type C Fibre Nerve Conduction Velocity in Polyneuropathies.

Sympathetic skin response (SSR) is one of the simplest means to register the electrophysiological activity of small size nerve fibres. It corresponds to the emission of an electrical potential by sweat glands due to stimulation by type C nerve fibres. These fibres belong to vegetative and somatic nervous systems which can be activated by various stimuli.

Medical applications were considered in the context of polyneuropathies with dysautonomia but SSR did not yield a satisfactory diagnostic value. Parameters used were SSR latency and amplitude. Latency showed little variation with pathology and it is considered that as long as fibres are present their conduction velocity is respected as an all-or-nothing phenomenon. On the contrary, amplitude is very variable, in particular between subjects, which prevents from applying a confidence interval to a given subject. Only unilateral suppression of the response seems to be a reliable criterion and gave results in the context of peripheral nervous pathologies.

The good results obtained with "response suppression" shows SSR sensibility. The discredit of "conduction velocity" variable seems to come from a publication using microneurography. However with this technique only one fascicle is investigated and preferably one giving a good signal, so not representative of all nerve fibres.

Consistent results were achieved with a technique consisting in recording SSR at two points of a same path, separated by a known distance. Knowing the difference of response latency at these two points, velocity could be deduced on the path. This technique was tested in healthy patients in 1988 taking as measurement sites the hand stuck on the body and the ground as a reference for foot plantar. It gave a velocity equivalent to the result found by another team with the same method. The purpose of the study is to apply this technique to pathology.

It should be noted that the velocity measured in this way depends on superior and inferior limb paths, on a medullar portion between C7 et D12 and on pre-ganglionic neurone portion. It has consequently no lesion focalisation capacities and is more appropriate for polyneuropathies with diffuse damage. Nevertheless, it has two advantages. First sweet follicles are in the same functional state in hand and on foot since stimulation intervals are the same at both levels. Secondly the influence of cerebral trunk centres on SSR emission and latency may be bypassed. Thus the study hypothesis is that conduction velocity determination of SSR constituting fibres will better characterize their functional state than response latency measurement which is subject to central excitability variations.