Sensory Neurography and S2PD, a Test-Retest Study

A neurophysiological examination can objectively measure remaining function in an injured digital nerve. Using this method, axons that are still able to conduct an action potential are assessed.

The response can be measured as the amplitude (the summed action potentials from the nerve's axons) and as the area (the amount of functioning axons). The area is a somewhat more precise measurement method, as it includes all axons, both those with low and high conduction velocities. Amplitude measurements may give falsely low values if nerves with poorer degrees of myelination are included in the measurement. Amplitude is measured in µV (microvolt) and area in µV/ms (microvolt per millisecond).

The method currently used to assess discriminative sensation is the S2PD-test. In this test two pins are pressed perpendicular and longitudinally on each side of the fingertip. The pins can consist of an unfolded paper clip or a factory-made discriminator. The distance between the two pins, where it is possible to discriminate between one or two pins, is the S2PD. The test is influenced by many external factors, such as the clinician's level of experience, the patient's desire to give the correct answer, language barriers, and similar issues. Furthermore, this examination method does not clarify what is actually being measured beyond the patient's subjective discriminative sensation.

If neurophysiological assessment is used as a complement to S2PD and can provide an objective measurement tool with high reliability and reproducibility, it could become an important addition to the clinical arsenal for follow-up after nerve injury. Such knowledge would make it easier to clinically evaluate, understand, and monitor the healing process of the injured nerve. It would also facilitate the assessment and systematic follow-up of treatment outcomes.

The participants will be examined at the Hand Surgery Clinic at South General Hospital at two separate occasions separated. Each visit is estimated to last approximately 60 minutes. It will include assessment using S2PD and neurophysiological examination of the radial side of the second and fourth fingers on both hands.

Finger temperature will be measured using an infrared thermometer (Testo 826-T2), and, if necessary, the fingers will be warmed (using a water bath, heating pad or something similar) to a target temperature of a minimum of 32 °C. The hands will then be gently abraded using a preparation gel (NuPrep). Conductive gel (Ten20) will be applied to the electrodes.

A recording electrode will be placed volarly over the wrist, a ground electrode in the palm, and the stimulating electrode on the radial side of the finger (digits II and IV). Electrical stimulation will be delivered in an orthodromic direction, from the finger toward the wrist. The stimulus intensity will be gradually increased until no further increase in amplitude is observed/amplitude is stable. At the first visit, the examination begins with S2PD followed by sensory neurography. At the second visit, the order of the assessments is reversed to minimize potential crossover effects.