Sistema para avaliação de fibras nervosas periféricas utilizando corrente elétrica senoidal: estudo de caso em hanseníase

Abstract

The Perception Sensory Threshold (ST) for sinusoidal current stimuli at 5, 250, and 2,000 Hz is commonly used in the assessment of peripheral nerve fibers (C, A, and A, respectively). However, the neuroselectivity of these frequencies is far from consensus. In addition, Reaction Time (RT) measurements suggest that 2,000 Hz stimuli excite A-fibers, 250 Hz A- or A-fibers, as well as 5 Hz A-, A- or C-fibers. Therefore, we suppose that the sinusoidal current neuroselectivity may be better observed if ST and RT parameters are jointly evaluated. In addition, we have investigated whether there are other sets of frequencies that could be used. Thus this work investigates ST and RT for stimuli with frequency ranging from 1 to 3,000 Hz, on 28 healthy subjects aged from 19 to 44 years old (27.1 ± 5.49). ST and RT dissimilarity among different frequencies was evaluated applying bi-dimensional Fisher Quadratic Discriminant. The lowest classification error (3.6%) was obtained for 1, 250, and 3,000 Hz. Error for 5, 250, and 2,000Hz was 16.7%. Stimulation frequency at 1 Hz evoked more sensations related to C-fibers (53% of reports) than to A-fibers (36%). However, this behavior did not repeat itself at 5 Hz (only 21% of perceptions were related to C-fibers against 64% to A-fibers). Sensations related to A-fibers prevailed for the highest frequencies presented to the subjects (2,000 Hz 82% and 3,000 Hz 93%). Mean RT values showed a decreasing trend with frequency. The results with healthy subjects suggests that frequencies of 1, 250, and 3,000 Hz are more neuroselective than 5, 250, and 2,000 Hz and that RT parameter show usefulness for the evaluation of peripheral sensitive fibers. The new set of frequencies was then evaluated at 18 leprosy-diagnosed patients or under investigation, comparing LS and RT values with touch-pressure thresholds test, measured using Semmes-Weinstein monofilaments (SWM) of 0·05 g, 0·2 g, 2 g, 4 g, 10 g and 300 g. The results with the leprosy patients showed a strong correlation (r = 0.89 to 2,000 Hz and r = 0.93 to 3,000 Hz) between the SWM tests and high-frequency stimuli. Furthermore, this may indicate that the CPT can also be used as a screening tool on diagnosis of cutaneous form of leprosy and selection of patients who should be submitted to a more detailed examination.