Neurophysiological mechanism of the unloading reflex as a prognostic factor in the early stages of idiopathic adolescent scoliosis.

Abstract

thoracic and lumbar erector spinae electromyographic (EMG) responses to a postural perturbation are abnormal in progressive idiopathic scoliotic patients, and that these responses could be used for early prognosis of this affection. The perturbation used was a rapid asymmetrical lowering of a platform on which subjects were standing, eyes closed, and with the neck and knees immobilized. As for the responses of the thoracic paraspinal muscles, she mainly describes a “rebound” followed by a “silent period” and by rhythmical bursts. The “silent period” is interpreted as an unloading reflex, although it follows the rebound rather than preceding it. Considering the potential importance of this study, we made a further analysis of the responses induced in subjects with fast progressive idiopathic scoliosis, referred to as FPIS children (n=2), and in control subjects, that is children who were either non scoliotic (n=3) or non-FPIS (n=15), using the same device and protocol. Confirmation of the existence of several components in the response to the postural perturbation was obtained. However, the results only partly correspond to those published by Dobosiewicz. The initial component of the response, which is present in all subjects, lies between 30 and 200 ms. It presents one or two maxima, but we never observed a “silent period” between two bursts of short duration (about 50 ms). This initial activity is generally similar on both sides, but larger on the side of the thoracic convexity. Its amplitude, latency and time course are variable across subjects, and across trials in a given subject. No constant difference could be seen between thoracic and lumbar muscles. The absence of any significant difference in the responses of the paravertebral thoracic and lumbar muscles to a right or a left tilt, that is, on either side of the convexity, was confirmed. These results indicate that the stimulus may mainly induce a response to stretch in the paraspinal muscles in the concave parts of the spine, as they tend to be lengthened during the lowering phase of the stimulus. This is probably what happens on either side in the lumbar paravertebral muscles, leading to a similar (and more or less symmetrical) pattern of response to right and left tilts. In scoliosis, the same phenomenon is likely to take place on the concave side, leading to reciprocal responses on each side (and possibly different responses to right and left tilt). We found larger responses on the side of the thoracic convexity; this might be due to the fact that, on this side, the muscles in which the response is initiated are mainly located in the lumbar area and not in the thoracic area. LETTER TO THE EDITOR Eur Spine J (2001) 10 :363–364 DOI 10.1007/s005860100279