The plexus brachialis consists of a network of nerve fibres which terminates in the main nerves of the upper extremities. If parts of the plexus brachialis are damaged during birth, it may lead to palsy of various muscles of the affected arm and hand. In rare cases, an operative revision of the injury is necessary. A method which provides an estimation of the degree of reinnervation in the affected muscles, would offer valuable help for preoperative decisions and postoperative control of the reinnervation progress. The conventionally used method of needle EMG is invasive and therefore especially unpleasant for children. Here the high-spatial-resolution electromyography (HSR-EMG), developed at the Helmholtz Institute, is a suitable alternative.
The onset and conduction of stimuli in muscles is controlled by electrical currents on the cell surfaces, i.e. action potentials (AP). These currents cause a time-varying potential distribution which can be measured on the surface of the skin. The non-invasive high-spatial-resolution electromyography is based on a two-dimensional array of electrodes. This facilitates the detection of the potential on the skin. By spatial filtration of the so acquired potential distribution, it is possible to visualize the excitation of single motor units (MU) (fig.1). These motor units form the functional units of the musculature. The recorded HSR-EMG data curves show typical variations for patients with obstetric plexus lesions.
In the case of reinnervation of musculature paralysed by plexus lesion, the supply of affected myofibres is taken over by the nerves of still intact or already reinnervated MUs. Consequently the size of the MUs enlarges. By this, the signal amplitude measured on the skin surface, increases. At the same time, in comparison to unaffected muscles, the number of APs is reduced.
These variations in the HSR-EMG can be detected quantitatively using the frequency distribution of the signal amplitudes (FS). In case of reinnervation, a typical biphasic curve is observed, while for a healthy muscle a normal distribution results. In a study with 37 children suffering from obstetric plexus lesions it was proven that the (²-value, calculated from the deviation of the FS from normal distribution, provides a good estimate of the degree of reinnervation (fig.2). Therefore, it can be used as a parameter which helps to control the postoperative state of children with severe plexus lesions.