PubMed · 11463046
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W O Roberts. 2001. Which side?. https://doi.org/10.1046/j.1365-2044.2001.02137-35.x
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Recently, transcranial magnetic stimulation of the motor cortex (TMS) revealed impaired voluntary activation of muscles during maximal efforts. Hence, we evaluated its use as a measure of voluntary activation over a range of contraction strengths in both fresh and fatigued muscles, and compared it with standard twitch interpolation using nerve stimulation. Subjects contracted the elbow flexors isometrically while force and EMG from biceps and triceps were recorded. In one study, eight subjects made submaximal and maximal test contractions with rests to minimise fatigue. In the second study, eight subjects made sustained maximal contractions to reduce force to 60 % of the initial value, followed by brief test contractions. Force responses were recorded following TMS or electrical stimulation of the biceps motor nerve. In other contractions, EMG responses to TMS (motor evoked potentials, MEPs) or to stimulation at the brachial plexus (maximal M waves, Mmax) were recorded. During contractions of 50 % maximum, TMS elicited large MEPs in biceps (> 90 % Mmax) which decreased in size (to approximately 70 % Mmax) with maximal efforts. This suggests that faster firing rates made some motor units effectively refractory. With fatigue, MEPs were also smaller but remained > 70 % Mmax for contractions of 50-100 % maximum. For fresh and fatigued muscle, the superimposed twitch evoked by motor nerve and motor cortex stimulation decreased with increasing contraction strength. For nerve stimulation the relation was curvilinear, and for TMS it was linear for contractions of 50-100 % maximum (r2 = 1.00). Voluntary activation was derived using the expression: (1 - superimposed twitch/resting twitch) x 100. The resting twitch was measured directly for nerve stimulation and for TMS, it was estimated by extrapolation of the linear regression between the twitch and voluntary force. For cortical stimulation, this resulted in a highly linear relation between voluntary activation and force. Furthermore, the estimated activation corresponded well with contraction strength. Using TMS or nerve stimulation, voluntary activation was high during maximal efforts of fresh muscle. With fatigue, both measures revealed reduced voluntary activation (i.e. central fatigue) during maximal efforts. Measured with TMS, this central fatigue accounted for one-quarter of the fall in maximal voluntary force. We conclude that TMS can quantify voluntary activation for fresh or fatigued muscles at forces of 50-100 % maximum. Unlike standard twitch interpolation of the elbow flexors, voluntary activation measured with TMS varies in proportion to voluntary force, it reveals when extra output is available from the motor cortex to increase force, and it elicits force from all relevant synergist muscles.
The charts of the 173 consecutive patients who underwent microsurgical reconstruction for obstetrical brachial plexus palsy from 1988 to 1999 (inclusive) in the Division of Plastic Surgery at the Hospital for Sick Children were analyzed. The overall complication rate was 33.5 percent, and there was no mortality in this series. The most significant intraoperative complication was accidental extubation, which occurred five times in the first 84 patients (6 percent of this early group; 2.9 percent of the whole series). This complication was addressed by suturing the endotracheal tube to the membranous septum and by using a transparent drape to allow direct visualization of the tube in all 89 subsequent patients. There have been no further accidental extubations. Postoperative fluid overload occurred in 14 patients (8.1 percent), three (1.7 percent) of whom developed pulmonary edema. Intensive care unit admission was required in two of those patients. Diuretic treatment was required in seven patients. No patient receiving less than or equal to 4 ml/kg/hour developed fluid overload, whereas 50 percent of the patients receiving greater than or equal to 10 ml/kg/hour did. Currently, the authors' policy is to strictly limit intravenous maintenance fluids to 4 ml/kg/hour or less. Despite the long and complex procedure required to reconstruct obstetrical brachial plexus palsy, the incidence of significant complications can be minimized with simple precautions, such as suturing the endotracheal tube to the septum or reducing the amount of fluids administered during the operation.
Tumours of the brachial plexus are rare lesions and may be classified as benign or malignant. Within each of these groups, they are further subdivided into those that are neurogenic in origin (schwannoma, neurofibroma and malignant peripheral nerve sheath tumour) and those that are non-neurogenic. Careful pre-operative diagnosis and staging is essential to the successful management of these lesions. Benign neurogenic tumours are well characterized with pre-operative MRI, appearing as well-defined, oval soft-tissue masses, which are typically isointense on T1-weighted images and show the "target sign" on T2-weighted images. Differentiation between schwannoma and neurofibroma can often be made by assessing the relationship of the lesion to the nerve of origin. Many benign non-neurogenic tumours, such as lipoma and fibromatosis, are also well characterized by MRI. This article reviews the imaging features of brachial plexus tumours, with particular emphasis on the value of MRI in differential diagnosis.