CN EMG findings due to denervation and reinnervation

Fig. 5.3 Comparison of normal (above) and pathological (below) motor unit potentials (MUPs) sampled by multi-MUP analysis from the right halves of the subcutaneous parts of the external anal sphincter (EAS) muscles. On the right, logarithm amplitude vs duration plots of the MUPs are shown; the inner rectangle presents the normative range for mean values, and the outer rectangle for outliers. Below the MUP samples values are tabulated. Three plots on the bottom were obtained by turn/amplitude analysis of the interference pattern (IP) in a patient with a cauda equina lesion. Delineated areas (clouds) present the normative range, and dots individual IP samples. The normal subject was a 45-year-old woman. Results of MUP and IP analysis were normal. The pathological sample was obtained from a 36-year old man with cauda equine lesion caused by central herniation of the intervertebral disc 13 months before the examination, with perianal sensory loss. Mean values for MUP amplitude and area are above the normative range, and polyphasicity is increased. In addition, for all MUP parameters shown, individual values of more than 2 MUPs are above the outlier limits. Note that IP analysis in the patient is within the normative range despite marked MUP abnormalities.

In PFM and perineal muscles, complete or partial denervation may be observed after lesions to its nerves. The changes occurring in striated muscles after denervation are in principle similar. After complete denervation all motor unit activity ceases and there may be electrical silence for several days; 10–20 days after a denervating injury, ‘insertion activity’ becomes more prolonged and abnormal spontaneous activity in the form of short biphasic spikes, ‘fibrillation potentials’, biphasic potentials with prominent positive deflections, and ‘positive sharp waves’ appear. With successful axonal reinnervation MUPs appear again; first short bi- and triphasic, soon becoming polyphasic, serrated and of prolonged duration. In partially denervated muscle some MUPs remain and mingle eventually with abnormal representation of the motor unit potential (MUP) to demonstrate different components, and parameters analysed. Initial part Terminal part Negative afterward Spike part duration Slow part duration 0 5 10 15 20 ms Small positive afterwards Turn Phase 100 ìV Satellite spontaneous activity. In longstanding partially denervated muscle a peculiar abnormal insertion activity appears, so-called ‘repetitive discharges’. This activity may be found in the striated urethral sphincter without any other evidence of neuromuscular disease. In partially denervated muscle, collateral reinnervation takes place. Surviving motor axons will sprout and grow out to reinnervate denervated muscle fibres. This will result in a change in the arrangement of muscle fibres within the motor unit. Following reinnervation several muscle fibres belonging to the same motor unit come to be adjacent to one another; this is reflected in changes of MUPs (increased duration and amplitude). In the late stage, after reinnervation has been completed, CN EMG as a rule fi nds a reduced number of remaining motor units (i.e. the IP of MUPs is reduced). The MUPs are of higher amplitudes, and longer duration, and the percentage of polyphasic MUPs is increased. Such a finding may be taken as proof of previous denervation and uccessful reinnervation. The function of the reinnervated muscle will depend on the number (and size) of remaining motor units. The relative amount of remaining motor units can only be estimated.