EMG may help to differentiate between normal, denervated and reinnervated and myopathic muscle. In pelvic floor muscles (PFM) and sphincter muscles, ‘neurogenic’ changes are sought as a rule because only patients with suspected denervation injury are routinely referred for assessment. One or several muscles may be examined, according to the clinical problem in the individual patient. The levator ani, anal and urethral sphincter, and bulbocavernosus are the muscles routinely examined, but if a rather equal involvement of PFM is suspected, examination of the external anal sphincter (on one or both sides) suffices.
Concentric needle EMG
Single-use disposable concentric needle EMG (CN EMG) electrodes are used as a rule to diagnose striated muscle denervation/reinnervation. The CN EMG electrode records spike (or ‘near’) activity from about 20 muscle fibres in the vicinity of its active recording surface at the beveled tip. The number of motor units recorded depends both upon the local arrangement of muscle fibres within the motor unit and the level of contraction of the muscle.
CN EMG can provide information on insertion activity, abnormal spontaneous activity, MUPs, and interference pattern. In healthy skeletal muscle initial placement of the needle (and any movement of the tip) elicits a short burst of ‘insertion activity’ due to mechanical stimulation of excitable membranes. Absence of insertion activity with an appropriately placed needle electrode usually means a complete denervation atrophy of the examined muscle. At rest, tonic MUPs are the only normal bioelectrical activity recorded. In partially denervated sphincter muscle there is – by definition – a loss of motor units, but this is difficult to estimate. Normally, MUPs should intermingle to produce an ‘interference’ pattern on the oscilloscope during strong muscle contraction, and during a strong cough. The number of continuously active MUPs during relaxation can be estimated by counting the number of continuously firing low-threshold MUPs.
In patients with lesions of peripheral innervations, fewer MUPs fire continuously during relaxation. In addition to continuously firing low threshold (‘tonic’) motor units, new motor units (‘phasic’) are recruited voluntarily and reflexly. It has been shown that the two motor unit populations differ in their characteristics: reflexly or voluntarily activated ‘high-threshold’ MUPs being larger than continuously active ‘low-threshold’ MUPs. Using the standard recording facilities available on all modern EMG machines, individual MUPs can be captured and their characteristics determined Typically MUP amplitude and duration are measured. To allow identification of MUPs and to be certain the ‘late’ MUP components of complex potentials are not due to superimposition of several MUPs, it is necessary to capture the same potential repeatedly. MUPs are mostly below 1 mV and certainly below 2 mV in the normal urethral and anal sphincter; most are less than 7 ms in duration, and few (less than 15%) are above 10 ms; most are bi- and triphasic, but up to 15–33% may be polyphasic. Normal MUPs are stable – their shape on repetitive recording does not change, Vodušek & Light.
There are indeed two approaches to analyzing quantitatively the bioelectrical activity of motor units: either individual MUPs are analyzed, or the overall activity of intermingled MUPs (the ‘interference pattern’ – IP) is analyzed. Generally three techniques of MUP analysis (‘manual- MUP’, ‘single-MUP’ and ‘multi-MUP’) and one technique of IP analysis (turn/amplitude – T/A) are available on advanced EMG systems. By either method a relevant sample of EMG activity needs to be analyzed for the test to be valid. In the small half of the sphincter muscle collecting ten different MUPs has been accepted as the minimal requirement for using single-MUP analysis. Using manual-MUP and multi-MUP techniques sampling of 20 MUPs (standard number in limb muscles) from each EAS poses no difficulty in healthy controls and most of patients. Normative data obtained from the external anal sphincter (EAS) muscle by standardized EMG technique using all three MUP analysis techniques (multi-MUP, manual- MUP, single-MUP) have been published. There are several technical differences in the methods. The template based multi-MUP analysis of MUP is fast, easy to apply, and allows little examiner bias. Use of quantitative MUP and IP analyses of the EAS is facilitated by the availability of normative values that can be introduced into the EMG system software. It has been shown that normative data are not significantly affected by age, gender, number of uncomplicated vaginal deliveries, mild chronic constipation and the part of the external anal sphincter muscle (i.e. subcutaneous or deeper) examined. This makes quantitative analysis much simpler and results from different laboratories easily comparable. Similar in-depth analyzed normative data by standardized technique for other pelvic floor and perineal muscles are not yet available, but individual laboratories use their own normative data.