(1)
Functional Urology Unit, Casa Madre Fortunata Toniolo, Bologna, Italy
Electromyography (EMG) is a study of bioelectrical activity of striated muscles.
The unit of measure is in microvolts (µV).
Pelvic floor muscle EMG is most commonly recorded as part of urodynamic studies to obtain information about the kinesiological behavior of pelvic floor structures during filling and voiding.
According to AUA/SUFU guidelines on adult urodynamics, EMG of pelvic floor muscles is recommended in patients with relevant neurological disease at risk for neurogenic bladder.
EMG may also be useful in the evaluation of neurologically intact individuals with obstructive pressure-flow studies in the absence of an anatomical obstruction (dysfunctional voiding).
The study may be quantitative or qualitative:
The quantitative EMG (kinesiological EMG) allows an assessment of the state of relaxation or contraction of the muscle investigated, and that is what is normally detected with an equipment of urodynamics. For this type of recording, surface electrodes are appropriate.
The qualitative EMG (neurophysiological EMG) is the recording of the action potentials of muscle cells investigated through needle electrodes and recording oscilloscope with sound recording. Its use is limited to cases of neurological injuries in which it is necessary to assess the extent of denervation and reinnervation phenomena.
8.1 Types of Electrodes and Setup
Three main types of electrodes for EMG measurement are available (Fig. 8.1):
Figure 8.1
EMG electrodes (a) surface (b) needle (c) wire.
Surface
Needle
Wire
Surface electrodes consist of small patches with shielded cables or unshielded wires with reusable snaps.
The active plates are placed as close as possible to the anal sphincter at 3 and 9 o’clock position, while the ground electrode is usually placed in the medial aspect of the thigh (Fig. 8.2). The wires are then fixed on symphysis pubis before their connection with the recording device. Attempts should be made to avoid any possible wetting of the electrodes during voiding.
Figure 8.2
Surface and needle electrodes placement. Ground is usually placed on the buttock or medial aspect of the thigh
Footnote 1
When surface electrodes are used, the skin should be shaved and cleaned with abrasive paste prior to their placement.
Footnote 2
EMG activity is detected, for convenience, from the anal sphincter. In fact, the anal sphincter, such as urethral sphincter, is a part of pelvic floor musculature with which shares both innervation and functional activity. However, in cauda equina lesions as well as in the peripheral nerves injuries, there should be a discrepancy in behavior between the urethral sphincter and anal sphincter, so it is preferable to detect activity directly from the urethral sphincter by a needle electrode.
The major advantages of surface electrodes are their noninvasiveness. The main drawback is that they use, by convention, perianal muscle activity as surrogate for urethral sphincter activity. Several studies have demonstrated separate innervation of levator ani and urethral sphincter. This suggests that levator ani activity may not accurately reflect urethral sphincter activity. This condition is the rule in sacral neurologic lesions and in peripheral nerve lesions, in which it is more appropriate to record electrical activity directly from urethral sphincter.
Concentric needle electrodes consist of a hollow steel needle within which there is a fine wire.
The potential difference between the outer and inner core is measured while the patient is grounded with a separate surface electrode. The electrodes record electrical activity within 0.5-mm radius of the tip.
Concentric needles are placed into the periurethral muscles near the urethral meatus in women and advanced parallel for 1–2 cm (Fig. 8.3) into the bulbocavernous muscle in men. Correct placement can be verified by an audio monitor which will demonstrate a poppy noise that increases when the patient is asked to contract the muscles. The major limit of needle electrodes is patient discomfort and limitation of mobility during urodynamic testing to avoid their dislodgement. The advantage is the possibility to analyze single-action potentials through an oscilloscope monitor.
Figure 8.3
Needle electrode placement in female
An alternative to concentric needle electrodes are the wire electrodes which consist of platinum or copper wires with a small hook on the tip which are inserted into the pelvic floor using 25-gauge needles. Once inserted, the needle is withdrawn leaving the hook within the musculature. Like concentric needles, wire electrodes are placed into periurethral muscles in female and in the bulbocavernous muscle in male. Again, audio monitor is recommended while placing the wires.
Once inserted, wire electrodes cannot be manipulated and when necessary they must be removed and replaced. However they permit a better movement of the patient during urodynamic testing.
8.1.1 Checking the Correct Position of the Electrodes
The correct position of the electrodes can be verified by asking the patient to contract and release the anal sphincter: the contraction results in an increase of activity, while relaxation causes a reduction of the activity. The cough (increased activity) can be used with the same purpose.
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