Fig. 7.1
Generation and propagation of an action potential (AP) in a cylindrical muscle fiber. (a) The axonal branch releases acetylcholine at the neuromuscular junction (NMJ), and triggers the AP. The underlying membrane depolarizes generating current loops that extend the depolarization to nearby regions of the cell. (b) The current “poles” corresponding to this situation at time t1 are depicted. Upward arrows indicate currents exiting the muscle fiber and depolarizing the membrane, downward arrows indicate currents entering the muscle fiber and repolarizing the membrane. (c) Two APs are being generated in space and propagate in opposite directions as the NMJ area repolarizes (time t2). (d) The current “poles” corresponding to the situation at time t2 are depicted: two tripoles are generated. (e) The two APs are fully generated and travel in opposite directions towards the tendon junctions. (f) Two separate current tripoles describe the propagating APs at time t3 (Reproduced with permission from Fig. 3.4 of [10])
Let us now consider two depolarized regions (two current tripoles) moving along a muscle fiber parallel to the skin as indicated in Fig. 7.2a. The two potential distributions resulting on the skin, due to these sources, are depicted in Fig. 7.2b.
Fig. 7.2
Generation of multichannel EMG. (a) Single muscle fiber with two depolarized zones (current tripoles) propagating in opposite directions starting from the neuromuscular junction (see Fig. 7.1e, f). (b) Analog differential voltage detected on the skin, due to the propagating tripoles, and instantaneous samples in space provided by the amplifier array at a fixed time instant t = tk. (c) Array of differential amplifiers connected to an array of electrodes placed on the skin (A-N). Note that signals propagating in the right direction see the non-inverting input first and then the inverting input while the opposite happens for signals propagating in the left direction. (d) Set of real EMG signals acquired with a linear electrode array (such as the A-N array) from the biceps brachii muscle. The 1-15 channels correspond to the 1-15 output signals of the differential amplifiers versus time. Many propagating potentials, generated by many motor units, can be identified. It is interesting to observe that all the motor units are innervated under channel 7 and extend over the entire muscle length. Channels 14 and 15 are over the distal tendon area. The dashed lines represent the “signature” of a motor unit. Inter-electrode distance = 10 mm
Consider the array of electrodes A-N and differential amplifiers with outputs 1-15 depicted in Fig. 7.2c. The propagating action potential generated by one fiber and detected by this set of differential amplifiers, at a given time instant, is depicted in Fig. 7.2b. The fibers innervated by branches of the same axon, forming a motor unit, generate a motor unit action potential (MUAP) which is the sum of the single fiber action potentials associated to each discharge of the neuron. Figure 7.2d shows a 0.2 s segment of multichannel EMG detected by the array of Fig. 7.2c placed on a biceps brachii muscle. An example of MUAP is identified by dashed lines in Fig. 7.2d, together with many other MUAPs generated by motor units of the same muscle with fibers parallel to the skin. All motor units are innervated under channel 7 and terminate beyond channel 1 and at channel 13. Channels 14 and 15 are above one of the tendons. The other tendon is beyond channel 1. Figure 7.2d provides a space-time (y versus t) representation of “firings” of many motor units (MUAPs) as detected by an electrode array placed on the skin along the muscle fiber direction. Each trace represents the signal detected by one electrode pair (1-15).
In the case of the anal sphincter muscle the motor units are arranged in a circular pattern and so are the electrodes, as depicted in Figs. 7.3 and 7.4. Figure 7.3a shows a motor unit spanning channels 14 to 4 of a circular electrode array. The IZ is “under” electrodes 15-16 and propagation is detected up to past electrodes 14 (counterclockwise) and 4 (clockwise). Five motor units are schematically depicted in Fig. 7.3c, each with different length and innervation zone location. If motoneurons MN1 and MN2 are cut by the episiotomy surgery (“lesion” in Fig. 7.3c) the corresponding motor units are denervated and may or may not be re-innervated after surgery. A commercially available EAS probe is depicted in Fig. 7.4a and an example of experimental signals is depicted in Fig. 7.4c. It is evident, from Fig. 7.4c, that in this EAS muscle there are two main IZs: one under channel 14-15 with clockwise propagation and one under channel 12-13 with counterclockwise propagation (channel 1 is anterior and channel 9 is posterior).
Fig. 7.3
Schematic diagram of the arrangements of motor units of the external anal sphincter (EAS). (a) One motor unit (motorneuron 1 and muscle fibers). (b) Differential signals detected by electrode pairs placed on the circumference of a cylindrical probe inserted in the anal canal (see Fig. 7.4). (c) Schematic diagram of the arrangements of five motor units of the EAS. Two of these motor units are innervated by motor neurons (MN1 and MN2) comprised in the right-ventral quadrant (RV) of the EAS, two are innervated by motor neurons (MN3 and MN4) comprised in the left dorsal quadrant (LD) of the EAS and one by a motor neuron (MN5) in the left ventral quadrant of the EAS. A right mediolateral episiotomy would likely damage or interrupt MN1 and MN2 and denervate the corresponding motor units. A left right mediolateral episiotomy would likely damage or interrupt MN5
Fig. 7.4
(a) Intra-anal probe with 16 circumferential electrodes. (b) Cross-sectional view of the electrode array and schematic representation of two motor units. (c) Example of anal sphincter EMG differential signals. Two motor units can be identified (among others): the first is innervated at one end under channel 13 and extends to channel 6, the second is innervated at one end under channel 15 and extends to channel 4. Other motor units can be identified and their innervation zone can be localized (Permission from [9], Fig. 1, page 114)
It has been shown that the innervation of the EAS sphincter is very variable from subject to subject [1, 4, 5, 9]. Examples of schematic and real propagating MUAPs are provided in Fig. 7.5.
Fig. 7.5
(a) Schematic representation of innervation zones and propagation directions of two motor units, one in the right and one in the left hemisphincter. (b) Schematic representation of innervation zones and propagation directions of two long motor units, one innervated under channel 8 and one innervated under channel 1-16. (c) Experimental signals showing motor units innervated under channel 2-3, under channel 12, under channel 1-3 and under channel 15. Consider that, due to the circular probe, channel 1 is next to channel 16
Figure 7.5a schematically depicts the MUAP pattern of motor units innervated under channel 5 and under channel 12-13 while Fig. 7.5b shows a case of innervation under channel 8 and a case of innervation under channel 1-16. Figure 7.5c depicts a set of experimental signals showing a number of MUs innervated under channels 2, 12, 1-3, 1-16. Figure 7.6 shows a circular histogram of the distribution of IZs of 2,748 motor units observed in 500 pregnant women [4, 5] indicating that the ventral side of the EAS is the least innervated while the right and left hemisphincters have rich innervation which is highly variable from person to person [1].
Fig. 7.6
Circular histogram of the location of innervation zones of 2748 motor units identified in the external anal sphincter of 500 women. The distribution is bimodal with higher likelihood of innervation zones in the right and left hemisphincter (Reproduced with permission from [4])
7.3 Results of Recent Studies on the Innervation of the External Anal Sphincter and Puborectalis Muscles
Since the distribution of IZs of the EAS shows a very high inter-subject variability [1] no general rule can be proposed in favor of right or left or midline episiotomy. Figure 7.7 depicts four examples of EAS innervation and related episiotomy risks. Figure 7.7a depicts a case of bilateral hemisphincter innervation implying a medium risk for a short incision, Fig. 7.7b depicts a case of high risk due to innervation being concentrated in the rectovaginal septum (perineum), Fig. 7.7c depicts a case of dorsal innervation of the EAS and therefore low risk, while Fig. 7.7d depicts a case of innervation in the left-ventral quadrant implying a high risk of a left mediolateral episiotomy and a much lower risk of a right mediolateral episiotomy; of course the opposite holds for a single innervation region in the right-ventral quadrant of the EAS.
Fig. 7.7
Four innervation modalities of the anal sphincter with four different risks levels related to episiotomy. (a) Innervation zones concentrated in the right and left hemisphincter: risk is moderate and depends on the pattern of the axonal branches. (b) Innervation zones in the right and left ventral quadrants of the sphincter: the risk of partial denervation of the sphincter due to episiotomy is high, regardless of the side. (c) Innervation zones in the right and left dorsal quadrants of the sphincter: the risk of partial denervation of the sphincter due to episiotomy is low, regardless of the side. (d) Innervation zones in the left ventral quadrant of the sphincter: the risk of denervation of the sphincter is high if a left mediolateral episiotomy is performed and low if a right mediolateral episiotomy is performed. A mirror-like situation implies high risk of denervation if a right mediolateral episiotomy is performed reprinted with permission from Di Vella G, Riva D, Merletti R. Incontinenza dello sfintere anale sterno da episiotomia e prevenzione del danno iatrogeno. Riv It Med Leg. 2015; 37(2):473–489
The technique described in [12–14] and summarized in Sect. 7.2.2 of this chapter, was adopted in the double blind study (“Technologies for anal sphincter analysis and incontinence (TASI)”) [5] carried out on 331 pregnant women, 86 of which underwent mediolateral episiotomy (82 on the right side, 4 on the left side). One of these cases is outlined in Fig. 7.8. Figure 7.8a shows the arrangement of nine motor units and of their IZs, identified as described in Sect. 7.2.2, 6 weeks before delivery and 6 weeks after delivery. For methodological details see [14]. Despite the evidence of major denervation and of EAS EMG asymmetry, this woman was not incontinent at the time of the second evaluation.
Fig. 7.8
(a, b) Distribution of nine motor units and of their innervation zones before and after delivery with right mediolateral episiotomy. (c, d) The thickness of the black arcs indicate the average rectified value of the EMG. Despite the major denervation the woman was not incontinent 6 weeks after delivery (Reproduced from [5])
Depending on the individual EAS innervation pattern, episiotomy may or may not imply alterations of such pattern. In some cases, random factors, posture or different contraction modalities might even cause the involvement of a greater number of motor units in the postpartum condition. Statistical considerations are therefore required. Figure 7.9a shows the prepartum distribution of IZs of 82 women who underwent episiotomy. The postpartum distribution of IZ is depicted in Fig. 7.9b. The decrement on the number of IZ in the RV quadrant of the EAS is evident and statistically significant (p < 0.05) whereas it is not in the other quadrants (see also Fig. 7.10b and [5] for issues related to EAS tears). Figure 7.10 compares the changes between pre- postpartum number of IZs in each EAS quadrant in the case of cesarean delivery (60 cases) and episiotomy (82 cases). No significant changes are observed in the first case while a statistically significant change is observed in the RV quadrant of the EAS in the second case. Nevertheless, cesarean delivery is not necessarily preventive of EAs damage [15].
Fig. 7.9
Circular histogram of the distribution of innervation zones in the anal sphincter of 82 women before and after child delivery with right mediolateral episiotomy. A reduction of innervation zones in the right-ventral (RV) quadrant of the sphincter is evident and is statistically significant (see Fig. 7.10b) while changes in other quadrants are not reprinted with permission from Di Vella G, Riva D, Merletti R. Incontinenza dello sfintere anale sterno da episiotomia e prevenzione del danno iatrogeno. Riv It Med Leg. 2015; 37(2):473–489
Fig. 7.10
Mean and standard deviation of the number of motor units identified with innervations in each of the four quadrants of the anal sphincter. (a) Pre- and postpartum with cesarean delivery (60 subjects). (b) Pre- and postpartum with right mediolateral episiotomy (82 subjects). The only pre–post difference that is statistically significant concerns the number of IZ in the right-ventral (RV) quadrant in the episiotomy cases (Reproduced from [5])
The puborectalis muscle is the second most important muscle providing continence. This muscle is not affected by episiotomy but may be stretched by pregnancy. It might easily be investigated, with the same minimally invasive technique, with an electrode array applied on a gloved finger [3].
7.4 The Controversial Issue of Episiotomy: Analysis of the Literature
Episiotomy is a controversial practice which is applied routinely in some countries (Latin America and Eastern Europe) and more rarely in others (Scandinavian Countries) with the purpose of reducing the stress of the newborn and the likelihood of perineal and sphincter tears which are difficult to suture. Right mediolateral episiotomy is by far more common than left mediolateral episiotomy because it is more easily performed by a right-handed person. Mediolateral episiotomies angled closer to the midline are significantly associated with anal sphincter injuries (26° vs. 37°, p = 0.01) [17, 18]. Standard obstetric and midwifery texts indicate that a mediolateral episiotomy should be at least 40° from the midline but doctors and midwifes follow different rules [16]. If episiotomy is performed at the proper angle (45° to 60° from the midline) [16–18] it is unlikely that the EAS is directly damaged, nevertheless the axons and their terminal branches innervating the muscle may be cut, resulting in more or less extensive EAS denervation which may or may not be followed by reinnervation in the first few weeks after delivery [34]. This damage may lead to increased likelihood of fecal incontinence (FI) in later years [19].
Women with anal sphincter tears are more than twice as likely to report postpartum fecal incontinence than women without sphincter tears [20]. Anal sphincter tears occur in 2–19 % of vaginal deliveries [21], however limited information is available on episiotomy-related EAS denervation which may have similar consequences on voluntary muscle control.
Stay updated, free articles. Join our Telegram channel
Full access? Get Clinical Tree
