When anal sphincter weakness is detected clinically and/or at manometry in patients with fecal incontinence, anal sphincter EMG recordings are included in the pelvic floor assessment in order to identify anal sphincter neurogenic injuries resulting from damage to the sacral spinal cord, cauda equina, S2–4 spinal nerves, or pudendal nerve (pudendal neuropathy), and endoanal ultrasound imaging is used to detect anal sphincter anatomic lesions.

Several studies, including two controlled studies, using quantitative EMG analyses have shown that there is a significant prolongation of motor unit duration in the external anal sphincter and puborectalis muscles in fecally incontinent patients (Bartolo et al. 1983; Sorensen et al. 1991). The results of quantitative EMG recordings of the anal sphincter have been correlated with anal pressure during voluntary contractions (Sorensen et al. 1991; Cheong et al. 1995). Gregory et al. performed a quantitative analysis of anal sphincter EMG recordings in patients with fecal incontinence, with or without anal incontinence, after vaginal delivery (Gregory 2008b). The interference pattern analysis of the group of postpartum women with anal incontinence symptoms showed evidence of denervation and subsequent reinnervation (Gregory et al. 2008b). This study and others prompted the American Gastroenterological Association to recommend that concentric needle EMG recordings be used to look for denervation due to peripheral nerve lesions when assessing patients with fecal incontinence (Barnett et al. 1999).

Dyssynergia is defined as a paradoxical increase in anal sphincter pressure (anal contraction) of less than 20 % relaxation of the resting anal sphincter pressure or inadequate abdomino-rectal propulsive forces leading to difficult defecation. In two-thirds of subjects, dyssynergia is a consequence of faulty toilet habit, painful defecation, obstetric or back injury, or brain-gut dysfunction (Rao et al. 2005). The diagnostic criteria for dyssynergic defecation are as follows (Rao 2008): patients must satisfy the symptomatic diagnostic criteria for chronic constipation (Rome III) and must show a dyssynergic pattern on manometry, imaging, or EMG of defecation during repeated attempts to defecate. Anorectal dyssynergia is suspected when there is no relaxation (EMG activity stable or higher) of the external anal sphincter and/or puborectalis muscles on EMG testing during attempts to defecate. Surface EMG recordings appear to be more useful than needle EMG recordings in providing evidence of non-relaxation of the anal sphincter or puborectalis muscle during defecation in constipated patients (Pfeifer et al. 1998). Only a few studies have investigated the predictive value of EMG recordings for excluding a diagnosis of pelvic floor dyssynergia in constipation. Bordeianou et al. studied constipated patients with and without pelvic floor dyssynergia and compared the results of anal sphincter EMG and the balloon expulsion test to the results of defecography, which is considered the reference method for diagnosing pelvic floor dyssynergia (Bordeianou et al. 2011). They reported that 84.1 % of the patients with abnormal EMG results did not expel the balloon. However, the presence of these abnormalities, in isolation or together, did not predict the presence of dyssynergia on defecography (Bordeianou et al. 2011). Consequently, it not clear which of the three tests (anorectal manometry, EMG, imaging) most accurately diagnose pelvic floor dyssynergia.

Multiple system atrophy is a neurodegenerative disease presenting with a combination of Parkinsonian, cerebellar, and autonomic (including cardiovascular, urinary, and anorectal) dysfunction. While it is pathologically defined, there is no definitive clinical diagnostic test. The majority of patients with probable multiple system atrophy have an abnormal sphincter EMG because of the selective loss of anterior horn cells in Onuf’s nucleus (Gilad et al. 2001; Palace et al. 1997). However, patients with idiopathic Parkinson’s disease do not show marked sphincter EMG abnormalities. As such, these abnormalities can be used to distinguish multiple system atrophy from idiopathic Parkinson’s disease in the first 5 years after disease onset. In contrast, similar sphincter EMG abnormalities are found in some, though not many, patients with dementia with Lewy bodies, pure autonomic failure, progressive supranuclear palsy, or spinocerebellar ataxia type 3 (Winge et al. 2010). The limitations of sphincter EMG recordings should thus be kept in mind. Sphincter EMG recordings associated with sacral autonomic tests are also used as diagnostic tools for autonomic disorders (Winge et al. 2010).

Kiff and Swash (1984) developed a stimulating electrode known as St. Mark’s pudendal electrode (Fig. 20.3). A bipolar stimulating electrode is mounted on the tip of the gloved index finger, which is inserted into the rectum. Recording electrodes located 3 cm proximally at the base of the fingers pick up the contraction response of the anal sphincter. The ischial spine is located on transrectal examination, and electrical stimuli are applied at that site to stimulate the pudendal nerve where it leaves the pelvis through the greater sciatic notch and before it branches into the inferior rectal nerve (to the anal sphincter) and the perineal nerve (to the periurethral striated muscle). Fowler (1995) applied a stimulation on both sides of the pelvis and reported that the mean latency of the response from the anal sphincter is 2.1 ± 0.2 ms (Fowler 1995).

The pudendal nerve latency technique is quite reliable (Tetzschner et al. 1997a). However, because of the very short latency value (less than 2.5 ms) and because of a frequent marked motor artifact, alternative stimulation and recording techniques have been developed. In women, the pudendal nerve can be stimulated through the vaginal wall using a St. Mark’s electrode, with a recording surface or needle electrode on the anal sphincter (Tetzschner et al. 1997b). The stimulation can also be performed through the anus, but the response of the bulbo(clitorido)-cavernosus muscle must be recorded using a needle electrode in order to avoid the motor artefact and obtain easier-to-interpret responses. Lastly, magnetic shocks can be applied over the sacral roots at the sacral foramina (see chapter below).

Pudendal nerve terminal motor latency is the time lapse between the stimulation of the pudendal nerve and the response of the anal sphincter. Terminal motor latency is prolonged if the nerve between the site of stimulation and the muscle is demyelinated, as occurs with mechanical nerve injuries or diabetes (Fowler 1995).

Numerous studies have reported prolonged pudendal nerve terminal motor latencies in various anorectal disorders, including fecal incontinence (Cheong et al. 1995; Lefaucheur 2006), perineal descent (Lefaucheur 2006), and constipation (Vaccaro et al. 1994). Snooks et al. reported that terminal motor pudendal latency was prolonged for 2–3 days after vaginal delivery in 42 % of the women and that the abnormalities were more marked in multiparous women and in those who had a prolonged second stage of labor and forceps delivery. Two months later, the abnormality was resolved in 60 % of the women, but recovery was poorer in multiparous women (Snooks et al. 1984). A follow-up study 5 years later involving some of the multiparous women showed that the occult damage to the pudendal nerve persisted and became more marked with time (Snooks et al. 1990). An initial pudendal nerve injury at the time of the childbirth or during chronic straining at stools, such as with constipation, may explain abnormal pudendal nerve latencies associated with anorectal disorders that worsen with succeeding deliveries and/or repeated straining at stools, with traction on the pelvic floor leading to further stretch-induced injury to the pudendal nerve.

Pudendal nerve latency has been proposed as a predictive factor for the clinical outcome of biofeedback therapy (Leroi et al. 1999) and anal sphincter repair (Gilliland et al. 1998), but not for sacral nerve stimulation (Gallas et al. 2011). However, the conduction velocity of a nerve may have little bearing on its functional integrity. The prevalence of prolonged pudendal nerve terminal motor latency in patients presenting for anorectal neuropathies has been reported to be 20–28 % with unilateral neuropathy and 11–12 % with bilateral neuropathy (Ricciardi et al. 2006; Gurland and Hull 2008). The majority of incontinent patients with intact sphincters have a normal pudendal nerve terminal motor latency (Ricciardi et al. 2006). Bilateral neuropathy, but not unilateral pudendal neuropathy, is associated with diminished sphincter function and higher incontinence scores (Ricciardi et al. 2006). As such, there is no consensus on the significance of pudendal nerve terminal latencies, and the American Society of Gastroenterology does not recommend the use of this test for the routine assessment of patients with anorectal disorders (Barnett et al. 1999). However, the diagnosis of pudendal neuropathy should not be limited to an assessment of pudendal nerve terminal motor latency since pudendal nerve conduction velocity measurements are abnormal only when the largest and most heavily myelinated nerves are lost. This may explain the lack of sensitivity of pudendal nerve terminal motor latency measurements for detecting anal sphincter denervation.

Sacral reflexes are reflex contractions of striated pelvic floor muscles that occur in response to stimulations of the perineum or genital region. One of the reflexes most commonly used in research is the bulbo(clitorido)-cavernosus reflex , which can be elicited by electrical stimulations of the dorsal nerve of the penis or clitoris. A needle electrode is used to record the responses of the bulbo(clitorido-)cavernosus muscle (Fig. 20.4). Following a stimulation of the dorsal nerve, afferent impulses are conveyed via the pudendal nerve to the sacral spinal cord through the posterior roots. After a variable synaptic delay, efferent impulses traveling in the pudendal nerve give rise to contractions of the bulbocavernosus muscle. There are two components to this reflex: the first response, which is used clinically, has a latency of the order of 35 ms while the second later response occurs after approximately 60–70 ms (Fowler 1995).

Sacral reflex latencies are influenced by age and gender (Pradal-Prat et al. 1998). Since it is often easier to elicit the reflex in males than in females, no significance should be given to its absence in women (Fowler 1995). A double-shock stimulation rather than a single shock can be used to decrease the rate of failure (Podnar 2014). The sacral reflex latency test should be part of the diagnostic armamentarium for investigating neurogenic pelvic floor disorders. The results, however, should be interpreted with caution since a normal latency does not exclude the possibility of a partial axonal lesion while, on the other hand, an abnormal latency may not be clinically relevant.

Sacral reflex latency investigations are indicated for suspected lesions of the conus medullaris or cauda equine, sacral radiculopathies, and sacral plexus. Only extreme pudendal nerve demyelination can cause a significant delay in peripheral conduction leading to an abnormal bulbocavernosus reflex latency (Podnar 2011). The few studies that have investigated the sacral reflex latency of patients with anorectal disorders seem to confirm the usefulness of this test for diagnosing sacral reflex arc lesions (sacral afferent fibers, sacral spinal cord, sacral efferent fibers) in the case of anorectal disorders. Ismael et al. described perineal electrophysiologic findings in 19 women with pelvic floor disorders (urinary and/or fecal incontinence, dysuria and/or dyskesia, sexual dysfunctions) after vaginal delivery (Ismael et al. 2000) and reported no associated lower limb sensory or motor deficits. However, perineal electrophysiologic examinations revealed signs of denervation with abnormal bulbo(clitorido)-cavernosus reflexes in all cases (Ismael et al. 2000). This study highlighted the value of bulbo(clitorido)-cavernosus reflex latency measurements in patients with suspected lumbo-sacral plexopathies but no lower limb deficits. In another study, Podnar assessed the sensitivity, specificity, positive predictive value, and negative predictive value of quantitative concentric needle EMG recordings of the external anal sphincter muscles and neurophysiologic measurements of the bulbo(clitorido)-cavernosus reflex, individually and in combination, for diagnosing sacral neuropathic lesions in 24 women with chronic cauda equine lesions with bladder, bowel, and/or sexual dysfunction (Podnar 2014). Podnar reported a high sensitivity and negative predictive value (98–100 %) and a reasonably high specific and positive predictive value (50–75 %) of the bulbocavernosus reflex associated with anal sphincter EMG recordings for confirming or excluding sacral neuropathic lesions (Podnar 2014).

It is possible to study central motor pathway conduction to the external anal sphincter by recording motor evoked potentials (MEPs) by the transcranial magnetic stimulation of the motor cortex (Lefaucheur 2006) (Fig. 20.5). Magnetic shocks can also be applied over the lumbar and sacral roots at the sacral foramina to evoke anal sphincter compound muscle potentials (Lefaucheur 2006). Central conduction along the pyramidal tract is calculated by subtracting the response latency to sacral root stimulation from the total conduction time to motor cortex stimulation (Fig. 20.5). Sacral magnetic stimulation has a number of advantages for measuring peripheral motor conduction time to the anal sphincter since it is less uncomfortable than intra-rectal stimulation and can be used to study the pudendal nerve along its entire length (Remes-Troche et al. 2007).

However, the anal MEP technique requires the use of a double-cone coil rather than a circular coil for the cortical stimulation to obtain interpretable results. The electromagnetic field induced by a double-cone coil is better adapted for stimulating the cortical representation of the anal sphincter, which is deep within the interhemispheric fissure (Lefaucheur 2006). Needle electrodes, surface electrodes, or anorectal pressures can be used to measure the anal sphincter response following cortical or sacral stimulation (Lefaucheur 2006). However, failure rates of 14–25 % have been observed for sacral magnetic stimulation because the long recovery period of the stimulus artifact interferes with latency measurements, especially in the case of needle EMG recordings (Jost and Schimrigk 1994; Loening-Baucke et al. 1994; Sato et al. 2000). Intrarectal placement of the ground electrode substantially reduces the stimulus artifact and improves the reliability of the technique (Lefaucheur 2005). Normal MEP values depend on the stimulation (type of coil, stimulation parameters) and recording techniques used.

The relevance of investigating MEPs in patients with anorectal disorders remains to be confirmed. Such investigations may facilitate the diagnosis, understanding, and follow-up of anorectal diseases in which the brain-gut axis is involved (Lefaucheur 2006). For example, trans-lumbar and trans-sacral MEPs have been used to reveal significant lumbo-sacral neuropathies in 90 % of subjects with various levels of spinal cord injury (Tantiphlachiva et al. 2011). A study of 65 fecally incontinent patients with no known neurologic disease revealed abnormal anal cortical MEP latencies in 21.5 % of the patients (Paris et al. 2013). More recently, Rao et al. described abnormal lumbo-anal, lumbo-rectal, sacro-anal, and sacro-rectal MEPs in 44 (88 %) of 50 subjects with fecal incontinence (Rao et al. 2014). In these cases, abnormal MEP latencies might reveal undetected lesions of pelvic floor motor pathways that could help in the management of fecally incontinent patients. For example, patients with normal cortical MEP latencies might benefit more from treatments such as biofeedback than patients with abnormal motor responses who are unable to voluntarily contract their external anal sphincter muscle. However, this is pure conjecture and needs to be confirmed by other studies.