PELVIC NEUROANATOMY

 

INTRODUCTION

The sensory information about the pelvic organs travels from the periphery to the central nervous system (afferent fibers) and the motor fibers, either somatic (voluntary red muscles) or sympathetic and parasympathetic innervate the the glands, smooth involuntary white muscles. The autonomic nerves are either sympathetic coming through the the anterior roots of T1-L1, or parasympathetic coming through the anterior roots of S2-5. In the inferior hypogastric plexus all autonomic nerves are interconnected. The somatic innervation to the pelvic organ running through the lumbar and sacral plexuses. The hypogastric plexus and the sympathetic trunk are the contributors for autonomic innervation to the pelvis.

THE SACRAL PLEXUS

 The sacral plexus is formed from the lumbo-sacral trunk and ventral rami of S1 down to S5. Anatomical variations are present in population with lumbarization or sacralization of the area. The pelvic parasympathetic splanchnic nerves arise from S2,3,4, which innervate the descending colon, the rectum, urinary bladder and genital organs.

2.1 NEURAL CONTROL OF THE LOWER URINARY TRACT
The lower urinary tract (LUT) is innervated by three sets of peripheral nerves. The pelvic parasympathetic nerves arise at the sacral level of the spinal cord (they excite the bladder and relax the urethra). The sympathetic nerves arise from the upper lumbar segments and inhibit the bladder body, modulate transmission in bladder parasympathetic ganglia, and excite the bladder base and urethra. The somatic efferents and afferents from the S2–S4 sacral roots innervate pelvic floor muscles (levator ani) both through direct branches and by the pudendal nerve, which innervates also the perineal muscles, including the anal and urethral sphincter.
All of these nerves contain both efferent and afferent nerve fibers that are controlled by centers in the brain and particularly important centers in the brainstem. Long tracts in the spinal cord subserve the spinobulbospinal reflex pathway, which is relevant for coordinated detrusor-sphincter function and normal micturition. The dorsal pontine tegmentum is established as an essential control center for micturition (with a close anatomical relationship with the locus coeruleus). While different types of sensation of the lower urinary tract travel both in the anterolateral and the dorsal part of the spinal cord, the descending (motor) pathways lie within the lateral aspects of the spinal cord.

2.2 ANORECTUM
Touch, pin-pricks, and hot and cold stimuli can be perceived in the anal canal to a level of up to 15 mm above the anal valves. The epithelium in the area from about 10–15 mm above the valves has a rich sensory nerve supply made up of both free and organized nerve endings. The sensory endings in the hairy perianal skin are similar to those in hairy skin elsewhere. The afferent nerve pathway for anal canal sensation is by the inferior hemorrhoidal branches of the pudendal nerve. Sensory pathways from the rectum and the bladder travel in the pelvic visceral nerves to the sacral cord, but some afferent information is probably also related to hypogastric nerves entering the spinal cord at the thoracolumbar level.
Functionally, the most important part of the smooth musculature of the anorectum is the internal anal sphincter, which is responsible for about 85% of the resting pressure in the lumen of the canal. The smooth musculature of rectal walls (and of the detrusor) receives extrinsic motor innervation from the sacral parasympathetic outflow arising in the intermediolateral cell columns of sacral cord segments S2–S4. These first-order neurons send axons that emerge with the ventral spinal nerve roots to synapse with second-order neurons lying within the pelvic plexus or the visceral walls. The sympathetic nerve supply arises from the thoracolumbar chain and travels in the hypogastric nerve to innervate visceral smooth muscle directly, and also via a modulatory influence on parasympathetic function at the level of the pelvic plexus. The internal anal sphincter is probably controlled both by sympathetic (hypogastric) and sacral parasympathetic pathways, but the inhibition brought about by rectal distention (the important rectoanal inhibitory reflex) is predominantly an intramural one.
The external anal sphincter is innervated by the pudendal nerve and occasionally also by a perineal branch of S4. The neurons of the sphincter motor nucleus (Onuf’s nucleus) are under voluntary control via corticospinal pathways.
Normal defecation is probably triggered by filling of the rectum from the sigmoid colon, and the signals from stretch receptors in the rectal wall and pelvic floor muscles are interpreted at the conscious level as a desire to defecate. The extension of the rectum causes reflex relaxation of the smooth internal sphincter muscle. Voluntary relaxation of the striated sphincter muscle permits defecation, which is assisted by colonic pressure waves and abdominal straining. If defecation is to be deferred, brief conscious contraction of the voluntary sphincter allows time for recovery of internal sphincter tone and relaxation of the rectum to accommodate filling. Conscious appreciation of the desire to defecate and intentional control over defecation are conferred by suprasacral neural influences. The precise way in which the autonomic, pyramidal, extrapyramidal, and sensory pathways integrate to achieve a reliable and predictable anorectal function is not yet fully understood [11].

2.3 SEXUAL ORGANS
Of the sexual functions affected by neurogenic lesions, research has centered on the male functions, and particularly on erection. Erection can be initiated in the brain and/or follow genital stimulation; in sexual activity a combination of both is probably involved.
Neurogenic erectile dysfunction due to peripheral lesions can be secondary to the disruption of sensory nerves contributing to the afferent arm of reflex erection or to the disruption of autonomic nerves that mediate arterial dilatation and trabecular smooth muscle relaxation. Erectile dysfunction can occur from disruption of the relevant pathways in centers within the spinal cord (both suprasacral and sacral), cauda equina, the sacral plexus, the pelvic plexus, the cavernosal nerves, and the pudendal nerves. Particular pelvic surgeries such as radical prostatectomy or cystoprostatectomy lead to a high percentage of mostly neurogenic erectile dysfunction; the lesion occurs in the pelvic plexus or in the cavernosal nerves located in the posterolateral aspect of the prostate.
Ejaculation can be abolished by a lesion to the sympathetic innervation of the bladder neck (leading to a retrograde ejaculation) and by disruption of the sensory and (particularly) motor nerves innervating the perineal muscles, whose contraction leads to expulsion of the semen. It can also be abolished by central lesions.
A disturbed sexual response in females is due to (1) afferent lesions leading to loss of sensitivity of the perineal area, and (2) efferent lesions leading to a loss of lubrication, loss of clitoral erection, and pelvic floor muscle denervation.

3 CLINICAL NEUROPHYSIOLOGICAL TESTS IN DIAGNOSTICS
Since the function of all the aforementioned systems relies on neural control, clinical neurophysiological tests have been introduced to support and supplement clinical evaluation in patients. The tests comprise electrophysiologic methods of testing conduction through motor and sensory pathways (both peripheral and central) and electromyographic methods. Traditionally, in testing both the lower urinary tract and anorectal function, the EMG signal obtained from sphincter muscles has been used to delineate the sphincter activity patterns in relationship to micturition or defecation. In addition to that, electromyographic methods have been used to distinguish between normal and neuropathic pelvic floor muscles. Conduction tests have been introduced to evaluate the integrity of different reflex pathways (sacral reflexes), the individual motor pathways (pudendal nerve terminal latency, MEP), and sensory pathways (penile sensory neurography, SEP). In addition, autonomic tests have also been introduced (sympathetic skin response, corpus cavernosum EMG). For diagnostic purposes a single testing is performed without knowledge of the previous status of the investigated structure. In this diagnostic situation, results have to be compared to values obtained from healthy subjects. The tests of conduction have been found to be relatively insensitive to axonal lesions because amplitudes of responses vary widely in the control population (particularly due to technical reasons), and conduction may remain normal in partial lesions.
Thus, in the diagnostic situation, the ability of the concentric needle EMG to detect abnormal spontaneous activity as an indicator of denervation, and changes of motor unit potentials as indications of reinnervation, has been found to be particularly helpful. EMGs and recordings of the bulbocavernosus reflex (indicating the potency of the lower sacral reflex arc) have been proposed as the basic battery of tests for evaluation of patients with sacral dysfunctions and suspected neurogenic involvement [12]. From conduction tests, only recordings of the sacral reflex and SEP after dorsal penile or clitoral nerve stimulation have been suggested since they have been validated by extensive clinical studies.
They may be of value in selected patients with suspected peripheral (i.e., bulbocavernosus reflex testing) and central nervous system (i.e., SEP testing) lesions [3, 13]. The other neurophysiological tests have been suggested as useful in further research. The corpus cavernosum EMG is the most controversial of the tests so far described. It is not yet well clarified whether the signal really originates from penile smooth muscle; validation of the method would offer a most important source of information on penile innervation status, which is necessary for erection.

4 INTRAOPERATIVE CLINICAL NEUROPHYSIOLOGY
With appropriate modifications of methods it is technically feasible to record in anesthetized patients (a) dorsal root action potentials (DRAPs) after pudendal nerve stimulation, (b) pudendal somatosensory evoked potentials over the conus, the spinal cord, and the scalp, (c) sphincter muscle EMG responses to sacral ventral root stimulation and motor cortex stimulation, and (d) the bulbocavernosus reflex. Only one of the aforementioned techniques has been used extensively enough to gather pertinent information regarding the practical relevance of sacral nervous system monitoring during surgical interventions. However, the techniques are expected to be valuable safeguards against inadvertent lesioning of nervous structures that would lead to some (neurogenic) dysfunction of micturition, defecation, or the sexual response. Further studies are required to clarify these issues.

4.1 BASIC TECHNICAL ASPECTS OF STIMULATION FOR INTRAOPERATIVE SACRAL MONITORING
In order to obtain bioelectrical signals useful for monitoring purposes in the different segments of the sacral neuromuscular system, it is necessary to depolarize the nervous system at particular segments. Up to now only electrical stimulation has been appropriate for this purpose. Stimulation can be applied to either the sensory part or the motor part of the system (afferent versus efferent events; Fig. 9.1). At present, most intraoperative monitoring of the sacral system has relied on responses evoked from stimulation of the sensory system, apart from recording of anal sphincter muscle responses upon stimulation of ventral spinal roots or the motor cortex.

FIGURE 9.1 Neurophysiological events used to intraoperatively monitor the sacral nervous system. Left, “afferent” events after stimulation of the dorsal penile or clitoral nerves and recording over the spinal cord: (1) pudendal SEPs, traveling waves, (2) pudendal DRAPs, and (3) pudendal SEPs, stationary waves, recorded over the conus. Right, “efferent” events: (4) anal M wave recorded from the anal sphincter after stimulation of the S1–S3 ventral roots, (5) anal motor-evoked potentials recorded from the anal sphincter after transcranial electrical stimulation of the motor cortex, and (6) bulbocavernosus reflex obtained from the anal sphincter muscle after electrical stimulation of the dorsal penile or clitoral nerves. Reprinted from Deletis, V. (2001). Neuromonitoring. In “Pediatric neurosurgery,” 4th ed. (D. MacLeone, ed.), pp. 1204–1213. W.B. Saunders, Philadelphia.

FIGURE 9.2 Lower left, position of the electrodes over the clitoris and labia majora for the stimulation of the dorsal clitoral nerves. Lower right, position of the electrode for stimulating the dorsal penile nerves. R1 = recording BCR from anal sphincter. Upper right, schematics of recording DRAPs with a hand-held hook electrode (R2) over the exposed dorsal sacral roots of the cauda equina. Upper left, intraoperative picture of DRAP recordings.

The appropriate peripheral sensory structures that are available for stimulating purposes are the two adjacent dorsal penile or clitoral nerves. These nerves are stimulated by silver/silver chloride cup EEG-type electrodes (EEG electrodes) placed on the dorsal surface of the penis or clitoris (this electrode representing the cathode). The other electrode (anode) is placed either distally on the penis (1–2 cm apart from the proximal electrode) or on the adjacent labia (Fig. 9.2). In small children with short penises, the anode may be attached on the ventral side of the penis. The dorsal side of the penis must be scrubbed gently with Nuprep (D. O. Weaver & Co., Aurora, CO) before placing electrodes in order to avoid stimulus artifacts. The electrodes are filled with electrode cream and secured appropriately (Tegaderm; Smith and Nephew Medical Limited, Hull, England). The electrode sites are then bandaged with a few layers of gauze to prevent them from being displaced when the patient is moved onto the
operating table. Electrode impedances should be kept below 5 kΩ.
As a general rule, stimuli of 20 mA intensity at 0.2 ms duration have been delivered in procedures involving stimulation of the penis or clitoris (at various frequencies for different measurements, up to 13.3 Hz).
Stimulation can also be performed at the level of the spinal roots; a handheld sterile monopolar electrode can be placed under the appropriate roots (or rootlets, after the root is freed from neighboring roots and lifted outside the spinal canal). Square wave pulses of 1 to 2 mA intensity and 0.2 ms duration are delivered (Fig. 9.1, right).

4.2 BASIC TECHNICAL ASPECTS OF RECORDING FOR INTRAOPERATIVE SACRAL MONITORING
Bioelectrical activity from the sacral neuromuscular system has up to now been recorded from the sacral dorsal spinal roots, the spinal cord, the somatosensory cortex, and the anal sphincter muscle.
Recordings from dorsal spinal roots (dorsal root action potentials, DRAPs) are obtained by hand-held sterile bipolar hook electrodes (after the root is freed from neighboring roots and lifted outside the spinal canal). In this case, the electrode closer to the point of stimulation is the G1 (active) electrode. Epoch lengths of 0 to 50 ms are used for these recordings (Figs. 9.1, 9.2).
Recordings of pudendal spinal somatosensory evoked responses (SSEPs, stationary wave) are obtained by a spinal epidural electrode placed over the conus (S2–S4). These potentials are generated by interneurons of the grey matter within the S2–S4 segments of the spinal cord. Typically 100 responses are averaged together; epoch lengths of 0–50 ms are used (Fig. 9.1).
Recordings of pudendal spinal somatosensory evoked responses (SSEPs, traveling wave) are obtained by a spinal epidural electrode inserted anywhere over the dorsal column of the spinal cord. These potentials are generated by pudendal afferents traveling within the dorsal columns. Typically 100 responses are averaged together; epoch lengths of 0–50 ms are used (Fig. 9.1).
To obtain pudendal cerebral somatosensory evoked responses (CSEPs), Screwtype recording electrodes are placed on the scalp 2 cm behind CZ (G1 or active electrode) and at FZ (G2 or reference electrode), according to the somatotopic representation in the primary somatosensory cortex related to the International 10–20 System of scalp electrode placement. The active electrode is placed in the
midline because the sacral segments are represented deep within the medial longitudinal interhemispheric fissure. For CSEPs, 100–200 traces are typically averaged together. Epoch lengths of 0–200 ms are used.
To record anal sphincter (EMG) responses, either surface-type electrodes or hook wire electrodes can be used. Given the close anatomical relationship between the small sphincter muscle and neighboring larger muscle groups, recording must be selective (e.g., with intramuscular hook wire electrodes) if the stimulation technique is “nonselective” (such as in the case of ventral root stimulation, as a consequence of which neighboring muscles are also excited).
When the stimulation procedure is more specific (e.g., in bulbocavernosus or pudendoanal reflex monitoring), the recording may be obtained with properly attached surface-type electrodes. Sterile hook wire recording electrodes (Teflon-coated, 76 μm diameter wire with a 3 mm bare tip) are introduced into the left and right sides of the external anal sphincter with sterile needles; these are immediately removed carefully from the sphincter (the hooked wires remaining in place). The integrity of the electrodes can be tested by passing a short train of 50 Hz current at 10 mA and observing sphincter contraction (if the patient is not paralyzed at the time of electrode placement). The electrode impedances of these electrodes should be checked, although clean recordings are usually still possible with high electrode impedances.
The epoch length used for anal sphincter EMG recordings varies according to the type of response; either single or few averaged responses can be obtained upon stimulation (of course, the patient should not be under the influence of a muscle relaxant during this procedure).

4.3 SPECIFIC SACRAL NEUROMUSCULAR SYSTEM MONITORING PROCEDURES

4.3.1 Pudendal Dorsal Root Action Potentials (DRAPs)

In the treatment of spasticity (e.g., in cerebral palsy), the sacral roots are increasingly being included during rhizotomy procedures. Lang [14] demonstrated that children who underwent L2–S2 rhizotomies had an 81% greater reduction in plantar/flexor spasticity compared to children who underwent only L2–S1 rhizotomies. But, as more sacral dorsal roots have been included in rhizotomies, neurosurgeons have experienced an increased rate of postoperative complications, especially with regards to bowel and bladder functions.
In order to spare sacral function, we attempted to identify those sacral dorsal roots that were carrying afferents from pudendal nerves. To do this, we used recordings of dorsal root action potentials (DRAPs). Patients were anesthetized with isoflurane, nitrous oxide, fentanyl, and a short-acting muscle relaxant introduced only at the time of intubation. The cauda equina was exposed through a T12–S2 laminotomy or laminectomy and the sacral roots were identified using bony anatomy. The dorsal roots were separated from the ventral ones, and DRAPs were recorded by a hand-held sterile bipolar hooked electrode (the root being lifted outside the spinal canal) (Fig. 9.2). The DRAPs were evoked by electrical stimulation of the penile or clitoral nerves. One hundred responses were averaged together and filtered between 1.5 and 2100 Hz. Each average response was repeated to assess its reliability. Afferent activity from the right and left dorsal roots of S1, S2, and S3 was always recorded, along with occasional recordings from the S4–S5 dorsal roots. DRAP recording was successfully obtained in the majority of patients, the DRAPs were present in the S2 and S3 roots bilaterally in 19 patients, whereas in 7 patients DRAPs were also present in both S1 roots (in 8 patients they were present in the S1 root unilaterally). However, the response in the S1 root was never larger than the S2 or S3 root responses. The range of amplitudes was 2.9–18.3 μV in S1 roots, 3.2–129.9 μV in S2 roots, and 4.6–333 μV in S3 roots.
Of special relevance was the finding that in 7.6% of these children, all afferent activity was carried by only one S2 root (Fig. 9.3, C and F). These findings were confirmed by a later analysis of the results of mapping in 114 children (72 male, 42 female, mean age 3.8 years) [5]. Mapping was successful in 105 out of 114 patients; S1 roots contributed 4.0%, S2 roots 60.5%, and S3 roots 35.5% of the overall pudendal afferent activity. The distribution of responses was asymmetrical in 56% of the patients (Fig. 9.3, B, C, and F). Pudendal afferent distribution was confined to a single level in 18% (Fig. 9.3, A), and even to a single root in 7.6% of patients (Fig. 9.3, C and F). Fifty-six percent of the pathologically responding S2 roots during rhizotomy testing were preserved because of the significant afferent activity, as demonstrated during pudendal mapping. None of the 105 patients developed long-term bowel or bladder complications.
All our results in the early series of dorsal root mapping with 19 patients [4] have been confirmed by analysis of the larger series of 105 patients [5]. With this series we showed that selective S2 rhizotomy can be performed safely without an associated increase in residual spasticity, while at the same time bowel and bladder function are preserved by performing pudendal afferent mapping [14].
Therefore, we suggest that the mapping of pudendal afferents in the dorsal roots should be employed whenever these roots are considered for rhizotomy in children with cerebral palsy without urinary retention. Preoperative neurourological investigation of the children should help in making appropriate decisions; for example, in children with cerebral palsy with hyperreflexive detrusor dysfunction, sacral rhizotomy may be considered to alleviate the problem. In any case, intraoperative mapping of sacral afferents should make selective surgical approaches possible and provide the maximal benefit for children with cerebral palsy.

FIGURE 9.3 Six characteristic examples of DRAPs showing the entry of a variety of pudendal nerve fibers to the spinal cord via S1–S3 sacral roots. (A) Symmetrical distribution of DRAPs confined to one level (S2) or three levels (D). Asymmetrical distribution of DRAPS confined to the right side (B), only one root (C or F), or all roots except right S1 (E). Recordings were obtained after electrical stimulation of bilateral penile/clitoral nerves.

Our results so far support the hypothesis that the root distribution of afferent fibers that are important for the control of micturition may be similar to the distribution of mucocutaneous afferent fibers from the pudendal nerve. Only further studies will clarify the complex functional anatomic issues involved.

Recordings were performed in four children of both sexes, 2.5 to 7.0 years of age. Small-amplitude (up to 1 μV) SSEPs, which were very stable, could be recorded with subdurally placed electrodes over the thoracic spinal cord (traveling waves), while a stationary wave could be recorded with a much higher amplitude (up to 10 μV) over the conus region. The latencies of the spinal SEP over the conus region ranged from 6.0 to 10.4 ms (Fig. 9.1, left). The recordings were made as a pilot study, and thus far only demonstrate the ability to obtain such recordings intraoperatively. Further employment of this technique showed that traveling waves are difficult to record, and successful recording of a stationary wave necessitates that the electrode be placed strictly over the S2–S4 sections of the spinal cord.

4.3.3 Pudendal Cerebral Somatosensory Evoked Potentials (CSEPs)
Well-formed cerebral SEPs with amplitudes of 0.5–0.7 μV were recorded on dorsal penile nerve stimulation throughout spinal neurosurgical procedures in two adult male patients, 50 and 78 years of age. Stable P40 peaks were obtained.
The recordings were made as a pilot study and thus far only demonstrate the ability to obtain such recordings intraoperatively. Further employment of this method showed that this potential is very sensitive to anesthetics; a formal feasibility study has not yet been performed.

4.3.4 Anal Sphincter Motor Response Monitoring
In five children of both sexes, 2.5 to 9.0 years old, anal sphincter muscle EMG responses were recorded by stimulation of the ventral spinal roots (L5, S1, S2, S3, and S4) to identify ventral roots carrying motor fibers to the sphincter muscle. Recordings were obtained by surface conductive rubber electrodes (applied para-anally) and intramuscular hooked wire electrodes. In surface recordings, no unilateral responses could be identified, and responses were also obtained on stimulation of the L5 and S1 roots (on stimulation of L5 and S1 roots no adequate responses could be discerned from simultaneous recordings from intramuscular electrodes). The surface recorded responses were recognized as “nonspecific” (derived from neighboring muscles, most probably glutei [15]).

The latency of surface recorded responses was, as a rule, shorter than the latency of responses obtained from intramuscular electrodes, which was between 5 and 8 ms. On electrical stimulation of the motor cortex, anal sphincter responses were recorded in a large group of anesthetized patients without pyramidal involvement. Because of polysynaptic connections of the corticospinal tract to the α-motoneuron of S2–S4, these responses are moderately sensitive to anesthetics (Fig. 9.1).

4.3.5 Bulbocavernosus Reflex (BCR) Monitoring
Intraoperative recordings were first performed in 15 neurosurgical patients (11 males, 4 females, 2–6 years old). Patients were without sacral dysfunction and were anesthetized with fenatyl and propofol or nitrous oxide without the influence of a muscle relaxant. Recordings from the anal sphincter were obtained by hooked wire electrodes and were recorded as a single response.
Very reproducible responses could be obtained on double-pulse stimulation, the optimum interstimulus interval being found to be 3 ms and the optimum stimulation rate 2.3 Hz. Continuous periods of stimulation and recording for up to 10 min were repeatedly performed with very reproducible results (Fig. 9.4). The reflex response was suppressed by the administration of isoflurane and nitrous oxide and was completely abolished by muscle relaxants [16]. After this pilot study, 119 patients were tested, 38 of which underwent surgery without risk and 81 of which underwent surgery with risk of damage to sacral structures. In all, 51 adults (19 to 64 years old, 32 male and 19 female) and 68 children (24 days to 17 years old, 30 male and 38 female) took part in the study. Patients were anesthetized with propofol, fenatyl, or nitrous oxide with a short-acting relaxant. Clinically, most patients had mild to moderate upper motor neuron deficits in the lower extremities, and no patient had major urinary problems. In all patients it was possible to record reproducible reflex response with the previously described method. In patients without risk to the sacral system, only a few minutes of the responses were recorded to test their feasibility, whereas in the patients at risk, continuous monitoring was conducted.

FIGURE 9.4 Continuous monitoring of the BCR for a period of 10 min showing the stability of the BCR’s appearance.	FIGURE 9.5 The influence of isoflurane (Iso) on the BCR. Note that the response was almost completely blocked when the concentration of 1.25% was administered for 15 min and did not recover until almost 30 min after the isoflurane was discontinued. Reprinted from [6].

The influence of volatile anesthetic was tested in 6 patients. In 3 patients, BCR was suppressed by the administration of 1.25% isoflurane for 15 min (Fig. 9.5). Administration of nitrous oxide (60% inspired concentration) for 15 min reduced the BCR in 3 patients, and the response was completely abolished by muscle relaxant in 2 patients [6].
Preliminary results conducted in 50 patients with a lumbosacral tethered cord showed no clear correlation between BCR and postoperative sphincter control. The authors concluded that the complexity of sphincter innervation and segmental or suprasegmental control probably accounts for this discrepancy [17].

Up to now, close to 250 patients have been monitored using this method. In the last 100 patients, a train of four consecutive stimuli [7] was used rather than a double stimulus, providing even more robust responses. Some problems have been encountered in female patients in whom the stimulation method is not as robust as required; this problem needs particular attention from the technician.

5 DISCUSSION AND CONCLUSIONS

The impetus to include intraoperative monitoring of sacral nervous structures came from concerns over optimizing care for children with cerebral palsy. These were successfully treated for their spasticity by performing selective dorsal rhizotomy; surgeons have always sought to minimize the side-effects of this procedure while maintaining its benefits (benefits being a reduction in muscle hypertonia, the side-effects being a partial or complete loss of sensory modalities). The first sacral dorsal roots have always been a target for rhizotomy in this procedure, but the S2 dorsal roots have become increasingly considered for rhizotomy. Not to treat the S2 dorsal roots is to leave potentially abnormal reflexive circuits that will continue to drive spasticity in the musculature of the leg. For this reason, the lesion zone was extended to include S2 dorsal roots, and the result was a greater reduction in spasticity as compared with children in whom the lesion was extended only to the S1 segment. Unfortunately, the extension to include S2 roots was also associated with disorders in micturition.
In one group of patients, before DRAP mapping was introduced, 24% experienced urinary retention (which was only transient in most children). Although most of these children had both the left and right S2 roots cut, 2 of the children had only one S2 root cut, and they also experienced retention. The selective dorsal rhizotomy procedure should be performed in young children who are certainly too young to assess sexual function; they are even too young to be completely confident that all symptomatic complaints regarding micturition or defecation and perineal sensation were being relayed. It was this concern over the preservation of genitourinary afferent function that led us to develop the technique of intraoperative neurophysiological identification of the sacral roots responsible for perineal sensation. In the first 31 children, neurophysiological identification of roots and rootlets carrying afferent activity from the penile or clitoral nerves led to zero micturition disturbances and allowed for rhizotomy of S2 roots or rootlets not carrying such afferent activity. Therefore, maximum possible antispastic effects could be achieved. The particularly important lesson we learned by doing very systematic recordings in S1–S2 and S3 roots bilaterally (and in some children also S4 and S5 roots) is that although most of our patients showed evidence of pudendal afferents in S2 and S3 roots bilaterally, about half of them also showed evidence of some afferent activity in S1 (either unilaterally or bilaterally). In slightly more than half of our patients, the root potentials were symmetrical, but the pudendal afferents of many were irregularly distributed across the sacral roots. In a few of the children the afferentactivity was confined to a single root (either S2 or S3 root). Since this may be the most important afferent contribution from the genitourinary area, even if only the S2 dorsal roots are checked for relevant afferent activity, they should not be sacrificed if they show any such activity. The finding of asymmetrical distribution of fibers that may be confined to a single root is consistent with the work of Junemann et al. [18], who found that the majority of motor fibers for the urethral sphincter are carried by a single variable lower sacral root. Therefore, for some patients undergoing an operation in the area of the cauda equina, the sacrifice of one single root may have dire functional consequences.
For other neurosurgical procedures in the lumbosacral spinal canal (e.g., the release of a tethered cord or the removal of a tumor), other authors have proposed, and in a small series have performed, the identification of motor and sensory nerve roots. This has been achieved through continuous monitoring of electromyographic activity in muscles innervated by lumbosacral segments, and through monitoring of tibial nerve somatosensory evoked potentials [19]. In addition, monitoring pudendal SEPs should provide very relevant complementary information, and, as we have demonstrated, should not be technically demanding if the structures are preserved preoperatively. Kothbauer et al. [19] claim that intraoperative recordings saved operating time by allowing the surgeon more rapid and decisive preparation than would be possible on an anatomical basis alone, and they also gave the impression that the procedures were safer. The recordings of spontaneous anal sphincter electromyographic activity during such operations have been described earlier [20]. To accomplish the two goals of neurophysiology (i.e., first, immediate identification of structures as functional nervous tissue and their distinction from other tissue; and second, continuous monitoring of the function of the relevant nervous structures), a battery of methods needs to be applied, and a whole set of structures needs to be assessed bilaterally. Therefore, both the lower sacral segments and sphincter muscles, and also the upper sacral segments and the lumbar segments, need to be included. Also, all these segments and several functional modalities need to be monitored more or less simultaneously. The appropriate setup for each surgical situation would need to be selected on the basis of anatomical and physiological considerations, and a compromise between the possible and the necessary would be sought.
Other authors have claimed that continuous EMG recording of bursts or trains of motor unit potentials or repetitive neurotonic discharges elicited by injury to the peripheral motor fibers have correlated with postoperative transient or permanent neurological deficit [21, 22] in the area of facial nerves. The predictive value of these “manipulation-evoked” discharges is only based on empirical data, but has been proposed to also be of value in the lumbosacral segments [19]. The electrophysiological identification of motor nerves is already an integral part of cranial nerve surgery [22] and should also provide a similar service in the region of the cauda equina.
Up to now, only recordings of DRAPs have been made in a large number of subjects (to identify sacral roots carrying genitourinary afferents), and the electrophysiological procedure decreased postoperative voiding disturbances [4, 5].
We propose, however, that the other intraoperative electrophysiological recordings of the sacral neuromuscular system that have been described (spinal SEP, CMAP of sphincter muscles, bulbocavernosus reflex recordings) should prove relevant in surgeries involving the sacral roots, the cauda equina, and the conus and should aid the surgeon in preventing inadvertent damage to these structures.
These other procedures have not, however, been performed in an adequate number of patients, and their relevance cannot yet be appraised. The most interesting question is whether monitoring of the bulbocavernosus reflex for conus and cauda equina surgery could, as we are proposing, replace and even improve upon separate monitoring of motor and sensory fibers of the lower sacral roots.
As for surgeries involving the spinal cord above the conus, the procedures of pudendal SSEPs, cortical SEPs, and (possibly) motor evoked potentials of the anal sphincter muscle might be interesting in some selected patient groups.
These would include groups in whom the preservation of sacral function may be particularly important (for instance, in scoliosis surgery in patients with advanced neuromuscular diseases who have heavily compromised motor function but no sacral dysfunction).
In other contexts, authors have argued that recordings from the anal sphincter cannot be taken as completely adequate information on the functional status of the urethral sphincter [18]. Since innervation of both sphincters originates from the same sacral segments (which also provide innervation for the detrusor), the monitoring of sphincter ani should generally mirror the function of relevant urethral structures.
As previously stated, the relatively limited experience with the monitoring of sacral structures cannot as yet prove that surgeries accompanied by such monitoring are easier and safer. For the time being, there is anecdotal evidence that difficult surgical decisions that have relied on the results of intraoperative neurophysiological measurements have not resulted in unexpected neurological deficits. Although the techniques described clearly provide the surgeon with additional information about nerve location and function, the value of these techniques must be further defined and documented. As Daube [23] suggests, it will be necessary to demonstrate that these techniques indeed save operative time, save anesthesia time, or—most importantly—improve outcomes.
It will be difficult to demonstrate this without doing a randomized study of patients who are undergoing surgery with and without such monitoring techniques.