Bulbocavernosus Reflex - an overview (2023)

Spinal Column and Spinal Cord Surgery (Monitors: Somatosensory-Evoked Potentials, Motor-Evoked Potentials, Electromyogram, And Bulbocavernosus Reflex)

Intraoperative monitoring of SSEPs has been used most extensively in patients undergoing surgical procedures involving the spinal column or spinal cord, or both. Extensive experience has been gained in patients who havedecompressive laminectomies or who have undergone corrective procedures for scoliosis. Intraoperative changes in SSEPs have been noted in 2.5% to 65% of patients undergoing surgical procedures on the spine or spinal cord.^118-121 When these changes are promptly reversed either spontaneously or with interventions by the surgeon or anesthesiologist (e.g., lessening the degree of spine straightening in scoliosis surgery or increasing arterial blood pressure), the patients most often have preserved neurologic function postoperatively. When these changes persisted, however, the patients most often awakened with worsened neurologic function.

False-negative (rare) and false-positive (common) results have been reported with SSEP monitoring during spine surgery. Patients with intact SSEPs throughout the procedure have awakened with a new significant neurologic deficit, but the total reported incidence of this finding is far less than 1% of all cases monitored. Patients with no postoperative neurologic deficit commonly experience significant changes in intraoperative SSEPs.⁷⁰ This monitoring pattern is most commonly caused by failure to control for other, nonpathologic factors that may alter the SSEP. Overall, the reliability of properly performed SSEP monitoring to predict the postoperative sensory and motor function has been reported to be excellent.^41,121,122 Motor tracts are not directly monitored by SSEPs, however. In addition, the blood supply to the dorsal columns of the spinal cord, which carries all of the upper extremity SSEPs and at least a portion of the lower extremity SSEPs, is derived primarily from the posterior spinal arteries. The blood supply to motor tracts and neurons is derived primarily from the anterior spinal artery. It is possible for a significant motor deficit to develop postoperatively in patients with intact SSEPs throughout the operative course. Such events have been reported.^123,124

In operations on the spinal column and after acute spinal cord injury, the sensory and motor changes generally correlate well;⁴¹ however, in patients with neurologic dysfunction after thoracic aortic vascular surgery, frequently posterior spinal cord function (proprioception, vibration, light touch) is left intact when motor and other sensory functions (pain, temperature) are impaired. This result occurred in 32% of patients with neurologic injury after aortic aneurysm repair in one series,¹²⁵ with similar results in many other series. Intraoperative SSEP monitoring in these patients carries a significant risk for false-negative results, and as a result, such monitoring is not widely used.

Bulbocavernosus reflex

The bulbocavernosus reflex (BCR) is an oligosynaptic sacral reflex that can be used to assess the integrity of sacral sensory and motor fibers as well as the sacral spinal cord segments, S2–S4. Intraoperatively, repetitive stimulation of the dorsal penile or clitoral nerve generates an EMG response obtained from hooked wire electrodes placed in the external anal sphincter. Feasibility of BCR monitoring under general anesthesia has been demonstrated in small cohorts of patients (Rodi and Vodusek, 2001; Skinner et al., 2007); however, correlation of intraoperative changes with postoperative function of micturition still has to be established. Whether the application of BCR monitoring is useful in terms of improvement of postoperative outcome has to be deduced from larger studies. Nevertheless, there is no doubt that during tumor resection in the conus or within the cauda equina, information about functional changes of the sacral reflex circuitry is relevant for the preservation of functional tissue.

Sacral Evoked Response: Bulbocavernosus Reflex Latency

This test is used to assess the somatosensory reflexogenic mechanism of penile erection. Testing consists of a direct-current stimulator, which delivers square-wave impulses via two stimulating ring electrodes placed around the penis, one secured near the corona and the other secured 3 cm more proximally, and a recorder that gauges responses via concentric needle electrodes placed in the right and left bulbocavernous muscles. Latency period is measured as the interval from the beginning of each stimulus to the beginning of each response. An abnormal latency time, defined as a value more than three standard deviations above the mean (30 to 40 ms), indicates a high probability of neuropathology (Padma-Nathan, 1988). However, the use of this test has been questioned, and it has been shown that a full battery of electrophysiologic tests evaluating limb nerve function is more sensitive in diagnosing neuropathy than such tests specific to pudendal nerve function alone (Ho et al., 1996;Vodusek et al., 1993).

B Clinical Significance

The bulbocavernosus reflex is one of the few ways to test the conus medullaris (distal end of the spinal cord) and the S2 to S4 pelvic nerves (the only other bedside test of this region is testing sensation in the perineal, or “saddle,” area).^60-62 This reflex is particularly important in patients with urinary retention, which may be caused by disease of the pelvic nerves or cauda equina. In one study of consecutive patients referred for urodynamic studies,⁶⁰ most of whom had difficulty with urination, an absent reflex predicted disease in the S2 to S4 segments only modestly in women (LR = 2.7) but much better in men (LR = 13). The modest accuracy of the sign in women may reflect damage to the pudendal nerve from prior childbirth or pelvic surgery.⁶⁰ In this study the presence of a bulbocavernosus reflex was unhelpful; although the positive response is expected in patients with urinary retention from common disorders like prostate hypertrophy, it also is commonly found in incomplete lesions of the sacral nerves.

In spinal cord injury above the S2 to S4 level (i.e., lesion of upper motor neurons innervating the S2 to S4 segment), the bulbocavernosus reflex also disappears, but only temporarily for a period of 1 to 6 weeks.⁶⁰

Monitoring of the Bulbocavernosus Reflex

The bulbocavernosus reflex is an oligosynaptic reflex mediated through the S2–S4 spinal cord segments that is elicited by electrical stimulation of the dorsal penis/clitoris nerves with the reflex response recorded from any pelvic floor muscles. The afferent paths of the bulbocavernosus reflex are the sensory fibers of the pudendal nerves and the reflex center in the S2–S4 spinal segment. The efferent paths are the motor fibers of the pudendal nerves and anal sphincter muscles. In neurophysiologic laboratories, the bulbocavernosus reflexis usually recorded from the bulbocavernosus muscles, and this is where it gets its name. We have described an intraoperative method for recording the bulbocavernosus reflex from the anal sphincter muscle, with improvement in methodology reported by others.^103-105 The advantage of bulbocavernosus reflex monitoring is that it tests the functional integrity of the three different anatomic structures: the sensory and motor fibers of the pudendal nerves and the gray matter of the S2–S4 sacral segments (seeFig. 4.10). Because of a lack of published statistical data collected for large groups of patients, the reliability of monitoring for conus and cauda equina surgeries remains unclear.

Sacral Reflex Testing

The bulbocavernosus reflex is the most frequently used sacral reflex test and typically involves electrical stimulation of the dorsal penile nerve with recording of the subsequent motor response in the bulbocavernosus muscle.²⁰ Both the afferent and efferent responses of this reflex travel via the pudendal nerve. In patients with sacral cord (S2-S4) lesions or pudendal nerve lesions, latency of this reflex may be prolonged, or the reflex may be absent altogether. However, the sensitivity of this test is less than optimal,⁷⁷ and its value in evaluating erectile dysfunction has been questioned.⁷⁹

Sacral reflexes

Two reflexes are commonly elicited clinically in the lower sacral segments: (1) the BCR (also penilocavernosus); and (2) the anal reflex. EMG recording of the sacral reflex has been shown to be more reliable than the clinically assessed response (e.g., observing and palpating the contraction) (Wester et al., 2003). The reflex recorded by EMG can be elicited by mechanic, electric, or magnetic stimulation. Electric stimuli can also be applied in the perianal region, and, using a catheter-mounted ring electrode, to the bladder neck/proximal urethra (Vodušek, 2006).

Electric stimulation of the dorsal penile nerve elicits (somatosomatic) reflexes in perineal muscles with a typical latency of about 33ms in men (Podnar, 2007a), traditionally called the BCR. In addition to single-pulse electric stimulation, two identical electrical pulses separated by a 3-ms interval can be used (i.e., double-pulse electric stimulation), which is more efficient in eliciting sacral reflexes (Podnar, 2007a). In men, values of 40, 36, and 36ms have been suggested as the upper limit of normal for the shortest latency obtained on eliciting a series of BCR responses using single, double, and mechanic stimulation, respectively (Podnar, 2007a). Sacral reflex responses recorded with needle or wire electrodes can be analyzed separately for each side from the EAS or bulbospongiosus muscle. Using unilateral dorsal penile nerve blocks, the existence of two unilateral BCR arcs has been demonstrated. Thus, by detection from the left and right bulbospongiosus (and also the EAS) muscles, separate testing of right and left reflex arcs can be performed. In cases of unilateral (sacral plexopathy, pudendal neuropathy) or asymmetric lesions (cauda equina), a healthy reflex arc may obscure a pathologic one on clinical elicitation, but not on neurophysiologic measurements of the sacral reflexes.

The BCR was shown to be a complex response, often forming two components. The first component, with a typical latency of about 33ms, is the response that has been most often called the BCR. It is stable, does not habituate, and has other attributes of an oligosynaptic reflex response (Vodušek and Janko, 1990).

In men with cauda equina lesions BCR could not be elicited in 64% / 47% of patients on single/double electric stimulation, respectively. Measurement of the reflex latency increased the sensitivity to record abnormalities for 17% and 36%, respectively. BCR measurement increased sensitivity of quantitative EMG of the EAS muscles from 73% to 83% (Podnar, 2007b). In those subjects in whom BCR is difficult to elicit, double electric stimuli should be used. A complete reflex arc lesion should not be inferred by absence of a response if only single pulse is used for stimulation (Podnar and Vodušek, 2012).

“Simple” electrophysiologic BCR testing has been studied extensively and is used in many laboratories in everyday practice to demonstrate objectively the integrity of the S2–4 reflex arc. BCR testing is suggested as a complementary test to concentric needle EMG (CNEMG) examination of pelvic floor muscles in patients with suspected peripheral nervous lesions (Tubaro et al., 2013).

In addition to latency, a number of other parameters can also be measured using electric stimulation, such as, for instance, the reflex threshold, thus evaluating the excitation level of the sacral reflex pathway. Normative data are available (Podnar, 2007a). The excitation level of the sacral reflex pathway changes physiologically during voiding; BCR cannot normally be elicited during detrusor contraction but in the presence of spinal cord lesions such as myelodysplasia this normal suppression is lost. Recording of BCR during the voiding cycle has been called “dynamic BCR recording” (Walter et al., 1994). It is an interesting concept, revealing the underlying changes of reflex threshold, which has, however, so far no established clinical usefulness.

Continuous intraoperative recording of BCR on penile stimulation is being performed in specialized centers to protect innervation of pelvic organs during particular surgeries (Sala et al., 2013).

Stimulation of the perianal skin, bladder neck, or proximal urethra elicits sacral reflexes with latencies significantly longer than BCR. The reflex responses elicited from on bladder neck or proximal urethra stimulation have a visceral afferent reflex limb (fibers accompanying the pelvic nerves). With visceral denervation (e.g., following radical pelvic surgery) the viscerosomatic reflexes (from both bladder and urethral stimulation) may be lost while the bulbocavernosus (penilocavernosus) reflex is preserved.

The Critical Information in More Detail

Sacral Reflexes

Sacral reflexes refer to electrophysiologically recordable responses of perineal or pelvic floor muscles to electrical stimulation in the urinary-genital-anal region. Two reflexes, the anal and the bulbocavernosus reflex, are commonly clinically elicited in the lower sacral segments. Both have the afferent and efferent limb of their reflex arc in the pudendal nerve, and both are centrally integrated at the S2 to S4 cord levels.^49,53 In women, the bulbocavernosus reflex is clinically elicited by squeezing or taping of the clitoris and observing movement of the perineum or anal sphincter. It is, however, much less reliable than in men,^53,54 and in our opinion, is not useful. The anal reflex is elicited by a pinprick of the perianal skin, producing an anal wink.

Electrophysiologic correlates of these reflexes have been described using electrical, mechanical, and magnetic stimulation. Whereas the latter two modalities have been applied only to the clitoris, electrical stimulation can be applied to other sites, such as the dorsal clitoral nerve and perianal area. Responses are usually detected by needle electrode inserted into the EAS or bulbocavernosus muscle. The bulbocavernosus detection site is preferred because traces do not contain continuously firing, low-threshold MUPs.

The bladder neck or proximal urethra can be stimulated using a catheter-mounted ring electrode, and reflex responses can be obtained from perineal muscles. With visceral denervation, such as after radical hysterectomy, these reflexes may be lost while the sacral reflex mediated by pudendal nerve is preserved. Loss of vesicourethral reflex with preservation of vesicoanal reflex has been described for patients with urethral afferent injury after recurrent urethral operations.

Reports of sacral reflexes obtained after electrical stimulation of the clitoral nerve give consistent mean latencies of between 31 and 39 ms (see Fig. 10-3). Sacral reflex responses obtained on perianal, bladder neck, or proximal urethra stimulation have latencies between 50 and 65 ms.⁴⁹ This more prolonged response is thought to be caused by the afferent limb of the reflex being conveyed by thinner myelinated pelvic nerves with slower conduction velocities than the thicker myelinated pudendal afferents. The longer-latency anal reflex, the contraction of the EAS on stimulation of the perianal region, may also have thinner myelinated fibers in its afferent limb because it is produced by a nociceptive stimulus.⁴⁹

Types