Ugur Yilmaz, Claire C Yang
Neurophysiologic testing of female genital innervation is an extension of the genital physical examination, and is used to search for evidence of motor and sensory nerve disruption. The nervous system is intricately involved with female sexual function, but how the central nervous system and peripheral nerves mediate the sexual response is still largely unknown (see Chapters 4.1-4.4, 5.3, 10.5, 16.5, and 16.6 of this book). Existing electrodiagnostic methods for evaluating the integrity of the components of the nervous system, such as nerve conduction studies, somatosensory evoked potentials, and electromyography, can be used in clinical and research settings not only to define genital neuropathology, but also to clarify the physiology of the sexual response.
In this chapter, we describe (a) basic principles used in electrodiagnostic tests, (b) anatomic correlates of the procedures, (c) extant genital neurophysiologic tests, and (d) future applications in clinical female sexual function research.
Basic technical principles in clinical neurophysiology
Most clinical neurophysiology tests require sophisticated electronic instruments that include computer-based software programs. A standard electromyography/evoked potential unit comprises amplifiers, display, loudspeaker, and data storage device, and can deliver stimuli and record specific signals through electrodes. The electrodes, surface/cutaneous, needle,
or affixed to probes, can be used for both neural stimulation and recording of bioelectric signals in accordance with the specific testing procedure. Surface electrodes are usually square or round metal plates made of platinum or silver, applied to the skin with adhesive plates or fixed with tape and conducting jelly. A surface electrode records signals from a wider region (recording radius of 20 mm), whereas a needle electrode is useful to select signals from a smaller area (recording radius of 0.5 mm).1 Needle electrodes, such as concentric (coaxial), monopolar, bipolar, or single-fiber needles, vary according to procedure.
Electrical activity is recorded through electrodes, and the bioelectrical potentials are amplified and displayed for visual analysis. Bioelectrical potentials can be “evoked”, or generated in response to a stimulus (e.g., nerve conduction studies or somatosensory evoked potential tests). Potentials can also be measured in muscle during rest or during contraction, as in electromyography.
The recording settings of the instrument vary with the parameters of the bioelectrical signal, and they should have appropriate gain, sweep (time-base), and frequency filters. For instance, a slow-frequency signal, such as a sympathetic skin response, can be recorded with a longer duration of sweep (1 s/div) and a narrower frequency window (0.1-100 Hz) than a somatosensory evoked potential test, which involves a shorter sweep (100 ms/div) and wider frequency limits (10-3000 Hz). For responses with small amplitudes (e.g., somatosensory evoked potential recordings), the evoked potentials must be averaged to obtain a better signal-to-noise ratio. Typically, at least 200 stimuli are delivered and the corresponding époques are averaged. In tests of sympathetic activation, the response amplitudes are usually large and subject to habituation (fading); therefore, averaging is not employed. In most situations, tests should be repeated at least twice to demonstrate the reproducibility of the response.
Responses are characterized by the shape of waveforms, latency, and amplitude measurements. Response latencies are measured either from the onset of response (e.g., sacral reflex tests) or from the individual peaks of the potentials (e.g., somatosensory evoked potentials). Amplitudes are measured relative to the baseline or “peak to peak” (Fig. 10.6.1).
Stimulation of a nerve should evoke a recordable, reproducible, and defined response. Several different types of stimuli, such as electrical, mechanical, or magnetic, can be delivered to depolarize a nerve. In clinical neurophysiology applications, electrical stimulus is the most commonly applied stimulus type, and it is defined by several parameters characterizing its duration, intensity, and pattern of delivery. In somatosensory evoked potential tests, an electrical stimulus with a rectangular pulse, 0.1-ms duration, two to three times of sensory threshold intensity, and recurrent pattern is typically employed. An example of stimulus parameters for the sympathetic skin response is a rectangular pulse of 0.2-ms duration, 10-15-mA intensity, and nonrecurrent pattern.2
Figure 10.6.1. The waveform of a pudendal somatosensory evoked potential (SEP) test. After the onset, the response follows a downward deflection (PI: first positive peak) and an upward deflection (N1: first negative peak). The second positive (P2) and negative (N2) peaks follow. The latency of the responses is usually measured at P1 response. The amplitude of the response is measured between the first peak-to-peak distance (P1-N1).
Female genital neuroanatomic correlates of neurophysiologic tests
The female genitalia are innervated by both somatic and autonomic nerve fibers (Fig. 10.6.2). Somatic nerves are myelinated, and thus of large diameter, and conduct neural impulses relatively rapidly. Somatic motor fibers mediate impulses to skeletal muscles (under voluntary control), and somatic sensory fibers mediate information from the skin, skeletal muscles, and joints. Autonomic fibers are of small diameter, and poorly myelinated or nonmyelinated, and conduct neural impulses relatively slowly. Their motor impulses mediate involuntary responses such as sweat gland release, blood flow, and gut peristalsis. Autonomic sensory fibers are known as visceral afferents, and conduct sensory information from the viscera and blood vessels (see Chapter 4.2).
Figure 10.6.2. Innervation of female genitalia. Thoracolumbar sympathetic fibers (T11-L2), coursing through the hypogastric nerve, join with the sacral parasympathetic fibers (S2-S4) to form the pelvic plexus on either side of the bladder. The pelvic plexus supplies the autonomic innervation of genital organs, including the uterus, cervix, proximal vagina, clitoris, bulbs, and bladder. Somatic sensory branches innervating the distal vagina, clitoris, and perineum join to form the pudendal nerve (S2-S4), the motor branches of which innervate the pelvic floor and perineal muscles.
The somatic sensory and motor innervation of female genitalia is carried via branches of the pudendal nerve. The pudendal nerve arises from sacral spinal segments 2-4 (S2-S4). The nerve enters the perineum after traveling through Alcock’s canal.3 There are three branches of the pudendal nerve (dorsal clitoral, perineal, and inferior rectal), each carrying sensory input to the S2-S4 spinal levels from the genital structures, which is relayed through the central nervous system via ascending spinal tracts.4,5 The skeletal pelvic floor muscles receive somatic innervation mainly from the motor branches of the pudendal nerve (perineal nerve, inferior rectal nerve).6,7
The exact pathways of the genital autonomic fibers within the human have not been documented, but they generally follow the major arteries of the abdomen and pelvis. The pelvic plexus is the main crossroad for autonomic nerves, consisting of parasympathetic fibers from the sacral nerve roots (S2-S4) and sympathetic nerve fibers from the thoracolumbar sympathetic nerve roots (T11-L2). The plexus lies on either side of the bladder and rectum, and supplies all the pelvic viscera, including the bladder and urethral sphincter, the uterus and upper vagina, and the female genital tissue.8 These autonomic fibers are especially important since they regulate the blood flow to the vagina, clitoris, and bulbs.9-11
Neural testing of somatic nerves
This section will briefly review the neurophysiologic testing applicable to the somatic innervation of the female genitalia. Neurophysiologic testing involves the assessment of both the afferent (sensory) and efferent (motor) nerves. Sensory innervation is usually tested by stimulating the sensory branch and recording either from the somatosensory cortex (e.g., somatosensory evoked potentials) or from the distal end of the sensory nerve, as in nerve conduction studies. The motor component is evaluated with electrodes from the innervated muscle or muscle groups, with or without nerve stimulation.
Somatic sensory tests
Somatosensory tests are evaluations of nerve conduction, which primarily evaluate large myelinated nerve fibers, without regard to the smaller fibers. The genital somatosensory pathway can be evaluated along the whole neuraxis with pudendal somatosensory evoked potentials and, in theory, on certain segments of the nerve pathway with lumbar evoked potentials to localize a neural lesion.
Somatic sensory nerve pathways can also be assessed by quantitative sensory tests, which are based on subjective reports of sensory experience (see Chapter 10.5). Although it is possible to evaluate small-diameter, unmyelinated nerve fibers by quantitative sensory testing, this method depends on the psychophysical/cognitive factors of the subject tested and therefore lacks the objectivity of nerve conduction studies. Quantitative sensory testing measures the integrity of the entire sensory pathway without the capacity to localize abnormal findings.12
Pudendal somatosensory evoked potentials
The technique of pudendal somatosensory evoked potentials involves stimulation of a pudendal nerve branch, such as the dorsal clitoral nerve or perineal nerve, with recurrent electrical square wave stimuli, and recording multiple responses with averaging techniques in order to increase the signal-to-noise ratio. The recording is done with scalp electrodes placed over the somatosensory cortex where genital sensation is mapped. At this point, the highest amplitude response can be obtained (Cz -2 cm: Fz of the International 10-20 electroencephalogram System).4
The primary method to perform pudendal somatosensory evoked potential testing is to depolarize the dorsal clitoral nerve. It can be stimulated with self-adhesive surface electrodes placed bilaterally on each side of the clitoral body, allowing assessment of each clitoral nerve branch separately.13 The stimulus intensity is usually two to four times the sensory threshold. A vaginal probe mounted with electrodes can depolarize the perineal nerve in the distal vagina and labia minora to elicit somatosensory evoked potentials similar to those obtained with depolarization of the dorsal clitoral nerve (Fig. 10.6.3).14
Figure 10.6.3. Vaginal probe used to depolarize the perineal nerve in female subjects. The cathode electrode (distal) is positioned to the introitus, and the anode (1 cm proximal) is in contact with the labia minora.
The response in healthy women is a W-shaped potential, and the latency is measured at the first positive (downward) peak, called P1. The P1 response occurs at approximately 40 ms. This first response is followed by an upward deflection of the waveform, defined as the first negative peak at about 55 ms (N1 response). The amplitude of the response is measured between the onset and first positive peak or between the first positive and negative peaks4,15,16 (Figs 10.6.1 and 10.6.4).
The amplitude of an evoked potential is a reflection of the axon density of the nerve population, while the latency is a reflection of the myelin content and number of synapses through the afferent pathway. However, in most neurophysiology laboratories, somatosensory evoked potential latency measurements are regarded as more informative and reliable than the amplitude measurements, which are highly variable and are dependent on the intensity of stimuli delivered.17
The clinical utility of the pudendal somatosensory evoked potential test is that it is an assessment of the genital somatic innervation. Genital somatic sensation is crucial to the generation and maintenance of male sexual reflexes,18,19 and this paradigm, although not yet demonstrated, is also presumed to apply to women. The only study correlating neurophysiologic findings with sexual function in women was by Yang et al., evaluating women with multiple sclerosis.13 They found that women with abnormal dorsal clitoral nerve somatosensory evoked potentials had a very high incidence of orgasmic dysfunction. These abnormal somatosensory evoked potentials also were associated with subjective loss of genital sensation.
Figure 10.6.4. The pudendal somatosensory axis. After depolarization of the pudendal nerve, the signal is carried through the dorsal column in the spinal cord to the somatosensory cortex. The recording is usually made with surface electrodes placed on the scalp. A normal pudendal somatosensory evoked potential test demonstrates the integrity of the sensory axis from the dorsal clitoral nerve to the sensory cortex (DNC dorsal clitoral nerve; SEP sensory evoked potential).
Perineal nerve somatosensory potentials evoked with a vaginal probe were less consistently obtained than the dorsal clitoral nerve somatosensory evoked potentials (69% vs 90%), probably because of a decreased axon density of the perineal nerve at the level of the introitus.14 Therefore, the presence of a normal perineal somatosensory evoked potential can confirm the integrity of perineal innervation, but an absent response is not necessarily an indication of neuropathology.
Lumbar evoked potentials
Several authors have attempted to record evoked potentials from the spinal level by the stimulation of the dorsal clitoral/penile nerve.4,20 In theory, knowing the latencies of lumbar evoked potentials and somatosensory evoked potentials can help to identify a neuroanatomic lesion along the sensory neuroaxis (Fig. 10.6.5). However, the amplitudes of the potentials are small with low signal-to-noise ratio, and it is difficult to record lumbar evoked potentials in women and in healthy, obese men. Therefore, the technique is not feasible for routine clinical application. A combination of sacral reflex latency measurements (see below) and pudendal somatosensory evoked potentials gives ample information about the integrity of afferent pudendal pathways.
Somatic motor tests
The pelvic floor skeletal muscles are important in the female sexual response. Inability to contract the pelvic muscles during arousal and orgasm appears to affect a woman’s sexual respon- sivity.21 Thus, methods to evaluate the nerves to the pelvic muscles and their function may be helpful in the diagnosis of certain sexual dysfunctions.
Genital somatic motor pathways can be tested by (a) sacral reflex measurements, (b) directly stimulating motor nerves (e.g., pudendal terminal motor latency measurement), (c) anterior sacral root stimulation, (d) stimulation of the motor cortex with a transcranial cortical stimulator, or, (e) electromyography.
Sacral reflex measurements
The integrity of the sacral reflex arc is assessed by sacral reflex latency measurement, which typically involves stimulating the sensory branches of the pudendal nerve and recording the reflex response from the bulbocavernosus (bulbocavernous reflex) or external anal sphincter muscles (pudendal-anal reflex), both of which are innervated by branches of the pudendal nerve22-24 (Fig. 10.6.6). Motor potentials from the external anal sphincter can be recorded with a concentric needle electrode, anal plug electrode, or surface electrodes on the perineal skin.17
There are several different applications of sacral reflexes, and each gives information about the reflex pathway tested. Several types of stimuli such as electrical, mechanical, or magnetic stimulation can be used to elicit sacral reflexes.25 Electrical stimulation can be applied at the dorsal clitoral nerve,26 the perineum,27 and the vesicourethral junction,28 and to bladder mucosa with a catheter-mounted ring electrode.29 If the vesicourethral junction or bladder mucosa is stimulated and the reflex is recorded from the bulbocavernosus muscle, the reflex arc involves a viscerosomatic pathway, meaning that the afferents are visceral and the efferents are somatic nerve pathways. If the dorsal clitoral nerve is stimulated and the response is recorded from the bulbocaver- nosus muscle or external anal sphincter - the most commonly applied technique - the reflex arc is a somatosomatic pathway.
Figure 10.6.5. Pudendal lumbar evoked potentials. Although it is theoretically possible to assess the sensory branches distal to the spinal cord with this technique, the low-amplitude responses in female subjects are difficult to record (DNC dorsal clitoral nerve).
Figure 10.6.6. Sacral reflex arc. After depolarization of the dorsal clitoral nerve (DNC), the sacral reflexes are carried through the perineal branch of the pudendal nerve to the bulbocavernosus and external anal sphincter muscles, where the recording can be made.
The latency of sacral reflexes after stimulation of the dorsal clitoral nerve is consistently obtained at 31-38.5 ms.17,22,30-33 The latency of the viscerosomatic reflex pathway is longer (50-65 ms) than the somatosomatic reflex pathway, and this is probably due to the differences in myelin content of the afferent branches and the number of spinal synapses involved in the reflex arc.28,34
Most studies of electrophysiologic sacral reflex latency measurements in women were in the context of voiding function, not sexual function.22,35,36 Even though the importance of sacral reflexes is well understood in the evaluation of male sexual dysfunction,23 there is no neurophysiologic study in women for the evaluation of sacral reflexes in the context of sexual function. One study involving a clinical (physical examination only) assessment of the bulbocavernosus reflex in men and women found deficiency of a clinically obtained bulbocavernosus reflex in a substantial proportion of cases of primary anorgasmia in both sexes.37 However, no reliable conclusion can be drawn from that study, since electrophysiologic bulbocavernosus reflex testing is more sensitive to the clinically assessed bulbocavernosus reflex response.38 Sacral reflex latency measurements can potentially be used in the evaluation of deficient orgasmic pelvic contractions.
Sacral reflex measurements, as with other tests of nerve conduction, are not sensitive to partial axonal lesions. In a neurophysiologic evaluation protocol, the combination of sacral reflex latency measurements with pudendal somatosensory evoked potentials gives more reliable information about the somatic innervation of the pelvic floor and external genitalia than either test alone. However, subtle, partial lesions are not always detectable.
Pudendal terminal motor latency
Pudendal nerve terminal motor latency measurement assesses the distal motor branches of the pudendal nerve, similar to the technique routinely used in the evaluation of limb motor nerves: a motor nerve is stimulated proximally, and a compound motor action potential is measured in the appropriate muscle or muscle groups.39 The technique can be employed by recording with a concentric needle electrode from the bulbocavernosus, the external anal sphincter, or the urethral sphincter muscles in response to bipolar surface stimulation in the perianal/perineal region. It is easier, however, to use a special surface electrode assembly fixed on a gloved index finger, known as the St Mark’s electrode, consisting of a bipolar stimulating electrode on the tip of the gloved finger and a recording electrode pair placed 8 cm proximally on the base of the finger.40 The pudendal nerve can be accessed through the rectum or vagina with the tip of the finger placed close to the ischial spine. With the St Mark’s electrode, the distal motor latency for the anal sphincter is approximately 2 ms.41,42
Most of the studies involving pudendal nerve terminal motor latency measurements were directed to assess the urinary and fecal continence functions after delivery, but the value of the test in clinical practice is still controversial.43 There is no study explaining the role of pudendal nerve terminal motor latency in female sexual dysfunction. Theoretically, such a study may provide information on pelvic floor dysfunction and how it relates to the female sexual response.
Other somatic motor nerve tests
Transcutaneous stimulation of the anterior sacral nerve roots can be performed, with responses recorded in the muscles of the pelvic floor.27 Transcortical stimulation of the motor cortex also results in recordable responses in the pelvic floor musculature.25,44, 45 Both these tests are difficult to perform, and their clinical utility in sexual function evaluations has not been determined.
Pelvic floor electromyography
Pelvic floor electromyography can evaluate the integrity of muscle innervation, as well as identify primary muscle pathology. The primary means of evaluation is the recording of motor unit potentials through needle electrodes. A motor unit is represented by a single motor neuron and the muscle fibers innervated by its branches.
The motor unit potentials are analyzed for their amplitude and duration (Fig. 10.6.7). The innervation status of a skeletal muscle can be assessed by motor unit potential characteristics. After complete denervation, no motor unit potentials can be recorded for several days. With time, reinnervation occurs, and is reflected as abnormal motor unit potentials. Needle electromyography has frequently been used to assess peripheral nerve injury, multiple system atrophy, neuropathic conditions involving spinal segments, and cauda equina lesions.46 Even though pelvic floor electromyography has been classically directed to the assessment of voiding function, it may have value in the evaluation of female sexual disorders involving the pelvic floor. In one study involving women with vaginismus and healthy control subjects, pelvic floor electromyography activity was increased both at rest and on induction of pelvic floor spasm in women with vaginismus.47 Hypertonicity of the pelvic floor muscles is one of the recognized pathophysiologic processes in genital pain disorders; therefore, needle electromyography may be a valuable diagnostic tool before the initiation of pelvic floor training programs. Electromyographic monitoring with surface electrodes for biofeedback training could be useful in women with genital pain disorders.48,49
Figure 10.6.7. A normal motor unit potential (MUP). The amplitude of the MUP is measured from peak to peak, and the duration is measured from the first deflection to the return to baseline.
Neural testing of autonomic nerves
The autonomic nervous system plays an important role in the female sexual response. Sacral parasympathetic (S2-S4) and thoracolumbar sympathetic (T11-L2) spinal segments represent the autonomic pathways to female genital tissue, as shown in animal studies with retrograde labeling techniques.50 The similarity of autonomic nervous system responses between males and females has also been demonstrated with animal studies.10,11,51 Autonomic neuropathy, such as occurs in diabetes, can result in sexual dysfunction in both genders.52,53 Therefore, the assessment of afferent and efferent autonomic pathways has critical importance in the evaluation of sexual dysfunction. Autonomic nerve assessments differ in technique from somatic nerve electrodiagnostic tests, although the neurophysiologic principles are essentially the same as for somatic innervation.
Autonomic (visceral) afferent pathway assessment
Since the sensory innervation of the proximal urethra, bladder neck, and bladder wall is mediated by visceral afferent fibers sharing the same derivation as those fibers innervating the genital tissue, cerebral evoked potentials after stimulation of these areas theoretically gives direct information about the visceral afferent pathways to the genitalia28,54-56 (Fig. 10.6.8). The recording technique is similar to the pudendal somatosensory evoked potentials tests except for the use of a transurethral catheter for stimulation. There has been no study demonstrating the utility of visceral somatosensory evoked potentials in the evaluation of female sexual function.
Autonomic efferent pathway assessment
Female genital tissue receives both parasympathetic and sympathetic innervation. This is homologous to the innervation of the male corpus cavernosum and corpus spongiosum by the cavernous nerves,57 although the patterns of innervation are not well defined in the human female. The sexual arousal response with pelvic organ engorgement is similarly mediated by the autonomic component of the cavernous and pelvic nerves.9-11 In males, corpus cavernosum electromyography and evoked cavernous activity have been proposed for the assessment of genital autonomic innervation.58,59 These tests presumably assess the sympathetic autonomic pathways to the corpus cavernosum. In men with erectile dysfunction, corpus cavernosum electromyography has been found to have deteriorated.
Figure 10.6.8. Evaluation of the autonomic (visceral) afferent pathways. Since the bladder wall and vesicourethral junction (VUJ) are innervated by the autonomic fibers, evoked potentials by the stimulation of these areas can give information about the integrity of pelvic visceral afferent pathways. The depolarization is carried via the pelvic plexus to the sacral spinal cord, and recording can be made from the somatosensory cortex (SEP sensory evoked potential).
Currently, information on female autonomic genital innervation can be obtained by indirect clinical assessment tools, such as vaginal blood flow or vaginal pulse amplitude measurements, which are end-organ response autonomic impulses.60-62 However, work is being done with neurophysiologic techniques, such as clitoral electromyography and evoked bulbar/clitoral activity, for direct assessment of autonomic innervation. We developed a technique similar to the male autonomic neurophysiologic methods to record both spontaneous and evoked electrical activity from the clitoris.63 In a subsequent study, we also showed that electrical activity could be obtained from the bulbs, which are large erectile structures on either side of the urethra.64 The bulbar evoked activity is more robust and easier to elicit than clitoral evoked activity. Spontaneous electrical activity recordings of both structures are highly variable and subject to artifacts, and thus are not as reliable as evoked recordings (Fig. 10.6.9).
Briefly, the technique involves placement of concentric needle electrodes in the clitoral body and bulb. The median nerve is electrically stimulated to evoke a generalized sympathetic discharge, as in the procedure applied in sympathetic skin response testing. The frequency filters for recording are set within 0.2 and 100 Hz. The recorded responses are much slower than the evoked potential and sacral reflex tests, with an onset latency of 410-3080 ms, requiring a long sweep of 1 s/div. The test appears promising in the assessment of female genital autonomic innervation.
Figure 10.6.9. Evaluation of autonomic efferent pathways. A startling electrical stimulus to the median nerve activates the sympathetic nervous system, resulting in evoked electrical activity in the clitoris and bulbs, similar to sympathetic skin responses. The activities can be recorded with concentric needle electrodes with a sweep of 1 s/div.
Sympathetic skin response
Sympathetic skin response tests are one of the most commonly employed autonomic neurophysiologic assessments of sweat gland activity, which is mediated by the sympathetic nervous system. Sympathetic skin response tests have been applied to evaluate the autonomic innervation of the genital skin. Changes in sweat gland activity are recordable as spontaneous oscillating waveforms with surface electrodes placed on the palmar, plantar, and dorsal sides of the hands and feet. Noxious stimulation, such as a sudden noise or an electrical impulse, creates a potential shift in the sweat gland activity, and the recorded response is known as the sympathetic skin response of the corresponding area. Sympathetic skin response tests are useful in the assessment of small-fiber neuropathy.2 The reflex arc involves a myelinated somatosensory nerve, the central autonomic network, and a sympathetic efferent limb with postganglionic, nonmyelinated C fibers.
Sympathetic skin responses are recordable from the perineum and the penis, and abnormal responses are associated with male erectile dysfunction.65-68 A few studies assessed the value of the test in autonomic neuropathy and voiding dysfunction.69-71 Shorter latency and higher amplitude measurements of genital sympathetic skin response have been found in women with vaginismus than in healthy individuals. This finding suggests that there is increased sympathetic activity within the pelvis (unpublished data). Further work is needed to substantiate the value of female genital sympathetic skin responses in sexual pain disorders.
General comments and the future
Neural testing in the clinical assessment of female sexual function is still in its infancy. Very few tests are indicated on a routine basis, and those that are, are typically indicated for those with a neurologic disease or injury. Furthermore, the number of persons trained (and willing) to perform such tests is limited. This is not to say that these methods have no place in the study of female sexual function. Neural testing is limited only because of our finite knowledge of the role of the central and peripheral nervous system in female sexual function. The integrity of the nervous system is critical for healthy sexual function, but the details of sexual reflex arcs and central nervous system integration still need to be unraveled. To this end, the role of neural testing as a research entity has much promise: it is one of the means by which these problems can be solved. Neurophysiologic examinations can help to determine which neural pathways are involved with the female sexual response, and, once this is determined, whether that pathway is intact. As with all human conditions, only a better understanding of the physiology can lead us to better diagnosis and treatment of female sexual dysfunction.
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