The Role of Genital Afferent Nerve Stimulation for the Treatment of Erectile Dysfunction and Urinary Disorders
Kambiz Tajkarimi, MD, Summit Urology Group, Chambersburg, Pennsylvania
Arthur L. Burnett, MD, MBA, Johns Hopkins Medical Institutions, Baltimore, Maryland
Published May 2012, AUANews. Click HERE to view full issue.
Introduction
Recent interest in the potential benefits of genital afferent neuromodulation and penile vibratory stimulation is long overdue. There is expanding clinical and experimental support for the role of genital afferent neurophysiology in the mechanisms of sexual behavior, erectile function and micturition. Multiple studies have exposed many profound and unanticipated benefits of penile vibratory stimulation, especially in men with spinal cord injury: relief of lower extremity spastic muscle contractions, increased bladder capacity, decreased detrusor irritability and improved continence, ejaculation, and reflexogenic erection.1 Data have emerged as well showing similar benefits in non-spinal injured men and women for a host of conditions, including erectile dysfunction (ED), overactive bladder, orgasmic dysfunction, and stress urinary and fecal incontinence.2
This article serves to introduce the concept of genital afferent nerve stimulation as a new therapeutic approach with a potential role in the treatment of pelvic genitourinary disorders with an emphasis on management of ED and erectile rehabilitation.
Erectile Neurophysiology
Penile erection is a neurovascular event controlled by spinal autonomic centers, the activity of which is dependent on input from higher centers and the genitalia. From a neurophysiological standpoint, penile erection is a culmination of multiple successful nerve reflexes (e.g., nitric oxide mediated pudendo-cavernous reflex, somatic bulbocavernosus reflex) that lead to a vascular and mechanical event.3 Similar somatic-somatic and somatic-autonomic urinary reflexes also exist between genital afferents and detrusor function and pelvic floor sphincters. In humans and higher mammals, the glans penis and clitoris are critical sources of sensory information to the CNS for induction and maintenance of sexual and urinary reflexes. The human penis has dual somatosensory nerve innervation. The dorsal and ventral surfaces are supplied by the dorsal nerve of penis (DNP) and the perineal branch of the pudendal nerve (PerN), respectively.
Millions of nerve receptors exist on the surface of the human penis and clitoris. Free nerve endings (FNE), the most numerous, express polymodality or dissociated sensibility. FNE and other mechanoreceptors can translate multiple sensory inputs to excitatory signals. This information is rapidly transmitted by genital afferents via the pudendal nerve (PN) to the spinal cord and higher centers to produce sexual arousal, erection, ejaculation, and pleasure.
PN circuitry is increasingly being recognized as a mixed nerve, composed of both somatic (sensory, motor) and autonomic components. This stems from evidence in humans and animals that both the DNP and PerN are neuronal nitric oxide synthase (n-NOS) positive in the penis and not in their scrotal skin distribution. NOS activity and its neurotransmission is considered autonomic in origin and a critical component of sexual and eliminative reflexology. Interestingly, multiple studies have shown extensive neuronal anastomotic bundles between PN and CN branches at regular intervals. The functional importance of such nerve communications is not known and needs further investigation.
The PN and its branches take a different anatomical path from the CN on their way to and from the penis and pelvic skeletal muscles. Due to the close proximity of the CN to the prostate and rectum, one or both of its trunks may be injured during pelvic surgery. During CN neuropraxia, the PN becomes the only reliable neuronal communication between the penis and CNS.
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| Figure 1: Anatomical relationship of penile corpora with ischiocavernosus (ICM) in red color andbulbospongiosus (BCM) muscles. Gray's Anatomy. |
Importantly, the bulbocavernosus reflex (BCR) is thought to remain intact after pelvic surgery, because it does not involve CN activity in its reflex loop. The BCR is a spinal reflex involving somatic afferent and efferent neurons of the PN. The afferent limb is the DNP and PerN, which combine fibers that relay messages to the sacral spinal Onuf's nucleus S2-4. The efferent motor neurons then travel via the PN branches that supply the ischiocavernosus (ICM), bulbospongiosus (BCM), external urethral sphincter, and external anal sphincter muscles. Stimulation of the glans penis activates the BCR. Human electrophysiological studies have demonstrated that PN activity is responsible for contraction of the ICM during the rigid phase of erection and rhythmic contraction of the BCM during ejaculation.3
Genital afferent nerves also relay critical sensory information from the genitalia to supraspinal sexual centers, which include medial pre-optic area (MPOA) and paraventricular nucleus (PVN) of the hypothalamus, hippocampus, medial amygdala, thalamus, medial reticular formation, and several areas of the forebrain. There is growing support for their role in initiation of sexual behavior, penile erection, orgasm, and ejaculation at the supraspinal level. Males and females share a comparable pattern of genital afferent neuroanatomy and neurophysiology, and sexual and micturition reflexes are similar in both genders.3
Use of Vibratory Devices in the United States
Many female-specific non-medical vibratory devices are available today. For men, two FDA cleared therapeutic penile vibrators include the Ferticare® [Multicept, Denmark], cleared to provoke ejaculation in men with spinal cord injury, and the Viberect®[Reflexonic, USA], cleared to provoke penile erection in men with ED and to provoke ejaculation in men with spinal cord injury.4 (Figure 2)
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| Reflexonic Viberect® | Multicept Ferticare® |
| Figure 2 | |
Vibrators are commonly recommended by clinicians as an adjunct for the treatment of female sexual dysfunction. A 2009 cross-sectional survey of vibrator use by 3,800 American women reported prevalence of 52.5%. Vibrator use among women was associated with health-promoting behaviors and positive sexual function, and rarely associated with side effects. The same authors also investigated the prevalence of vibrator use in a representative sampling of 1047 American men. 44.8% reported using vibratory devices during their lifetimes, with 10.0% having done so in the past month. Men with recent vibrator use scored higher on four of the five domains of the International Index of Erectile Function (erectile function, intercourse satisfaction, orgasmic function, and sexual desire).4 In a small representative series of urology patients with ED, the Viberect® device was reported to be safe, easy to use, tolerable, and highly satisfying.5
Based on our current understanding of the functional importance of genital afferent neurophysiology and pelvic reflexes, a proposal can be made for well-controlled studies to validate a potentially exciting role for this modality in erection therapy, treatment of pelvic floor disorders, and a possible neuromodulatory benefit for restoration of erection and urinary control after pelvic surgery. Well-designed and dedicated studies in each population will shed light on its therapeutic prospects in the near future.
References
1. Ohl DA, Menge AC, SonskenJ. Penile vibratory stimulation in spinal cord injured men: Optimized vibration parameters and prognostic factors. Arch Phys Med Rehabil 1996;77:903-5.
2. Sonsken, J, Ohl, DA, Bonde B. Transcutaneous mechanical nerve stimulation using perineal vibration: A novel method for the treatment of female stress urinary incontinence. J Urol 2007; 178:2025-2028.
3.Tajkarimi K, Burnett AL. The role of genital afferents in the physiology of the sexual response and pelvic floor function. J Sex Med 2011; 8: 1299-1312.
4. Reece M, Herbenick D, Sanders SA, Dodge B, Ghassemi A, and Fortenberry JD. Prevalence and characteristics of vibrator use by men in the United States. J Sex Med 2009;6:1867-1874.
5. Tajkarimi, K, Burnett AL. Viberect® use by men with erectile dysfunction: safety, ease of use, tolerability, and satisfaction. SMSNA 2011 moderated poster, presented in Las Vegas, Nevada.
