BACKGROUND AND INTRODUCTION
It is estimated that more than half of men aged 40–70 in the United States are unable to attain or maintain a penile erection sufficient for satisfactory sexual performance. Advances in pharmacologic therapy for erectile dysfunction (ED), coupled with a better understanding of male sexual dysfunction, have resulted in greater numbers of patients seeking care for sexual concerns. Oral phosphodiesterase type 5 inhibitors have emerged as the preferred first-line treatment of ED worldwide because of their efficacy, ease of use, and patient safety. Erectile function can now be evaluated by the response to these agents at home or by assessment of response to intracavernous injection (ICI) of vasoactive agents in the office, and improved diagnostic tests can differentiate among types of impotence. Patient satisfaction with penile prostheses is high, as the latest generation of devices is more sophisticated and durable than ever. Current treatments continue to evolve and new therapies, such as low-intensity extracorporeal shockwave, stem cell, and gene therapies, may represent the next generation of more physiologic and disease-specific solutions to various types of ED (Melman et al, 2007; Lin et al, 2017; Bahk et al, 2010).
PHYSIOLOGY OF PENILE ERECTION
The autonomic spinal erection center is located in the intermediolateral nuclei of the spinal cord at levels S2–S4 and T12–L2. Nerve fibers from the thoracolumbar (sympathetic) and sacral (parasympathetic) spinal segments join to form the inferior hypogastric and pelvic plexuses, which send branches to the pelvic organs. The fibers innervating the penis (cavernous nerves) travel along the posterolateral aspect of the seminal vesicles and prostate and then accompany the membranous urethra through the genitourinary diaphragm (Figure 39–1). Some of these fibers enter the corpora cavernosa and corpus spongiosum with the cavernous and bulbourethral arteries. Others travel distally with the dorsal nerve and enter the corpus cavernosum and corpus spongiosum in various locations to supply the middle and distal portions of the penis. The terminal branches of the cavernous nerves innervate the helicine arteries and trabecular muscle, and are responsible for vascular events during tumescence and detumescence.
Location of cavernous nerves in relation to urethra.
The center for somatic motor nerves is located at the ventral horn of the S2–S4 segments (Onufs nucleus). The motor fibers join the pudendal nerve to innervate the bulbocavernosus and ischiocavernosus muscles. The somatic sensory nerves originate at receptors in the penis to transmit pain, temperature, touch, and vibratory sensations. The brain has a modulatory effect on the spinal pathways of erection, specifically the medial preoptic area and paraventricular nucleus of the hypothalamus, the periaqueductal gray area of the midbrain, and the nucleus paragigantocellularis of the medulla. Positron emission tomography (PET) and functional magnetic resonance imaging (MRI) have allowed for greater understanding of brain ...