CHAPTER 23 Male Sexual Function and Dysfunction
What is the anatomic arrangement of the penis?
The dorsal penis contains 2 erectile bodies, the corpora cavernosa. Each corpus cavernosum contains a central cavernosal artery that is principally responsible for penile engorgement during erection. There is a single erectile body (the corpus spongiosum) on the ventral portion of the penis that contains the urethra. Each corporal body is enclosed in a sheath of tough connective tissue called the tunica albuginea.
What is Buck’s fascia?
Buck’s fascia is the layer of connective tissue that covers both corpora cavernosa and the corpus spongiosum in separate fascial compartments. The dorsal artery, deep dorsal vein, and dorsal nerves of the penis lie beneath Buck fascia.
From which artery is the deep penile blood supply derived?
The cavernous arteries responsible for erection of the corpora cavernosa receive blood from the internal pudendal artery, which in turn is a branch of the hypogastric artery, which is a branch of the internal iliac artery.
What is the anatomic relationship of the cavernous nerves to the prostate?
The cavernous nerves, which are branches of the pelvic plexus, travel posterolateral to the apex of the prostate at the 5-o’clock and 7-o’clock positions.
What is the mechanism of penile erection?
During sexual stimulation, the neurotransmitter nitric oxide (NO) is produced by the action of neuronal nitric oxide synthase (nNOS) and released from the cavernous nerves to smooth muscle in the cavernous arteries and the corpora cavernosa. NO activates guanylate cyclase, which cleaves cellular guanosine triphosphate (GTP) into cyclic guanosine monophosphate (cGMP). Through a complex cascade of events including activation of protein kinase G (PKG), cGMP leads to decreased intracellular concentration of calcium ions; this prompts smooth muscle relaxation in cavernosal arteries and the corpora cavernosa.
With smooth muscle relaxation, the cavernous arteries and erectile tissues dilate, leading to increased penile blood flow and engorgement of the corpora. Shear forces with vasodilation stimulate the production of NO from the endothelium by action of endothelial nitric oxide synthase (eNOS). Engorgement of the corporal bodies with blood compresses the emissary veins (which drain the corpora) against the tunica albuginea. Compression of these veins prohibits blood from escaping the corpora, producing a firm penile erection (full erection phase). With increasing sexual excitement, there is concomitant contraction of the bulbospongiosus and bulbocavernosus muscles. This leads to a pressure increase in the distal corpora cavernosa which may exceed systolic blood pressure. Blood is also forced into the corpus spongiosum, causing engorgement of the ventral shaft and glans of the penis (rigid erection phase).
The NO/cGMP/PKG pathway is the best understood factor in promoting penile erection, but other pathways also play a role. Norepinephrine, endothelin, and sympathetic stimuli tend to promote vasoconstriction. Acetyl choline, vasoactive intestinal peptide, and prostaglandins tend to promote vasodilation. Calcium sensitization via the RhoA/ROCK pathway modulates responsiveness of the actin/myosin contractile proteins in vascular smooth muscle. These other pathways may become increasingly important in future treatments for erectile dysfunction (ED).
What is the mechanism of penile detumescence?
After sexual excitement has passed NO release from the cavernous nerves and endothelium declines. Concomitantly, the enzyme phosphodiesterase type 5 (PDE5) cleaves cGMP into 5’ GMP, which is not metabolically active. Sympathetic tone is restored, which tends to further promote arterial constriction. With the decline in cGMP concentration, intracellular calcium levels in the vascular smooth muscle rise and contraction occurs, leading to reversal of arterial dilation and drainage of blood from the corpora.
What is the physical sequence of events of ejaculation and which portion of the nervous system controls it?
Ejaculation is divided into 2 phases, emission and ejection.
Emission is the phase during which the bladder neck closes and seminal fluid (from the prostate and seminal vesicles) is mixed with sperm from the epididymis in the posterior urethra. This phase of ejaculation is under sympathetic nervous system control and may be disrupted by injury to sympathetic nerves (most commonly from retroperitoneal surgery or injury), sympatholytics, and/or bladder neck surgery.
Ejection is expulsion of semen from the penis by rhythmic contractions of the bulbospongiosus muscle. Ejection is mediated by the somatic nervous system. The force of ejection may decline with muscle weakness/age.
Which areas in the brain control erection and ejaculation?
The medial preoptic area and the paraventricular nucleus of the hypothalamus have been identified as important supraspinal centers for erection and ejaculation. In the central nervous system, the neurotransmitter serotonin typically has an anti-ejaculation effect whereas dopamine typically has a pro-ejaculation effect. Our current understanding of the neurophysiology of sexual function is poor.
What is erectile dysfunction (ED)?
ED is the persistent/recurrent inability to attain and/or maintain a penile erection rigid enough for satisfactory sexual intercourse.
According to data from the Massachusetts Male Aging Study, ED affects how many men older than 50 years?
More than 50%. The prevalence of ED increases with age. Less than 10% of men in their 20s and 30s complain of ED, but the condition has been reported in men of all ages.
What are the most common risk factors for ED?
ED may be divided into organic, psychogenic, or mixed. Vascular risk factors (hypertension, hypercholesterolemia, diabetes, tobacco use, and coronary artery disease) are the greatest risk factors for ED in the Western world. Nerve injury from pelvic surgery and/or neuropathic conditions may also contribute to ED. Deficiency of testosterone may lead to tissue atrophy in the penis as well as diminished libido. Certain medications are also known to increase risk of ED by a variety of mechanisms.
Psychogenic ED may result from depression, anxiety, relationship stress, or other nonorganic factors. Psychogenic factors frequently occur in men with organic ED and may complicate medical treatment. Attention to nonorganic factors is important in management of ED of any etiology.
What classes of antihypertensive medication are most often associated with ED?
β-Blockers and thiazides. While all anti-hypertensives have been asosciated with some degree of ED, there is some evidence that angiotensin converting enzyme (ACE) inhibitors calcium channel blockers, and angiotensin receptor blockers may have a neutral or even positive effect on erectile function in some hypertensive men.
Are there subtypes of vasculogenic ED?
Yes. Arterial insufficiency is defined as inadequate arterial response to sexual stimulation, leading to inadequate blood flow into the penis. This may be due to inadequate arterial vasodilation or occlusion of the arteries themselves with atherosclerotic plaques. Venous leak is a failure of the occlusive mechanism in which the emissary veins are compressed between the corporal body and the tunica. Mixed vascular ED is the coexistence of both venous leak and arterial insufficiency.
Why is diabetes a particularly severe risk factor for ED?
Neuropathy and vascular disease are common in men with diabetes. Diabetes is also associated with decreased levels of circulating testosterone and damage to endothelial and smooth muscle cells. Ergo, diabetes impacts many of the biological processes necessary for penile erection.
Spinal cord injury patients may have reflex erections from tactile stimulation or psychogenic erections. What cord level of injury separates the 2 groups?
Patients with injuries above T12 may have reflexogenic erections. These erections are mediated by efferent tactile signals transmitted from the penis via the pudendal, pelvic, and hypogastric nerves and afferent signals carried from the sacral spine cord via the pelvic nerve to the pelvic nerve and then the cavernous nerves. However, disruption of transmission via higher spinal cord segments may limit signaling regions of the brain responsible for interpreting and responding to erotic stimuli.
Patients with a level of injury below T12 may experience psychogenic erections (brought on by erotic stimuli processed in the brain) mediated by suppression of sympathetic tone (which promotes vasoconstriction) from the thoracolumbar spinal cord. These patients may however have disruption of the sacral reflex arc described above.
What other sexual problems may be confused with and/or associated with ED?
Premature ejaculation (PE): Persistent and/or recurrent ejaculation before or shortly after penile penetration associated with a lack of feeling of control over the process and leading to significant patient and/or partner distress. This condition may be lifelong or acquired. PE has been associated with differences in brain processing of serotonin, thyroid hormone disturbances, lower urinary tract symptoms, and/or psychosocial issues.
Hypoactive sexual desire disorder: Decreased interest or receptivity to sexual activity. This condition has been associated with hypogonadism, relationship stress, and psychological issues.
Retarded/delayed ejaculation: Persistent and recurrent difficulty ejaculating and/or attaining orgasm despite ostensibly sufficient sexual stimulation. This condition may be related to neuropathy, changes in sexual activities, and/or psychological issues.
Which commonly used drugs have been associated with impairment in ejaculation and orgasm?
Alcohol, tricyclic antidepressants (amitriptyline and imipramine), and selective serotonin reuptake inhibitors (SSRI) are the mostly commonly used drugs associated with impairment in ejaculation and orgasm. SSRI have been used with some success as a treatment for PE.
What is an appropriate initial evaluation for a man with ED?
The timing and nature of the concern must be explicitly confirmed. An assessment should be made for vascular and/or neurological risk factors as well as psychological comorbidities. Medications should be reviewed. A basic electrolyte panel, hemoglobin A1c, resting blood pressure, and serum lipid profile are adequate for initial evaluation in most cases. An AM serum testosterone (free and total) should be considered, particularly if there are other symptoms of hypogonadism and/or if the patients has previously failed oral erectogenic therapy.
What are the Princeton criteria?
The Princeton criteria are a set of algorithms for stratifying ED patients with respect to their cardiac risk factors. Men with severe cardiac disease should undergo cardiac optimization prior to initiation of any therapy for ED. Men with no or mild cardiac disease may safely start ED therapy without further investigation. Men in the intermediate category may need further evaluation by a cardiologist prior to starting ED therapy.
What is dynamic infusion cavernosometry (DIC)?
DIC is a procedure to diagnose venous leak as a cause of ED. A needle is placed into the corpora to inject a vasodilating agent. After 10 minutes and/or after a penile erection has developed, sterile saline is infused. The rate of inflow necessary to maintain a set degree of axial rigidity in the penis is a measure of the integrity of the veno-occlusive mechanism of the penis; a high flow rate implies greater leak and suggests venous leak ED.
Normal veno-occlusive function is suggested by what maintenance flow rate on DIC testing?
A maintenance flow rate of 6 mL/min or less represents normal veno-occlusive function if maximal smooth muscle relaxation has been achieved.
What is the role of cavernosography in the evaluation of ED?
In a patient in whom venous leak is clinically suspected, contrast injection into the corpora demonstrates the sites of abnormal leaking veins. This test is seldom indicated in the modern era but still is useful for preoperative planning if venous surgery for the treatment of ED is contemplated in carefully selected young patients.
What is nocturnal penile tumescence (NPT)? When do nocturnal erections occur and how is the rigidity of these erections measured?
NPT is the normal cycle of nighttime erections that occur during rapid eye movement (REM) sleep in healthy males of all ages. Testing for NPT can be performed using a pressure monitor that fits around the penis. NPT testing is typically utilized to determine whether ED is psychogenic or organic; if normal nocturnal erections occur in a patient with ED it somewhat more likely that there is a psychological component to ED. NPT may have some utility in assessment of ED; men with normal NPT and ED should consider psychosexual therapy as an alternative or adjunct to medical treatment options.
What role does penile color Doppler ultrasound play in the diagnosis of ED?
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