Residency · Residency · Urology

Erectile Dysfunction: Pathophysiology and Management

Definition and Epidemiology

Erectile dysfunction (ED) is defined as the persistent inability to attain and/or maintain an erection sufficient for satisfactory sexual performance. It affects approximately 10-20% of all men, with prevalence increasing dramatically with age. Around 20% of men in their 40s experience ED, rising to about 30% in their 50s, 50% in their 60s, and up to 70% in their 70s. There is a strong association between ED and systemic conditions such as cardiovascular disease, diabetes, and metabolic syndrome. Importantly, ED may serve as the first clinical manifestation of systemic vascular disease, acting as a sentinel symptom. On average, ED precedes coronary artery disease (CAD) events by 3 to 5 years, supporting the artery size hypothesis, which suggests that smaller penile arteries show symptoms of atherosclerosis earlier than larger coronary arteries.

Penile Anatomy for Erection

The penis contains two corpora cavernosa, which are paired erectile bodies surrounded by a tough fibrous layer called the tunica albuginea. Within the corpora cavernosa lies sinusoidal smooth muscle, which plays a critical role in erection. Blood supply to the penis comes from the helicine arteries, which are branches of the cavernosal arteries originating from the internal pudendal artery. Venous drainage occurs through the subtunical venous plexus, emissary veins, and the deep dorsal vein. Innervation involves three key components: parasympathetic fibers from S2-S4 via the pelvic and cavernous nerves promote erection; sympathetic fibers from T11-L2 via the hypogastric nerve mediate detumescence; and somatic fibers via the pudendal nerve provide penile sensation and control contraction of the bulbocavernosus and ischiocavernosus muscles.

Physiology of Erection

Erection begins with sexual stimulation, which activates parasympathetic pathways. This leads to the release of nitric oxide (NO) from non-adrenergic non-cholinergic (NANC) nerve terminals and endothelial cells. NO activates guanylate cyclase, increasing cyclic guanosine monophosphate (cGMP) levels within smooth muscle cells. Elevated cGMP causes relaxation of the smooth muscle in the sinusoidal spaces and helicine arteries, allowing increased arterial inflow that fills the sinusoidal spaces. As these sinusoids expand, they compress the subtunical venous plexus against the tunica albuginea, effectively trapping blood within the corpora cavernosa—a mechanism known as veno-occlusion. This results in a rigid erection. Detumescence occurs when sympathetic activation leads to phosphodiesterase type 5 (PDE5) degrading cGMP, causing smooth muscle contraction and resumption of venous outflow.

<image>Diagram of the NO-cGMP pathway in penile erectile physiology showing nitric oxide release, guanylate cyclase activation, cGMP-mediated smooth muscle relaxation, and the site of PDE5 inhibitor action</image>

Etiology

Vascular (Most Common)

The most common cause of ED is vascular in origin, primarily due to atherosclerosis affecting the pudendal and cavernosal arteries, leading to arterial insufficiency. Endothelial dysfunction impairs the NO pathway, reducing smooth muscle relaxation. Venous leak, or corporoveno-occlusive dysfunction, occurs when the penis fails to trap blood effectively. Risk factors for vascular ED include hypertension, hyperlipidemia, diabetes, smoking, obesity, and a sedentary lifestyle.

Neurogenic

Neurogenic causes can be central or peripheral. Central causes include stroke, multiple sclerosis, Parkinson disease, and spinal cord injury. Peripheral causes are most commonly diabetes-induced neuropathy, cavernous nerve injury from radical prostatectomy, pelvic radiation, pelvic surgery, and other peripheral neuropathies.

Hormonal

Hormonal abnormalities such as hypogonadism, characterized by low testosterone levels, primarily reduce libido but also affect erectile function. Hyperprolactinemia and thyroid dysfunction can also contribute to ED.

Anatomic/Structural

Structural causes include Peyronie disease, which involves fibrous plaques in the tunica albuginea, penile fracture from trauma, and priapism, which is ischemic injury to the sinusoidal tissue.

Psychogenic

Psychogenic ED is often related to performance anxiety, depression, or relationship conflicts and is more common in younger men. It is characterized by situational ED, preservation of normal nocturnal erections, and sudden onset.

Medication-Induced

Certain medications can induce ED, including antihypertensives such as beta-blockers and thiazides, antidepressants like SSRIs and SNRIs, antiandrogens including GnRH agonists and 5-alpha reductase inhibitors, opioids, and substances such as alcohol and marijuana.

Evaluation

History (IIEF-5/SHIM Questionnaire)

The International Index of Erectile Function (IIEF-5) is a validated five-question tool used to assess ED. A score below 21 indicates the presence of ED, with scores under 7 reflecting severe dysfunction. History should characterize the onset (gradual versus sudden), whether the dysfunction is situational or global, libido status, and ejaculatory function. Cardiovascular risk assessment is critical since ED often signals underlying vascular disease. Psychosocial factors should also be evaluated.

Physical Examination

Physical examination includes cardiovascular assessment with blood pressure measurement, evaluation of peripheral pulses, and body mass index calculation. Genitourinary examination looks for penile plaques indicative of Peyronie disease, testicular size, and phimosis. Neurologic evaluation includes testing the bulbocavernosus reflex and perineal sensation. Endocrine assessment involves checking secondary sexual characteristics and signs such as gynecomastia.

Laboratory

Laboratory testing should include fasting glucose or HbA1c to assess for diabetes, a lipid panel, and morning fasting testosterone levels if low libido, suspected hypogonadism, or ED unresponsive to PDE5 inhibitors is present. Thyroid-stimulating hormone (TSH) and prolactin levels should be checked if clinically indicated.

Specialized Testing (Selected Cases)

Nocturnal penile tumescence (NPT) testing can differentiate psychogenic from organic ED. Penile duplex Doppler ultrasound assesses arterial inflow and venous leak after intracavernosal injection of vasoactive agents. A peak systolic velocity (PSV) greater than 25 cm/s indicates normal arterial inflow, while PSV below 25 cm/s suggests arterial insufficiency. An end-diastolic velocity (EDV) above 5 cm/s indicates venous leak. These tests are rarely needed for initial management.

<image>Penile duplex Doppler ultrasound showing normal arterial waveform with peak systolic velocity measurement after intracavernosal injection of alprostadil</image>

Management

Lifestyle Modifications and Risk Factor Management (All Patients)

All patients should be counseled on lifestyle modifications, including weight loss and engaging in at least 30 minutes of moderate exercise daily. Smoking cessation is essential, as is optimizing diabetes control and treating hypertension and dyslipidemia. Alcohol intake should be reduced, and psychological factors addressed, potentially through sex therapy or counseling. A medication review should be performed to identify and change any offending drugs if possible.

First-Line: PDE5 Inhibitors

Phosphodiesterase type 5 (PDE5) inhibitors are the first-line pharmacologic treatment. Sildenafil (Viagra) has an onset of 30 to 60 minutes and lasts 4 to 6 hours; it should be taken on an empty stomach as food delays absorption. Tadalafil (Cialis) also has a 30 to 60-minute onset but lasts up to 36 hours, with food having no effect on absorption. It is available as a daily low-dose formulation (2.5-5 mg). Vardenafil (Levitra) shares a similar onset and duration to sildenafil but should be avoided with fatty meals. Avanafil (Stendra) has the fastest onset (15-30 minutes) and is the most selective for PDE5, lasting about 6 hours.

Second-Line: Intracavernosal Injection Therapy (ICI)

Intracavernosal injection therapy involves injecting vasoactive agents directly into the corpora cavernosa. Alprostadil (prostaglandin E1) is used as monotherapy in doses of 10-40 mcg. The most common combination, Trimix, includes alprostadil, papaverine, and phentolamine, and is more effective than monotherapy with approximately a 90% erection rate. The lower doses of each component reduce side effects. Bimix, consisting of papaverine and phentolamine without alprostadil, is another option. The injection is administered into the lateral aspect of the proximal penile shaft, avoiding the dorsal neurovascular bundle and urethra. Onset occurs within 5 to 15 minutes, with effects lasting 30 to 60 minutes. Complications include priapism (prolonged erection over 4 hours) occurring in 1-5% of cases, more common with higher doses, penile fibrosis or plaque formation in 5-10% of long-term users, and hematoma or pain at the injection site. Patients require training for self-injection, typically provided in an office teaching session.

Second-Line: Intraurethral Alprostadil (MUSE)

The medicated urethral system for erection (MUSE) involves inserting an alprostadil pellet (125-1,000 mcg) into the urethra using an applicator. The drug is absorbed through the urethral mucosa. This method is less effective than intracavernosal injection, with efficacy around 30-40%. Penile pain is common, but the risk of priapism is lower. It serves as an alternative for patients unwilling to self-inject.

Second-Line: Vacuum Erection Device (VED)

Vacuum erection devices create negative pressure that draws blood into the corpora cavernosa. A constriction ring placed at the base of the penis traps the blood to maintain the erection. This non-invasive method requires no medications and has a satisfaction rate of 60-70%. Limitations include a cold or cyanotic penis, pivoting at the base, numbness, and petechiae. VEDs are useful for penile rehabilitation after radical prostatectomy. The constriction ring should be removed within 30 minutes to avoid ischemic injury.

Third-Line: Penile Prosthesis Implantation

Inflatable Penile Prosthesis (IPP) -- Three-Piece

The inflatable penile prosthesis is considered the gold standard surgical treatment for ED. It consists of paired intracavernosal cylinders, a scrotal pump, and an abdominal fluid reservoir. This device provides a rigid erection on demand while remaining concealed when deflated. Patient satisfaction rates range from 85-95%, the highest among ED treatments, with partner satisfaction between 80-90%. Surgical implantation is performed via penoscrotal or infrapubic incisions. Common brands include the AMS 700 (Boston Scientific) and Coloplast Titan. Complications include infection (1-3% incidence with antibiotic-coated devices), which is the most serious and often necessitates device removal; mechanical failure occurring in 5-15% of cases over 10 years, requiring revision or replacement; erosion of the cylinder through the urethra, tunica, or skin; and autoinflation. Infection prevention strategies include antibiotic irrigation, no-touch surgical technique, and use of devices coated with InhibiZone (rifampin/minocycline) or hydrophilic Titan coatings.

Semi-Rigid (Malleable) Prosthesis

Semi-rigid prostheses consist of paired bendable rods implanted into the corpora cavernosa. These rods remain semi-rigid at all times and can be bent upward for intercourse or downward for concealment. The surgery is simpler, less costly, and involves fewer mechanical parts than inflatable devices. This option is suitable for patients with limited manual dexterity, such as those with spinal cord injury, or in settings where cost is a concern. The main disadvantage is the constant partial rigidity, which can cause concealment issues.

<image>Components of a three-piece inflatable penile prosthesis (cylinders, pump, reservoir) in deflated and inflated states, with anatomic positioning diagram</image>

Penile Rehabilitation After Radical Prostatectomy

The goal of penile rehabilitation after radical prostatectomy is to preserve erectile tissue by promoting early return of oxygenated blood flow. Although evidence is limited, common strategies include daily PDE5 inhibitor use, such as tadalafil 5 mg starting 2 to 4 weeks postoperatively, and use of vacuum erection devices without the constriction ring two to three times daily beginning 4 to 6 weeks after surgery. If there is no response to PDE5 inhibitors by 6 to 9 months, intracavernosal injection therapy may be initiated. Spontaneous return of erections can take 12 to 24 months, especially with bilateral nerve-sparing procedures. Nerve recovery depends on factors such as patient age, preoperative erectile function, and the quality of nerve sparing.

Clinical Pearls

Erectile dysfunction often serves as a sentinel symptom of cardiovascular disease, so all men presenting with new-onset ED should undergo cardiovascular risk assessment. PDE5 inhibitors require sexual stimulation to be effective because they potentiate nitric oxide rather than generate it; patients should be counseled on proper use and given an adequate trial of at least five attempts at the optimal dose. The absolute contraindication for PDE5 inhibitors is concurrent nitrate use due to the risk of potentially fatal hypotension. Tadalafil daily dosing at 5 mg is unique in that it eliminates timing concerns and simultaneously treats lower urinary tract symptoms and benign prostatic hyperplasia. Intracavernosal injection therapy with Trimix achieves approximately 90% efficacy, making it the most effective non-surgical treatment and a reliable option when PDE5 inhibitors fail. Penile prosthesis implantation offers the highest satisfaction rates among ED treatments (85-95%) but is irreversible and carries a risk of infection. In cases of post-prostatectomy ED, patience is essential because nerve recovery can take 12 to 24 months, and treatment should proceed through escalating options sequentially.

References

• AUA Guideline on Erectile Dysfunction, 2018 (amended 2023)
• EAU Guidelines on Sexual and Reproductive Health, 2024 Update
• Burnett AL, et al. "Erectile dysfunction: AUA guideline." J Urol. 2018;200(3):633-641.
• Montorsi F, et al. "Penile rehabilitation after radical prostatectomy." Eur Urol. 2010.
• Mulcahy JJ, et al. "Penile prosthesis implantation." J Sex Med. 2016.
• Campbell-Walsh-Wein Urology, 12th Edition, Chapter on Erectile Dysfunction

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