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01Pathophysiology

Erection is fundamentally a vascular event regulated by the parasympathetic nervous system. During sexual arousal, parasympathetic signals travel via the pelvic splanchnic nerves (S2 through S4) to the cavernous nerves, which release acetylcholine at the endothelium and stimulate the release of nitric oxide (NO) from both endothelial cells and non-adrenergic, non-cholinergic (NANC) nerve terminals within the corpus cavernosum.

Nitric oxide activates the enzyme soluble guanylate cyclase, which converts GTP to cyclic guanosine monophosphate (cGMP). Rising intracellular cGMP triggers smooth muscle relaxation in the trabecular arterioles and sinusoidal spaces of the corpora cavernosa. As smooth muscle relaxes, arterial inflow increases and the sinusoidal spaces fill with blood. The expanding sinusoids compress the subtunical venules against the rigid tunica albuginea, effectively trapping blood within the corpora. This compression is called the veno-occlusive mechanism, and it is what sustains the erection.

The erection terminates when the enzyme phosphodiesterase type 5 (PDE5) degrades cGMP back to GMP, allowing smooth muscle contraction and venous outflow to resume. This is the exact molecular target of PDE5 inhibitor therapy: by blocking PDE5, these drugs prevent cGMP breakdown and prolong the vasodilatory signal. A critical teaching point is that PDE5 inhibitors require the presence of sexual stimulation to work, because they amplify existing NO/cGMP signaling rather than initiating it.

The most common cause of ED is vasculogenic, driven by the same endothelial dysfunction and atherosclerotic process that underlies coronary artery disease. Damaged endothelium produces less NO, reducing the vasodilatory response. This is why ED is now considered a sentinel marker and cardiovascular risk equivalent: the smaller penile arteries (1 to 2 mm) become symptomatic from atherosclerosis before the larger coronary arteries (3 to 4 mm). Studies suggest that ED may precede a cardiovascular event by 3 to 5 years. Any man presenting with new-onset ED, especially with vascular risk factors, should receive a full cardiovascular risk assessment.

In neurogenic ED, damage to the autonomic pathways (from diabetes, spinal cord injury, radical prostatectomy, or multiple sclerosis) disrupts the nerve-mediated release of NO. In hormonal ED, low testosterone reduces libido and may also impair NO synthase expression, though the primary complaint is often decreased desire rather than inability to achieve erection. Drug-induced ED is extremely common and frequently tested, with offending agents including thiazide diuretics, beta-blockers (especially non-selective), spironolactone, SSRIs, and anti-androgens.

Psychogenic ED is distinguished by its sudden onset, situational nature, and preservation of nocturnal and early-morning erections. This contrasts with organic ED, which is gradual in onset, consistent across all situations, and associated with loss of nocturnal erections.

02Classification and Clinical Manifestation

By Etiology

Severity Classification (IIEF-5 / SHIM Score)

The International Index of Erectile Function (IIEF-5), also known as the Sexual Health Inventory for Men (SHIM), is a validated 5-question self-report tool scored from 1 to 25.

03Diagnostic Workup

The best initial step in evaluating ED is always a thorough history and physical examination. The history should focus on onset pattern (gradual vs. sudden), presence or absence of nocturnal/morning erections, situational vs. consistent nature, libido, psychosocial stressors, surgical history (especially pelvic or prostate surgery), and a complete medication review. The physical exam should include assessment of secondary sexual characteristics, genital exam (penile plaques, testicular size and consistency), peripheral pulses, and a focused neurological exam including perineal sensation and the bulbocavernosus reflex.

Laboratory workup begins with a morning total testosterone level (drawn between 8:00 and 10:00 AM when levels peak). If the initial level is low (below approximately 300 ng/dL), it must be confirmed with a repeat measurement, along with free testosterone, LH, FSH, and prolactin to distinguish primary from secondary hypogonadism. All patients should also undergo screening for diabetes (fasting glucose or HbA1c) and dyslipidemia (lipid panel) because of the strong association between ED and cardiovascular disease.

The nocturnal penile tumescence (NPT) test is the key study for differentiating psychogenic from organic ED. Healthy men experience 3 to 5 erections during REM sleep lasting 25 to 35 minutes each. In organic ED, these erections are absent or diminished. In psychogenic ED, nocturnal erections are preserved. This test is not routinely needed but becomes valuable when the clinical picture is ambiguous.

Penile duplex Doppler ultrasonography performed after intracavernosal injection of a vasoactive agent (such as alprostadil) is the most accurate test for vasculogenic ED. It measures peak systolic velocity (PSV) and end-diastolic velocity (EDV) in the cavernous arteries. A PSV below 25 cm/s suggests arteriogenic insufficiency. An EDV above 5 cm/s suggests venous leak (failure of the veno-occlusive mechanism). This test is typically reserved for patients being considered for vascular surgical intervention.

04Management and Treatment

Lifestyle modification is the foundation and should be addressed in every patient. Weight loss, regular aerobic exercise, smoking cessation, and optimization of blood glucose, blood pressure, and lipids not only improve erectile function directly but also reduce the associated cardiovascular risk. If an offending medication is identified (e.g., a thiazide or beta-blocker), substitution with a less sexually impairing alternative (such as an ACE inhibitor, ARB, or nebivolol) should be considered.

PDE5 inhibitors are the cornerstone of pharmacologic management. These agents work by blocking the degradation of cGMP in cavernosal smooth muscle, thereby enhancing the vasodilatory response to sexual stimulation. Important counseling points:

• They require sexual stimulation to be effective (they do not cause spontaneous erections).

• They should be taken on an empty stomach for optimal absorption (except tadalafil, which is less affected by food).

• Common side effects include headache, flushing, nasal congestion, dyspepsia, and visual changes (blue-tinted vision with sildenafil due to cross-reactivity with PDE6 in the retina).

• Absolute contraindication: concurrent use of nitrates (nitroglycerin, isosorbide mononitrate/dinitrate). The combination causes profound, potentially fatal hypotension due to excessive cGMP-mediated vasodilation. This is one of the highest-yield pharmacology points tested. A patient must wait at least 24 hours after sildenafil or vardenafil (48 hours for tadalafil) before nitrate administration.

• Relative contraindication with alpha-1 blockers (e.g., tamsulosin, doxazosin) due to additive hypotensive effect. If coadministration is necessary, the patient should be on a stable alpha-blocker dose, and the PDE5 inhibitor should be started at the lowest dose.

Tadalafil has a uniquely long half-life (approximately 17.5 hours, with a clinical duration of up to 36 hours), which allows for a daily low-dose regimen (2.5 to 5 mg). This daily dosing is preferred in men who have frequent sexual activity, and tadalafil is also FDA-approved for concurrent treatment of ED and benign prostatic hyperplasia (BPH).

If PDE5 inhibitors fail (after adequate trial of at least 4 to 6 attempts with proper technique), second-line options include:

• Intracavernosal injection of alprostadil (prostaglandin E1), which acts directly on smooth muscle independent of the NO pathway. The onset is 5 to 15 minutes. The primary risk is priapism (erection lasting more than 4 hours), which requires emergent aspiration and injection of phenylephrine.

• Vacuum erection devices, which are non-invasive and can be combined with a constriction ring. The ring should not remain in place for more than 30 minutes to avoid ischemic injury.

Third-line therapy is the penile prosthesis, typically an inflatable three-piece device implanted surgically. This is reserved for patients who have failed or cannot use pharmacologic and mechanical therapies. Patient satisfaction rates are high (over 85%), but the procedure is irreversible in terms of destroying natural erectile tissue.

Testosterone replacement is indicated only when there is biochemically confirmed hypogonadism with two separate low morning testosterone measurements. It is not appropriate to prescribe testosterone for ED in a eugonadal man (normal testosterone levels), as it will not improve erectile function and carries risks including erythrocytosis, worsened sleep apnea, and theoretical concern for prostate cancer stimulation (though recent data have moderated this concern). Before starting testosterone, a baseline PSA and hematocrit should be obtained.

05Differential Diagnosis and Distractors

06Traps and High-Yield Pearls

The single most tested concept in ED questions is the absolute contraindication of PDE5 inhibitors with nitrates. The classic vignette presents a man with ED who also has stable angina managed with sublingual nitroglycerin. Students who reflexively choose sildenafil as first-line without checking the medication list will fall into this trap. The correct answer in this scenario is to switch the patient off nitrates first (to a non-nitrate antianginal such as amlodipine or ranolazine) before prescribing a PDE5 inhibitor, or to proceed directly to an alternative therapy like intracavernosal alprostadil or a vacuum device.

A second common trap involves confusing decreased libido with erectile dysfunction. When a vignette describes a man who has "no interest in sex," the answer is not a PDE5 inhibitor. The correct next step is to check a morning testosterone level and evaluate for hypogonadism or depression. PDE5 inhibitors treat the vascular mechanism of erection; they do nothing for desire.

A third high-yield point is the recognition that ED is a cardiovascular risk equivalent. When a middle-aged man with newly diagnosed ED presents to the exam, the question may not be asking about treating the ED at all. Instead, the correct answer may be to pursue a cardiovascular workup (lipid panel, fasting glucose, blood pressure screening, exercise stress test), because the ED is a harbinger of coronary artery disease.

Finally, students should remember the pharmacologic distinction between PDE5 inhibitors: tadalafil stands apart with its long duration of action and daily dosing option, and it is the only PDE5 inhibitor also approved for BPH. When a vignette describes a man with both ED and lower urinary tract symptoms (hesitancy, frequency, nocturia), tadalafil 5 mg daily is the targeted answer.

The core competency being tested across ED questions is the ability to identify the underlying etiology (vascular, neurogenic, hormonal, drug-induced, or psychogenic), select the appropriate first diagnostic and therapeutic step, and recognize critical drug interactions that make standard therapy unsafe.