Erectile Dysfunction: What Could Be Causing It

How Common Is Erectile Dysfunction, and Why Does It Matter?

Erectile dysfunction is not rare. The Massachusetts Male Aging Study (MMAS), one of the largest community-based investigations of male sexual health, found that 52% of men aged 40 to 70 reported some degree of ED [1]. That figure rises sharply with age. But age alone does not explain the problem.

A Signal, Not Just a Symptom

ED frequently serves as an early warning sign of systemic vascular disease. A 2005 analysis published in JAMA demonstrated that ED preceded coronary artery disease diagnosis by a mean of 38.8 months [2]. The Princeton III Consensus guidelines now classify ED as an independent cardiovascular risk factor, recommending that any man presenting with new-onset ED and no obvious cause receive cardiovascular risk stratification [3].

The Numbers Behind the Risk

The connection between ED and heart disease is dose-dependent. Men with severe ED carry a 65% higher risk of coronary events compared to men without ED, according to a meta-analysis of 12 prospective studies involving over 36,000 participants published in the Journal of the American College of Cardiology [4]. The shared mechanism is endothelial dysfunction: the same process that narrows coronary arteries also restricts penile blood flow. Penile arteries, at 1-2 mm in diameter, show symptoms before the larger 3-4 mm coronary vessels do.

Vascular Causes: The Leading Driver

Reduced arterial inflow to the corpora cavernosa is the single most common cause of ED. Vascular disease accounts for an estimated 70% of organic ED cases [5].

Atherosclerosis and Endothelial Dysfunction

The erection process depends on nitric oxide (NO) released from endothelial cells lining the penile arteries. NO activates cyclic GMP, which relaxes smooth muscle and allows blood to fill the erectile tissue. Atherosclerotic plaque reduces arterial diameter. Endothelial damage decreases NO production. Both processes starve the tissue of blood flow.

Risk Factors That Damage Vessels

Diabetes, hypertension, dyslipidemia, and smoking all accelerate endothelial injury. The MMAS data showed that treated heart disease, hypertension, and diabetes each independently predicted ED after adjusting for age [1]. Smoking alone doubles the odds. A 2015 meta-analysis in BJU International found that current smokers had a pooled odds ratio of 1.51 for ED compared to never-smokers, and heavy smokers (over 20 cigarettes per day) had an OR of 2.01 [6].

Venous Leak

Less commonly, blood enters the corpora cavernosa normally but drains too quickly through incompetent veins (corporeal veno-occlusive dysfunction). This condition produces erections that cannot be maintained. Diagnosis often requires duplex Doppler ultrasonography after intracavernosal injection of a vasoactive agent [5].

Hormonal Causes: Testosterone and Beyond

Low testosterone does not explain most ED. But it is a contributing factor in 15-20% of cases, and it is the most treatable hormonal cause [7].

Low Testosterone (Hypogonadism)

The Endocrine Society defines male hypogonadism as a total testosterone level below 300 ng/dL on two morning samples, combined with symptoms [8]. ED can be one of those symptoms, though low libido is often more prominent. Testosterone supports erection physiology at multiple levels: it maintains NO synthase expression in penile tissue, modulates central arousal pathways, and preserves corporal smooth muscle structure.

Hyperprolactinemia and Thyroid Disorders

Elevated prolactin suppresses gonadotropin-releasing hormone, which in turn reduces testosterone. Prolactinomas are uncommon but worth screening for when testosterone is low and LH is inappropriately low or normal. Both overt hypothyroidism and hyperthyroidism alter sex hormone-binding globulin (SHBG) levels and can impair erectile function. The European Association of Urology (EAU) 2024 guidelines recommend checking prolactin and thyroid function when testosterone is low or when clinical suspicion warrants it [9].

When to Test Hormones

Any man with ED should receive at minimum a morning total testosterone level. If that value falls below 400 ng/dL, a repeat test with free testosterone, LH, FSH, and prolactin is warranted. The AUA/SMSNA 2018 guideline on ED evaluation states: "Serum testosterone testing should be offered to all men with erectile dysfunction" [10].

Neurological and Structural Causes

Erection requires intact neural pathways from the brain through the spinal cord to the pelvic nerves. Damage at any point in that chain impairs function.

Diabetes-Related Neuropathy

Diabetic neuropathy is the most common neurogenic cause of ED. Approximately 50% of men with diabetes develop ED within 10 years of diagnosis [11]. The mechanism is twofold: peripheral nerve damage to the cavernosal nerves and concurrent microvascular disease in the penile vasculature. A 2017 cross-sectional study published in Diabetes Care found an ED prevalence of 52.5% among 1,359 men with type 2 diabetes, with HbA1c above 7% independently predicting severity [12].

Surgical and Traumatic Injury

Radical prostatectomy severs or stretches the cavernosal nerves that run along the posterolateral surface of the prostate. Even with bilateral nerve-sparing technique, 30-70% of men experience ED post-operatively, depending on age, baseline function, and surgical skill [13]. Pelvic fractures, spinal cord injuries at or above T10, and radiation therapy for prostate or rectal cancers also cause neurogenic ED.

Multiple Sclerosis and Parkinson Disease

Both conditions disrupt central and peripheral neural signaling. ED prevalence in men with MS ranges from 50% to 75% in published series [14]. Parkinson disease affects dopaminergic pathways that modulate sexual arousal, and the medications used to treat it (particularly SSRI antidepressants added for concurrent depression) can worsen the problem.

Medications That Cause or Worsen ED

Drug-induced ED accounts for roughly 25% of all cases seen in primary care [15]. Identifying an offending medication is one of the fastest paths to improvement.

Antidepressants

SSRIs (fluoxetine, sertraline, paroxetine) and SNRIs (venlafaxine, duloxetine) cause sexual dysfunction in 30-70% of users. Paroxetine carries the highest incidence. The mechanism involves serotonergic inhibition of dopamine and norepinephrine pathways that drive arousal. Bupropion, which lacks serotonergic activity, shows significantly lower rates of sexual side effects and is sometimes used as a switch or augmentation strategy [15].

Antihypertensives

Thiazide diuretics and older beta-blockers (atenolol, propranolol) are the most frequently implicated classes. A 2015 network meta-analysis in Hypertension showed that nebivolol, a third-generation beta-blocker with NO-potentiating properties, had a neutral or mildly favorable effect on erectile function compared to atenolol [16]. ACE inhibitors and ARBs are generally considered ED-neutral or mildly beneficial.

Other Common Culprits

5-alpha reductase inhibitors (finasteride, dutasteride) cause ED in 3-16% of users according to labeled trial data [17]. Antiandrogens (spironolactone at high doses, GnRH agonists), opioids, and first-generation antipsychotics round out the list. When a medication is suspected, a trial discontinuation or substitution under physician guidance is the diagnostic test.

Psychological and Psychogenic Causes

Psychological factors are the primary cause in an estimated 10-25% of men with ED and a contributing factor in many more [18]. Performance anxiety, relationship conflict, depression, and generalized anxiety disorder all impair arousal.

How to Distinguish Psychogenic from Organic ED

The key differentiator is nocturnal erections. Men with psychogenic ED typically retain normal nocturnal penile tumescence (NPT), because the psychological inhibitory input is absent during REM sleep. Sudden onset, situational pattern (present with one partner but not another, or absent during intercourse but present during masturbation), and age under 40 all raise suspicion for a psychological component [18].

Depression and ED: A Bidirectional Relationship

Depression causes ED, and ED worsens depression. A 2018 systematic review in The Journal of Sexual Medicine found that men with ED had a 2.3-fold increased risk of concurrent depressive symptoms [19]. Treating the depression pharmacologically with an SSRI can paradoxically worsen ED, creating a clinical dilemma. Psychotherapy, particularly cognitive behavioral therapy (CBT) focused on sexual performance anxiety, has shown efficacy in randomized trials as both a standalone and adjunctive treatment [18].

How Erectile Dysfunction Is Diagnosed

Diagnosis begins with a thorough sexual, medical, and medication history. The International Index of Erectile Function (IIEF-5), a five-question validated questionnaire, quantifies severity and tracks treatment response [20].

Laboratory Workup

The AUA recommends the following baseline tests for men presenting with ED [10]:

• Fasting glucose or HbA1c
• Lipid panel (total cholesterol, LDL, HDL, triglycerides)
• Morning total testosterone (8-10 AM draw)

If testosterone is low or borderline, add free testosterone, LH, FSH, prolactin, and TSH. A comprehensive metabolic panel and CBC may identify renal or hepatic disease contributing to ED.

Specialized Testing

Duplex Doppler ultrasonography of the penile arteries (after intracavernosal injection of alprostadil or trimix) measures peak systolic velocity (PSV). A PSV below 25 cm/s suggests arterial insufficiency. A PSV above 35 cm/s with persistent venous drainage suggests veno-occlusive dysfunction [5]. Nocturnal penile tumescence testing, though less commonly used today, can confirm the presence of physiologic erections during sleep when psychogenic ED is suspected.

Treatment Approaches Based on Cause

The correct treatment depends entirely on what is driving the dysfunction. Treating every case of ED with a PDE5 inhibitor alone misses the underlying disease.

PDE5 Inhibitors

Sildenafil, tadalafil, vardenafil, and avanafil remain first-line pharmacotherapy. A Cochrane review of 82 trials (N=47,626) found that sildenafil improved erections in 67% of men versus 32% with placebo [21]. Tadalafil offers a 36-hour half-life, which some men prefer for spontaneity. These drugs do not work without sexual stimulation, and they are less effective in men with severe vascular disease or post-prostatectomy nerve injury.

Testosterone Replacement

When ED accompanies documented hypogonadism, testosterone replacement therapy (TRT) can restore erectile function. The TRAVERSE trial (N=5,204), published in The New England Journal of Medicine in 2023, established that TRT did not increase major adverse cardiovascular events in men aged 45-80 with hypogonadism and pre-existing or high risk for cardiovascular disease [22]. TRT is most effective for ED when combined with a PDE5 inhibitor in men who fail PDE5 monotherapy.

Second-Line and Third-Line Options

Intracavernosal injection therapy (alprostadil, trimix) produces erections independent of neural input and works in 85-90% of men, including post-prostatectomy patients [23]. Vacuum erection devices offer a non-pharmacologic alternative. Penile prosthesis implantation, the definitive surgical treatment, carries satisfaction rates above 90% in published long-term series and is reserved for men who fail or cannot tolerate other therapies [24].

Lifestyle Modification

Weight loss, exercise, smoking cessation, and alcohol moderation each independently improve erectile function. A randomized trial published in JAMA found that 2 years of intensive lifestyle intervention (diet plus 195 minutes per week of moderate exercise) restored normal erectile function in 31% of obese men with ED, compared to 5% in the control group [25]. Dr. Kevin McVary, former chair of the AUA Sexual Medicine guidelines panel, has stated: "Lifestyle modification should be recommended for all men with ED, regardless of whether pharmacotherapy is also prescribed" [10].

When to See a Specialist

Primary care physicians manage most ED cases. Referral to a urologist specializing in sexual medicine is appropriate when initial PDE5 inhibitor therapy fails, when Peyronie disease or penile curvature is present, when post-surgical ED does not respond to rehabilitation protocols, or when the patient is considering a penile prosthesis. Men under 40 with sudden-onset ED and no risk factors should receive psychological evaluation in addition to medical workup.

A cardiologist referral is warranted for any man with new ED who has two or more cardiovascular risk factors (diabetes, hypertension, dyslipidemia, smoking, family history of premature coronary disease, or BMI above 30) and has not had recent cardiac evaluation [3].