What causes low semen volume?

Low Semen Volume: Causes and Diagnostic Approach

Low semen volume (hypospermia, defined as <1.4-2.0 mL) results from either structural abnormalities of the seminal vesicles/ejaculatory ducts, functional ejaculatory disorders, or hormonal deficiencies affecting accessory gland function. 1, 2

Primary Etiologic Categories

Obstructive/Structural Causes

• Ejaculatory duct obstruction (EDO) is the most important structural cause, characterized by acidic semen (pH <7.0), low volume (<1.4 mL), and azoospermia or severe oligospermia with very low motility 1, 2
• Congenital bilateral absence of vas deferens (CBAVD) presents with very low volume and is diagnosed by physical examination showing absent vas deferens bilaterally 1, 2
• CBAVD is frequently associated with CFTR gene mutations (cystic fibrosis transmembrane conductance regulator), making genetic screening essential 2, 3
• Seminal vesicle hypoplasia or agenesis reduces the contribution of seminal vesicle fluid, which normally comprises 60-70% of ejaculate volume 4

Functional/Ejaculatory Disorders

• Partial retrograde ejaculation causes low antegrade volume, diagnosed by post-ejaculatory urinalysis showing sperm in urine 2, 3
• Post-ejaculatory urinalysis is mandatory when volume is <1 mL (except in patients with bilateral vasal agenesis or hypogonadism) 5, 2
• Incomplete ejaculation from neurologic dysfunction, medications (alpha-blockers, antipsychotics), or diabetes can reduce volume 4

Hormonal/Endocrine Causes

• Hypogonadism (low testosterone) impairs prostate and seminal vesicle function, reducing their secretory contribution to semen volume 1, 2
• Dihydrotestosterone (DHT) specifically regulates semen volume and viscosity through its effects on prostate and seminal vesicle development and function 6
• Men with 5-alpha-reductase deficiency demonstrate extremely low semen volumes (0.05-1.0 mL) due to rudimentary prostates and small seminal vesicles 6

Medication-Related Causes

• Finasteride 5 mg/day is associated with reduced semen volume, though data for 1 mg/day are inconclusive 1
• Exogenous testosterone suppresses gonadotropin secretion and can reduce accessory gland function 7, 2

Collection/Technical Issues

• Incomplete collection during masturbation is a common artifactual cause that must be excluded by repeat analysis 4, 3
• Short abstinence intervals (<2 days) result in lower volumes, as repeated ejaculation significantly decreases semen volume 8

Diagnostic Algorithm

Initial Assessment

• Confirm true hypospermia with at least two semen analyses after 2-3 days of abstinence, as single measurements are unreliable 5, 2
• Check semen pH immediately: acidic pH (<7.0) with low volume strongly suggests EDO or CBAVD 1, 2
• Palpate for vas deferens bilaterally: absence indicates CBAVD 2
• Assess testicular size and consistency: normal-sized testes suggest obstruction, while atrophic testes indicate primary testicular failure 5, 2
• Perform digital rectal examination to evaluate prostate size and consistency 2

Laboratory Evaluation

• Measure serum testosterone and FSH: low testosterone with low/normal FSH indicates hypogonadotropic hypogonadism, while elevated FSH (>7.6 IU/L) suggests primary testicular failure 2
• Post-ejaculatory urinalysis when volume <1 mL to diagnose retrograde ejaculation (except in bilateral vasal agenesis or hypogonadism) 5, 2

Imaging (Selective Use Only)

• TRUS or pelvic MRI should be reserved for suspected EDO when clinical picture shows: acidic semen, volume <1.4 mL, azoospermia or severe oligospermia with very low motility, normal testosterone, and palpable vas deferens 1, 2
• Do not perform TRUS or pelvic MRI as part of initial evaluation—this is a common pitfall 1, 2

Genetic Testing When Indicated

• CFTR gene testing is mandatory for patients with CBAVD before proceeding with assisted reproduction, as female partners require screening 2, 3
• Karyotype and Y-chromosome microdeletion analysis are required for azoospermia or severe oligospermia (<5 million/mL) 2

Critical Clinical Pearls

• The combination of low volume, acidic pH, and azoospermia/severe oligospermia with very low motility is pathognomonic for distal genital tract obstruction 1
• Fructose testing is relatively unreliable and unnecessary when clinical suspicion for EDO or CBAVD is high 1
• Normal semen volume and pH distinguish non-obstructive azoospermia from obstructive causes 7
• Age correlates with decreased semen volume even in men with high sperm counts, as older patients with high sperm counts tend to have lower volumes 9