What is the differential diagnosis for penile discharge following a bowel movement?

Differential Diagnosis for Penile Discharge After Bowel Movement

The most critical diagnosis to consider is a perirectal or prostatic abscess with fistulous communication to the urethra or corpus cavernosum, which requires urgent imaging and surgical consultation. 1, 2

Immediate Life-Threatening Conditions

Perirectal Abscess with Urogenital Extension

• Ischiorectal or perianal abscess can present with penile discharge as the primary symptom when the abscess extends into the corpus cavernosum or creates a fistulous tract to the urethra 2
• The discharge occurs specifically during bowel movements due to increased intra-abdominal pressure forcing purulent material through the fistulous connection 2
• A digital rectal examination is mandatory and may reveal a fluctuant mass, tenderness, or induration in the perirectal space 3, 2
• CT imaging with contrast should be obtained urgently if a perirectal abscess is suspected, as it can demonstrate the abscess cavity and any extension into adjacent structures 2
• Patients may lack typical abscess symptoms such as fever, perianal pain, or systemic signs, making this diagnosis easily missed 3, 2

Prostatic Abscess

• Prostatic abscess can present with milky-white penile discharge during defecation without fever, dysuria, or perineal pain 1
• The discharge occurs during bowel movements because rectal pressure compresses the enlarged, fluid-filled prostate, forcing purulent material through the urethra 1
• CT of the abdomen/pelvis will show a heterogeneous, enlarged prostate with fluid collection 1
• Culture of the discharge is essential and commonly grows multidrug-resistant organisms such as E. coli 1
• Treatment requires transurethral unroofing via cystourethroscopy plus prolonged intravenous antibiotics (e.g., ertapenem for resistant organisms) 1

Common Non-Emergent Causes

Sexually Transmitted Urethritis

• Chlamydia trachomatis and Neisseria gonorrhoeae are the predominant organisms causing urethral discharge in sexually active men under 35 years 4, 5
• Typical symptoms include urethral discharge, penile itching or tingling, and dysuria—though discharge may be the only presenting symptom 5
• The temporal relationship to bowel movements may be coincidental or related to increased awareness during hygiene activities 5
• Diagnosis requires urethral swab or first-void urine testing for gonorrhea and chlamydia, with treatment consisting of azithromycin or doxycycline plus ceftriaxone or cefixime 5
• A positive leukocyte esterase test or ≥10 white blood cells per high-power field in urine sediment supports the diagnosis 5

Chronic Prostatitis

• Chronic bacterial prostatitis can cause intermittent penile discharge that may be more noticeable during or after bowel movements due to prostatic massage effect 1
• Patients may have minimal or absent dysuria, fever, or perineal pain, particularly in chronic cases 1
• Expressed prostatic secretions or post-prostatic massage urine should be cultured 1

Less Common but Important Differentials

Rectourethral Fistula

• A fistulous communication between the rectum and urethra can cause passage of fecal material, gas, or purulent discharge through the penis 3
• This may result from Crohn's disease, prior pelvic surgery, radiation therapy, or trauma 3
• The discharge would likely contain fecal material or have a distinctly fecal odor 3
• Diagnosis requires CT imaging, MRI, or contrast studies to identify the fistulous tract 3

Atypical Urethritis Pathogens

• Mycoplasma genitalium, Ureaplasma species, Trichomonas, adenovirus, and herpes simplex virus can cause urethritis with discharge 5
• These should be considered when standard gonorrhea/chlamydia testing is negative and symptoms persist 5

Diagnostic Algorithm

Step 1: Immediate Assessment

• Perform a thorough digital rectal examination specifically palpating for fluctuance, mass, tenderness, or induration suggesting perirectal abscess 3, 2
• Assess for systemic signs including fever, tachycardia, or hemodynamic instability that would indicate necrotizing infection requiring emergency surgery 3, 4
• Examine the penis for erythema, warmth, swelling, or tenderness that might suggest cellulitis or other local pathology 6

Step 2: Obtain Discharge Culture

• Collect a sample of the discharge for Gram stain and culture before initiating antibiotics 1, 5
• The presence of gram-negative diplococci suggests gonorrhea, while white blood cells without organisms suggest nongonococcal urethritis 5

Step 3: Imaging

• Order CT of the abdomen/pelvis with IV contrast if digital rectal examination reveals abnormalities or if prostatic abscess is suspected 1, 2
• Ultrasound with Doppler can be used as an adjunct to assess for penile cellulitis or other soft tissue pathology if the clinical picture is unclear 4, 6

Step 4: Laboratory Testing

• Obtain urinalysis with microscopy looking for pyuria (≥10 WBC/hpf) 5
• Send first-void urine or urethral swab for gonorrhea and chlamydia nucleic acid amplification testing 5
• Check serum glucose and hemoglobin A1c, as diabetes is a major risk factor for both prostatic and perirectal abscesses 3, 4

Critical Pitfalls to Avoid

• Never assume penile discharge is solely due to sexually transmitted infection without performing a digital rectal examination, as perirectal abscess with urogenital extension is easily missed 2
• Do not delay imaging if the rectal examination is abnormal or if the patient has risk factors for abscess (diabetes, immunosuppression) 3, 1
• Avoid empiric treatment for STIs without obtaining cultures first when the presentation is atypical (discharge only with bowel movements), as this may mask a more serious underlying condition 1, 2
• Remember that patients with prostatic or perirectal abscesses may lack classic symptoms such as fever or pain, particularly in early presentations 3, 1
• If the patient fails to improve within 3 days of appropriate antibiotic therapy for presumed urethritis, reevaluate the diagnosis and consider imaging 4