What are the appropriate diagnosis and treatment options for a patient presenting with watery rectal discharge, potentially accompanied by abdominal pain, fever, or blood in the stool?

Watery Rectal Discharge: Diagnosis and Management

Begin with a focused history and digital rectal examination to identify the underlying cause, as watery rectal discharge can indicate serious conditions ranging from infectious proctitis to inflammatory bowel disease, anorectal abscess, or fistula. 1

Initial Clinical Assessment

Key historical elements to obtain:

• Duration and frequency of discharge 1
• Associated symptoms: fever, abdominal pain, blood in stool, tenesmus, or perianal pain 1
• Recent antibiotic use (risk for Clostridioides difficile colitis) 2
• Sexual history, particularly anoreceptive intercourse (risk for sexually transmitted infections causing proctitis) 3
• History of inflammatory bowel disease, particularly Crohn's disease 1
• Immunosuppression status, diabetes, or HIV infection 1, 3

Physical examination priorities:

• Vital signs to assess for systemic infection or sepsis 1
• Careful inspection of the perineum for surgical scars, anorectal deformities, signs of perianal Crohn's disease, cellulitis, or external fistula openings 1
• Digital rectal examination (may require sedation if intense pain present) to detect tender indurated areas, masses, or deeper abscesses 1

Laboratory Investigations

For patients with signs of systemic infection or suspected abscess:

• Complete blood count 1
• Serum creatinine and inflammatory markers (C-reactive protein, procalcitonin, lactates) 1
• Serum glucose, hemoglobin A1c, and urine ketones to identify undetected diabetes mellitus 1

For patients with suspected infectious proctitis:

• Rectal cultures for bacterial, viral, and protozoal pathogens 3
• Consider HIV testing if risk factors present 3

Imaging Studies

CT scan with IV contrast is indicated when:

• Atypical presentation suggests deeper pathology 1
• Suspicion of occult supralevator abscess, complex anal fistula, or perianal Crohn's disease 1
• Signs of systemic infection with unclear source 1

MRI is preferred for:

• Detailed evaluation of complex fistulas and perianal Crohn's disease (higher accuracy than CT) 1
• When available and clinical scenario permits 1

Endoanal ultrasound (EUS) with color Doppler:

• Useful for detecting deep abscesses and evaluating complex fistulas 1
• Requires special skills and may be precluded by intense anal pain in emergency settings 1

Endoscopic Evaluation

Anoscopy or flexible sigmoidoscopy should be performed as first-line diagnostic tool when:

• Suspected proctitis or inflammatory bowel disease 1
• Need to visualize rectal mucosa and obtain biopsies 3
• Rule out neoplastic causes in patients with risk factors 1

Full colonoscopy is indicated for:

• Risk factors for colorectal cancer 1
• Suspicion of more proximal source of symptoms 1
• Evaluation for inflammatory bowel disease 1

Critical Differential Diagnoses

Infectious causes:

• Bacterial proctitis (gonorrhea, chlamydia, syphilis) 3
• Viral proctitis (HSV, CMV in immunocompromised) 3
• Clostridioides difficile colitis (pseudomembranous colitis) - can occur 2+ months after antibiotic use 2

Inflammatory/structural causes:

• Anorectal abscess with spontaneous drainage 1
• Anal fistula 1
• Perianal Crohn's disease 1
• Rectovaginal or rectovesicular fistula 1

Neoplastic causes:

• Anal or rectal carcinoma (11% of fistulas are malignancy-related) 1
• Must maintain high index of suspicion and biopsy suspicious lesions 1, 3

Management Approach

For suspected infectious proctitis:

• Empiric antibiotics covering gram-positive, gram-negative, aerobic and anaerobic bacteria if systemic signs present 1
• Specific antimicrobial therapy based on culture results 3

For anorectal abscess:

• Timely surgical incision and drainage to prevent serious complications 3
• Empiric broad-spectrum antibiotics if signs of systemic infection 1

For C. difficile colitis:

• Discontinue offending antibiotic if possible 2
• Contact healthcare provider immediately if watery diarrhea develops during or after antibiotic therapy 2

For inflammatory bowel disease:

• Multidisciplinary management with gastroenterology 1
• Medical therapy directed at underlying condition 1

Critical Pitfalls to Avoid

• Do not delay imaging in patients with atypical presentations or signs of systemic infection - deeper abscesses can be challenging to diagnose on examination alone 1
• Do not miss undiagnosed Crohn's disease - mandatory to exclude in every patient with anorectal abscess, especially if recurrent 1
• Do not overlook malignancy - obtain biopsy of suspicious lesions, as inflammatory disease may be complication of or associated with malignancy 1, 3
• Do not underestimate severity in immunocompromised patients - symptoms are frequently absent or diminished in elderly, debilitated, diabetic, or immunosuppressed patients 1
• Do not ignore antibiotic-associated diarrhea - pseudomembranous colitis can occur up to 2+ months after completing antibiotics 2