What is the likely diagnosis for a patient with throbbing anal pain, fever, and pain during a digital rectal examination (DRE) without significant discharge or bleeding, and a normal colonoscopy result?

Diagnosis: Intersphincteric Abscess

The most likely diagnosis is an intersphincteric abscess (Option A), given the combination of throbbing anal pain, fever, pain on digital rectal examination, and normal colonoscopy findings. 1

Clinical Reasoning

Why Intersphincteric Abscess is Most Likely

• Throbbing pain is the hallmark symptom of anorectal abscesses, distinguishing them from other anorectal conditions 1

• Fever without significant systemic signs ("no soaking of clothes") suggests a contained infectious process rather than extensive perianal sepsis 1

• Pain on digital rectal examination (DRE) is characteristic of deeper abscesses, particularly intersphincteric and supralevator types, where DRE reveals a tender, indurated area above the anorectal ring 1

• Normal colonoscopy effectively rules out mucosal pathology, inflammatory bowel disease, and malignancy that could present similarly 1

• Intersphincteric abscesses are often occult on external examination but cause significant pain on rectal examination, making them challenging to diagnose clinically 1

Why Other Options Are Less Likely

Hemorrhoids (Option B):

• Hemorrhoids typically present with bleeding and prolapse, not fever 1, 2
• While thrombosed hemorrhoids cause acute pain, they do not cause fever and are visible on external examination 3, 2
• The presence of fever strongly argues against uncomplicated hemorrhoidal disease 1

Anal Fissure (Option C):

• Fissures cause sharp, tearing pain during and after defecation, not throbbing pain 4, 3
• Fissures are best visualized by effacing the anal canal with opposing traction on the buttocks, not through DRE 4
• DRE may actually be contraindicated in anal fissures due to severe pain it causes 4
• Fissures do not cause fever unless complicated by secondary infection 4, 3
• Most fissures (90%) are located posteriorly in the midline and are visible on external examination 4

Submucosal Abscess (Option D):

• Submucosal abscesses are high abscesses that may have few local symptoms but significant systemic symptoms 1
• They would typically present with more pronounced systemic signs than described 1
• The clinical presentation better fits an intersphincteric location 1

Anatomical Classification Context

Anorectal abscesses are classified by anatomical location: intersphincteric, perianal, ischiorectal (ischioanal), and supralevator 1. The intersphincteric space is the most common site of origin, as anorectal abscesses result from infection of the intersphincteric anal glands 1.

Critical Diagnostic Pitfalls

• Deeper abscesses (intersphincteric, supralevator) can be challenging to diagnose because they may be occult on physical examination, with DRE revealing only a tender, indurated area 1

• Symptoms are frequently absent or diminished in older, debilitated, diabetic, or immunosuppressed patients, requiring a high degree of suspicion 1

• Anal pain is often mistakenly attributed to hemorrhoids when it may actually be due to anal fissure, perianal abscess, or anal fistula 4

Recommended Next Steps

• Check serum glucose, hemoglobin A1c, and urine ketones to identify undetected diabetes mellitus 1

• Request complete blood count, serum creatinine, and inflammatory markers (C-reactive protein, procalcitonin, lactates) to assess severity 1

• Consider imaging (MRI, CT scan, or endoanal ultrasound) for suspected intersphincteric or supralevator abscesses, as these are difficult to diagnose clinically 1

• Prompt surgical drainage is indicated once the diagnosis is confirmed, as undrained anorectal abscesses can expand into adjacent spaces and progress to systemic infection 1