What is the most likely diagnosis for a rectal fistula without an external perianal opening, with a computed tomography (CT) scan showing a pelvic collection in the posterior aspect?

Most Likely Diagnosis: Intersphincteric Fistula with Lower Rectal/Pelvic Abscess

The most likely diagnosis is D: Intersphincteric fistula with lower rectal/pelvic abscess, given the absence of an external perianal opening combined with a posterior pelvic collection on CT imaging. 1, 2

Key Diagnostic Features

Absence of External Opening Points to High Intersphincteric Disease

• High intersphincteric fistulas characteristically have no external opening in the majority of cases and are frequently associated with high intersphincteric and/or supralevator abscesses. 2
• The presence of a fistulous tract without external manifestation distinguishes this from simple perianal or ischiorectal pathology, which typically present with visible external openings or lateral perianal swelling. 1, 3
• High intermuscular abscesses present with no external swelling, induration, or opening, but demonstrate high extension with a palpable mass or induration above the levator ani. 3

Posterior Pelvic Collection Localizes the Pathology

• The posterior pelvic location on CT is consistent with an intersphincteric abscess extending upward in the intersphincteric plane, potentially reaching or exceeding the puborectal muscle level. 2, 3
• This anatomical distribution excludes ischiorectal abscess (option C), which would penetrate through the external sphincter into the lateral ischioanal/ischiorectal space, not remain in the posterior midline intersphincteric plane. 1, 4

Why Other Options Are Less Likely

Option A: Pelvi-rectal Fistula

• This term is not a standard classification in the Parks system or contemporary anorectal disease nomenclature. 4
• The specific anatomical features described better fit established intersphincteric pathology. 1, 4

Option B: Intersphincteric Abscess Alone

• While partially correct anatomically, this option fails to account for the fistulous tract component explicitly mentioned in the clinical scenario. 1
• Approximately one-third of anorectal abscesses are associated with anal fistulas, and the majority of anal fistulae arise from preexisting abscesses. 1

Option C: Ischiorectal Abscess

• Ischiorectal fistulas penetrate through the external anal sphincter into the ischioanal space laterally, which is inconsistent with a posterior pelvic collection without external opening. 1, 4
• These would present with lateral extension and typically have external manifestations. 4

Clinical Pearls and Management Implications

Preoperative Imaging is Essential

• Since most high intersphincteric fistulas have no external opening and are frequently associated with abscesses, preoperative imaging with MRI or endoanal ultrasound is mandatory for surgical planning. 2
• CT provides important information in diagnosing the underlying etiology and detecting the course and locations of fistulae, though MRI shows higher accuracy for complex fistulae and secondary extensions. 5

Look for Internal Opening at Dentate Line

• The internal opening at the dentate line is pathognomonic for cryptoglandular fistulas and should be identified during examination under anesthesia. 1, 3
• In high intermuscular cases, passing a cannula from the primary opening into the cavity confirms the diagnosis. 3

Exclude Crohn's Disease

• It is mandatory to exclude underlying Crohn's disease, especially with recurrent presentations, by assessing for inflammatory bowel disease symptoms including diarrhea, weight loss, and abdominal pain. 1, 4

Treatment Approach

• Transanal advancement flap repair combined with adequate drainage of associated abscesses achieves 100% overall healing in high intersphincteric fistulas, though primary healing may be lower (79%) when supralevator extension is present. 2
• Simple fistulotomy risks diminished fecal continence when dividing large amounts of internal anal sphincter in high disease. 2