What is the recommended approach for a digital rectal examination (DRE) on a patient with a fistula in ano, considering their medical history and potential underlying conditions like inflammatory bowel disease?

Digital Rectal Examination in Fistula in Ano

Perform a complete digital rectal examination as part of the initial clinical assessment, but exercise caution to avoid probing for occult fistulas, as this can cause iatrogenic complications. 1

Initial Clinical Assessment

DRE should be performed as part of a focused medical history and complete physical examination in patients with suspected or known fistula in ano. 1 The examination provides critical information about:

• Palpation of a cord-like structure between the internal and external sphincter muscles, which is pathognomonic for intersphincteric fistula 2
• Location of the internal opening at the dentate line, which defines cryptoglandular fistulas 2
• Assessment of sphincter tone, which has prognostic implications for surgical planning and risk of postoperative incontinence 3, 4
• Detection of induration or tender areas that may indicate deeper abscesses (intersphincteric or supralevator) that are occult on external examination 5

Critical Limitations and Contraindications

DRE has significant diagnostic limitations compared to imaging modalities:

• Digital examination correctly classifies only 61% of fistula tracks, compared to 81% for endosonography and 90% for MRI 4
• Agreement with surgical findings is only fair (kappa = 0.38) for DRE versus good (kappa = 0.68) for endosonography and very good (kappa = 0.84) for MRI 4
• Examiner experience matters significantly: experienced colorectal surgeons achieve 0.7-0.96 agreement with manometry for sphincter assessment, while junior examiners achieve only 0.52 3

Avoid probing to search for occult fistulas during DRE in patients with anorectal abscess and no obvious fistula, as this risks creating iatrogenic fistula tracts. 1

Specific Examination Technique

When performing DRE in fistula patients:

• Assess resting and squeeze sphincter pressures using systematic palpation, though recognize this is subjective and less accurate than manometry 3
• Palpate for cord-like structures running between sphincter layers, which indicate intersphincteric fistula 2
• Identify tender, indurated areas above the anorectal ring that suggest deeper abscess collections 5
• Note any palpable internal opening at the dentate line 2

When DRE May Be Contraindicated or Modified

Severe pain may preclude adequate examination:

• If acute abscess is present with significant pain, DRE findings may be limited and examination under anesthesia may be warranted 6
• In patients with acute anal fissure, DRE may be contraindicated due to severe pain it causes 6

Mandatory Additional Workup

Always obtain focused history to exclude inflammatory bowel disease, particularly Crohn's disease, which occurs in approximately one-third of patients with anorectal abscess and has markedly reduced surgical success rates. 1, 2 Look specifically for:

• Diarrhea, weight loss, abdominal pain 2
• Recurrent perianal sepsis 2
• History of previous failed fistula repairs 7

Check metabolic parameters:

• Serum glucose, hemoglobin A1c, and urine ketones to identify undetected diabetes mellitus 1, 5
• Complete blood count, creatinine, and inflammatory markers (CRP, procalcitonin, lactate) if systemic infection or sepsis is present 1

Imaging Is Essential for Surgical Planning

DRE alone is insufficient for preoperative planning. 4

Obtain MRI or endoanal ultrasound for:

• Suspected intersphincteric or supralevator abscesses (difficult to diagnose clinically) 1, 5
• High intersphincteric fistulas requiring surgical planning 2
• Recurrent fistulas 2
• Suspected inflammatory bowel disease 1, 2
• Complex fistulas with secondary extensions 4

MRI shows higher accuracy (90% correct classification) than endosonography (81%) or clinical examination (61%) for fistula classification, though endosonography is a viable alternative for identifying the internal opening (91% vs 97% for MRI). 4

Common Pitfalls to Avoid

• Do not rely solely on DRE for fistula classification or surgical planning—imaging is mandatory 2, 4
• Do not probe for occult fistulas during acute abscess drainage, as this creates iatrogenic tracts 1
• Do not miss Crohn's disease: approximately one-third of Crohn's patients develop anorectal abscess, and failure to identify this leads to poor surgical outcomes 1, 2
• Do not assume normal external examination excludes deep abscess: intersphincteric and supralevator abscesses are often occult externally but cause significant pain on DRE 5
• Symptoms may be absent or diminished in older, debilitated, diabetic, or immunosuppressed patients—maintain high clinical suspicion 5