Differential Diagnosis for Chronic Anal Discharge with No External Opening

Single Most Likely Diagnosis

• Intersphincteric fistula: This is the most likely diagnosis given the patient's symptoms of chronic anal discharge and the MRI findings showing a tract from 2 o'clock to 6 o'clock, about 5 cm away from the anal orifice. Intersphincteric fistulas are a type of anal fistula that passes through the intersphincteric plane, which is between the internal and external anal sphincters. The location and description provided in the MRI are consistent with this type of fistula.

Other Likely Diagnoses

• Ischiorectal fistula: Although less common, an ischiorectal fistula could also present with similar symptoms. This type of fistula passes through the ischiorectal fossa, which is located lateral to the anal canal. The distance from the anal orifice and the direction of the tract could potentially align with an ischiorectal fistula, making it a plausible alternative diagnosis.
• Intersphincteric abscess: An abscess in the intersphincteric plane could cause chronic discharge if it has formed a fistulous tract to the skin or mucosa. However, the primary complaint of discharge without an external opening and the specific MRI findings make a fistula more likely than an abscess.

Do Not Miss Diagnoses

• Levator ani abscess: Although less likely given the specific location and nature of the symptoms, a levator ani abscess could potentially cause significant morbidity if not promptly diagnosed and treated. It is located higher up in the pelvis and could potentially cause more severe symptoms, including systemic illness.

Rare Diagnoses

• Perineal abscess: A perineal abscess would typically present with more localized signs of infection, such as swelling, redness, and pain in the perineal area, and might not fit as well with the description of chronic anal discharge without an external opening.
• Prolapsing piles (hemorrhoids): While prolapsing hemorrhoids can cause discomfort and discharge, they are less likely to cause chronic anal discharge without any visible external signs, especially when coupled with the specific MRI findings described.
• Other rare conditions such as rectovaginal fistulas (in females), Crohn's disease, or other inflammatory bowel diseases could also present with anal discharge but would typically have additional symptoms and findings that distinguish them from the primary diagnosis of an intersphincteric fistula.