What is a fistula?

Understanding Fistulas: Definition and Types

A fistula is an abnormal connection between two epithelial-lined surfaces, creating a pathological communication that should not normally exist. 1, 2, 3

General Definition and Pathophysiology

• Fistulas form when there is a loss of wall integrity from an underlying insult, leading to penetration into an adjacent organ or epithelialized surface. 1
• The abnormal tract connects two epithelial surfaces, which can be between two portions of intestine, between intestine and another hollow organ, or between intestine and skin. 2, 3
• Histopathological analysis reveals acute and/or chronic inflammation due to the underlying pathology causing the fistula. 1

Classification Systems

Anatomical Classification

• Fistulas are named based on the two surfaces or lumens they connect, such as enterocutaneous (bowel to skin), enterovesical (bowel to bladder), or enterovaginal (bowel to vagina). 1, 3
• Arteriovenous fistulas are autogenous connections between an artery and vein, commonly created surgically for hemodialysis access (e.g., Brescia-Cimino fistula connecting radial artery to cephalic vein at the wrist). 4
• Perianal fistulas connect the anus or rectum to the perianal skin or other organs, classified by their relationship to the anal sphincter complex. 4

Complexity Classification

• Simple fistulas have a single tract with one external opening, no associated stricture, and no active surrounding inflammation. 5, 6
• Complex fistulas have multiple external openings, multiple tracts or extensions, are associated with abscesses, involve high anatomical positions, or have active inflammation in surrounding tissues. 4, 5

Perianal Fistula Anatomical Types

• Intersphincteric: tract between internal and external anal sphincter. 4
• Transsphincteric: tract crossing through the anal sphincter muscle (can be low or high depending on level). 4
• Suprasphincteric: tract passing upward in intersphincteric plane above puborectalis muscle then laterally into ischioanal fossa. 4
• Extrasphincteric: tract passing directly from rectum to perineal skin through ischioanal fossa without originating in anal canal. 4
• Superficial: involves distal anal canal without involving sphincters. 4

Common Etiologies

• Post-surgical complications account for approximately 80% of enterocutaneous fistulas in developed countries, making surgical intervention the most common cause. 1, 7, 2
• Inflammatory bowel disease, particularly Crohn's disease, where deep ulcers penetrate through the bowel wall creating connections to adjacent structures. 4, 7
• Infection and abscess formation, where perianal abscesses rupture or are drained, leaving a fistulous tract. 4, 6
• Malignancy through direct invasion of adjacent structures. 5, 7, 2
• Radiation injury causing tissue damage and necrosis following pelvic radiation therapy. 5, 7, 2
• Foreign bodies, bowel diverticula, and congenital anomalies are less common causes. 1, 2

Imaging Characteristics

Cross-Sectional Imaging Features

• On MRI, fistulas appear as tubular structures with central high T2 signal intensity, restricted diffusion, and peripheral contrast enhancement. 4
• On CT, fistulas manifest as tubular structures with fluid or air content exhibiting peripheral contrast enhancement. 4
• On ultrasound, fistulas appear as hypoechoic duct-like structures with or without internal gaseous artifacts, with cross-sectional diameter <2 cm (distinguishing from abscess). 4

Key Imaging Features to Report

• Number, location, and length of fistulous tracts should be documented. 4
• Type of fistula based on organs involved (enteroenteric, enterocutaneous, enterovesical, enterovaginal, etc.). 4
• Presence of extensions or secondary tracts, which indicate complexity. 4
• Associated abscesses or collections, including size and location. 4
• Degree of active inflammation assessed by wall thickness, T2 signal, and enhancement pattern. 4

Clinical Significance

• Fistulas cause serious complications including sepsis, malnutrition, metabolic imbalance, fluid and electrolyte disturbances, with mortality rates up to 30%. 1, 7, 2
• Enterovesical, enteroureteral, or enterobiliary fistulas are at increased risk of septic complications and may require urgent surgery. 4
• Management requires multidisciplinary approach with imaging playing a crucial role in detection, characterization, and treatment planning. 1, 7, 3

Visual Representation Concept

While I cannot provide an actual diagram, a fistula should be conceptualized as an abnormal tubular tract or tunnel connecting two epithelial surfaces that should not normally communicate. 2, 3 For example, an enterocutaneous fistula would show a tract extending from the bowel lumen through the abdominal wall to the skin surface, while a perianal fistula would show a tract from the anal canal through or around the sphincter complex to the perianal skin. 4, 1