Aetiology

Majority are thought to be secondary to impaired drainage of the anal glands (according to the "cryptoglandular hypothesis");

• impaired drainage and infection of the anal gland will result in acute perianal abscess. With time, inadequately or incompletely drained abscess will result in the infection spreading through the pathway of least resistant creating fistulous tracts
• around 87% of patients with acute perianal abscess may subsequently develop a fistula (1)

Other associated conditions include:

• Crohn's disease
  • cumulative incidence of anal fistula in patients with Crohn's disease is 20-25%.
  • complex and multiple fistulas are often seen in these patients (2)
• tuberculosis
• hidradenitis suppurativa
• HIV infection
• previous surgery
• radiotherapy
• trauma
• foreign bodies
• anorectal cancer (2)

Note:

• perianal abscess and perianal fistula are considered to be acute and chronic manifestations of the same clinical disease (1)

References:

1. O'Malley RB et al. Rectal imaging: part 2, Perianal fistula evaluation on pelvic MRI--what the radiologist needs to know. AJR Am J Roentgenol. 2012;199(1):W43-53
2. Simpson JA, Banerjea A, Scholefield JH. Management of anal fistula. BMJ. 2012;345:e6705