Ideal investigation for fistula-in-ano is: March 2008

Ans. B: MRI A fistula in ano is a tract lined by granulation tissue which connects the anal canal mucosa with the skin around the anus. The fistula continues to discharge and the constant reinfection from the anal canal prevents healing of the fistula. Identification of fistulous tract: Physical examination - Digital Rectal Examination, Proctoscopy, Probe test Injection technique - Dye injection (methylene blue or Indigo carmine), Hydrogen peroxide Fistulography Endoanal ultrasound gives impoant information about sphincter integrity. Endoanal ultrasound, especially with hydrogen peroxide, can also be used to delineate fistulae, although definition of sepsis outside or above the sphincters is limited by the probe's focal range and scarring makes interpretation difficult. MRI is acknowledged to be the gold standard for fistula imaging but it is limited by availability and the cost and is usually reserved for difficult recurrent cases. CT scan and fistulography both have limitations but are useful if extrasphincteric fistula is suspected.