Introduction:
Caused by drainage of an inflammatory process (eg. perirectal, on the neck and others). In tuberculosis the term scrophuloderma is used (on the neck).
Sometimes this term is used for congenitally based communication.
Histology:
Chronic inflammation with neutrophils, draining sinus, often lined by squamous epithelium.
Pictures
Fistula, inflammation:
Fistula, 20x (5111) [zoomify]
Perianal fistula:
Perianal fistula, HE 20x (14049)