Page 100 - Clinical Anatomy

ECA2 7/18/06 6:42 PM Page 85

The gastrointestinal tract 85

Fig. 65◊The anatomy of
perianal fistulae and
abscess.

The so-called ‘thrombosed external pile’ is a small tense haematoma at
the anal margin caused by rupture of a subcutaneous vein and is much
better termed a perianal haematoma.

Perianal abscesses (Fig. 65)
These may be localized beneath the anal mucosa (submucous), be beneath
perianal skin (subcutaneous) or occupy the ischiorectal (-anal) fossa, or,
more anatomically accurate, the isschio. Occasionally, abscesses lie in the
pelvirectal space above levator ani, alongside the rectum and deep to the
pelvic peritoneum.

Fistulae (Fig. 65)
Anal fistulae usually result from rupture of perianal abscesses. They are
classified anatomically and may be:
•◊◊submucous —confined to the tissues immediately below the anal
mucosa;
•◊◊subcutaneous—confined to the perianal skin;
•◊◊low-level— passing through the lower part of the superficial sphincter
(most common);
•◊◊high-level—passing through the deeper part of the superficial
sphincter;
•◊◊anorectal —which has its track passing above the anorectal ring and
which may or may not open into the rectum.
In laying open fistulae in ano, it is essential to preserve the anorectal ring
if faecal incontinence is to be avoided. The lower part of the sphincter, on
the other hand, can be divided quite safely without this risk.

Fissure in ano
This is a tear in the anal mucosa; over 90% occur posteriorly in the midline.
The anatomical basis for this probably lies in the insertion of the superficial