Perineal abscesses and fistulae

ANORECTAL ABSCESSES

As with any abscess, this is a collection of pus, just in the anal/rectal region. They are more common in men than women and often result in perianal fistulas which we will discuss soon.

Pathophysiology

Anal ducts help secrete mucus to allow faeces to pass easily. When these become blocked, they cause anorectal abscesses due to fluid stasis which allows for infection to occur.

The most common causative agents: E. coli, bacteriodes spp., enterococcus spp..

We have 4 main types of anorectal abscess to consider:

1. Perianal - by far the most common
2. Ischiorectal
3. Intersphincteric
4. Supralevator

😷 Presentation

Severe pain in the perianal area (worse when sat down or with pressure)

Perianal discharge or bleeding

Erythematous and tender mass on examination.

Severe abscesses may lead to systemic symptoms (fever, rigors, malaise, features of sepsis if allowed to progress).

🔍 Investigations

⭐️ Clinical diagnosis is usually the case. However, to prep for surgical drainage routine bloods are needed (FBC, U&Es, clotting + G&S).

Patients with anorectal abscesses without any fistulae or known underlying causes should have an HbA1c to assess for diabetes mellitus.

For more complicated cases with atypical presentation (deeper abscesses and fistulae) may need and MRI or a CT if MRI is unavailable).

🧰 Management

• Antibiotics
• Analgesia

ANAL FISTULAE

Pathophysiology

An anal fistula is essentially a small tunnel developing between the anus and perianal skin. An anal fistula often develops due to complications of perianal, ischiorectal and supralevator abscesses. It is essentially an abnormal connection between 2 epithelial surfaces (columnar in rectum → squamous perianally) with a chronically infected tract that may epithelialise later on. It extends from an opening at the level of the dentate line and passes through a site of a previous abscess to an external opening on the perianal skin near the old scar.

Other factors that may precipitate an anal fistula are: IBD, DM, anal trauma, radiation therapy to anus.

🔢 Classification

We may use Park’s classification system to classify anal fistulae into 4 types:

1. Inter-sphincteric - most common
2. Trans-sphincteric
3. Supra-sphincteric - least common
4. Extra-sphincteric

😷 Presentation

• ⭐️ Discharge onto perineum (mucus, blood, pus, faeces).
• ⭐️ External opening on perineum upon examination with a fibrous tract felt under skin upon DRE.

We can predict the trajectory of a fistula by using the Goodsall Rule:

1. A fistula that opens posterior to the transverse anal line will follow a curved course towers the posterior midline
2. A fistula opening anterior the transverse anal line will follow a straight path to the dentate line.

🔍 Investigations

🏆 MRI pelvis scan - allows for surgical planning.

EUA is used for assessment where a probe is used to identify the internal orifice.

Management

🔪

Surgical management Fistuolotomy is performed. This means an incision is made between the fistula and the anal margin (cutting through the sphincters). It is then allowed to heal through secondary intention. Continence will not be lost, but flatus may be poorly controlled. This is more for low-lying fistulas as opposed to higher fistulas.

• If the fistulotomy will involve >50% of the anal sphincters then it is difficult to operate on as we need to preserve continence.
• Often in more severe cases, a Seton thread is placed through the fistula and into the anus and then is tied off like a ring. It allow the pus to drain while preventing abscess formation.
• In most severe cases, the only cure is perineal excision of the anal canal + lower rectum entirely with an end colostomy being placed. This is true in Crohn’s patients who tend to have the most severe manifestations of fistulae (“pepper-pot perineum” with multiple fistulae).
• Management in women is more nuanced as their sphincter complex may be damaged due to childbirth which could leave them incontinent