**6. Abscess and anal fistula**

Abscesses and anal fistulas are common anorectal problems, representing two different phases of the same infectious process. Perianal and perirectal abscesses

are acute infections, resulting in pus collection, mostly due to non-specific cryptoglandular obstruction [50]. Anorectal fistulas represent the chronic evolution of a suppurative process, characterised by an epithelised tract connecting two epithelised surfaces, in particular anal or rectal mucosa to the perianal skin or perineum [51, 52].

Estimated anal abscess incidence is about 2 cases per 10,000 population per year [53, 54], leading to fistula formation in about 25% of patients. Fistulas may present de novo, but in about 30–50% of patients, they follow a previous anorectal abscess. Both anal abscesses and fistulas affect men twice more than women, having a mean age of presentation of 40 years (ranging from 20 to 60 years). However, since most patients attribute proctologic symptoms to haemorrhoidal disease without referring to a specialist, abscesses and fistulas real incidence is unknown.

As previously said, both anal abscesses and fistulas usually originate from obstructed anal crypt glands. Less frequently they can be caused by inflammatory bowel disease (mainly Crohn's disease), infection such as actinomycosis, tuberculosis and lymphogranuloma venereum, human immunodeficiency virus, trauma (both in case of iatrogenic injuries and foreign rectal bodies), surgery, malignancy and irradiation [50, 55].

The commonest onset of a perianal abscess is constant severe pain, usually not related to bowel movements, that can be associated to general symptoms such as fever or generalised malaise [52]. On the contrary, intermittent perianal pain exacerbated by bowel movements and chronic purulent drainage are typical manifestations of fistulas [56].

On physical examination, abscesses present as erythematous, tender and fluctuant masses; purulent drainage, either from the overlying skin or from the rectum, can be present if the abscess has begun to spontaneously drain [52]. On the contrary, the presence of an external opening draining pus and a palpable cord leading from the detected external orifice to an internal orifice, are the most common findings in case of anal fistula [56].

Although collecting the medical history and performing a physical examination allow to detect the majority of abscesses and fistulas, sometimes patients may not have any physical finding on examination, and further instrumental exams may be required. Local examination may result difficult in case of deep abscesses, not appreciable on external examination nor by digitorectal examination, and in case of incomplete or blind-ended fistulas, lacking of the external orifice. Moreover, symptoms of both conditions may overlap with the clinical manifestations of other proctologic diseases. In these cases and in case of complex or recurrent fistulas, imaging is necessary [57, 58]. The most commonly performed imaging studies are magnetic resonance imaging (MRI), which is the gold standard imaging technique, and endosonography (EUS), having a specificity of 69% and 43% respectively, and the same sensitivity of 87% [58–60]. Instrumental investigation gives important information about abscess localisation, fistula anatomy and integrity of the sphincter muscles, allowing to establish the proper treatment.

In case of anal abscesses, the required treatment is always a prompt drainage, considering that all undrained abscess can expand toward adjacent spaces or progress to a systemic infection [56, 57]. Surgical approach may vary depending on imaging findings, that allow to classify an abscess as follow:


According to this classification, perianal abscesses treatment can be safely performed in an outpatient setting, while in case of complex, large or deep abscesses, drainage should be performed in the operating room, under general anaesthesia, sedation or local anaesthesia [61].

Anorectal fistula management is quite more difficult, considering the chosen surgical treatment depends on the relation between the fistula tract and the external anal sphincter, and on the amount of the sphincter complex involved with the fistulous tract. According to Parks' classification, firstly described in 1976, four different types of anal fistula can be identified based on the relationship between the primary track and the sphincter [62]:


A fistula can also be categorised as simple or complex. The former includes those with an intersphincteric or low transsphincteric track that involves less than 30% of the sphincter complex. A fistula in the presence of inflammatory bowel disease, malignancy, incontinence, chronic diarrhoea or previous irradiation should be considered complex as well as those with an anterior track in a female patient [63]. In some complex cases, a staged surgical procedure will be required.

Since perianal infection sequelae range from minor pain and social embarrassment due to smelly purulent drainage, to life-threatening sepsis, once a diagnosis is established, surgery is the mainstay of the treatment, aiming to resolve local infection, remove fistulous tracts, avoid recurrences and preserve sphincteric function.

### **6.1 Abscess drainage**

Abscess incision and drainage is a procedure that can be performed either under local anaesthesia, in an ambulatory setting, whenever the pus collection is small and superficial, either under general anaesthesia, sedation or local anaesthesia in the operating room, in case of more complex, larger or deeper abscesses [56, 57]. In these cases an office-based treatment can be performed only in referral centres if carried out by expert surgeons.

Incision should be performed as close as possible to the anal verge minimising the length of a potential fistula while still providing adequate drainage of the collection. As 30–70% of abscesses present with a concomitant fistula, surgeons question whether to perform or not a primary fistulotomy with the abscess drainage [64–67]. Data regarding this argument show primary fistulotomy reduces the risk of recurrence and persistence of the disease, but increase sphincter damage risk even leading to faecal incontinence [56, 57, 68]. Thus, primary fistulotomy is recommended only in case of simple anal fistula or high recurrence risk.

While perianal abscesses (simple abscesses) may be treated in an ambulatory setting, complex abscesses usually require an operating theatre, except for small intersphincteric abscesses, that can be managed without requiring an exploration under anaesthesia (EUA).

Regardless of the surgical setting, a course of empiric antibiotics is strongly recommended for all patients who went through incision and drainage of an anorectal abscess, in order to reduce the rate of fistula formation. Recommended drugs are amoxicillin-clavulanate or a combination of ciprofloxacin and metronidazole, administered for a four- to five-day course [69]. However, even if this treatment reduces fistula recurrence rate, it does not affect abscess recurrence rate [70].
