How to examine?
The external opening may be seen as an elevation of granulation tissue discharging purulent liquid. However the internal opening may not be apparent The number of the external openings and their location may be helpful in locating the internal opening and subsequently classifying the type of anal fistula present in the patient.
Digital examination of Fistula
As a rule of thumb, the greater the distance of the external opening from the anal margin, the greater the probability of a complicated extension.
Clinical examination may reveal an indurated cord-like structure underneath the skin in the direction of the internal opening with asymmetry between right and left sides.
It is important to note for perianal surgical scar from previous fistula surgery to detect fistula relapse or recurrence. This will then alert the clinician of a possibility of a complex fistula which may require specialist care.
Bidigital rectal examination must be performed to define the relationship of the tract with the anal sphincter muscles. It also provides information regarding pre-operative anal sphincter tone at rest and on squeezing because of the risk of fecal incontinence.
How to Investigate?
Investigations to be performed in patients with anal fistula isdivided into 2 categories -
- Diagnosis and classification of anal fistula
- Assessment of integrity and function of anal sphincters
Anoscopy must be performed to identify the internal opening. Sigmoidoscopy should be performed to locate the internal opening and to exclude underlying secondary pathology. Occasionally, total examination of the large bowel is required either using colonoscopy or barium enema in patients with atypical, complex, multiple or recurrent anal fistula who have symptoms suggestive of inflammatory bowel disease.
Endoanal ultrasonography is useful in the total assessment of anal fistula. It allows imaging of the fistula tract to provide information on the complexity of the anal fistula, detect the presence of secondary abscess cavity and establish relationship of the fistula to the anal sphincter complex.
Endoanal Ultrasound Fig. 1 Endoananl Ultrasound Fig. 2
Other modalities such as fistulography and magnetic resonance imaging are available to further delineate 'difficult' and complex recurrent anal fistula.
Pre-operative assessment of the anal sphincter function is not generally required. However, it may be useful as an adjunct to planning operative approach in patients with recurrent fistula, women with previous obstetric trauma and elderly patients. Resting and squeezing anal pressure can be measured preoperative to aid in the discussion and planning of the management of 'difficult' anal fistula.