**6.3 Clinical application of the natural pattern and the role of adjunct imaging modalities**

Understanding the pathogenesis and natural pattern helps in management of fistula-in-ano. Lessons from early publications showed that successful treatment of fistula-in-ano lies on the ability of surgeons to eradicate the source of infection, which is the infected anal crypt/gland and the intersphincteric abscess/tract [1, 2, 10, 19]. Recent publications further emphasized on eradicating secondary tracts or abscesses to prevent recurrences [15, 20, 21]. Therefore, objective clinical assessment should assist clinicians to:


In the author's view, using the knowledge and classification of the Natural Patterns of Anal Abscess and Fistula [7], the above information can be actively sought after using a combination of clinical assessment and imaging modalities.

## *6.3.1 Clinical examination or examination under anesthesia*

In cases of acute abscess, clinical examination generally elicits tenderness and fluctuation around perianal or ischioanal fossa. However, detailed assessment is usually informative with sedation, local or regional anesthesia. In high intersphincteric abscesses or Infralevator abscesses, tenderness is elicited on digital rectal examination at the anorectal ring. Examination under anesthesia may reveal pus discharge from internal opening upon insertion of anoscope. Perianal abscess is typical of type 1 (Intersphincteric) and type 2 (Low Transphincteric) patterns, and internal opening usually corresponds with the location of abscess. Ischioanal fossa abscess is the usual presentation of type 3 and 4 (high transphincteric) patterns. However, it should also be remembered that type 4 pattern produces Infralevator abscess, where internal opening is almost always posterior. Type 5 pattern produces high intersphincteric abscess and internal opening is usually posterior [6, 7].

In cases of chronic fistula, location of external opening and course of fistula tract should direct clinicians to the possible patterns. Low fistulas are clinically palpable as thickened fibrous cord extending from the external opening towards the infected anal crypt (internal opening). In high fistulas, tracts are usually not palpable subcutaneously. Digital examination may reveal chronic induration over the anorectal ring adjacent to lateral wall of rectum. External tracts usually runs deep and parallel with the anal canal on probing [6].

In cases where internal opening is not apparent, there are several techniques described to facilitate the identification of internal openings [6, 15].

a. Hard, board like changes to the deep surface of the internal sphincter usually represents the location of infected anal crypt.


#### *6.3.2 Imaging modalities as adjunct to classify the abscess/fistula pattern*

Magnetic resonant imaging (MRI) and Endoanal ultrasound (EAUS) are the 2 most reliable imaging modality to delineate anorectal abscess and fistula. Conventionally, both modalities are equally sensitive in detecting anal fistula, but MRI has slightly superior specificity compared to EAUS [23]. MRI is not readily available in all institutions, whereas EAUS is operator dependant and requires significant learning curve.

Kim et al. in 2009 reported that 3 dimensional endoanal ultrasound is the preferred method, and use of hydrogen peroxide contrast may increase the detection rate of anal fistula. Sensitivity in detecting primary fistula tract is 84.4%, 81.8% for secondary extension and 84.2% for localizing the internal opening [22].

Recently, the interest in MRI has surged, in line with renewed efforts from various institutions to produce new classifications [16, 18]. With the availability of MRI scan, the fistula could be assessed in all three dimensions (axial, coronal and sagittal) [14]. The sensitivity and specificity of MRI in diagnosing fistula tracts were 98.8 and 99.7%, and in identifying internal opening were 97.7 and 98.6% respectively [14]. In addition, MRI is able to reclassify simple fistula based on clinical assessment to complex fistula, as it has the extra benefit of detecting additional secondary tracts, horseshoe tracts and supralevator extensions [18].

Clinical assessment and imaging adjunct helps clinicians to identify internal opening and intersphincteric tract/abscess, location of abscess, external tracts and secondary tracts. It also helps to define low and high fistula. This information will assist clinicians to recognize the type of anal fistula/abscess, thus allowing stratification and planning for appropriate surgical treatment. Surgical treatment will be discussed in the next segments.

#### **7. Definitive surgical treatment in acute abscess stage**

Eisenhammer wrote: '*single stage definitive surgery during the acute abscess phase is the correct timing to provide definite treatment and is associated with remarkably high healing rate, as long as the offending anal crypt is correctly identified and dealt with*.' [6] The idea of definitive surgery for fistulous anorectal abscess is not a recent concept, but one which never took off for the past few decades due to concerns of incontinence [24].
