**4.3 Transphincteric fistula**

Low or high? This represents the level where infection extends through external sphincter into ischiorectal space. In clinical practice, we define low transphincteric

fistula as those involving <1/3 of external sphincter, and high transphincteric fistula if >1/3 involved [12, 13]. Intersphincteric infection can pass through the external sphincter [1, 2, 8, 11], at junctions of each external sphincter portions [8]. If the infection passes through junction between levator ani and deep external sphincter, abscess may present as a Infralevator abscess, and the resulting fistula is a Suprasphincteric type as described by Park [5]. This typically occurs posteriorly and leads to horseshoe pattern (described in 2.2.5). On the other hand, if infection spread at the junction between superficial and deep external sphincter, it will cause ischioanal abscess and a high transphincteric fistula. A low transphincteric fistula results from infection spreading between the junction of superficial and subcutaneous external sphincter.

### **4.4 Anterior glands or posterior glands**

Infection originating from anterior glands or posterior glands will results in typical patterns. Various authors reported internal openings found mainly at the anterior or posterior anal canal, which corresponds well with infected anal gland/crypt [1, 6].

Anterior gland infection that spreads via transphincteric route have predictable patterns. A low transphincteric pattern will tract along the subcutaneous tissue and below transversalis fascia in a linear fashion. A high transphincteric pattern will tract along the perineal space, in male, it extends into the scrotum. In female, it may result in ano-vaginal fistula or opens around the labia majora or causes perineal abscesses. Anterior horseshoe pattern has also been reported. It extends into the ischioanal space at 11 and 1 o'clock position [2, 6, 7].

Posterior gland infections are as described in 4.3 and 4.5.

#### **4.5 Anatomy of the posterior perineum and deep posterior anal space**

Hanley described the horseshoe pattern in detail; Infected anal glands originated from posterior midline of the anal canal, spreading along the longitudinal muscle cranially, passing superior or inferior to deep external sphincter (transphincteric extension) into the space known as deep postanal space. Deep postanal space communicates with both ischiorectal spaces above the surface of the superficial external sphincter. Pus will extend through the plane of least resistance into one or both ischiorectal spaces [10, 11].

In 2006, Kurihara made further anatomical discovery regarding posterior horseshoe pattern. Ischiorectal space is divided into 2 compartments by the septum of ischiorectal space, which starts at the Alcock's canal to border between puborectalis (part of levator ani) and deep external anal sphincter. This septum is important as the inferior rectal vessels and nerve runs along this fascia layer to penetrate the upper anal canal wall at the deep external sphincter level. At the point where inferior rectal vessels and nerve enters the external sphincter, tissue is loose. Infection spreads upwards along the intersphincteric plane, forms a nidus at the level of deep external sphincter within the intersphincteric space, which is termed as posterior deep space. It can extend via the weak points into either above or below the septum of ischiorectal space, spread either unilaterally or bilaterally to form horseshoe abscesses/fistulas [8]. Both authors however agreed that the internal opening is usually situated at the mid-anal canal posteriorly [8, 10]. Rojanasakul reports that the posterior high transphincteric fistula can occurs at 5 and 7 o'clock position of the anal canal [7].
