**3. Relevant updates in anorectal anatomy**

Quoting Kurihara et al. in 2006, 'To be able to successfully treat cryptoglandular anorectal abscesses and fistulas, we need to understand the exact anatomy and extension course' [8]. Secondly, as mentioned before, we need to understand that infection will spread along the least resistant plane, along the planes of anorectal muscles and fascia to reach the respective anorectal spaces [7].

#### **3.1 Review of relevant anorectal anatomy**

Important anatomical structures are depicted in **Figure 1a** and **b**. The internal sphincter and the longitudinal muscle are continuation of the circular and longitudinal smooth muscles of rectum respectively in the anal canal. There are 3 components of external sphincters, subcutaneous, superficial and deep external sphincters, whereas puborectalis is a component of the levator ani [1, 2]. Recent publications suggest that puborectalis is also known to be the same entity as deep external sphincter [7, 9]. Perianal space and Ishio-rectal fossa were described by Parks as the 2 most common spaces for abscess formation [1]. However, his postulation that the source of infection was between internal sphincter and longitudinal muscle was later updated [1].

**Figure 1.**

*Coronal view of the anorectal anatomy. Potential space for abscess to form; ISA: ischioanal space, IFL: Infralevator space, SL: Supralevator space, DPA: deep postanal space, PDS: posterior deep space (intersphincteric), IS: intersphincteric space, PRA: perianal space, SP: Superficial perineal space. SIF: septum of ischioanal fossa,TF: transversalis fascia, DPM: deep perineal membrane, ACL: anococcygeal ligament, IAS: internal anal sphincters. EAS: external anal sphincters, components: deep, sup (superficial) and sub (subcutaneous). Deep EAS is interchangeably termed puborectalis muscle. Sagittal view shows significant difference between anterior and posterior perineum. Deep perineal space lies above deep perineal membrane (DPM). Yellow lined arrows show postulated paths for intersphincteric sepsis to traverse the sphincter complex into respective anorectal spaces. Detailed explanation in segment 4.*

### **3.2 Several updates in anorectal anatomy are summarized below**

Internal sphincter circular muscles and longitudinal muscle layer are fused together, and the intersphincteric plane is a potential space between the longitudinal layer and the fascia of striated muscle external sphincters [4, 7] (see **Figure 1a** and **b**).

Deep external sphincter overlaps with puborectalis (part of levator ani), and superficial external sphincter overlaps with deep external sphincter, implicating that the external sphincter is not a continuous sheet of striated muscles. The author made a clear distinction between puborectalis and deep external anal sphincter as 2 separate entities, with weak connective tissue between each group [8].

However, other view states that the vertical portion of the levator ani's striated muscles around the anorectal ring is the puborectalis muscle, interchangeably known as the deep external sphincter [7]. This is supported by previous study by Shafik in 1975 confirming that puborectalis muscle and deep external sphincter are actually fused and functions as a single loop termed the top loop [9].

Both authors stipulate that there is a potential point of weakness between the vertical group and the horizontal group of striated muscles at the level of anorectal ring, allowing infection in the intersphincteric space to spread into the Infralevator space [7, 8].

The emerging terms of deep postanal space, posterior deep space and septum of ischiorectal fossa which will be explained next (refer to segments 4.2 & 4.5) [7, 8, 10, 11].

The anatomy of anterior perineum, especially superficial and deep perineal space are equally important to explain anterior patterns of abscesses and fistulas. Anterior perineum lacks puborectalis/deep external sphincter component. Posteriorly, there is a complex interconnection between intersphincteric space, supralevator space, posterior deep space and deep postanal space. (Shown in **Figure 1b**) Deep postanal space communicates with both ischioanal space and Infralevator space laterally and deep perineal space anteriorly (refer to segment 4.2) [7].
