Keywords
intersphincteric plane sepsis - management of perianal fistula - anorectal abscess
- pathogenesis of perianal fistula - transperineal ultrasound
Introduction
The intersphincteric plane (ISP) is a potential space between the external and the
internal anal sphincters. It communicates with the perianal space inferiorly and the
supralevator space superiorly; it is separated from the ischiorectal fossa by the
external anal sphincter and the puborectalis muscle, and from the deep postanal space
by the upper part of the external anal sphincter and the proximal part of the puborectalis
muscle. The anal glands are spread along the walls of the anal canal at the level
of the dentate line, which separates the squamous epithelium distally from the columnar
epithelium proximally, being situated in the submucosal region and within the internal
sphincter.[1]
About 90% of the perianal sepsis is caused by an obstruction of the ducts of anal
glands with subsequent secondary bacterial infection. Types of anorectal abscesses
according to their common anatomical locations are perianal abscess, ISP abscess,
ischiorectal fossa abscess, and supralevator abscess.[2]
[3]
The main four types of perianal fistula are the intersphincteric fistula, transsphincteric
fistula, suprasphincteric fistula, and extrasphincteric fistula with many subtypes.[4]
About 10% of anorectal abscesses were not cryptoglandular in origin due to other reasons,
among them Crohn’s disease, trauma, malignancy, HIV infection, and tuberculosis.[2]
[3]
[4]
The imaging modalities used to diagnose perianal sepsis are the ultrasound via endoanal
and transperineal routes using two-dimensional (2D) or three-dimensional (3D) image
reconstruction, and magnetic resonance imaging (MRI) through various image sequences
and multiplanar image analysis.[5]
[6]
An abscess appears as closed cavity containing proteinaceous fluid inside (Pus); the
fistula tract is a tract with external and internal openings, while a sinus tract
is a tract with only one opening and blind end. Fistula tract with an abscess formation
is a tract with internal and external openings seen communicating with an abscess
cavity; also, a sinus tract with an abscess is a blind end tract with only one opening
seen communicating with an abscess cavity. Longstanding sinus or fistula tract showed
thick echogenic walls in an ultrasound exam with little or no surrounding fat tissue
edema and appeared as a tract with low luminal signal intensity in T1- and T2-weighted
images in MRI study, while an active tract showed ill-defined hypoechoic walls with
marked surrounding fat tissue edema in ultrasound exam and with increased luminal
signal intensity using T2 and fat suppression image sequences in MRI study.[5]
[6]
The treatment of anorectal abscess depends on incision and drainage, while the treatment
of simple perianal fistula depends on fistulotomy or fistulectomy.
The ISP had gained great significance in the pathogenesis and management of various
types of anorectal abscesses and perianal fistulas.
Objective
Descriptive study aimed to identify the various patterns of the ISP sepsis and their
incidence among our work group and to clarify the significance of ISP in the pathogenesis
of anorectal abscesses and various types of fistula tract, to optimize the way of
surgical management.
Methodology
Retrospective descriptive study of 57 patients with an ISP sepsis, in whom the final
diagnosis was based on combining the image findings of the ultrasound, the results
of digital rectal examination under anesthesia by an experienced colorectal surgeon,
and the operative findings.
All were admitted to the radiology department of our institute from March 2018 to
October 2019; among them were 7 females and 50 males, and their age ranged from 20
to 48 years, with the mean age being 37 years.
All were referred from the department of colorectal surgery where they were evaluated
by obtaining a full medical history and clinical examination.
Transperineal ultrasound was aided with 3D capability, and for female patients an
additional transvaginal ultrasound was performed by an expert radiologist in transperineal
and endovaginal ultrasound, using linear (5–12 MHz) and transvaginal (4–9 MHz) multifrequency
probes with SonoAce X8 Ultrasound Machine (Medison, Korea).
A 2D transperineal ultrasound was done with the patient in left lateral decubitus
position; the knee and hip joints were flexed; the anal canal was examined in the
sagittal planes through obtaining midline, left paramedian, and right paramedian sagittal
images, and obtaining axial planes at low, mid, and high anal levels, followed by
3D reconstruction of the anal canal with multiplanar image reconstruction and volume
rendering. Female patients were further subjected to transvaginal ultrasound through
obtaining multiple axial images of the anal canal from low to high anal levels.
The anal canal wall was formed of three layers in an ultrasound exam: the inner echogenic
mucosa, the middle hypoechoic internal anal sphincter, and the outer moderate echogenic
external anal sphincter ([Fig. 1]). The abscess was identified as localized thick wall encysted turbid content fluid
collection. Fistula tract was identified as hypoechoic cord-like structure that communicates
between an external opening around the anal verge and an internal opening situated
in the submucosa of the anal canal wall. The sinus tract was identified as hypoechoic
tract with only an internal opening and no external opening. The extrasphincteric
course and the relations of abscesses, sinuses, and fistula tracts to the anal canal
sphincters were evaluated and the ultrasound images were recorded.
Fig. 1 Normal anatomy of anal canal axial image showing the anal mucosa (arrow), internal
sphincter (IS), and the external sphincter (ES) with companion small intersphincteric
plane abscess (AB).
Results
Overall, 57 patients with an ISP sepsis were included in the study: among them were
50 males (88%) and 7 females (12%), with male to female ratio being 7:1; also among
them, 13 patients (23%) were detected with an ISP abscess ([Fig. 2]), 5 patients (9%) with free sepsis within the ISP along the anterior, posterior,
or the lateral sides of the anal canal wall, 32 patients (56%) with an intersphincteric
fistula tract ([Fig. 3A]
[B]), and 7 patients (12%) with sinus tracts ([Fig. 4]).
Fig. 2 Transperineal ultrasound three-dimensional multiplanar image analysis with volume
rendering of an intersphincteric plane abscess (arrowhead, AB) and the internal anal
sphincter (arrow).
Fig. 3 (A) Two-dimensional transperineal ultrasound of an intersphincteric plane fistula extending
to mid ⅓ of anal canal wall (arrow) and the external anal sphincter (arrowhead). (B) Transperineal ultrasound three-dimensional volume rendering of an intersphincteric
plane fistula (F).
Fig. 4 Transperineal ultrasound three-dimensional multiplanar image analysis with volume
rendering of an intersphincteric plane sinus (S).
Regarding the site of sepsis: Thirty-five patients (61.5%) were detected with posterior ISP sepsis, 17 patients
(30%) with an anterior sepsis, and 5 patients (8.5%) with lateral sepsis.
Regarding the longitudinal extension of fistula/sinus tracts: Among 39 patients with fistula and sinuses, the tract was seen extending to the
upper ⅓ of the ISP in 12 patients (30.5%), to the mid ⅓ of the ISP in 21 patients
(54%), and to the lower ⅓ in 6 patients (15.5%) using an imaginary line, which divides
the length of anal canal wall into three divisions.
Regarding fistula/sinus with abscess formation: An intersphincteric fistula with an abscess formation was detected in one patient
(2.5%), an intersphincteric sinus with an abscess formation in one patient (2.5%),
and transsphincteric sinus with the ISP abscess formation was detected in two patients
(5%).
Regarding the shape and site of the abscesses: Among 13 patients with ISP abscesses, 4 patients showed posterior horseshoe intersphincteric
abscesses ([Fig. 5]), 2 patients had large abscesses extending into the supralevator space with an associated
distension of the ISP below the site of the abscesses, and 7 patients showed localized
abscess within the ISP.
Fig. 5 Transvaginal two-dimensional ultrasound showing large posterior horseshoe intersphincteric
plane abscess (arrow) seen locally, destroying the posterior aspect of the internal anal sphincter (arrowhead).
Regarding complex fistulas: Among the patients with ISP fistula extended to the upper ⅓ of ISP, two patients
showed straight nonbranching transsphincteric components seen reaching an external
opening away from the anal verge and one patient was detected with posterior ischiorectal
fossa horseshoe component ([Fig. 6]) reaching two bilateral external openings located posterior to the anal verge.
Fig. 6 Transvaginal two-dimensional ultrasound of posterior horseshoe ischiorectal fossa
fistula (arrow) seen crossing the external and internal sphincters posteriorly at
6 o’clock (the site of the internal opening, arrowhead).
Discussion
The current study described many patterns of the ISP sepsis and estimated the incidence
of each pattern among our study group. Such patterns could be in the form of fistula
tracts, sinus tracts, abscesses, intersphincteric abscess with supralevator extension,
sinus with abscess formation, fistula with abscess formation, distension of the ISP
with free pus, and other complex forms like horseshoe abscess, abscess with transsphincteric
sinus, and ISP fistula/sinus with transsphincteric branches reaching an external opening
away from the anal verge.
Such many patterns demonstrated that sepsis can spread from the initial septic focus
to all directions of the ISP and from the ISP through the external sphincter to the
ischiorectal fossa to form transsphincteric sinuses and fistulas of high and low types;
it can also spread from the ISP to the supralevator space and from ISP to the deep
postanal space traversing the external sphincter and puborectalis muscle to form horseshoe
abscess and subsequent fistula. Such patterns will give an idea about the significance
of the ISP in the pathogenesis of cryptoglandular perianal sepsis with subsequent
great impact on the way of management.
An obstruction of the anal gland ducts leads to stasis and subsequent secondary bacterial
infection.[2]
[3]
[4] The pathogens start to destroy tissue cells, which attracts macrophages to the site
of infection, leading to a fight between the leukocytes and pathogens with resultant
liquefactive tissue necrosis and pus formation.[7] The sepsis extends to the intersphincteric space after localized destruction to
the submucosa and the internal sphincter at the site of the infected glands, which
leads to the formation of an internal opening mostly at the level of the dentate line
(the site of anal glands). The pus that extends to ISP is formed of dead tissue cells,
dead and viable bacteria, and dead macrophages. The pus at early stages may be free
within the ISP but thereafter becomes walled off with fibrin and viable tissue cells
to form a localized abscess. The pus may escape from the intersphincteric space and
extend to the perianal space and acquire walls to form perianal abscess or extend
to the supralevator space to form supralevator abscess. The infection may spread across
the external anal sphincter through localized liquefactive necrosis in the muscle
or through the holes of inferior rectal vessels as they penetrate the external sphincter
to the ischiorectal fossa to form ischiorectal fossa abscess. The infection may cause
an extensive liquefactive necrosis in the ischiorectal fossa and spread to the contralateral
side, likely across the deep postanal space, to form ischiorectal fossa horseshoe
abscess.[8] The pus may spread through the ISP in any direction or to its both sides to form
an intersphincteric horseshoe abscess.
An external drainage of an abscess, either spontaneous or surgical, may lead to the
formation of fistula tract, and insufficient drainage through the internal opening
without an external drainage will lead to the formation of sinus tract. So, an external
drainage of perianal abscess, which is located inferior and in direct continuity with
ISP, may lead to the formation of lower ⅓ ISP fistula. An external drainage of ischiorectal
abscess may lead to the formation of transsphincteric fistula, which, according to
the site of spread of ISP sepsis across the external sphincter, will be either high
or low transsphincteric fistula. An external drainage of supralevator abscess through
the ischiorectal fossa will lead to the formation of suprasphincteric fistula tract,
which extends to the external opening through the ischiorectal fossa and to the internal
opening through the initial site of spread of infection within the ISP. Horseshoe
fistula occurs subsequent to external drainage of horseshoe abscess. Extrasphincteric
fistula is rarely cryptoglandular in origin unless it shows an extension through the
ISP to an internal opening at the dentate line, which can be explained by iatrogenic
false passage of a suprasphincteric fistula that creates another external opening
in the wall of the rectum. Fistula side branches are either due to false passage or
an extension of infection to the surrounding soft tissue planes.
Understanding the pathogenesis will help to reach the optimum way of management. All
patterns of cryptoglandular sepsis were assumed to have start point at the internal
opening at level of the dentate line, from which it extends initially to the ISP and
from there it extends in many directions and through the anal canal walls to the surrounding
soft tissue planes and potential spaces. Since the best way to treat cavities that
distend with pus (abscess, sinus, and fistula) is adequate drainage, so if the cavity
was confined to the ISP, drainage will be through adequate opening of the ISP through
the internal opening and the internal sphincter overlying the cavity. If the cavity
lies outside the anal canal wall and communicates with the ISP to reach the internal
opening, the drainage will be through opening the ISP and laying open the cavity through
an external drainage. The role can be applied to different types of cryptoglandular
abscesses, fistulas, and sinuses.
Fistulotomy is the best treatment for simple fistula. We lay open the tract from the
external to the internal opening and the best treatment for ischiorectal fossa abscess
is through incision and drainage with fistulotomy to drain the abscess and associated
ISP sepsis. However, fistulotomy cannot be performed for complex fistula that involves
a large part of the anal sphincters for the fear of anal incontinence. Applying aforementioned
role, the treatment of complex fistula can be achieved through laying open the extrasphincteric
tract, opening the ISP overlying the intersphincteric fistula component, and widening
of the internal opening. A study by Garg concluded that the supralevator fistula almost
had intersphincteric component and was managed successfully through laying open the
transanal route.[9] The same principle was applied in the procedure of transanal opening of the intersphincteric
space, which was associated with high success rate in managing the complex perianal
fistula and in preserving the anal continence, as it preserves the external sphincter.[10] On the contrary, any procedure that leads to closure of the internal opening like
fistula plug, fibrin glue, rectal advancement flap, and ligation of the intersphincteric
fistula tract procedures will have a low success rate in managing the complex fistula,
since it does not lead to the eradication of the extended sepsis within the ISP that
harbors viable pathogens, which by time multiply and lead to fistula recurrence.[10]
[11]
[12]
[13]
[14]
The deep postanal space lies outside the anal canal wall and is not in direct continuity
with the initial sepsis involving the anal glands; it can serve as a route for the
extension of posterior ischiorectal fossa sepsis to the contralateral site to form
posterior ischiorectal fossa horseshoe abscess or fistula tract. The deep posterior
ISP, which is situated in direct continuity with the ISP, plays a more significant
role in the pathogenesis of complex fistula tract.[15]
The reported sensitivity of ultrasound in the evaluation of perianal fistula in one
study was comparable to that of MRI (87% for each), with MRI being more specific.[16] In another study, ultrasound showed high sensitivity in evaluating different types
of perianal fistula, with reported sensitivity of 100% for low transsphincteric fistula,
92% for high transsphincteric fistula, 96% for intersphincteric fistula, and of lower
sensitivity for extrasphincteric fistula, 78%.[17]
The limitation of ultrasound in the evaluation of perianal sepsis is that it is operator
dependent and has lower image resolution than that of MRI.
Conclusion
Understanding the role of the ISP in the pathogenesis of anorectal abscesses and perianal
fistula tracts can help in reaching the optimum way of management.
