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222
inevitably affect continence. The distal aspect
of the anal canal is the anal verge, defi ned as
the area where anoderm becomes true skin
characterized by pigmentation, hair, sweat
glands and a ring of apocrine glands, which
may contribute to the condition hidradenitis
suppurativa.
Anal Glands
Because of the diminishing size calibre from rec-
tum to anus, the undulated dentate line area of the
anal canal is characterized by a pleated appear-
ance, which is referred to as the columns of
Morgagni. At the base of each column, at the
Internal sphincter
Valve of Houston
Internal rectal plexus
Column of Morgagni
Dentate line
Anal gland
Muscularis
submucosa ani
Transverse septum
External hemorrhoidal plexusAnoderm
Deep external sphincter
and puborectalis
Conjoined longitudinal
muscle
Superficial external sphincter
Subcutaneous external sphincter
b
Transitional
zone
Anal canal
Column of Morgagni
Dentate line
Anal crypt
Anal gland
Anoderm
a
Fig. 17.1 ( a ) The anal canal [ 1 ]. ( b ) The anorectum [ 1 ]
S.M. Goldberg and P.S. Tawadros
223
level of the dentate line, there is a small pocket or
crypt. About half of these crypts communicate
with the drainage ducts of anal glands that are
located in the internal sphincter muscle area.
These anal glands, which were fi rst described by
Hermann and Desfosses in 1844 [ 3 ], are located
in the submucosal space, the internal sphincter
muscle itself, and about half also communicate
with the intersphincteric space, without involv-
ing the external sphincter muscle. Anal crypts are
therefore of signifi cant importance in the patho-
physiology of anal sepsis as foreign material
lodging itself in a crypt can obstruct the drainage
path of anal glands and result in sepsis. The
importance of this mechanism was fi rst described
by Parks in 1961 [ 4 ].
Perianal Spaces
The complex of muscles which surround and
form the anal canal create a network of spaces
that are of paramount importance in the manage-
ment of perianal sepsis (Fig. 17.2a ).
The perianal space is in the immediate area of
the anal verge surrounding the anal canal. The
lateral aspect is continuous with the subcutane-
ous fat of the buttocks, while medially, it extends
into the anal canal to the dentate line. It includes
the distal most aspect of the external sphincter
muscle and communicates freely with the
intersphincteric space. The space is itself divided
by an array of radiating elastic septa that limit
communication within the space and account for
the severe pain produced by the accumulation of
even a small amount of pus.
The intersphincteric space is the space
between the internal and external sphincter mus-
cles. Proximally it terminates in the rectal wall,
while distally, it communicates with the perianal
space. This is the space that is often fi rst seeded
by sepsis of cryptoglandular origin.
The ischiorectal space or fossa is delineated
proximally by the levator ani muscle and extends
distally to the skin of the perineum. Laterally,
along the obturator foramen, it contains Alcock’s
canal which carries the internal pudendal vessels
and nerve.
The supralevator spaces are located proximal
to the levator muscle on either side of the rectum.
They extend proximally or superiorly to the peri-
toneal refl ection and are limited laterally by the
pelvic wall. Sepsis in this area may come from
upward extension of perianal sepsis or downward
extension of pelvic sepsis. Differentiating the
source of sepsis in this space is important for
deciding the appropriate route of drainage.
The postanal space is directly posterior to the
anal canal and altogether distal to the levator ani
muscle (Fig. 17.2b ). It is divided into deep and
superfi cial postanal space based on whether it is
deep or superfi cial to the anococcygeal ligament.
This space is of signifi cant importance for under-
standing the pathophysiology of bilaterally com-
municating ischiorectal space sepsis, also known
as a horseshoe abscess.
Pathophysiology
Anorectal sepsis is a common condition that can
occur in healthy, normal individuals. Since ano-
rectal abscesses often lead to fi stulae formation,
it is reasonable to consider both on the same
pathological continuum.
Aetiology
There are several reasons that can account for the
development of anal sepsis. Most commonly, the
cause is of cryptoglandular origin, whereby anal
crypts are obstructed and the drainage path of
anal gland secretions is blocked. As a result, pus
accumulates and builds in a retrograde fashion
often seeding the intersphincteric space [ 5 ].
Other notable causes of perianal sepsis include
perianal trauma and anal foreign bodies, which
can cause abscess formation by the same mecha-
nism as inspissated faecal material in anal crypts.
Iatrogenic causes such as surgical wounds, haem-
orrhoid banding or injection have also been
reported to occasionally cause perianal sepsis.
More recently, two reviews have been published
on stapled hemorrhoidectomy as another poten-
tial cause of severe perianal, pelvic and even
17 Anal Sepsis: Anatomy, Pathophysiology and Presentation
224
intra-abdominal sepsis [ 6 , 7 ]. Lastly, skin lesions
such as hidradenitis suppurativa, localized pyo-
derma or infection of a sebaceous adenoma are
also possible causes of perianal sepsis. Crohn’s
disease, while not itself a cause, is a strongly pre-
disposing factor in the development of perianal
abscesses, usually via the cryptoglandular
mechanism.
Fig. 17.2 ( a ) Perianal and perirectal spaces (frontal view) [ 1 ]. ( b ) Perianal and perirectal spaces (lateral view) [ 1 ]
Levator ani muscle
Puborectalis and deep
external sphincter
Superficial external
sphincter
Subcutaneous external
sphincter
Internal sphincter
Transverse septum
Peritoneum
Ureter
Supralevator space
Ischiorectal
space
Perianal
space
a
Retrorectal space
Rectosacral fascia
Supralevator space
Levator ani
Deep postanal space
Superficial postanal
space
b
S.M. Goldberg and P.S. Tawadros
225
Anal fi stulas are also predominantly of cryp-
toglandular origin and will develop in approxi-
mately one-third of patients who undergo
drainage of an anorectal abscess. A recent retro-
spective cohort study of 148 patients with ano-
rectal abscesses showed a 37 % overall rate of
fi stula formation over a mean follow-up period of
38 months and a twofold increased risk of fi stula
formation in patients under the age of 40 [ 8 ].
While about 90 % of fi stulas have a cryptoglan-
dular aetiology, not all fi stulas come from ante-
cedent abscesses, and not all abscesses lead to
fi stula formation. Indeed a previous history of
abscess may not be elicited in one-third of
patients with anal fi stula [ 9 ]. The remaining 10 %
of anal fi stulas have the following specifi c aeti-
ologies (Table 17.1 ): anorectal disease, infl am-
matory bowel disease, infection, malignancy and
trauma. It is important to distinguish the aetiol-
ogy of an anal fi stula in order to provide adequate
treatment. Indeed, perianal fi stulizing disease
may be the initial presentation in 10 % of Crohn’s
patients, preceding the diagnosis by several
years, and remains the only manifestation of dis-
ease in about 5 % of patients [ 10 ].
Bacteriology
Many studies have been performed looking at the
bacteriology of pus in anorectal sepsis. A mixed
fl ora of anaerobes, gut-derived gram negative
organisms and some aerobic Staphylococcal spe-
cies are found. Interesting studies have been per-
formed to look at whether the bacteriology profi le
could be used to predict fi stula formation. One
conclusive such study was performed by Grace
et al. in 1982 [ 11 ]. A prospective study of 165
patients presenting for incision and drainage of
perianal abscess over 4 years had their pus sent
for cultures. If a fi stula was unapparent at the
time of the fi rst operation, another examination
under anaesthesia was performed 10 days later
for a second look, blinded to the culture results.
The results were as follows: 54 % of the 114
patients who grew gut organisms were found to
have a fi stula, while none of the 34 patients who
grew skin-derived organisms were found to have
one. Other studies since then have confi rmed this
trend. The clinical implication from these studies
is that all perianal abscesses should be cultured
and only those that grow gut-derived organisms
should be subjected to a second look for fi stula
formation if none was observed at the initial
operation.
Spread of Infection
Perianal sepsis will spread using the path of least
resistance. Commonly originating in the inter-
sphincteric space, infection may spread distally
into the perianal space, laterally through the
external sphincter into the ischiorectal space or
proximally along the intersphincteric space caus-
ing a supralevator abscess (Fig. 17.3 ). Pus can
then originate from any of these locations to form
complicated tracts that may result in fi stula for-
mation. Fistula trajectory can be very diffi cult to
predict in patients with Crohn’s disease; how-
ever, the majority of ordinary fi stulas follow
Goodsall’s rule whereby tracts with external
openings around the anterior half of the anus tend
to have straight trajectories toward the nearest
crypt, while those with an external opening
around the posterior half of the anus will typi-
cally have a curved trajectory toward a posterior
midline crypt (Fig. 17.4 ).
Horseshoe Abscess
Perianal sepsis may also spread to affect the con-
tralateral side. This circumferential extension of
Table 17.1 Aetiology of anorectal fi stulas
Nonspecifi c (90 %) Cryptoglandular
Specifi c (10 %) Crohn’s disease
Chronic ulcerative colitis
Trauma
Foreign body
Carcinoma (anal Ca, rectal Ca)
Leukaemia/lymphoma
Radiation
Tuberculosis
Lymphogranuloma granulosum
Actinomycosis
Others
17 Anal Sepsis: Anatomy, Pathophysiology and Presentation
226
Longitudinal
muscle
Internal
sphincter
Puborectalis
External
sphincter
Abscess
Fig. 17.3 Avenues of extension [ 1 ]
Posterior
(curved tracts)
Transverse anal line
Anterior
(straight tracts)
3cm
E
x
c
e
p
t
i
o
n
Fig. 17.4 Goodsall’s rule [ 1 ]
S.M. Goldberg and P.S. Tawadros
227
infection can occur at any level within the inter-
sphincteric, ischiorectal or supralevator spaces,
resulting in a so-called horseshoe abscess
(Fig. 17.5 ). The ischiorectal horseshoe abscess
courses through the deep postanal space, while
the supralevator horseshoe wraps around the rec-
tum in the retrorectal space.
Presentation
Clinical Presentation
Acute perianal sepsis typically presents with
new onset of pain and swelling in the affected
area. The pain is aggravated by movement and
defecation and sometimes even coughing or
sneezing. Systemically, the pain is often accom-
panied by general malaise and fever. A clinical
history may also reveal an antecedent bout of
diarrhoea.
In the scenario of an intersphincteric abscess,
the pain can often persist undiagnosed. The pain
is described as throbbing in character through
day and night, and cause extreme pain on defeca-
tion to the point of causing secondary faecal
impaction. Occasionally, minor anal bleeding
and/or purulent discharge may be observed.
In the chronic scenario, the patient will often
describe a history of pain followed by spontane-
ous purulent discharge and subsequent temporary
relief. When drainage is incomplete, the abscess
cavity may re-accumulate and the internal open-
ing may bleed from chronic granulation tissue.
On examination, the cardinal signs of infl am-
mation are typically present and include redness,
swelling, warmth and tenderness. Pus may some-
times be seen or elicited at the internal opening.
Perianal abscesses are often more readily visible
than ischiorectal ones, which can be deeper in the
ischiorectal fat and harder to palpate. Digital rec-
tal examination is important when tolerated by
the patient. A careful rectal exam can sometimes
reveal an internal opening, and care should
always be taken to also palpate the side opposite
the swelling to gain information as to the possible
presence of a bilateral process. Intersphincteric
abscesses may be devoid of perianal signs but the
patient will be exquisitely tender to digital rectal
examination and the diagnosis must be differenti-
ated from a fi ssure. One additional sign, other
than a subtle mass-in-ano and/or pus extrusion, is
that an abscess may be associated with inguinal
lymphadenopathy whereas a fi ssure will not.
When the pain is severe but no abscess can be
readily detected on physical examination, further
Supralevator
space
Intersphinteric
space
Ischiorectal
space
Perianal
space
Fig. 17.5 Horseshoe
connections of anorectal
spaces [
1 ]
17 Anal Sepsis: Anatomy, Pathophysiology and Presentation
228
investigations in the form of imaging or examina-
tion under anaesthesia are indicated to avoid
abscess progression into deeper spaces and/or
development of a horseshoe abscess.
Classifi cations
Anorectal abscesses are classifi ed as listed in
Table 17.2 . This classifi cation follows from the
avenues of infection spread that were discussed
in Fig. 17.3 . In 1976, Parks and Gordon pub-
lished the classifi cation of fi stula-in-ano listed in
Table 17.3 [ 12 ].
Special Situation: Fournier’s
Gangrene
Fournier’s gangrene is a rare but potentially dev-
astating condition. It is a mixed microbial infec-
tion of aerobic and anaerobic fl ora, sometimes
referred to as a synergistic necrotizing cellulitis,
which can have its nidus in the genitourinary tract,
gastrointestinal tract or perineal skin. Risk factors
include immunocompromise, diabetes, alcohol
abuse and paraplegia. The onset can seem like that
of a perianal abscess, but the progression is much
quicker and far out of keeping with that of a local-
ized abscess. Pain is diffuse, scrotal involvement
is common and systemic symptoms and signs can
quickly evolve to septic shock and multi-organ
failure. Imaging often reveals gas tracking along
fascial planes. This diagnosis truly represents a
surgical emergency. Management includes a
combination of systemic antibiotics and wide sur-
gical debridement, often requiring a second look
and sometimes even faecal diversion. The goal of
surgical debridement is aggressive resection of
skin and underlying tissue back to healthy bleed-
ing tissue. Despite heightened alertness to this
condition and expedited care, this condition car-
ries a high mortality rate [ 13 , 14 ].
Special Situation: The
Immunocompromised Host
Anorectal sepsis in the immunocompromised
host represents a special circumstance with
uncommon presentation. The typical signs and
symptoms are predictably less apparent and
abscess progression can evolve more quickly. In
a recent review of 83 immunocompromised
patients with evidence of perianal sepsis, 28 %
were HIV positive, 34 % were on steroids for
IBD, 20 % had malignancies and 18 % had diabe-
tes. The average age was 44, and 76 % were
male. Most patients (40 %) had both an abscess
and fi stula at the time of presentation. The distri-
bution of fi stulas was similar to the general popu-
lation, although the incidence of horseshoe
abscess was higher at 14 %. Healing and compli-
cation rates were also comparable to those of the
normal population, suggesting that perianal sep-
sis can be safely managed in immunocompro-
mised patients, provided the diagnosis is made
[ 15 ]. Patients with neutropenia will often fail to
produce a pus-fi lled abscess and are therefore
preferentially treated with antibiotics, unless
there is a fl uctuant mass mandating incision and
drainage in addition to antibiotics [ 16 ] .
References
1. Goldberg SM, Gordon PH, Nivatvongs S. Essentials
of anorectal surgery. Philadelphia: J. B. Lippincott
Company; 1980.
2. Morson B, Dawson I. Gastrointestinal pathology.
Oxford: Blackwell Scientifi c Publications; 1972.
3. Hermann G, Desfosses L. Sur la muqueuse de la
region cloacale du rectum. C R Seances Acad Sci.
1880;90:1301–4.
Table 17.2 Anorectal abscess classifi cation
Perianal
Ischiorectal
Intersphincteric
Supralevator
Table 17.3 Parks’ classifi cation of fi stula-in-ano
Intersphincteric
Transsphincteric
Suprasphincteric
Extrasphincteric
S.M. Goldberg and P.S. Tawadros
229
4. Parks AG. Pathogenesis and treatment of fi stula-
in- ano. Br Med J. 1961;1(5224):463–9.
5. Goligher JC, Ellis M, Pissidis AG. A critique of anal
glandular infection in the aetiology and treatment of
idiopathic anorectal abscesses and fi stulas. Br J Surg.
1967;54(12):977–83.
6. Cirocco WC. Life threatening sepsis and mortality
following stapled hemorrhoidopexy. Surgery. 2008;
143(6):824–9.
7. van Wensen RJ, van Leuken MH, Bosscha K. Pelvic
sepsis after stapled hemorrhoidopexy. World J
Gastroenterol. 2008;14(38):5924–6.
8. Hamadani A, Haigh PI, Liu IL, Abbas MA. Who is at
risk for developing chronic anal fi stula or recurrent
anal sepsis after initial perianal abscess? Dis Colon
Rectum. 2009;52(2):217–21.
9. Read DR, Abcarian H. A prospective survey of 474
patients with anorectal abscess. Dis Colon Rectum.
1979;22(8):566–8.
10. Ingle SB, Loftus Jr EV. The natural history of perianal
Crohn’s disease. Dig Liver Dis. 2007;39(10):963–9.
11. Grace RH, Harper IA, Thompson RG. Anorectal sep-
sis: microbiology in relation to fi stula-in-ano. Br J
Surg. 1982;69(7):401–3.
12. Parks AG, Gordon PH, Hardcastle JD. A classifi ca-
tion of fi stula-in-ano. Br J Surg. 1976;63(1):1–12.
13. Rotstein OD, Pruett TL, Simmons RL. Mechanisms
of microbial synergy in polymicrobial surgical infec-
tions. Rev Infect Dis. 1985;7(2):151–70.
14. Thwaini A, Khan A, Malik A, et al. Fournier’s gan-
grene and its emergency management. Postgrad Med
J. 2006;82(970):516–9.
15. Munoz-Villasmil J, Sands L, Hellinger M.
Management of perianal sepsis in immunosuppressed
patients. Am Surg. 2001;67(5):484–6.
16. North Jr JH, Weber TK, Rodriguez-Bigas MA,
Meropol NJ, Petrelli NJ. The management of infec-
tious and noninfectious anorectal complications in
patients with leukemia. J Am Coll Surg. 1996;
183(4):322–8.
17 Anal Sepsis: Anatomy, Pathophysiology and Presentation
231
R. Cohen, A. Windsor (eds.), Anus,
DOI 10.1007/978-1-84882-091-3_18, © Springer-Verlag London 2014
Introduction
Fistual-in-ano describes an abnormal communi-
cation between the anal canal (or occasionally
the rectum) and the perianal skin. It is a common
condition that has a tendency to recur despite
apparently curative surgery. Recurrence after sur-
gery is almost always due to infection that has
escaped detection by the surgeon and thus gone
untreated. It is now increasingly recognised that
preoperative imaging, notably by MRI, is able to
identify fi stulas and associated abscesses that
would otherwise have been missed. Not only can
MRI elegantly display perianal fi stulas, but pre-
operative MRI has been shown to infl uence sub-
sequent surgery and signifi cantly diminish the
chance of recurrent disease as a result. Because
of this, preoperative imaging is becoming
increasingly routine, especially in patients with
recurrent fi stulas.
Aetiology, Classifi cation
and Treatment of Fistula-in-Ano
Relevant to Imaging
The previous chapter (Chap. 17 ) has described the
pathophysiology and presentation of anal sepsis.
Because cryptogenic fi stula-in-ano is predicated by
sepsis arising in the intersphincteric plane [ 1 , 2 ],
any useful imaging technique must be able to
image this region with precision (i.e. with high spa-
tial and contrast resolution). This is also the case
for the patient with an acute perianal abscess, since
87 % may subsequently develop a fi stula [ 3 ]; imag-
ing during the acute episode (before incision and
drainage) may be able to distinguish whether inter-
sphincteric infection underpins the abscess.
By defi nition a fi stula describes an abnor-
mal communication between two epithelial sur-
faces. The anatomical course of the fi stula will
be dictated by the location of the infected anal
gland and the anatomical planes and structures
that surround it. The internal opening of the fi s-
tula will usually be in the anal canal at the level
of the dentate line, i.e. at the original site of the
duct draining the infected gland. In the radial
plane, the internal opening is usually posterior at
6 o’clock, simply because anal glands are more
abundant posteriorly, especially in men. The den-
tate line cannot be identifi ed as a discrete ana-
tomical structure by any imaging technique but
its position can be approximated with suffi cient
accuracy by experienced radiologists – it lies
approximately 2 cm cranial to the anal verge on
coronal images.
S. Halligan , MBBS, MD, FRCR, FRCP (*)
S. A. Taylor , MBBS, BSc, MD, MRCP, FRCR
UCL , Centre for Medical Imaging,
University College Hospital,
University College London , 235 Euston Road ,
London NW1 2BU , UK
e-mail: s.halligan@ucl.ac.uk
1 8
Imaging of Anal Sepsis
Steve Halligan and Stuart A. Taylor
232
The fi stula can reach the perianal skin via a
variety of routes, some more tortuous than others
and thereby penetrating and involving the mus-
cles of the anal sphincter and surrounding tissues
to a variable degree. Fistulas are ‘classifi ed’
according to the route taken by this ‘primary
track’, which links the internal and external open-
ings. Although there have been a variety of dif-
ferent attempts to classify fi stula-in-ano, by far
the most widely used is that proposed by Parks
and colleagues in 1976 [ 4 ]. Parks carefully anal-
ysed a consecutive series of 400 patients referred
to the surgeons of St. Mark’s Hospital London, a
specialist hospital dealing with coloproctological
disease, and found that he was able to place all
fi stulas encountered into one of four broad
groups: intersphincteric, transsphincteric, supra-
sphincteric and extrasphincteric [ 4 ]. Importantly,
most of these groupings could be explained by
the cryptoglandular hypothesis. A major role for
imaging is the ability to distinguish between
these different fi stulas and so to arrive at an accu-
rate preoperative classifi cation for the operating
surgeon before he or she puts a knife to the skin.
Because of this, the interpreting radiologist must
be fully conversant with Parks’ classifi cation.
While most fi stulas probably start as a sim-
ple, single primary track, unabated infection
may result in ramifi cations (often multiple) that
branch away from this, generally, termed ‘exten-
sions’. Extensions are a major target for preop-
erative imaging because they frequently underpin
recurrent disease. This is often because they may
occur several centimetres away from the primary
track and frequently lie deep in surrounding tis-
sues, thus escaping easy detection. Extensions
may be intersphincteric, ischioanal or supraleva-
tor (pararectal), and their morphology may sug-
gest tracks or abscesses. Exactly when a ‘track’
becomes an ‘abscess’ has no precise defi nition on
imaging.
The ischioanal fossa is the commonest site for
an extension, especially one that arises from the
apex of a transsphincteric fi stula. The ischioanal
fossa lies lateral to the sphincter complex, is
fi lled with fat and is traversed by a network of
fi broelastic connective tissue . Because this space
lies adjacent to the anus (vs the rectum) and lies
immediately below (vs above) the levator plate
of the pelvic fl oor, the authors prefer the term
‘ischioanal’ fossa rather than ‘ischiorectal’,
which is commonly used by surgeons. However,
the two terms are interchangeable. Extensions
also occur in the horizontal plane and are known
as ‘horseshoes’ if there is ramifi cation of sepsis
on both sides of the internal opening.
Although surgical treatment of fi stula-in-ano
is usually straightforward, most frequently by
laying open the fi stula, this seemingly simple
procedure has many unexpected traps waiting for
the unwary. Injudicious incision and overenthusi-
astic exploration can very quickly convert a sim-
ple fi stula into a surgical nightmare by creating
additional extensions, tracks and communica-
tions, with disastrous consequences for the
patient. The surgeon’s prime objective is to iden-
tify the primary track and any associated exten-
sions and then eradicate these by draining all
associated infection all while simultaneously pre-
serving anal continence. Thus, there are two sur-
gical questions that should ideally be answered
preoperatively:
• What is the relationship between the fi stula
and the anal sphincter? That is, can the track
be safely laid open with only a low risk of
post-operative incontinence?
• Are there any extensions from the primary
track that need to be treated in order to prevent
recurrence? If so, where are they?
Although frequently used for this purpose, it is
now well established that examination under (gen-
eral) anaesthesia (EUA) is not infallible. At EUA,
the surgeon attempts to classify the fi stula via pal-
pation and probing, so as to determine the relation-
ship to the sphincter. However , the surgeon cannot
visualise underlying muscles directly, and general
anaesthesia and consequent loss of muscular tone
impair precise identifi cation further. The internal
opening may be diffi cult to identify, but probes
must not be advanced forcefully for fear of caus-
ing unintentional tracks and extensions. For exam-
ple, forceful probing of a transsphincteric fi stula
track in the roof of the ischioanal fossa can easily
rupture through the levator plate, thereby causing a
supralevator extension. In the worst instance, the
probe can even rupture into the rectum, converting
S. Halligan and S.A. Taylor