II.3.2. Endoanal Ultrasound in Perianal Disease

Dra. Susana Tabernero da Veiga
Hospital Universitario Príncipe de Asturias.Alcalá de HenaresHospital de Madrid Norte Sanchinarro. Madrid


Crohn’s Disease (CD) can manifest based on its behaviour in various patterns included, in turn, in the Montreal Classification1. It is traditionally divided into three types of behaviour: non-stenosing and non-penetrating (inflammatory), stenosing and penetrating (B1, B2 and B3 respectively, based on Montreal Classification). To the three types, we add a “p” if perianal impairment is concomitantly present. Patients with penetrating or fistulising CD usually have a more aggressive course. Fistulae can be external or internal. Internal fistulae between loops usually progress silently and are more difficult to diagnose. On the other hand, external (enterocutaneous or perianal) fistulae are usually associated with suppuration, pain and abscesses. Particularly, perianal fistulae can progress into anal incontinence, abscess formation and anal stenosis2.

The global incidence of the appearance of fistulae in patients with CD ranges between 17–50%, of which the great majority (54%) are perianal3. However, perianal impairment in CD includes both fistulising lesions (fistulae and abscesses) and another type of pathology (anal fissures, ulcers in the anal canal, rectal stenosis, skin tags, haemorrhoids and neoplasms). Although there is little information available in literature on the incidence of perianal manifestations other than fistulae, a recent study4 confirms that the probability of developing anal stenosis, deep ulcers in the anal canal and anal fissures in 10 years is 5.8%, 6.6% and 10.5% respectively. However, if we assess this same information in relation to perianal fistulae, the percentage rises significantly: up to 21% of patients with CD will develop this complication 10 years after diagnosis2. Therefore, perianal fistulae are the most significant pathology due to the percentage of patients affected and the severity of the complications that entail the development of such fistulae.



When a doctor requests an endoanal ultrasound for a CD patient with suspected fistulising perianal disease, the information that he or she should receive is based on three aspects. First, to contribute to the differential diagnosis to confirm the presence of perianal fistula. Second, classify it, since this is going to transcendentally influence the third aspect, which is the implication in the treatment that will be selected. Classification is the most important point and also the most difficult. However it has crucial implications when indicating the best therapeutic strategy for our patients5. This affirmation is likewise listed in the recommendations for managing perianal disease in the ECCO consensus6.

1. Confirm the presence of perianal fistula

Therefore, the first contribution of an endoanal ultrasound is the confirmation of the presence of a fistula tract, thus allowing for a differential diagnosis with other entities. As stated in the introduction, patients with CD can present (Table I) perianal lesions typical of their inflammatory bowel disease (IBD) (skin folds, fissures and ulcers, anorectal stenosis, anal incontinence and perianal fistulae) and others not related to their IBD (hidradenitis suppurativa and anal carcinoma). However, we must also not forget that patients with CD can also have perianal pathologies that are not secondary to IBD but pose a conflict in the differential diagnosis since they simulate typical manifestations of perianal disease in patients with CD (sexually transmitted disease, pilonidal sinus, perianal dermatitis, actinomycosis and tuberculosis). Therefore, endoanal ultrasound is a technique which provides vital help in the confirmation of fistulising perianal disease in patients with CD.


2. Classifying fistulae

Once the presence of perianal fistula is confirmed, the next contribution of an endoanal ultrasound is differentiation of a simple fistula from a complex fistula6, which is essential for managing fistulising perianal CD. In order to understand the classification of perianal fistulae, it is important to review the anatomy of the anal canal (Fig. 1).

The anal canal measures 2–4 cm in length and is made up of two circular layers of muscle7. The inner layer is made up of smooth muscle, which is the prolongation of the muscular layer of the mucosa of the rectal wall and forms the internal anal sphincter (IAS)8 in the anal canal. Beyond the IAS is another surrounding circular layer, in this case consisting of striated muscle extending from the puborectalis muscle, which goes on to form the external anal sphincter (EAS). The dentate line or pectinate line, located in the middle part of the anal canal, separates the columnar epithelium of the rectum from the squamous epithelium of the anal canal. This line enables the division of the anal canal into three levels:

Upper anal canal (Fig. 2): is the most proximal of the three levels and is located over the dentate line. It is formed outside of the puborectal muscle in a “U” or “V” shape on the posterior side of the anal canal, marking two right and left hyperechogenic loops or branches. Inside we observe the other circular layer of smooth muscle that forms a hypoechogenic circle that corresponds to the IAS.

Middle anal canal (Fig. 3): the middle anal canal is situated at the dentate line level, closed off by the two sphincters, forming two circular layers. The outermost layer is hyperechogenic and corresponds to the EAS. Inside the EAS there is another hypoechogenic circle formed by the IAS.

Lower anal canal (Fig. 4): lastly, the lower anal canal begins where the IAS disappears and we only find the hyperechogenic circular fibres of the EAS.

FIGURA 1. Anatomía y clasificación del canal anal.
FIGURA 2. Ecografía endoanal. Canal anal alto.
FIGURA 3. Ecografía endoanal. Canal anal medio.
FIGURA 4. Ecografía endoanal. Canal anal bajo.

The first classification of fistulae that we know of9 was described in 1934 by 

Milligan and Morgan. These authors based their classification on the relation of the fistula tract with the rectal ring. Thus, fistulae are classified as:


Lower anal (under the dentate line).

Upper anal (over the dentate line).

Anorectal under the levator ani muscle.

Anorectal over the levator ani muscle.


Subsequently, it was Parks who described another widely-known, more anatomically precise classification, which proposes to divide the various fistulae based on the relation of the fistula tract to the EAS10. This classification describes 5 different types of fistulae (Fig. 5):

Superficial: tract that runs under both the IAS and EAS.

Intersphincteric: tract that goes through the IAS and continues through the intersphincteric space between the EAS and IAS.

Transsphincteric: tract that goes through both sphincters.

Suprasphincteric: goes through the IAS to ascend through the intersphincteric space over the puborectal muscle and then goes through the levator ani muscle to end at the skin.

Extrasphincteric: tract that remains outside of EAS and penetrates the rectum by going  through the levator ani muscle.

FIGURA 5. Clasificación de Parks.

Presently, the classification for perianal fistulae in patients with CD for clinical practice recommended both by the American Gastroenterological Association (AGA)9 and the European Crohn’s and Colitis Organisation (ECCO)6 is to divide the fistulae into simple or complex. Ultimately, this classification is a mix of the two aforementioned concepts. Fistula tracts are divided based not solely on their location within the anal canal, but also on the relation of the fistula to the sphincters (Fig. 6):

Simple fistulae:

– Are located in the lower anal canal.

– Present a unique external orifice.

– Are not associated with pain or fluctuation.

– Are generally superficial and, therefore, less dangerous.

Complex fistulae:

– Are located in the upper or middle anal canal.

– Can present multiple external orifices.

– Can be associated with pain or fluctuation and anal stenosis, and is at a high risk for bringing about septic complications and anal incontinence.

– Include rectovaginal fistulae.

FIGURA 6. Fístulas simples y complejas.

3. Implication in the treatment

The contribution of information from the endoanal ultrasound when selecting various therapeutic options is determined in part as described in the paragraph above in relation to simple fistulae versus complex fistulae, the presence of complications and the integrity of the sphincter system.

The most common complication of perianal fistulae is the development of abscesses that transcendentally modify the treatments in these patients. Sphincter defects associated with fistula tracts generally secondary to previous surgical interventions also constitute indispensable information. It is important to bear in mind that, in patients with perianal fistulae that present any concomitant defect in the sphincters, subsequent surgical manipulation in the anal canal will increase the risk of anal incontinence, a crucial fact to consider when indicating treatments.



To perform an endoanal ultrasound, a rigid rectal probe or a radial echoendoscope can be used. Both provide a 360º image, but the advantage of using an endoscope instead of the rigid probe is that, with the former, we can assess on the fly whether there is any fistula-associated proctitis.

Ultrasound assessment of the anal canal used to be done done with 7-MZ transducers, but presently 10-MZ transducers are preferred. In this way, the fibres of the EAS are recognised more easily. A frequency of 7 MZ is only to be used for seeing a more distant area in detail. Patient preparation is as simple as taking a cleansing enema a few hours before examination. Although a significant proportion of subjects do not need sedation, some of them may need it when feeling an intense stinging upon the introduction of hydrogen peroxide through the fistula orifice; therefore, it is recommended that patients fast before the test.

The examination is performed in left lateral decubitus and consists of a few steps to follow on a routine basis8:

1. Physical examination: if the fistula has an external orifice, this will provide the first anatomic information on the location of the fistula tract. We have to focus on whether it is situated in the right or left gluteus and also whether it is anterior or posterior. Likewise, the physical examination can also alert to the existence of complications such as abscesses when visualising hot, erythematous and phlegmonous skin.

2. Introducing the transducer: next, we introduce the transducer up to the rectum where we have to align it in order to obtain the right orientation: for men, the prostate must be at 12 o’clock, and the vagina at the same localisation for women. In this way, the anterior part of the anal canal is located at 12 o’clock position, 3 o’clock represents the left part of the canal, 9 o’clock the right part and 6 o’clock the posterior part. From there, we slowly remove the transducer without rotating toward the sides, in order to not become disoriented until we visualise, in the posterior side of the rectum, the hyperechogenic fibres of the puborectal muscle that constitute our point of reference from the start of the anal canal. Images must be obtained at three levels: at the upper, middle and lower anal canal.

3. Canalisation of the external fistula orifice. In patients with an open external orifice, once the anal canal is examined, we remove the transducer and make attempts to canalise the external orifice with a thin Abbocath®. Provided that we find the external fistula orifice open, we must try to canalise it in order to inject hydrogen peroxide.

4. Injecting hydrogen peroxide. Lastly, we introduce the transducer again and inject 50% diluted hydrogen peroxide. It must be injected in two phases. In the first phase, we slowly inject a small amount (<1 ml) Once the fistula tract is visualised, it is injected more quickly in the second phase to see whether the secondary tracts refill.



In fistulae the following aspects must be reported:

1. Pathway of the fistula tract: we must describe the relation of the tract with the sphincters (superficial, intersphincteric, transsphincteric, suprasphincteric or extrasphincteric).

2. Anal canal level: the point where the main tract of the fistula passes through the sphincters defines the anal canal level (upper, middle or lower) where the fistula tract is located.

3. Internal orifice: it is extremely important for surgeons to plan surgical treatment and, if not located, this constitutes the most determining factor in post-surgery relapse. This is the point where the fistula tract passes through the mucosa-submucosa.

4. Secondary tracts: area always connected to the main tract.

5. Abscesses: we must assess their location and extent.

6. Assess the integrity of the sphincters.



1. Pathway of the fistula tract: echographically, the fistula tract appears as a linear or ovoid, more or less thick, hypoechogenic image. When administering hydrogen peroxide, it becomes a hyperechogenic tract with bubbles inside (Fig. 7 and Video 1). We must indicate whether they are positioned in the anterior or posterior quadrant and whether they are on the left or right side. Rectovaginal fistulae are, without fail, located in the anterior quadrant and after instilling hydrogen peroxide we will see how the vagina refills.

2. Anal canal level: the point where the main tract passes through sphincters defines the anal canal level at which the fistula is located (upper, middle and lower canal).

3. Internal orifice: is detected when identifying a defect in the mucosa-submucosa layer. Generally, it is more easily located after administering hydrogen peroxide.

4. Secondary tracts: have the same echographic characteristics as the main tract. They are differentiated because they fill a second time. Without hydrogen peroxide they are difficult to specify.

5. Abscesses: abscesses are viewed as a hypoechogenic area with poorly defined or slightly hyperechogenic edges. We have to indicate their location and relation to adjacent structures (Fig. 8).

6. Assessing the integrity of the sphincters: given that these sphincters in the anal canal adopt a two-ring shape, any interruption in its morphology is due to a defect in its structure. Defects are described by mentioning the height of the anal canal, the sphincter(s) affected, the quadrant in which it is situated and its extension, measured with an angle.

FIGURA 7. Ecografía endoanal. Fístula transesfinteriana.

FIGURA 8. Ecografía endoanal. Absceso.

Therefore, in an echographic test of the perianal fistulae, we must try to locate the primary and/or secondary fistula tract, as well as its relation to sphincters; identify the internal fistula orifice, find collections and assess the integrity of the sphincters.



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