I.1.6. Assessing perianal fistula disease

Dra. Ana Echarri Piudo
Complejo Hospitalario Universitario de Ferrol
Dra. Susana Tabernero da Veiga
Hospital Universitario Príncipe de Asturias. Alcalá de Henares Hospital de Madrid Norte Sanchinarro. Madrid
Dr. José Carlos Gallego Ojea
Complejo Hospitalario Universitario de Ferrol



Fistulae that appear or complicate Crohn’s Disease can be classified as internal or external. Internal fistulae are fistulae that spread from the digestive tract area with active disease up to the intestinal loop or adjacent organ, and external fistulae that spread from the digestive tract area with active disease (small intestine, colon, rectum or anus) to the skin (Table I).
One must take into account that the perianal impairment in CD includes both fistulising lesions (fistulae and abscesses) (Table II) and other pathologies (anal fissures, ulcers in the anal canal, rectal stenosis, skin tags, haemorrhoids and neoplasms) that may appear in up to 6–10% of the patients (Table III) at 10 years from the onset of the disease, although the frequency of perianal fistula is higher and is observed in up to 26% of the patients 20 years after diagnosis.

Distal impairment due to CD favours its appearance, and it has been observed that in up to 12% of patients with ileal CD the frequency of this has been progressively rising, up to 92% in cases in which the rectum is impaired.

In 10% of cases, fistulae can be the initial manifestation of CD and can even precede a CD diagnosis1-3.



Assessment of perianal fistulising disease begins by creating an adequate medical history, which includes the existence of drainage, anal pain, bowel rhythm, difficult bowel movement, incontinence, medical treatments or previous surgeries.

It is important to know the extension, phenotype and degree of activity of the bowel disease, especially rectosigmoid impairment, via the performance of relevant endoscopic and/or radiologic tests, which are of significant interest when planning treatment for fistulising disease (Table IV)4.



Next, we proceed to explore the anal and perianal region, as well as the perineum, via inspection, palpation and rectal exam.  There are several appropriate postures for perianal exploration, one of the most common is the Sims position, in left lateral decubitus, with the left leg stretched and the right bended over the abdomen or in genupectoral position.

It is important to assess the presence of dermatologic abnormalities such as scars related to injuries or a previous surgery, and to observe the existence of fistulising orifices with secretions such as fissures or anal ulcers and skin tags, as well as the presence of perianal abscesses or indurations. The observation of fistulising orifices and their number and location can indicate the complexity of the fistulae. Performing a rectal exam enables us to determine the tone of the anal sphincter and, by carefully palpating the wall of the anal canal, determine the presence of fistula tracts, indurations and abscesses (Tables V and VI)1,5.

Proper lesion identification and classification may be difficult to determine by rectal exam or even by exam under anaesthesia, given the presence of indurations or scars in these patients. When in the hands of an experienced surgeon, proper exploration and fistula classification can be achieved in 65–80% of cases, in which case it is necessary to complete the analysis via an imaging test (echo-endoscopy or resonance) that is considered to have high diagnostic precision in the initial assessment of these patients, which differentiates simple and complex fistulae and facilitates the management of perianal fistulising disease (Table IV)1,4-6.



There is very little difference among the great majority of ultrasound equipment: they usually come situated on a movable support with wheels, which includes a monitor, keyboard and printer. The option to connect different probes by way of connectors located on a side panel provides this equipment with great versatility.

The endoanal ultrasound is performed with an endoanal exploration probe, which is composed of a motor that enables turning, integrated into a rotating shaft 24 cm long, encased by a metal adapter that has a transducer at the end, with a plastic cap placed on top. This transducer rotates at 6 cycles/second, thus producing an image on its 360º transverse axis. The most utilised transducers are 7 and 10 MHz and offer a great definition capability7.

Exploration is performed in left lateral decubitus, beginning with a perianal exploration to localize the external fistulising orifice and assess the presence of abscesses. The transducer is introduced in the rectum, approximately 6 cm, aligning and guiding it in relation to the prostate gland in men, and vagina in women, that are viewed at 12 o’clock position, thus properly orienting the anal canal. When slowly removing the transducer, we will view, on the posterior side, the hyperechogenic fibres of the puborectal module, marking the start of the anal canal that is examined in its three segments: the upper part with bundles of puborectal muscle on the anterior side; medial, where external and internal sphincters are best viewed, and inferior, where the image of the internal anal sphincter disappears. The internal anal sphincter is viewed as a very well-defined, circular hypoechoic band, 1–3.5 mm thick. Outside is the external anal sphincter, with a higher refraction and thickness. It is generally poorly defined in its external border7,8.

When performing a surgical drainage and choosing adequate medical treatment, it is important to establish the direction of the fistula tract, its branches and secondary cavities. Oxygenated water instillation in the tract enables visualisation. Therefore, the external orifice is cannulated with a soft cannula, a 18-22-F (freinchs) Abbocath, and 2 ml of oxygenated water diluted to 50% is injected, obtaining a hyperechogenic image that highlights the tract and the internal fistulising orifice.



Pelvic magnetic resonance imaging is a radiologic imaging test, which in this case focuses on the perianal region. It is based on the changes produced in the tissues as a result of the activity of the magnetic fields. Resonance equipment is a powerful magnet, which is why patients must not enter the examination room with metal objects that are incompatible. In addition, powerful magnetic fields can interfere with the functioning of electronic devices such as pacemakers, defibrillators, etc., which is why the test is contraindicated in patients who carry these devices.

The patient is placed over the bed in supine position; it is important to place a receptor antenna and cover the pelvic region. It can be essential to catheterise a peripheral vein to administer intravenous gadolinium as contrast. Once the antenna is placed over the pelvis, the patient is introduced into the equipment, and image acquisition begins.

The sequences of images can be acquired in three spatial planes (sagittal, coronal and transverse). In transverse and coronal planes, the orientation of the planes must adjust to the direction of the anal canal, tilting approximately 45º (Fig. 1).

FIGURA 1. Elaboración de reconstrucciones multiplanares.
Sequences are obtained with several enhancements, with T2-weighted high-resolution sequences being the most useful in providing adequate contrast between fistula tracts and damaged anatomical structures (Table VII)9-11.
Once the test is performed, the images obtained are sent and stored in a workstation. The radiologist selects, analyses and prepares them by issuing a report on the findings (Fig. 2)12.
FIGURA 2. Clasificación de Saint James.

With perianal exploration and diagnostic explorations performed, we can determine the need for surgical drainage, collections or abscesses, or the appropriateness of placing drainage lines in complex or abscessificated fistula tracts.  In these cases, examination is completed with an assessment under anaesthesia by a specialised surgeon who will perform the necessary surgical therapy. After adequate drainage and diagnostic classification of the fistulising disease, it will be easier to make a more adequate therapeutic determination (Fig. 3)1,4,5.


FIGURA 3. Algoritmo de tratamiento en enfermedad fistulosa perianal.


The CD activity indexes (CDAI, Harvey) do not show a good correlation with the fistula activity, in which case it is important to use specific indices, with Perianal Disease Activity Index (Table VIII) being one of the most commonly used, especially in clinical trials and tests, although in medical practice, assessing drainage after soft compression of the tract is the most used assessment method5. Although it has not been completely validated, a resonance index that quantifies fistula activity has been described (Table IX)13.



Currently, the term “fistula closure” has been reserved for fistulae without external drainage and without activity data in the imaging tests13,14.



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