Intended for healthcare professionals

Endgames Case Review

A young woman with recurrent perianal sepsis

BMJ 2015; 350 doi: (Published 23 April 2015) Cite this as: BMJ 2015;350:h1969
  1. Tom L Kaye, radiology specialist registrar1,
  2. Anthony O’Connor, consultant gastroenterologist2,
  3. Dermot Burke, consultant in gastrointestinal surgery3,
  4. Damian J M Tolan, consultant radiologist4
  1. 1Department of Radiology, Leeds Teaching Hospitals NHS Trust, St James’s University Hospital, Leeds LS9 7TF, UK
  2. 2Department of Gastroenterology, Leeds Teaching Hospitals NHS Trust, St James’s University Hospital
  3. 3Department of Surgery, Leeds Teaching Hospitals NHS Trust, St James’s University Hospital
  4. 4Department of Radiology, Leeds Teaching Hospitals NHS Trust, St James’s University Hospital
  1. Correspondence to: T Kaye thomas.kaye{at}

A 23 year old woman had been experiencing cramping abdominal pain, alternating diarrhoea and constipation, and episodic blood per rectum for four months. An earlier colonoscopy showed proctitis, which was thought to be indicative of underlying inflammatory bowel disease, and she was started on oral and topical mesalazine. In the following months she was admitted twice with acute perianal sepsis, which required examinations under anaesthesia and drainage of an intersphincteric abscess. She later presented after feeling generally unwell for four days with “flu-like” symptoms and perianal pain. On examination she had tenderness and fluctuance in the region of the right ischial tuberosity, with an external fistula opening visibly in the perineum. Her blood tests show mild neutrophilia (9.40×109/L, reference range 2-7.5) and mildly raised C reactive protein (380.96 nmol/L (40 mg/L), 0-95.24 (0-10) only.


  • 1. What imaging modality should be used to investigate this clinical presentation?

  • 2. What is the underlying pathophysiology of perianal fistulation?

  • 3. What classification systems are used for perianal fistulas?

  • 4. What is the optimal management for a patient with a perianal fistula and abscess?


1. What imaging modality should be used to investigate this clinical presentation?


Magnetic resonance imaging (MRI) of the pelvis.


Although a variety of techniques can be used to image perianal fistulas, MRI is widely accepted as the technique of choice for patients who present with recurrent perianal sepsis or signs of complex fistulation and for those with underlying Crohn’s disease.

Perianal fistulography, in which the external opening of the fistula is catheterised and a water soluble contrast agent injected, is now rarely used. This is because both the primary tract and extensions may fail to fill with contrast and the sphincter muscles are not directly visualised, which hinders anatomical assessment. The test is less accurate and reliable than modern techniques.1 2

Computed tomography may depict perianal fistulas and associated abscesses but is not accurate enough to classify them.3

Endoanal ultrasound uses a small high frequency transducer to visualise the anal sphincter complex in detail, providing high resolution images.4 This valuable technique can correctly categorise perianal fistulas in 81% of cases,5 although it is operator dependent. It is ideally suited to identifying the internal fistula opening, although imaging of ischioanal and supralevator extensions is more limited owing to the distance from the probe.4 Unfortunately, endoanal ultrasound cannot reliably differentiate active infection from fibrosis, both of which have a hypoechoic appearance.6 Three dimensional endoanal ultrasound is a more recent development that takes images in multiple planes and may be able to discriminate fistulas caused by Crohn’s disease from those with other causes by detecting the Crohn’s ultrasound fistula sign.7

MRI outperforms other imaging modalities such as endoanal ultrasound—it correctly classifies 90% of fistulas and accurately identifies associated abscesses and extensions.5 A surgical approach guided by MRI can reduce the fistula recurrence rate by 75%.8 MRI should be performed using a specific fistula protocol in at least two planes aligned to the anal canal. T2 weighted fat suppressed sequences highlight tissues with a high water content, such as fluid collections, fistula tracts, and areas of oedema. T1 weighted sequences before and after gadolinium contrast are useful to delineate fistula anatomy and differentiate abscesses, which do not enhance centrally, from areas of enhancing granulation tissue.

2. What is the underlying pathophysiology of perianal fistulation?


Perianal abscesses and fistulas are thought to be acute and chronic manifestations of the same disease process. Primary disease results from obstruction of the anal glands, which then become infected (cryptoglandular hypothesis); a minority are secondary to Crohn’s disease (as in this case), cancer, previous radiotherapy, atypical infection (such as tuberculosis), or iatrogenic trauma.


The prevalence of perianal abscesses and fistulas is about 0.01% in the developed world, and these lesions are more common in young adults, with a male to female ratio of 2:1.9 They commonly present with perianal discharge, pain, and swelling, although they may be asymptomatic.

The “cryptoglandular hypothesis” is thought to explain about 90% of episodes of perianal sepsis.10 This involves development of infection and obstruction within the anal glands, which usually lie subepithelially above the dentate line and drain into the anal sinuses between the longitudinal columns of Morgagni.11 This leads to abscess formation, which if superficial may resolve through spontaneous discharge into the anal canal. Abscesses that are deep to the internal sphincter, or are incompletely and inadequately treated, eventually form additional drainage pathways, reaching the perineum through the formation of a fistulous tract. The anal canal is surrounded by the internal sphincter (formed from circular smooth muscle) and the external sphincter (composed of striated muscle) that superiorly blends with the puborectalis muscle and levator ani to form the pelvic floor (fig 1). Fistulous tracts often pass along the plane between these muscles (the intersphincteric plane) or may traverse both sphincter muscles to reach the perineum. Estimated recurrence rates after a first episode of perianal sepsis vary widely, and 37-87% of patients with acute perianal sepsis subsequently develop a fistula.4 12


Fig 1 Axial (top row) and coronal (bottom row) STIR (short TI inversion recovery) images showing the normal anatomy of the anal canal. The levator ani (Lev. A) blends with the puborectalis muscle (Pub) which inferiorly blends with the external sphincter (Ext). The internal sphincter (Int) is continuous with the muscularis propria of the rectum. Although not visible radiologically, the dentate line is about 2 cm from the anal verge (yellow oval). The intersphincteric space (red line) is the plane between the internal and external sphincters. Obt. Internus=internal obturator muscle

The remaining 10% of cases of perianal fistulation are secondary to a variety of conditions including Crohn’s disease, atypical infection (such as tuberculosis), cancer, iatrogenic trauma, and previous radiotherapy.10 Although the cryptoglandular hypothesis can explain many episodes of perianal sepsis in Crohn’s disease it cannot account for the complex extrasphincteric fistulas seen in this condition, which can communicate between the perineum, rectum, and other visceral structures without involving the anal canal. The cumulative incidence of perianal fistulation is 26% after 20 years in patients with Crohn’s disease.13 Because of the propensity for incomplete healing and recurrence, the condition can be debilitating and complex. Other manifestations of perianal Crohn’s disease include skin tags, anal fissures, and perianal abscesses (fig 2).


Fig 2 Axial STIR (short TI inversion recovery) and T1 weighted post-contrast magnetic resonance images showing a large perianal abscess involving the right labial fold in a patient with Crohn’s disease (solid arrows). A short sinus tract can be seen extending from the abscess towards the anal canal at 12 o’clock (dashed arrow)

3. What classification systems are used for perianal fistulas?


The most widely used system is the Parks classification, with the St James’s University Hospital classification also being commonly used.


Parks and colleagues’ classification of perianal fistulas is based on surgical findings, and it describes the course of the fistula in relation to the internal and external sphincters (fig 3).14


Fig 3 Coronal STIR (short TI inversion recovery) magnetic resonance image depicting the Parks fistula classification. Line A: intersphincteric fistula; line B: trans-sphincteric fistula; line C: suprasphincteric fistula; line D: extrasphincteric fistula

The Parks classification categorises fistulas into four groups: intersphincteric, trans-sphincteric (fig 4), suprasphincteric, and extrasphincteric (fig 5). Intersphincteric fistulas track along the intersphincteric plane inferiorly to reach the anal verge, and the lowermost fibres of the external sphincter may be affected. Trans-sphincteric fistulas cross the external sphincter to reach the skin through the ischioanal fossa. It is important to identify the level at which a trans-sphincteric fistula crosses the external sphincter because the risk of incontinence after fistulotomy increases with more proximal fistulas as more external sphincter is divided.


Fig 4 Axial T1 weighted post-contrast magnetic resonance image showing a trans-sphincteric fistula with internal opening at 12 o’clock (dashed arrow), which crosses the sphincter muscles and passes anteriorly on to the perineal skin (solid arrows)


Fig 5 Coronal STIR (short TI inversion recovery) magnetic resonance image showing a right sided extrasphincteric fistula passing superiorly from the perineum to the lower mesorectum (solid arrows) separately from the sphincter complex. A seton is in situ (dashed arrow) within a second partially healed fistula (not fully shown)

Although suprasphincteric fistulas are uncommon, it is important that they are identified correctly to facilitate appropriate surgical management. The fistula tracks superiorly in the intersphincteric plane before traversing the levator ani muscle and crossing the ischioanal fossa to reach the skin. In extrasphincteric fistulas, the anal canal is not affected, so these fistulas are not explained by the cryptoglandular hypothesis. In such cases it is important to exclude Crohn’s disease, complex diverticular disease, or cancer. In all groups secondary tracks (also known as extensions) and abscesses may complicate disease.

The increasing importance of MRI in delineating fistula anatomy and identifying hidden areas of undrained sepsis led to the development of the St James’s University Hospital classification, which is based on anatomical MRI findings.15 This system incorporates the primary fistulous track as well as secondary extensions and abscesses. There are five grading categories: grade 1—a simple linear intersphincteric fistula; grade 2—an intersphincteric fistula with an abscess or secondary track; grade 3—a trans-sphincteric fistula; grade 4—a trans-sphincteric fistula with an abscess or secondary track in the ischiorectal or ischioanal fossa; and grade 5—a supralevator or translevator fistula (extending above the insertion point of the levator ani muscle; translevator fistulas do not involve the anal canal). Both classification systems help standardise the description of disease and are useful for treatment planning and prognosis.

The Van Assche method is an alternative dynamic anatomical MRI scoring system, which combines information on the local extension of the fistula with signs of active inflammation and the presence of collections.16 It can be used to quantify radiological severity and is a useful tool for assessing response to treatment in patients with Crohn’s disease.

4. What is the optimal management for a patient with a perianal fistula and abscess?


Acute management involves surgery, with examination under anaesthesia to identify the fistula tract, incision and drainage of the abscess, and placement of a draining seton suture in the fistula. In patients with Crohn’s disease medical treatment can be started when the sepsis has settled.


Treatment of a perianal abscess involves early surgical drainage, ideally combined with an examination under anaesthesia performed by an experienced colorectal surgeon.17 There is little evidence to support primary antibiotic treatment, and a delay in surgical intervention may lead to further soft tissue damage and promote fistula formation.

Goodsall’s rule is traditionally used to predict the trajectory of a fistula tract and location of the internal opening, although it can be unreliable. It states that if a line is drawn transversely across the anus between 9 o’clock and 3 o’clock, an external opening lying anteriorly will lead to a straight radial tract. Conversely, an external opening lying posteriorly will lead to a curved tract and an internal opening posteriorly in the midline. If a fistula tract is identified during detailed clinical examination a variety of treatment options are available, although none is universally accepted or without risk.18

A seton is a thread that is placed through the fistula track to form a continuous ring between the internal and external openings. In the acute setting a draining seton can be used to maintain patency of the track, allow drainage, and reduce inflammation and sepsis.19 Subsequent definitive management of trans-sphincteric fistulas may use a cutting seton, which is regularly tightened to gradually cut through the sphincter muscle, thereby promoting tissue fibrosis and fistula healing.

Unfortunately the risk of sphincter damage and incontinence after treatment is high.20 Sphincter sparing methods include the ligation of the intersphincteric fistula track operation, in which an incision is made between the sphincter muscles, exposing the fistula, which is then ligated in the intersphincteric plane. One study found that this technique had a 57% success rate at a median follow-up of 20 months.21 Alternatively, endorectal advancement flaps can be used to close off the primary opening by mobilising a pedicled flap of rectal wall. Success relies on adequate flap vascularity and healing, and reported success rates range from 64% in patients with Crohn’s disease to 81% for cryptoglandular fistulas.22 Other treatment options include laying open the fistula track (fistulotomy), various glues, and the use of fistula plugs. In severe cases of uncontrolled perianal sepsis that are refractory to treatment, the distal intestine may need to be defunctioned by forming an upstream stoma to divert the faecal stream and promote healing. Proctectomy is occasionally needed in severe cases when all other treatment methods fail.

Management of perianal sepsis in patients with Crohn’s disease can be complex and a multidisciplinary approach may be needed. Fistulating perianal disease is unlikely to heal in patients with ongoing active proctitis, and medical therapy is usually the first line treatment. Two randomised controlled trials found that infliximab was beneficial in the treatment of perianal Crohn’s disease.23 24 Other agents such as antibiotics and thiopurines can be useful adjunctive treatments. More recent non-randomised studies have shown that combined treatment with infliximab and another immunosuppressant is associated with fistula closure.25 Combination treatment can be particularly useful in patients with proctitis.

The preservation of sphincter function is crucial in Crohn’s disease. Owing to poor wound healing and high fistula recurrence rates, surgery is usually reserved for undrained sepsis. Patients with Crohn’s disease and asymptomatic fistulas do not require aggressive surgical intervention, and complex fistulas can often be palliated with long term draining setons.26 In selected cases symptomatic simple fistulas can be definitively treated with fistulotomy.27

Patient outcome

An MRI scan identified a complex low trans-sphincteric fistula with a long supralevator intersphincteric extension on the right, an associated abscess (fig 6), and signs of active proctitis. Findings were suggestive of Crohn’s disease. Acute surgical admission was arranged after the scan. Our patient underwent an examination under anaesthesia, with incision and drainage of a complex intersphincteric abscess. A fistulous tract was identified corresponding to the MRI findings and a loose draining seton was inserted. She made a good postoperative recovery and was discharged shortly afterwards. She is due to start combination immunotherapy with infliximab and azathioprine once the sepsis has settled.


Figure 6 Coronal (A) and axial (B) post-contrast T1 weighted magnetic resonance image showing a complex trans-sphincteric fistula with an internal opening at 6 o’clock (dotted arrow in B), a long right sided supralevator intersphincteric extension (solid arrows), and a focal 15 mm abscess (dashed arrow in A)


Cite this as: BMJ 2015;350:h1969


  • Competing interests: We have read and understood BMJ policy on declaration of interests and declare that we have no competing interests.

  • Provenance and peer review: Not commissioned; externally peer reviewed.

  • Patient consent obtained.


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