Investigating perianal pain of uncertain cause
BMJ 2008; 336 doi: https://doi.org/10.1136/bmj.39455.393299.AD (Published 14 February 2008) Cite this as: BMJ 2008;336:387
- Rebecca Greenhalgh, radiology specialist registrar1,
- C Richard Cohen, consultant colorectal surgeon1,
- David Burling, consultant radiologist2,
- Stuart Andrew Taylor, senior lecturer in radiology and honorary consultant radiologist13
- 1University College Hospital, London NW1 2BU
- 2St Mark’s Hospital, Harrow HA1 3UJ
- 3University College London, London
- Correspondence to: S A Taylor stuart.taylor{at}uclh.nhs.uk
- Accepted 15 October 2007
Learning points
Imaging plays an important role in elucidating the cause of anal pain when the diagnosis is not clinically apparent
Endoanal ultrasound is an increasingly available, relatively inexpensive, quick and safe technique providing high resolution images of the anal sphincter complex, and intersphincteric space, but it is limited by a relatively small field of view
Magnetic resonance imaging, although not universally available, affords much greater anatomical coverage than endoanal ultrasound and can also image the perirectal tissues and lumbar-sacral spine, facilitating diagnosis of conditions beyond the anal canal
In the case of perianal sepsis, preoperative magnetic resonance imaging may be indicated in clinically suspected complex or recurrent disease to guide surgery, reducing the risk of future recurrence
The patient
A 29 year old woman presented with constipation, constant perianal burning, and pain on defecation. Digital rectal examination was uncomfortable with induration in the right posterior quadrant, but there was no evidence of anal fissure or skin tag, thrombosed haemorrhoid, perianal haematoma, anal mass, or palpable abscess or fistula. Perianal skin sensation was normal.
What is the next investigation?
The aim of further investigation is to identify those causes of perianal pain not always detected by direct clinical examination. Commoner causes to consider include occult perianal sepsis such as intersphincteric abscess (the prevalence of anal fistula in the general population is about 0.01%1), anal complications of inflammatory bowel disease, and anal cancer. Rarer causes include retrorectal developmental cysts, sacrospinal tumours, and sacral nerve tumours. Proctalgia fugax (intermittent severe anal or lower rectal pain of unknown aetiology) remains a diagnosis of exclusion.
Anal endosonography
Anal endosonography is a quick, safe, and cheap technique but requires operator experience. It provides detailed, high resolution images of the anal sphincter complex and intersphincteric space. Although its main clinical role is in investigating faecal incontinence, anal endosonography will usually reveal the presence of occult intersphincteric abscesses. It may also show anal tumours and developmental cysts, particularly if they communicate with the intersphincteric space, but data on its accuracy for these rarer conditions are limited.
In the case of perianal sepsis, an abscess is usually seen as a rounded, low echogenic defect between the internal and external anal sphincter, whereas the primary track of an established anal fistula is seen as a hypoechoic band (fig 1⇓). In a non-randomised study of preoperative imaging in patients with suspected perianal sepsis, anal endosonography had 82% sensitivity for the correct classification of simple primary perianal fistulas.2
However the technique is limited by its relatively small field of view (just a few centimetres), such that structures lateral to or above the anal sphincter complex are poorly visualised. For example, the presacral space, sacrum, and perirectal tissues are not reliably assessed, so sacrospinous tumours and pathological processes above the pelvic floor are not reliably excluded.
Fig 1 Results of anal endosonography. Top: image of the distal anal canal, showing an intersphincteric abscess (long arrow) as a local, low echogenic focus between the internal sphincter (short arrow) and external sphincter (arrowhead). Bottom: image distal to terminal fibres of the internal anal sphincter shows a low echogenic track (arrow) medial to the distal fibres of the external anal sphincter (arrowhead) in keeping with an intersphincteric fistula
Magnetic resonance imaging (MRI)
MRI provides high contrast and high resolution images of soft tissue, allowing exquisite depiction of the anal canal structures and surrounding tissues. Although generally a safe technique, it is more time consuming than anal endosonography, is best avoided in early pregnancy, and is contraindicated in some patients with implanted metallic devices including cardiac pacemakers (table⇓).4 5 Spatial resolution can be improved by placing a receiver endocoil in the anal canal itself,6 although in a comparative study of 30 patients an external MRI receiver coil placed over the pelvis was better tolerated than an endocoil and provided additional diagnostic information beyond the anal canal.7 MRI affords much greater anatomical coverage than anal endosonography and can show the perirectal tissues and sacrolumbar spine, thereby facilitating diagnosis of developmental cysts, sacrospinal tumours, and sacral nerve tumours.
Advantages and disadvantages of the use of anal endosonography and magnetic resonance imaging in the investigation of perianal pain of uncertain cause
In the case of suspected perianal sepsis, abscesses and fistulous tracks are seen as areas of bright high signal against the lower signal anatomical structures (fig 2⇓). One major advantage of MRI over anal endosonography is its ability to delineate secondary extensions and abscesses arising from the primary fistulous track, which often course beyond the ultrasonic field of view. Such extensions, if unrecognised, are responsible for most cases of fistula recurrence after surgical treatment. In one study MRI detected 90% of primary tracks and was superior to anal endosonography in the detection of secondary abscesses remote from the primary fistula (85% v 75%).2 In a non-randomised study of imaging in patients with recurrent anal fistula, use of preoperative MRI to guide surgery reduced future recurrence by 75% compared with standard surgery.8
Fig 2 Results of axial magnetic resonance imaging. Top: section through the distal anal canal shows the internal opening of the fistula (long arrow) as a high signal focus at the 7 o’clock position; there is no evidence of sepsis beyond the external sphincter (arrowhead) in the ischio-anal fossa (short arrow). Bottom: section distal to the anal sphincters shows the fistulous track (arrow) as high signal and forming a small abscess
Examination under anaesthetic
For patients who cannot tolerate digital rectal examination when awake, examination under anaesthetic is essential so that local causes of anal pain such as fissure or thrombosed haemorrhoids can be excluded. The procedure itself is well tolerated, but general anaesthesia carries a small risk. In patients with suspected perianal sepsis, examination may reveal the internal opening, secondary tracks, and abscesses, although in one study even experienced staff were able to identify only 61% of primary tracks, 78% of internal openings, and 33% of abscesses.2 The combination of normal results from examination under anaesthetic and from MRI allows a diagnosis of proctalgia fugax to be made with greater certainty.
Outcome
Given the patient’s relatively acute history and the findings from digital rectal examination, occult perianal sepsis was suspected. Anal endosonography, being quick and readily available, was performed on the same day as the outpatient assessment and confirmed a small intersphincteric abscess, probably associated with an intersphincteric fistula (fig 1⇑).
Further imaging would not usually be indicated for a simple perianal abscess or fistula before surgical treatment, but there was clinical concern that the palpable induration in the anal canal could indicate extension of sepsis beyond the primary abscess. Given the known risk of fistula recurrence caused by undiagnosed (and thus untreated) extensions, MRI was performed. This confirmed an intersphincteric fistula complicated by a small intersphincteric abscess, but excluded any secondary tracks or extensions (fig 2⇑). The patient then underwent examination under anaesthetic with definitive laying open of fistula and is now symptom-free.
Footnotes
This series provides an update on the best use of different imaging methods for common or important clinical presentations. The series advisers are Fergus Gleeson, consultant radiologist, Churchill Hospital, Oxford, and Kamini Patel, consultant radiologist, Homerton University Hospital, London.
Contributors: RG was responsible for the literature search, obtaining patient consent, collection and preparation of images, drafting the initial manuscript, and approving the final manuscript. CRC was responsible for the literature search, planning and drafting the initial manuscript, and approving the final manuscript. DB was responsible for the full literature search (using evidence based techniques), editing the manuscript, and approving the final manuscript. SAT was responsible for the clinical studies, the literature search, planning and drafting the initial manuscript, and editing and approving the final manuscript. SAT is also guarantor for the article.
Competing interests: None declared.
Provenance and peer review: Commissioned; externally peer reviewed.
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