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Endoanal ultrasound
Imaging
an axial resolution of 0.3 mm and a lateral resolution of <1.0 mm. Three-dimensional ultrasound uses a double crystal design with a 6-16 MHz frequency range. Both transducers produce a 360° axial image of high spatial resolution. The appearance of the image is dependent on the tissue characteristics: highly reflective tissues give a white hyperechoic image; poorly reflective tissues are hypoechoic and appear black.
Endoanal ultrasound Jeffrey W Wright
Technique Two-dimensional ultrasound A rotating transducer encased within a water-filled, hard sonolucent, parallel-walled plastic cone is used to accurately image the anal canal. Typically, this system has a diameter of 1.7 cm and can be moved freely within the anal canal, causing minimal discomfort or image distortion. Systems in which the transducer is enclosed within a water-filled balloon may distort the anatomy of the anal canal and are more suited to rectal imaging. Care should be taken when assembling the transducer system to ensure that all air bubbles are excluded in order to maintain acoustic coupling during imaging. The cone may be covered by a well-lubricated protective sheath if desired. The probe is inserted into the anal canal with the patient in the left lateral position. (There is an argument for using the prone position because better symmetry of images may be produced.) The probe is rotated so that the 12 o’clock position is anterior. The probe is inserted high into the anal canal before slowly withdrawing in increments of 1 cm or less. In general, insertion to a depth of 5 cm is sufficient to visualize the structures of the anal canal. Freeze-frame images at each increment (showing details of the deep, mid and superficial canal) should be taken.
Abstract Endoanal ultrasound is an essential investigation in the management of faecal incontinence. To accurately image the anal canal in two dimensions, a 10-MHz rotating transducer encased within a water-filled, hard sonolucent, parallel-walled plastic cone is used. In three-dimensional ultrasound the transducer moves slowly down a rigid probe positioned within the anal canal, a predetermined number of two dimensional images are digitized and converted to a single three-dimensional image. Endoanal ultrasonography produces an image dependent on tissue characteristics: highly reflective tissues give a white hyperechoic image; poorly reflective tissues are hypoechoic and appear black. Endoanal ultrasound identifies at least four layers within the anal canal. The innermost layer is the bright hyperechoic subepithelium tissue and is surrounded by the hypoechoic internal anal sphincter. The longitudinal muscle is hyperechoic and is often difficult to distinguish from the external sphincter. The final layer is the external anal sphincter, which has mixed echogenicity. Changes in normal sphincter morphology are apparent with ageing and gender. Endoanal ultrasonography is a method for the morphological diagnosis of damage to the anal sphincter and has brought new insights into the pathogenesis of anorectal disorders. It can differentiate between those individuals with faecal incontinence of idiopathic origin and those with sphincter damage who may be suitable for surgical repair, especially following obstetric or postoperative trauma. Three-dimensional ultrasound is particularly useful for defining the anatomy of the fistula tract in anal sepsis and for local staging of anal canal tumours.
Three-dimensional ultrasound A rigid probe containing the ultrasound transducer is positioned within the anal canal. The transducer moves slowly down the probe, capturing images at a predetermined rate and eliminating image changes due to movement artefact or anatomical repositioning. The two-dimensional ultrasound images are digitized and converted into a single three-dimensional image. Software allows instantaneous reconstruction and correction for geometric errors in the transducer. Images can be simultaneously viewed in the sagittal, transverse and coronal planes and rotated in order to view the anal canal from different angles.
Keywords anal canal; anus; diagnostic imaging; endosonography; faecal incontinence; imaging; perianal fistula; three dimensional; ultrasonography
Endoanal ultrasound is an essential investigation in the management of faecal incontinence in the UK.
Normal anatomy Endoanal ultrasound should reveal at least four layers within the anal canal (Figure 1).
Basic concepts
Subepithelium is the bright, hyperechoic innermost layer and is about 2.0 mm thick. Increases in thickness are unrelated to age, but can occur with excessive straining at stool and in those with solitary rectal ulcer syndrome.
Endoanal ultrasonography produces an image of the structures surrounding the anal canal by generating a ‘map’ of sound reflections from the different tissue surfaces. Conventional twodimensional ultrasound is generated by a mechanically rotated 10 MHz piezoelectric crystal with near-field focusing of 5–45 mm,
Internal anal sphincter is hypoechoic and surrounds the subepithelium. Usually, the internal anal sphincter is assessed for continuity, thickness and symmetry. Any break in the continuity of the internal anal sphincter is considered abnormal. In general, the thickness of the internal anal sphincter is about 2.2 mm, but
Jeffrey W Wright PhD is Chief Experimental Officer at the Division of Gastrointestinal Surgery, University Hospital, Nottingham, UK. Conflicts of interest: none declared.
SURGERY 26:6
247
© 2008 Elsevier Ltd. All rights reserved.
Imaging
External anal sphincter
Subepithelium
Longitudinal muscle
Internal anal sphincter
Transducer and water-filled cone assembly
Axial image of the anal canal. The outer edge of the plastic transducer cone is seen. The subepithelium is hyperechoic and is surrounded by the dark hypoechoic internal anal sphincter, which has a thickness of 2.2 mm (indicated by x-x on the scan). The longitudinal muscle is moderately hyperechoic and is bordered by the mixed echogenicity of the external anal sphincter. Figure 1
increases with age; values of >4.0 mm are abnormal. Thickness is unrelated to gender or sphincter length. Uniformity of echogenicity reduces with increasing age and may relate to sclerosis or degeneration of the muscle. The sphincter should be assessed in all quadrants for asymmetry.
factor. Prospective studies indicate that 30% of primiparous vaginal births result in some form of sphincter damage which may or may not be associated with postpartum faecal incontinence. Two-thirds of obstetric sphincter disruptions are asymptomatic in the post-partum period, but there is evidence that these individuals may be at increased risk for faecal incontinence with ageing compared with those without sphincter injury. Subsequent deliveries increase the risk of incontinence, particularly among women with a sphincter defect diagnosed after the first delivery.
Longitudinal muscle is the layer which is often difficult to distinguish from the external sphincter. The longitudinal muscle is hyperechoic (but less so than the external sphincter) with a thickness of 2.5 mm. There is no clear evidence for changes in thickness to occur with age or gender.
Postoperative trauma: anal endosonography is a valuable tool for characterizing structural damage to the sphincter, particularly when planning restorative surgery or for assessment after sphincter repair or direct anal trauma; it has high accuracy and
External anal sphincter has mixed echogenicity and a thickness of about 5.0 mm. In females, the external anal sphincter is thinner and shorter anteriorly and, in some women, there is a gap at the mid-to-high anal canal that must be considered in order to avoid misdiagnosis. Recent studies have identified decreases in thickness of the external anal sphincter with age. Additional landmarks: with the probe inserted deep into the anal canal, the puborectalis muscle is readily visualized as a U-shaped reflective structure forming a dorsal sling around the rectum. The anococcygeal ligament appears as an hypoechoic triangle posteriorly.
Indications Endoanal ultrasound helps to differentiate between those individuals with incontinence of idiopathic origin and those with sphincter damage who may be suitable for surgical repair. Obstetric trauma: injury to the anal sphincter sustained at vaginal delivery is a common predisposing cause of faecal incontinence in women (Figure 2). Disruption of the anal sphincter complex (third or fourth degree perineal tear) is the most important risk
SURGERY 26:6
Figure 2 Obstetric trauma in a 43-year-old female. Defects of the internal and external sphincter are seen anteriorly between the 10 o’ clock and 3 o’ clock positions.
248
© 2008 Elsevier Ltd. All rights reserved.
Imaging
the appearance of the tract from hypoechoic to bright hyperechoic. This is particularly beneficial for differentiating between simple and complex tracks. Three-dimensional ultrasound compares well with MRI for the assessment of complex anal fistulae (Figure 3) but when complex fistulae go beyond the field of view of the endosonography probe MRI becomes the first-line imaging modality. Constipation and anal pain: endoanal ultrasound is not routinely used for assessment of constipation. Occasionally, it may identify internal sphincter myopathy in patients with intractable constipation or proctalgia fugax. These patients often have grossly thickened internal sphincters. Anal carcinoma: the ability of endoanal ultrasound to visualize the layered structure of the anal canal enables depth of infiltration and local staging of tumours of the anal canal to be determined. Recent research indicates that three-dimensional endosonography may improve the staging of anal cancer. ◆
Further reading Bartram CI, Frudinger A. Handbook of anal endosonography. Petersfield: Wrightson Biomedical Publishing, 1997. Fuchsjager MH, Maier AG. Imaging fecal incontinence. Eur J Radiol 2003; 47: 108–16. Konerding MA, Dzemali O, Gaumann A, Malkusch W, Eckardt VF. Correlation of endoanal sonography with cross-sectional anatomy of the anal sphincter. Gastrointest Endosc 1999; 50: 804–10. Williams AB, Bartram CI, Halligan S, et al. Multiplanar anal endosonography—normal anal canal anatomy. Colorect Dis 2001; 3: 169–74. Felt-Bersma RJ, Cazemier M. Endosonography in anorectal disease: an overview. Scand J Gastroenterol Suppl 2006; 243: 165–174.
Figure 3 Three-dimensional endoanal ultrasound showing a transsphincteric fistula in ano (arrow), involving I internal and E external sphincters. Source: Charles Maxwell-Armstrong, Queen’s Medical Centre, Nottingham, UK.
e xcellent reproducibility. Tears of the anal sphincter are common in patients presenting with rectal prolapse and faecal incontinence. Knowledge of anal sphincter injury derived from endoanal ultrasound may guide surgical management in problematic cases of disordered continence particularly following haemorrhoidectomy and fistula repair. Long-term follow-up has also highlighted sphincter defects in patients who have undergone stapled low anterior resection for rectal carcinoma.
Acknowledgement
Anal sepsis: in perianal sepsis, endosonography assists in defining the anatomy of the fistula tract and is good for identifying fistulae that lie close to the anal lumen. The use of contrast agents has significantly increased the accuracy of endosonography in the assessment of perianal fistulae. The infusion of hydrogen peroxide into the fistula tract generates small air bubbles, changing
SURGERY 26:6
The author thanks Mrs Debbie Bush (Clinical Physiologist, Division of Gastrointestinal Surgery, University Hospital, Nottingham, UK) for assistance with the preparation of the figures.
249
© 2008 Elsevier Ltd. All rights reserved.
an axial resolution of 0.3 mm and a lateral resolution of <1.0 mm. Three-dimensional ultrasound uses a double crystal design with a 6-16 MHz frequency range. Both transducers produce a 360° axial image of high spatial resolution. The appearance of the image is dependent on the tissue characteristics: highly reflective tissues give a white hyperechoic image; poorly reflective tissues are hypoechoic and appear black.
Endoanal ultrasound Jeffrey W Wright
Technique Two-dimensional ultrasound A rotating transducer encased within a water-filled, hard sonolucent, parallel-walled plastic cone is used to accurately image the anal canal. Typically, this system has a diameter of 1.7 cm and can be moved freely within the anal canal, causing minimal discomfort or image distortion. Systems in which the transducer is enclosed within a water-filled balloon may distort the anatomy of the anal canal and are more suited to rectal imaging. Care should be taken when assembling the transducer system to ensure that all air bubbles are excluded in order to maintain acoustic coupling during imaging. The cone may be covered by a well-lubricated protective sheath if desired. The probe is inserted into the anal canal with the patient in the left lateral position. (There is an argument for using the prone position because better symmetry of images may be produced.) The probe is rotated so that the 12 o’clock position is anterior. The probe is inserted high into the anal canal before slowly withdrawing in increments of 1 cm or less. In general, insertion to a depth of 5 cm is sufficient to visualize the structures of the anal canal. Freeze-frame images at each increment (showing details of the deep, mid and superficial canal) should be taken.
Abstract Endoanal ultrasound is an essential investigation in the management of faecal incontinence. To accurately image the anal canal in two dimensions, a 10-MHz rotating transducer encased within a water-filled, hard sonolucent, parallel-walled plastic cone is used. In three-dimensional ultrasound the transducer moves slowly down a rigid probe positioned within the anal canal, a predetermined number of two dimensional images are digitized and converted to a single three-dimensional image. Endoanal ultrasonography produces an image dependent on tissue characteristics: highly reflective tissues give a white hyperechoic image; poorly reflective tissues are hypoechoic and appear black. Endoanal ultrasound identifies at least four layers within the anal canal. The innermost layer is the bright hyperechoic subepithelium tissue and is surrounded by the hypoechoic internal anal sphincter. The longitudinal muscle is hyperechoic and is often difficult to distinguish from the external sphincter. The final layer is the external anal sphincter, which has mixed echogenicity. Changes in normal sphincter morphology are apparent with ageing and gender. Endoanal ultrasonography is a method for the morphological diagnosis of damage to the anal sphincter and has brought new insights into the pathogenesis of anorectal disorders. It can differentiate between those individuals with faecal incontinence of idiopathic origin and those with sphincter damage who may be suitable for surgical repair, especially following obstetric or postoperative trauma. Three-dimensional ultrasound is particularly useful for defining the anatomy of the fistula tract in anal sepsis and for local staging of anal canal tumours.
Three-dimensional ultrasound A rigid probe containing the ultrasound transducer is positioned within the anal canal. The transducer moves slowly down the probe, capturing images at a predetermined rate and eliminating image changes due to movement artefact or anatomical repositioning. The two-dimensional ultrasound images are digitized and converted into a single three-dimensional image. Software allows instantaneous reconstruction and correction for geometric errors in the transducer. Images can be simultaneously viewed in the sagittal, transverse and coronal planes and rotated in order to view the anal canal from different angles.
Keywords anal canal; anus; diagnostic imaging; endosonography; faecal incontinence; imaging; perianal fistula; three dimensional; ultrasonography
Endoanal ultrasound is an essential investigation in the management of faecal incontinence in the UK.
Normal anatomy Endoanal ultrasound should reveal at least four layers within the anal canal (Figure 1).
Basic concepts
Subepithelium is the bright, hyperechoic innermost layer and is about 2.0 mm thick. Increases in thickness are unrelated to age, but can occur with excessive straining at stool and in those with solitary rectal ulcer syndrome.
Endoanal ultrasonography produces an image of the structures surrounding the anal canal by generating a ‘map’ of sound reflections from the different tissue surfaces. Conventional twodimensional ultrasound is generated by a mechanically rotated 10 MHz piezoelectric crystal with near-field focusing of 5–45 mm,
Internal anal sphincter is hypoechoic and surrounds the subepithelium. Usually, the internal anal sphincter is assessed for continuity, thickness and symmetry. Any break in the continuity of the internal anal sphincter is considered abnormal. In general, the thickness of the internal anal sphincter is about 2.2 mm, but
Jeffrey W Wright PhD is Chief Experimental Officer at the Division of Gastrointestinal Surgery, University Hospital, Nottingham, UK. Conflicts of interest: none declared.
SURGERY 26:6
247
© 2008 Elsevier Ltd. All rights reserved.
Imaging
External anal sphincter
Subepithelium
Longitudinal muscle
Internal anal sphincter
Transducer and water-filled cone assembly
Axial image of the anal canal. The outer edge of the plastic transducer cone is seen. The subepithelium is hyperechoic and is surrounded by the dark hypoechoic internal anal sphincter, which has a thickness of 2.2 mm (indicated by x-x on the scan). The longitudinal muscle is moderately hyperechoic and is bordered by the mixed echogenicity of the external anal sphincter. Figure 1
increases with age; values of >4.0 mm are abnormal. Thickness is unrelated to gender or sphincter length. Uniformity of echogenicity reduces with increasing age and may relate to sclerosis or degeneration of the muscle. The sphincter should be assessed in all quadrants for asymmetry.
factor. Prospective studies indicate that 30% of primiparous vaginal births result in some form of sphincter damage which may or may not be associated with postpartum faecal incontinence. Two-thirds of obstetric sphincter disruptions are asymptomatic in the post-partum period, but there is evidence that these individuals may be at increased risk for faecal incontinence with ageing compared with those without sphincter injury. Subsequent deliveries increase the risk of incontinence, particularly among women with a sphincter defect diagnosed after the first delivery.
Longitudinal muscle is the layer which is often difficult to distinguish from the external sphincter. The longitudinal muscle is hyperechoic (but less so than the external sphincter) with a thickness of 2.5 mm. There is no clear evidence for changes in thickness to occur with age or gender.
Postoperative trauma: anal endosonography is a valuable tool for characterizing structural damage to the sphincter, particularly when planning restorative surgery or for assessment after sphincter repair or direct anal trauma; it has high accuracy and
External anal sphincter has mixed echogenicity and a thickness of about 5.0 mm. In females, the external anal sphincter is thinner and shorter anteriorly and, in some women, there is a gap at the mid-to-high anal canal that must be considered in order to avoid misdiagnosis. Recent studies have identified decreases in thickness of the external anal sphincter with age. Additional landmarks: with the probe inserted deep into the anal canal, the puborectalis muscle is readily visualized as a U-shaped reflective structure forming a dorsal sling around the rectum. The anococcygeal ligament appears as an hypoechoic triangle posteriorly.
Indications Endoanal ultrasound helps to differentiate between those individuals with incontinence of idiopathic origin and those with sphincter damage who may be suitable for surgical repair. Obstetric trauma: injury to the anal sphincter sustained at vaginal delivery is a common predisposing cause of faecal incontinence in women (Figure 2). Disruption of the anal sphincter complex (third or fourth degree perineal tear) is the most important risk
SURGERY 26:6
Figure 2 Obstetric trauma in a 43-year-old female. Defects of the internal and external sphincter are seen anteriorly between the 10 o’ clock and 3 o’ clock positions.
248
© 2008 Elsevier Ltd. All rights reserved.
Imaging
the appearance of the tract from hypoechoic to bright hyperechoic. This is particularly beneficial for differentiating between simple and complex tracks. Three-dimensional ultrasound compares well with MRI for the assessment of complex anal fistulae (Figure 3) but when complex fistulae go beyond the field of view of the endosonography probe MRI becomes the first-line imaging modality. Constipation and anal pain: endoanal ultrasound is not routinely used for assessment of constipation. Occasionally, it may identify internal sphincter myopathy in patients with intractable constipation or proctalgia fugax. These patients often have grossly thickened internal sphincters. Anal carcinoma: the ability of endoanal ultrasound to visualize the layered structure of the anal canal enables depth of infiltration and local staging of tumours of the anal canal to be determined. Recent research indicates that three-dimensional endosonography may improve the staging of anal cancer. ◆
Further reading Bartram CI, Frudinger A. Handbook of anal endosonography. Petersfield: Wrightson Biomedical Publishing, 1997. Fuchsjager MH, Maier AG. Imaging fecal incontinence. Eur J Radiol 2003; 47: 108–16. Konerding MA, Dzemali O, Gaumann A, Malkusch W, Eckardt VF. Correlation of endoanal sonography with cross-sectional anatomy of the anal sphincter. Gastrointest Endosc 1999; 50: 804–10. Williams AB, Bartram CI, Halligan S, et al. Multiplanar anal endosonography—normal anal canal anatomy. Colorect Dis 2001; 3: 169–74. Felt-Bersma RJ, Cazemier M. Endosonography in anorectal disease: an overview. Scand J Gastroenterol Suppl 2006; 243: 165–174.
Figure 3 Three-dimensional endoanal ultrasound showing a transsphincteric fistula in ano (arrow), involving I internal and E external sphincters. Source: Charles Maxwell-Armstrong, Queen’s Medical Centre, Nottingham, UK.
e xcellent reproducibility. Tears of the anal sphincter are common in patients presenting with rectal prolapse and faecal incontinence. Knowledge of anal sphincter injury derived from endoanal ultrasound may guide surgical management in problematic cases of disordered continence particularly following haemorrhoidectomy and fistula repair. Long-term follow-up has also highlighted sphincter defects in patients who have undergone stapled low anterior resection for rectal carcinoma.
Acknowledgement
Anal sepsis: in perianal sepsis, endosonography assists in defining the anatomy of the fistula tract and is good for identifying fistulae that lie close to the anal lumen. The use of contrast agents has significantly increased the accuracy of endosonography in the assessment of perianal fistulae. The infusion of hydrogen peroxide into the fistula tract generates small air bubbles, changing
SURGERY 26:6
The author thanks Mrs Debbie Bush (Clinical Physiologist, Division of Gastrointestinal Surgery, University Hospital, Nottingham, UK) for assistance with the preparation of the figures.
249
© 2008 Elsevier Ltd. All rights reserved.