Peter Stewart1
(1)
Department of Colorectal Surgery, Concord Repatriation General Hospital, Sydney, Australia
Peter Stewart
Email: pstewart@bigpond.com.au
Anal ultrasound has been performed since 1987. It uses a modified technique developed for trans-rectal ultrasound.
Indications
1.
2.
3.
4.
Equipment
Endoanal ultrasound for evaluation of anorectal disease is often performed using an endocavity transducer with an element that rotates 360° radially allowing circumferential assessment of the sphincteric complex (Fig. 8.1). Older machines were only two dimensional but newer models have 3-D capability (Figs. 8.2 and 8.3) as well as colour Doppler and are useful for mapping anal sphincter defects, fistula tracks and extent of neoplastic spread in cases of anorectal cancer.
Fig. 8.1
Endocavity 2D transducer (BK 1850) with 360° rotating element (BK6005) in Hardcone for endoanal ultrasound
Fig. 8.2
3D endoanal transducer (BK 2052)
Fig. 8.3
BK Pro Focus 2202 ultrasound system
Technique of Endoanal and Endorectal Ultrasound
No oral bowel preparation is required. For endorectal ultrasound, a Fleet or similar enema is given just prior to the examination. No sedation or anaesthetic is needed.
Patients are positioned in the left lateral position with the buttocks close to or overhanging the edge of the examination bed. For endoanal ultrasound, the lubricated transducer is inserted a distance of about 8 cm. In the male, the distal prostate should be visualised; in the female, the cervix. Images are then obtained at the level of the puborectalis muscle, upper, mid- and lower anal canal (Figs. 8.4, 8.5, 8.6, and 8.7).
Fig. 8.4
Endoanal ultrasound- the sphincter is imaged at three levels (upper, mid and lower levels Figs. 8.5, 8.6 and 8.7) ES external sphincter, IS internal sphincter, PF pelvic floor/puborectalis, PN pudendal nerve
Fig. 8.5
Endoanal ultrasound image at level of upper anal canal T transducer, M mucosa, IS internal sphincter, ES external sphincter, Ant anterior, Post posterior
Fig. 8.6
Endoanal ultrasound image at level of mid anal canal
Fig. 8.7
Endoanal ultrasound image at level of distal anal canal
For endorectal ultrasound, the transducer is introduced to just proximal to the pelvic floor then the balloon is partly inflated with degassed water. The transducer can then be slowly manoeuvred proximally to the desired level. Alternatively the transducer can be introduced through a rigid sigmoidoscope.
Anatomy (Fig. 8.4)
The anal canal is usually examined at four levels (Figs. 8.5, 8.6, and 8.7):
· Levator ani
· Upper anal canal
· Mid anal canal
· Distal (lower) anal canal.
Indications for Endoanal Ultrasound
Evaluation of Sphincter Defects
Endo anal ultrasound is invaluable in evaluation of sphincter defects in patients with fecal incontinence. Sphincter defects may involve the internal sphincter, external sphincter or a combination. Most are obstetric related but may be traumatic (usually post surgical) or sepsis related. Defects may be discrete or multiple. Figure 8.8 illustrates an anterior external sphincter defect following obstetric injury and third degree tear. There is discrete separation of external sphincter fibres anteriorly. Severe pelvic floor injury during delivery can result in fourth degree tears and development of ano-vaginal fistula (Fig. 8.9). Endoanal ultrasound can be helpful following surgical treatment (Figs. 8.10, 8.11, and 8.12) and in assessment of rectal prolapse (Fig. 8.13).
Fig. 8.8
Anterior external sphincter defect. Discrete separation of external sphincter fibres anteriorly (arrows) following obstetric injury and third degree tear
Fig. 8.9
Combined anterior internal and external sphincter defects with ano-vaginal fistula. Obstetric injury with fourth degree tear. Arrow shows the ‘ring-down’ artifact indicating air in the fistula tract
Fig. 8.10
Lateral internal sphincterotomy. The distal fibres of the internal sphincter have been surgically divided to treat fissure in ano (arrow)
Fig. 8.11
Combined right posterior internal and external sphincter defect (arrow) post fistulotomy
Fig. 8.12
Appearance after overlapping sphincter repair
Fig. 8.13
Thickened rectal mucosa (arrow) consistent with full thickness rectal prolapse (about 8mm in this patient. Normally 2–3mm)
Evaluation of Perianal Pain and Sepsis
Ultrasound can be used to evaluate obscure anal pain and identify pathologies such as occult intersphincteric abscess (See Cases 1 and 2). In patients with fistula-in-ano, ultrasound may detect the internal opening of a fistula (Fig. 8.14) and confirm the trans-sphincteric nature of the fistula track (Fig. 8.15). An appreciation of the amount of sphincter contained by the fistula can be determined. The use of 3D ultrasound allows a measurement of the proportion of involved muscle (See Chap. 9). Spread of sepsis through the perianal tissues can be demonstrated on ultrasound, allowing planning for adequate drainage and treatment. Figure 8.16 demonstrates a posterior abscess cavity and Fig. 8.17 shows a horseshoe abscess.
Fig. 8.14
Anterior trans-sphincteric fistula (arrow)
Fig. 8.15
Left postero-lateral trans-sphincteric fistula
Fig. 8.16
Deep posterior abscess cavity
Fig. 8.17
Horseshoe abscess
Case 1
Male age 42 presented with over 12 months of dull anal pain. The pain was present constantly, a little worse on opening his bowels. There was no PR bleeding nor change in bowel habit. He was otherwise well.
Physical examination showed no obvious perianal pathology. No fissure or fistula identified. There was slight tenderness posteriorly on digital examination of the anal canal.
Ultrasound revealed a small posterior intersphincteric abscess (Fig. 8.18). The abscess was drained via a small posterior intersphincteric incision. A 1 cm diameter ‘mass’ of granulation tissue was removed. The patient had an uneventful recovery with remission of symptoms.
Fig. 8.18
Small posterior intersphincteric abscess (arrow)
These abscesses are thought to develop due to infection of a small anal crypt gland which passes into the intersphincteric plane where it becomes trapped.
Case 2
A 56 year old male presented with 18 months of chronic anal pain. The pain was constant, dull, worse at night and slightly worse with opening bowels. No bleeding or change in bowel habit. Physical examination revealed a tender mass posteriorly in the distal rectum. There was no anal tenderness.
Ultrasound revealed a 30 × 20 mm hypoechoic mass posterior to the rectum. An opening into the upper anal canal can be seen in this view (Fig. 8.19a, b).
Fig. 8.19
(a) Large chronic retrorectal abscess with opening into upper anal canal (arrow). (b) Same patient demonstrating large retrorectal component of the hypoechoic abscess cavity
This is an intersphincteric abscess which has tracked cranially above the sphincters to lie behind the rectum. It is usually drained by passing a probe through the internal opening directed cranially and then laying open the posterior rectal wall. The patient made an uneventful recovery.
Local Spread of Ano-rectal Tumours
Ano rectal ultrasound allows pre-operative T and N staging of ano-rectal malignancy. This assists with pre-operative decision making (local excision vs radical resection; pre-operative radiotherapy; involvement of adjacent tissues)
Case 3
Seventy-one year old male who had ultra-low anterior resection 2 years prior for Stage II adenocarcinoma of distal rectum. Presented with asymptomatic recurrence found at routine follow-up colonoscopy. The patient was unable to have a staging MRI scan due to cardiac pacemaker in situ.
Transrectal Ultrasound with an endocavity sector end-fire transducer shows absence of plane between tumour and prostate (Fig. 8.20). There was no metastatic disease. This patient underwent pelvic exenteration (cystoprostatectomy and abdomino-perineal excision of rectum) with curative intent.
Fig. 8.20
(a) Recurrent adenocarcinoma of distal rectum (T) adjacent to prostate (P). (b) Endoanal ultrasound with end-fire transducer showing absence of clear plane (arrow) between tumour and prostate
Case 4
Male age 71. Previous high anterior resection 18 months earlier for Stage I adenocarcinoma of proximal rectum. Routine follow-up CT showed lesion in posterior mesorectum 10 cm from anal verge. The lesion was glucose avid on PET scan. The patient underwent transrectal ultrasound guided core biopsy (Fig. 8.21a, b) which showed metastatic prostate cancer and was commenced on androgen deprivation therapy.
Fig. 8.21
(a) Sagittal image of mass in posterior mesorectum (T) obtained using end-fire transducer. (b) Transrectal ultrasound guided core biopsy (arrow) showed metastatic prostatic cancer