Anal Ultrasound

Rectoanal ultrasonography represents a significant advancement in the diagnostic approach and investigation of lesion extent within the anorectal canal. This examination is essentially the most important paraclinical investigation, which after a long period, complements many gaps left by clinical examination of the anal canal.

In the ultrasonographic investigation of anorectal diseases, 7 and 10 MHz transducers are used, offering very good resolution but limited focal depth (2-5 cm). In the examination of rectal diseases, the transducer is placed inside an air chamber filled with water, which excellently transmits ultrasound waves and ensures good acoustic contact within the wide lumen. The transducer rotates 360°, providing cross-sectional images of the area. For the examination of anal diseases, the air chamber around the transducer is replaced by a hard plastic cone with a diameter of 1.7 cm filled with water. This facilitates insertion into the anal canal and ensures full acoustic contact with its walls. The dimensions and shape of the transducer allow comfortable examination without distorting the anal walls.

During the ultrasonographic investigation of the rectum and lower sigmoid colon, the anatomical layers of the large intestine are visualized:

– Hypoechoic mucosa, in contact with the air chamber wall
– Hyperechoic submucosa
– Hypoechoic muscularis propria
– Hyperechoic perirectal tissues.

Endorectal Ultrasonography

Rectal neoplasms. Their staging can influence surgical technique, particularly in cases of minimal invasion or for high-risk surgical patients where local excision may be preferred. The method has very good sensitivity and specificity for determining local invasion. However, its sensitivity in detecting involved lymph nodes is significantly reduced.

Other indications include:

– Pelvic abscesses or upward extensions of perianal abscesses
– Less commonly, extramural tumors and retrorectal cysts.

Anal Ultrasonography

The anatomical structures visualized, from inner to outer, include:

1. Hyperechoic subepithelial layer, in contact with the acoustic cone
2. Hypoechoic internal sphincter. Thickness varies by sex (1.5-4 mm men, 2-4 mm women) and age (2.4-2.7 mm under 55; 2.7-3.4 mm over 55)
3. Hyperechoic longitudinal muscle layer, continuous with the external sphincter
4. Mixed echogenicity external sphincter.

The examination is performed at three levels:

– Deep level: puborectalis sling and deep part of external sphincter
– Mid-level: anococcygeal ligament, superficial external sphincter, internal sphincter, perineal body, and in females, the vagina
– Superficial level: superficial external sphincter.

Indications for Anal Ultrasonography

Perianal fistulas with recurrences or multiple branches and small abscess cavities:

• Appear as hypoechoic masses with possible small hyperechoic foci (pus). The internal opening is detected where the fistulous tract contacts the internal sphincter or subepithelial tissues.
• Identification of the internal opening and branches is essential for successful surgery. Recurrences often result from failure to locate this opening or clean out branches.
• Both sphincters and their relation to abscess cavities are imaged.
• Instilling a small amount of hydrogen peroxide through the external opening can facilitate fistula visualization.
• It is also useful postoperatively to monitor healing and detect residual infection.
• Comparison with MRI: MRI, though excellent for general pelvic assessment, cannot accurately determine fistula height or internal opening location, nor detect very small fistulas as effectively as EAUS.

Fecal Incontinence:

• Common in elderly, especially women, due to sphincter injury or progressive pudendal nerve degeneration.
• Traumatic causes include childbirth, excessive dilation, surgeries for fistula, fissure, hemorrhoids, or direct injury.
• Appears as localized sphincter defects. In neuropathic causes, no such defect is seen.
• Differentiation is critical for surgical planning. Sphincter defects are treated with overlapping repair, while pudendal neuropathy responds less to surgery and may require levatorplasty.
• Biofeedback therapy has shown satisfactory results in some studies.

Unexplained Perineal Pain:

• May be caused by coccygodynia, proctalgia fugax, or levator ani syndrome.
• EAUS may reveal internal sphincter hypertrophy in proctalgia fugax.
• Undiagnosed pain may also result from occult fistulas or small abscesses.

Unexplained Constipation:

• Sometimes associated with internal sphincter hypertrophy.

Assessment prior to ileoanal anastomosis:

• Also used to diagnose postoperative septic complications.

Congenital anomalies:

• E.g., anal atresia. Postoperatively, EAUS can precisely assess external sphincter position relative to anal lumen.

Diagnosis and staging of anal neoplasms:

• Allows precise measurement of lesion dimensions and depth of invasion into the external sphincter.

Reoperations:

• Up to 32% of operated patients show asymptomatic sphincter lesions postoperatively, rising to 40-60% after anal dilatation.
• Additional surgical trauma can significantly affect continence, especially after multiple surgeries.

Alternative Approaches

Vaginal or perineal approaches are used less often. Transvaginal scanning is useful in obstetric sphincter injuries or anterior perineal abscesses.

Comparison of Endoanal Ultrasound (EAUS) and MRI

Both are powerful diagnostic tools for anorectal disorders, each with advantages:

EAUS Strengths:

1. Detailed imaging of superficial structures: sphincters, fistulas, abscesses.
2. Quick and accessible: performed in-office with immediate results.
3. Superior for fistula mapping: accurately determines fistula height and internal opening, detects very small tracts.
4. Excellent for tumor staging: precise assessment of depth of invasion.

MRI Strengths:

1. Multidimensional imaging: 3D views of rectum, pelvic floor, perirectal fat, lymph nodes.
2. No radiation: safe for repeated studies.
3. Ideal for complex or extensive disease: Crohn’s disease, malignancies with deep tissue involvement.
4. Better for lymph node evaluation.

MRI Limitations compared to EAUS:

• Less accurate for fistula height and internal opening localization.
• Inferior resolution for superficial structures.

Conclusion

Choice between EAUS and MRI depends on clinical needs. EAUS excels for superficial structural assessment (fistulas, sphincters, small abscesses), while MRI offers comprehensive evaluation of deeper tissues and complex conditions. For fistula surgery planning, EAUS remains superior due to its fine anatomical detail.

References

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