From the Guidelines
The external anal sphincter (EAS) can be identified using high-resolution images achievable with external phased-array coils on MRI, as endoanal MRI is not routinely performed due to its invasive nature and potential to decrease patient acceptance and compliance 1. When identifying the EAS, it is essential to consider its anatomical location and structure. The EAS is a circular band of striated muscle surrounding the anal canal, located superficial to the internal anal sphincter.
- Key characteristics of the EAS include:
- Appearance as a darker, reddish-brown muscle with visible striations
- Location approximately 1-2 cm from the anal verge
- Firmer texture than the internal sphincter
- Voluntary contraction can be assessed by asking the patient to contract their anus as if holding back a bowel movement
- The EAS consists of three parts: subcutaneous, superficial, and deep portions, with the deep portion blending with the puborectalis muscle.
- Innervation of the EAS is provided by the pudendal nerve (S2-S4), allowing for voluntary control, unlike the internal sphincter which is involuntary. Understanding the structure and function of the EAS is crucial for diagnosing and treating conditions like fecal incontinence, anal fissures, and performing procedures such as anorectal manometry.
- The use of MRI with external phased-array coils can help visualize the EAS and surrounding structures, providing valuable information for clinical decision-making 1.
From the Research
Identification of the External Anal Sphincter (EAS)
The external anal sphincter (EAS) can be identified through various methods, including:
- Digital rectal examination (DRE) 2, 3
- Endoanal ultrasonography (EAUS) 2, 4, 5
- Endoanal magnetic resonance imaging (EAMRI) 4, 5
- External phased-array MRI 6
- MR fluoroscopy 4
Diagnostic Accuracy
The diagnostic accuracy of these methods varies:
- DRE has good sensitivity but poor specificity in discerning small from severe global anal sphincter defects and fair sensitivity and poor specificity in grading external anal sphincter defects 3
- EAUS and EAMRI are comparable in the diagnosis of EAS defects, with EAUS being superior in depicting defects <90 degrees 5
- External phased-array MRI is comparable to endoanal MRI in depicting external anal sphincter atrophy 6
Imaging Techniques
Imaging techniques can provide valuable information about the EAS:
- Endoanal ultrasonography can identify anal sphincter anatomy and defects 2, 4, 5
- MRI can visualize pelvic floor motion during simulated defecation and identify external anal sphincter atrophy 4, 6
- MR fluoroscopy can record pelvic floor contraction and relaxation during squeeze and simulated defecation 4