Imaging of the Internal Anal Sphincter: Study of Healthy Subject: Review Article

Introduction: The internal anal sphincter is a smooth muscle that works with other muscles to control defecation. The identification of morphological changes, defects or the precise definition of the level of tumor infiltration of muscle have significant importance in clinical practice. For these reasons the evaluation of shape and volume of muscle in healthy subjects has been studied for many years. The main used imaging techniques are the anal endosonografy, the endoanal coil magnetic resonance imaging and the phase-array coil magnetic resonance. The small size of muscle, the high irregularity in shape, the variability associated with factors such as age and sex, the use of different imaging techniques, including non-invasive ones, and the lack of a standardized method of measurement, can make difficult the correct comparison of the results. In this chapter we will discuss the results reported in the literature concerning the evaluation of muscle in healthy subjects and the advantages and disadvantages of different methods adopted. Normal IAS Variations: Age-related variations:the thickening of the muscle associated to aging is particularly noticeable when the measurements is performed with the EAU and MR with endoanal coil. Sex-related variations: The characterization of the shape of the muscle in relation to sex appears to be controversial with all methods. The only common result is the greater length of the anterior quadrant in males when measured with the EAU. Variations related to other factors: In a study of nulliparous women with the EAU, it is reported a positive correlation between the thickness, measured in the mid anal canal, and BMI. By contrast, in a similar study carried out with MRI without the introduction of endoanal coil the average thickness of the muscle does not appear to be correlated with BMI. The thickness of the IAS in relation to height appears to increase with both methods of MR. Conclusion: The MRI allows a better visualization of the entirety of the pelvic perineal floor compared to EAU, which is extremely effective in imaging the IAS. Even the lower cost and facility and speed of EAU are the basis of most of the research with anal endosonografy. Normal range have been specifically formulated in only a few works for thickness and length. We believe that new studies and a process of standardization of methods of measurement could provide significant advances in the study of muscle either in a state of normality or pathology.