AGA Abstracts of the rectal mucosa. Endoscopic mucosal resection (EMR) of rectal tissue could improve the diagnostic yield for HD when compared with traditional RSB because of the larger and deeper samples acquired for analysis. We sought to compare the performance characteristics of the two techniques and assess the relationship between the pathology results and the clinical presentations. Methods: In this prospective, single-center study, patients referred for RSB were offered enrollment in the trial and concurrent EMR. Specimens were analyzed pathologically for size, submucosal ganglionic tissue, and acetylcholinesterase or calretinin staining. Biopsy results were compared with transit studies, anorectal manometry, and constipation severity through validated questionnaires [Rome III criteria for functional disor- ders, Irritable Bowel Syndrome Quality of Life (IBS-QOL), Gastrointestinal Symptom Rating Scale (GSRS), and Bristol Stool Scale (BSS)]. Results: Seventeen patients (2 male, 15 female, mean age 35.8 years, range 22-61 years) were enrolled in the study from 2008-2014. By Rome III criteria, 10 subjects had IBS-C, 6 subjects had Functional Constipation, and all had at least moderate symptom severity by IBS-QOL and GSRS. All subjects underwent anorectal manometry (88% with anorectal dysfunction, 68% with outlet obstruction) and transit studies (41% with delayed transit). There were no reports of bleeding, perforations, or complications from the RSB and EMR procedures. The RSB sample volumes were signifi- cantly lower than EMR samples (0.05 cm 3 vs. 0.2 cm 3 , p=0.006). There was sufficient tissue for submucosal visualization by RSB in 53% (9/17) of cases compared to 100% (17/17) with EMR. No cases of aganglionosis were diagnosed by RSB; one patient had rare ganglions observed by EMR. In samples where sufficient tissue was obtained, Acetylcholinesterase staining was sufficient for interpretation in 75% of cases (3/4), Calretinin staining was sufficient for interpretation in 100% (13/13). There was no correlation between symptom severity, presence of ganglion cells on histology, acetylcholesterase or calretinin staining, or manometry findings. Conclusion: EMR providers greater tissue volumes and can improve the characterization of ganglion cells in rectal tissue compared to RSB in patients with moderate to severe constipation with suspected HD. Further studies involving larger patient populations are warranted to assess the clinical correlations with the additional histopatho- logic findings. Sa1635 HIGH FREQUENCY ULTRASOUND IMAGING OF THE EXTERNAL ANAL SPHINCTER TO IDENTIFY MICROSTRUCTURAL ANATOMY IN NORMAL SUBJECTS AND PATIENTS WITH FECAL INCONTINENCE Melissa M. Ledgerwood-Lee, David Kunkel, Ravinder K. Mittal Introduction: The current "gold standard" to identify anal sphincter anatomy and diagnose disruption utilizes 2D and 3D ultrasound (US) imaging with relatively low frequency (3-9 MHz) transducers. These US images provide general outline of the sphincter anatomy and disruptions. The high frequency (7-15MHz) US transducer has been used to study the microstructural details (muscle fascicles) of the muscle. Aims: The goal of our studies was to visualize microstructural anatomy of the anal sphincters, specifically the external anal sphincter (EAS) in healthy normals and patients with fecal incontinence (FI) using high frequency US. Methods: Studies were conducted in normal subjects and patients with symptoms of fecal incontinence. The 3D low frequency US images (3-9 MHz, endovaginal transducer) and high frequency US images (7-15MHz, hockey stick transducer) were obtained in 13 normals (8F, 5M) and 5 patients with FI who revealed disruption of the EAS between 11-2 o'clock position (perineal body, PB) in the low frequency US imaging. The high frequency probe is 2.5cm long and 2cm wide, it was placed intra-anally to image the anal canal at the 12 o'clock (PB), 9 o'clock (right), 3 o'clock (left) and 6 o'clock (posterior) positions. The US images of the PB was also recorded during voluntary contraction (squeeze) with high frequency US probe. US images were analyzed using the NIH ImageJ software and following measurements of the PB were made 1) frequency spectrum of echo-intensity and 2) mean grayscale intensity value. Results: high frequency US imaging reveals microstructural details (muscle fascicles and connective tissue) inside the EAS, which is not seen in the low frequency images, muscle fascicles are seen inside the PB of different echo-intensities (Figure 1). In FI patients, there was loss of muscle fascicles and alteration in the echo-intensity patterns in the region of damaged muscle. Grayscale intensity values are lower in patients compared to normal subjects, which most indicates a loss of muscle and increase in fibrosis. With voluntary contraction, the cranial end of the PB moves in ventral direction with change in the angle of PB with the US transducer, the above motion was reduced in the FI patients suggestive of disruption of muscle fibers. Conclusion: High frequency ultrasound imaging is a powerful method to visualize microstructural details of the anal sphincter muscles. Our studies show that damage to the EAS muscle results in alteration of muscle myoarchitecture and may indicate loss of muscle fascicles and increase in fibrous tissue inside the EAS. Figure 1 shows 3D low resolution US images and high resolution US images in a normal female (a & b) and a patient (d & e). Yellow arrows indicate the region of the perineal body in cross section. In the normal subject the perineal body is an oval shaped structure with muscle fascicles seen as a heterogeneous pattern of different echo-intensities in the high frequency US image (b). Note, in the patient image the PB is more homogeneous with S-314 AGA Abstracts a lower echo-intensity indicating a loss of muscle fascicles. Also note the difference in the frequency spectrum of the echo-intensity in the PB between the normal (c) and patient (f). Figure 2: Dynamic motion of PB in a normal (a & b) and patient (c & d). Note, with contraction the cranial end of PB (to the left) moves in the ventral direction. This motion is reduced in FI patients, a marker of damage to the EAS muscle. Sa1636 IMPAIRED RECTAL EVACUATION AND RECTAL STRUCTURAL ABNORMALITIES ARE COMMON IN CONSTIPATED PATIENTS WITH A NORMAL BALLOON EXPULSION TEST Tae Hee Lee, Su Jin Hong, Adil E. Bharucha, Joon Seong Lee Background & Aims: The balloon expulsion test (BET) is a useful screening test for defecatory disorders. However, a normal BET does not always exclude dyssynergic defecation (DD). There is limited information on the utility of other modalities (eg, defecography and anal surface EMG activity) for diagnosing DD in patients with a normal BET. Hence, our aims were to evaluate the utility of BET, anal EMG, and defecography for diagnosing DD in a consecutive series of patients in clinical practice. Methods: We reviewed the results of 253 consecutive constipated patients (95 males; mean age, 60.9 years) who had an anal EMG, BET and barium defecography. The EMG was performed with a non-invasive anal plug electrode with the plates in the long axis of the anal canal (Perry, Elan, SRS Medical System, Redmond, WA, USA). During attempted defecation, the failure to decrease anal EMG activity by at least 25% was considered abnormal. The BET was performed with a balloon filled with 50 mL of water; normal values are ≤ 60 seconds. During barium defecogra- phy, anorectal motion, rectal evacuation, and structural abnormalities (e.g., rectocele, rectal intussusception) were evaluated by two experienced gastroenterologists who were blinded to the results of the other tests. Results: Of 253 patients, 241 (95%) had impaired anal relaxation, 150 (59%) had a normal BET, 81 (32%) had a rectocele, 49 (19%) had rectal intussusception and 56 (22%) had descending perineum syndrome. Compared to patients with a normal BET, patients with an abnormal BET evacuated less barium within 30 seconds (40.2% ± 2.8% [Mean ± SEM]) vs. 9.2 ± 2.1%, p< 0.001) and 60 seconds (48.2 ± 2.9% vs. 13.2 ± 2.4%, p< 0.001), respectively. Compared with patients who had an abnormal BET, patients with a normal BET had greater rectoceles (1.7cm ± 0.1cm [Mean ± SEM] vs. 0.7cm ± 0.1cm, p< 0.001) and greater perineal descent (2.5 cm± 0.1cm vs. 1.5 cm ± 0.1cm, p< 0.001), respectively. The proportion of impaired rectal evacuation was greater in patients with an abnormal than a normal BET. However, rectocele, rectal intussusception, rectal prolapse, rectocele and descending perineum syndrome were observed more frequently in patients with a normal BET (Table) Conclusions: Impaired rectal evacuation and rectal structural abnormalities are common in constipated patients with a normal rectal BET. These findings highlight the need to conduct additional tests, even when the BET is normal, particularly when the clinical index of suspicion for a DD is high. Anal EMG disclosed impaired anal relaxation in 95% of patients, highlighting the need to assess normal values for this test.