Intra-anal intussusception: diagnosis by posteroanterior stress proctography.

Intra-anal intussusception was diagnosed in eight of 39 patients on evacuation proctography. Posteroanterior views revealed prolapse of the infolded rectum into the anal canal on staining in seven of eight patients, associated with splaying open of the anal canal and sudden distal movement of the fold during prolapse. Similar changes were seen in four of 31 patients in whom intussusception had not been diagnosed on lateral evacuation proctography. The pattern of the collapsed rectum was assessed for fold length, thickness, and angulation in relation to the midline of the rectum. Infoldings that prolapsed were closer to the anorectal junction on stress (mean 14.6: 42.4 mm, p < 0.0001) showed greater change in height between rest and strain (28.8: 14.6 mm, p < 0.05) and became more acutely angled during straining (41.9: 5.3 degrees, p < 0.01). Intra-anal intussusception may be missed in 33% (four of 12 patients) on routine evacuation proctography. Posteroanterior stress proctography is a simple supplementary examination to validate intussusception.