Trisomy 21 Etiology: congenitalComplications:— Moya Moya syndrome— O-C1, C1-C2 instability— Peripheral lung cysts— AV canal, ASD, VSD— Duodenal atresia— Annular pancreas— Hirschprung disease— Trident hands Radiology Cases of Atlanto Axial Instability in Trisomy 21 Lateral radiographs of the cervical spine show a mildly enlarged atlanto dens interval in neutral position (left), that increases significantly on flexion (center) and reduces on extension (right). Radiology Cases of Atlanto Occipital Instability in Trisomy 21 Lateral radiograph of the cervical spine in flexion (left) shows a normal atlanto dens interval and normal relationship of the occipital condyles to the lateral masses of C1 while on extension (right) there is posterior translation of the occipital condyles in relation to the lateral masses of C1. Radiology Cases of Duodenal Stenosis in Trisomy 21 AXR (above) shows the tip of the nasogastric tube within a very dilated duodenal bulb. There is extensive distal bowel gas. Spot image from an upper GI (below) shows a fixed area of narrowing between the first and second portions of the duodenum through which contrast passed slowly.