Anderson, J. R., Earnshaw, P. M., & Fraser, G. M. (1982). Extrinsic compression of the third part of the duodenum. Clinical Radiology, 33(1), 75-81. doi:10.1016/S0009-9260(82)80358-9 

            Notes: Study of 12 patients with radiological findings of SMAS. Prevalence of SMAS among patients referred for upper GI imaging (about 6000 total) was .2%. Includes clinical features, associated conditions, treatment and progress. Link: http://www.clinicalradiologyonline.net/article/S0009-9260(82)80358-9/abstract