Ahmad, K. K., A. Shaukat, M. Ali, Y. Amin, T. Qadeer, and S. Ahmad. “Is Secondary Achalasia a Reality? Primary Versus Secondary Achalasia: Role of Barium Swallow”. Annals of King Edward Medical University, vol. 9, no. 4, July 2016, doi:10.21649/akemu.v9i4.1375.