When this condition first became apparent to British doctors in 1915 it was thought that the cause was concussive force from being too near the explosions of artillery. Soon, however, this didn’t seem to be a comprehensive explanation, and in 1916 two conditions were described: Shell Shock (Wounded) and Shell Shock (Sick).

With data available from IED attacks in the middle eastern wars, doctors have now concluded that concussive force plays a significant role in all forms of this condition. You can read about this here, from the National Geographic, or here from the British popular press.

James (“Jim”) Patton BS BA MPA is a retired state official living in Shawnee, Kansas and a frequent contributor to several WW1 e-publications, including "Roads to the Great War," "St. Mihiel Tripwire," "Over the Top" and "Medicine in the First World War." He has spent many hours walking the WW1 battlefields, and is also an authority on British regiments and a collector of their badges. An Army Engineer during the Vietnam War, he does work for the US World War 1 Centennial Commission and has memberships in the WW1 Historical Association, the Western Front Association, the Indian Military Historical Society and the Salonika Campaign Society.