Source

When the sick wards began to fill up with wounded and sick people at the end of August [1914], many of them had nerve damage and brain injuries, but hardly any were mentally ill. Only when the great artillery battles began in the Champagne region in December 1914, when the artillery superiority of our Western opponents swelled to a devastating barrage, did the hospital trains bring us a larger number of physically unwounded officers and soldiers suffering from nervous disorders. From then on, the number grew at an ever more rapid pace. In the beginning, we improvised by placing the mental patients among other wounded and sick people, but soon it became clear that this method was inappropriate; special hospitals for mental patients had to be established. Their number has been increasing ever since; no sooner is a mental hospital opened than it is fully occupied and further beds must be provided, and now we have reached the point where the mental patients represent by far the major category of all the sick in our army in terms of numbers and where the mental hospitals of our country are probably the only ones that are always fully occupied. […]

Overall, there are several army corps in Germany today which are removed from front-line and garrison service due to nervous disorders. […]

The main causes are shock and fear at the explosion of enemy shells and mines, at the sight of mutilated and killed comrades, at the collapse of dugouts, causing the perception of one’s own wounding or physical injury by blunt force. The after-effects are the conditions with which you are familiar: sudden muteness, deafness or deaf-muteness, general trembling, inability to stand and walk, fits of fainting and convulsions. […]