Not to discourage other commenters, but there's a whole section on PTSD in the FAQ. You might find your answer there.

I took a history course in university which was based upon the relationship between war and mental health in the modern world, and covered the American Civil war right up to the Gulf War. Admittedly, these are much later conflicts then what your question pertains to, although this course did center in upon the very important point that PTSD is a mental health issue, and as a society, we tend to incorrectly think about it along the lines of a physical ailment. This is why concepts such as the 'rest-cure' were popularly employed during the First World War, as our understanding of PTSD, or Shell-Shock as it was then known, manifested largely in physical symptoms. This led to difficulty later on, as in Vietnam we began to see PTSD manifested as more of a mental issue. Why the difference? Well, prior to the First World War the field of mental health in general was sparse and unexplored. If one felt mentally ill, there were no treatments which a physician could prescribe, as there were often no diagnosable symptoms. The interesting part of this is that physical symptoms would seem to spontaneously manifest alongside the mental illness: We now understand this phenomenon far better with advancement in modern medicine, and we now see that mental health, unlike physical ailments, can actually adjust how it is presented to the societal and cultural practices of the individual.

So, a soldier in the First World War experiencing PTSD would express it physically, often through violent convulsion or shaking, not because those are the text-book symptoms of the metal illness, but because society only gives credence to illness with physical symptoms, and so in order to receive medical attention, the illness presented itself in a way which doctors recognized and understood.

Alternatively, with the growing understanding of mental health, soldiers returning from more current wars may experience PTSD in depression, flashbacks, ect. There are little physical symptoms compared to earlier modern war, as these men have been socialized in a society which understands that illness may extend past the physical form. While your best bet is to read the FAQ sections on PTSD, as it addresses most of your questions directly, I would encourage you to see PTSD and the nature of mental illness as inherently different then physical illness. It does not carry with it distinct and unchanging representations or symptoms, and is largely affected by culture, socializations, and time period. Thus, a Spartan and a Napoleonic soldier may have both experienced PTSD, though it may have manifested itself drastically differently in either individual.

A great source for further reading on the subject is Ben Shephard’s, A War of Nerves: Soldiers and Psychiatrists in the Twentieth Century. This was the course reading material and it is immensely in-depth in its discussion of soldier’s mental health in combat and at home. Hope that helps!

Modern society is alot more civilised and decadent than ancient society so the brutality and shock of war has a more severe effect on us now. Our minds aren't conditioned to hardship the way combat demands so when we experience the violence first hand it is more common nowawdays to have a severe psychological reaction to it and it manifests itself as PTSD or shell shock as they called it in the first world war