This is actually a pretty commonly asked question, and you can find some great answers at the top of the Military History FAQs page in the AH Wiki.

That being said, it is also extremely hard (nigh impossible) for historians to make a medical diagnosis, and arguably unethical. The records that do exist are often incomplete, signs and symptoms of disease can manifest differently, many diseases present themselves in a similar manner, and in the vast majority of cases we have no physical remains to analyze for evidence to support conclusions. Not to mention that most historians lack the training to make a medical diagnosis if all the evidence were available in the first place.

So the short answer is that we have no way of knowing if they suffered from something akin to Post-Traumatic Stress Disorder. But we can look at the available evidence, and point out that many records mention things that we see as signs of PTSD today.