My understanding is that Purple Heart is awarded to a person who is wounded in a battle-field setting – and again, my understanding, the wound is received while actively performing their combat-related duties. It would seem that one "purpose" of a PH is to acknowledge that the recipient may have lasting medical issues related to a wound received while defending our country. A piece of tin can never compensate for life-long loss of a limb, an eye, part of a liver, etc. but the recognition said injury occurred is of use to medical systems, the VA and other disability considerations.
TBI is not just a concussion. Concussion is caused by a hard shock to the head that moves the brain within its fluid cushion so that it hits the inside of the skull. The result is a temporary swelling that compresses blood vessels causing the a temporary impairment of brain function since it does not receive all of the blood flow that it needs (see my discussion of migraines here: http://teddysratlab.blogspot.com/2012/01/guide-migraine.html). The swelling subsides in a day or so and no permanent damage occurs.
TBI on the other hand is much more severe, and results from either a closed-head trauma (i.e. a hit, blow or fall) that cracks the skull, or an explosive shockwave directed at the head. Note that I am still talking about closed-head injuries here, a penetration or laceration injury that opens the skull is still TBI, but the consequences of such an injury are rather more obvious than the closed-head type. Critical differences between closed-head TBI and concussion are the severity and duration. The shock to the skull is enough to cause multiple back and forth impacts between brain and inside of the skull, and an explosive shock wave can cause distortions as it passes through the brain causing shearing or tearing of the nerve axons, blood vessels, and other tissues. The resultant bruising and/or hematoma again causes pressure and symptoms of a concussion, but the effects last much longer and may be permanent.
Doctors can tell whether TBI has occurred by brain scans such as MRI and CT scan or by looking for protein, blood, or immune system cells in the fluid from around the brain (the cerebrospinal fluid, CSF, obtained from a spinal tap http://teddysratlab.blogspot.com/2011/04/this-is-real-spinal-tap.html), but on the battlefield, there is not much that can be done to tell the difference. It is only by looking at the long-term consequences that the true picture of TBI occurs, such as long-term impairment of memory, personality changes, loss of vision, hearing - and in particular, sense of smell. The latter is important, because it is very easy for movement of the brain within the skull to separate the nerve endings of the nose from the olfactory nerve, so loss of smell is a key indicator of TBI, although it is not always present, nor is it exclusive.
So in many ways, TBI is long-term damage to the brain, it make take days to weeks for the real symptoms to show up, and months, years (or never) to go away. Unlike a traumatic amputation, the reality of TBI is much harder to see, but in my opinion, it is no less battlefield trauma than any of the other injuries that qualify for the Purple Heart. Fortunately, the trend in not acknowledging TBI was reversed over the past several years and I have been told that TBI is now recognized by the U.S.Army with a Purple Heart.
NOTE: They are wrong.
PTSD is not any of those things. PTSD is not aggression or attempts at combat. The issue comes from misguided attempts to put a clinical label on adaptive or maladaptive behaviors so that they can be excused or dismissed. This point is also why PTSD is over-diagnosed and can often be used to excuse malingering.
PTSD is a physiological fear reaction that is essentially triggered by memory. The heart beats faster, breathing is faster and shallower, adrenaline and other hormones triggered by stress are dumped into the blood. A key difference is in the reactions of the autonomic nervous system which controls many of the unconscious reactions of the body. However, the precise reactions are important - for example, pupil dilation (fear) vs. constriction (aggression), dry mouth (fear) vs. excess saliva (aggression - think "foaming at the mouth"), sweating (fear) vs. dry skin (aggression), muscle rigidity (fear) vs. a slight tremor or twitch indicating readiness to spring into action (aggression). The PTSD reaction can be triggered by nightmares, a sudden touch, sound or even a smell. The trigger is tightly tied to memory as I describe below, so I will need to drop into lecture mode a bit.
However, again I emphasize that all of these triggers can result in aggression - which is adaptive for combat, and even though inappropriate in a civilian setting, it *is* a survival skill set - but it is not PTSD. It is the fear reaction that is the hallmark of PTSD that is totally maladaptive and counter to survival (i.e. "freezing in the face of the enemy").
Memory is a key component of PTSD. Scientists who study memory know that there are instances in which memory is abnormally strong. Under normal conditions, items must be repeated in order to be remembered: addresses, phone numbers, SSNs, etc. Yet it is relatively easy to remember where you parked each day because you use all sorts of cues to help remember these single, nonrepeated, data points.
About a year ago, there were several reports of a blood test for PTSD. The utility and accuracy of such tests are still very much in doubt. One group was looking at the effects of stress hormones and their metabolites ("allostatic load") that could be quantified to give a green "healthy" reading or a red "unhealthy" reading indicative of unresolved/maladapted stressors. The inventors of the test - which dates back to research published in 2000 (B. McEwen, Allostasis and Allostatic Load, Neuropharmacology, 22:108-124, 2000) claim 85% correspondence between a "red" allostasis and PTSD. The Israelis also have a test that reveals levels of a particular type of immune cells (gamma delta T-lymphocytes) that also appear to be characteristic of PTSD. The problem with both of these tests is that many stressors also increase the stress hormones and immune response. In addition, T-cell sensitivity in particular varies between males and females, although the same is true for the stress hormone cortisol to a lesser extent. Prolonged illness, physical trauma and TBI can also result in positive indicators, but in the absence of those causes, a positive blood test could be a good indicator of unresolved stress or medical issues.
As for the original question of Purple Heart awarded for PTSD, I think I come down on the side of those who say no. At present we still don't have accurate tests, there is certainly abuse of the diagnosis, and we don't always have a definite link between the triggering event (which my understanding suggests should be honorable combat duty) and the disorder. On the other hand, we shouldn't stigmatize those who really do need medical intervention to get their lives back in order.
More research is needed, but then you probably knew I would say that.