and lowered tolerance for noise and light. One complication in diagnosis is that symptoms of PCS also occur in people who have no history of head injury, but who have other medical and psychological complaints. In one study 64% of people with TBI, 11% of those with brain injuries, and 7% of those with other injuries met the DSM-IV criteria for post-concussion syndrome. Many of these individuals with PCS were misdiagnosed as having other unrelated conditions due to commonality of symptoms. (see diagnosis above). Headache is one of the criteria for PCS, but it is notably undetermined where the headache comes from. Couch, Lipton, Stewart and Scher (2007) argue that headaches, one of the hallmarks of PCS, occur in a variety of injuries to the head and neck. Further, Lew et al. (2006) reviewed ample studies comparing headaches to post-traumatic headaches and found that there is wide heterogeneity in the source and causes of headaches. They point out that the International Headache Society lists 14 known causes of headaches, as well. Furthermore, the headaches may be better accounted for by mechanical causes, such as whiplash, which is often mistaken for PCS. An additional possibility is that posttraumatic stress disorder can account for some cases diagnosed as PCS, but for emotional regulation as well. Depression, posttraumatic stress disorder, and chronic pain share symptoms resembling those of PCS. One study found that while people with chronic pain without TBI do report many symptoms similar to those of post-concussion syndrome, they report fewer symptoms related to memory, slowed thinking, and sensitivity to noise and light than people with mTBI do. Additionally, it has been found that neuroendocrinology may account for depressive symptoms and stress management due to irregularities in cortisol regulation, and thyroid hormone regulation. Lastly, there is evidence that major depression following TBI is quite common, but may be better accounted for with a diagnosis of dysexecutive syndrome. In a syndrome, a set of symptoms is consistently present, and symptoms are linked such that the presence of one symptom suggests that of others. Because PCS symptoms are so varied and many can be associated with a large number of other conditions, doubt exists about whether the term "syndrome" is appropriate for the constellation of symptoms found after concussion. The fact that the persistence of one symptom is not necessarily linked to that of another has similarly led to doubt about whether "syndrome"