In the IHS Classification, Section 5.0 is for headache associated with head trauma. This is further subdivided into 5.1: acute post-traumatic headache, and 5.2: chronic post-traumatic headache. Post-traumatic headaches are a regular feature of the post-traumatic or post concussion syndrome which follows significant head injury. The syndrome is not necessarily associated with definable central nervous system injury and can occur whether or not unconsciousness occurred at the time of trauma. The syndrome is characterized by headache, vertigo, impairment of memory and concentration, and variable degrees of emotional impairment. Headache lasting more than two months occurs in up to 60 percent of patients hospitalized following head injury. The degree of apparent disability may seem to outweigh the amount of objective evidence of central nervous system or musculoskeletal injury. Some believe that persistent symptoms relate to the patient's desire to seek compensation, but most believe that this occurs only in a small percent. There is evidence that an organic mechanism is operative in a large proportion of these patients. Both common and classic migraine may have a significant exacerbation following closed head injury; uncommonly typical migraine attacks may follow injury in a previously headache free individual. The incidence of post-traumatic migraine is higher in people with a strong family history of migraine headache.