Hyperbaric Oxygen Treatment to Treat Mild Traumatic Brain Injury (mTBI)/Persistent Post-Concussion Syndrome (PPCS)

Inclusion Criteria: 1. Adults, 18-65 years old. 2. One or more mild TBI's due to blunt or blast injury. 3. Meets criteria for PPCS. 4. Ability to complete the NSI. 5. Ability to complete the PCL-M or C. 6. Ability to complete CAPS if needed. 7. Absence of acute cardiac arrest or hemorrhagic shock at time of TBI that would cause a global ischemic insult to the TBI. 8. Ability to complete the Michigan Alcohol Screening Test( MAST) and Drug Abuse Screening Test (DAST). 9. Ability to complete a urine toxicology screen for drugs of abuse. 10. Negative pregnancy test in females. Female subjects will need to have a negative urinalysis result in order to start or continue treatment. 11. Subjects must be legally responsible, speak and understand English fluently, and be able to sign their own consent documents. 12. Otherwise good health. Exclusion Criteria: 1. Pulmonary disease that precludes HBOT (e.g., bronchospasm unresponsive to medication, bullous emphysema). 2. Unstable medical conditions that are contraindicated in HBOT (e.g. severe congestive heart failure or heart failure requiring hospital emergency evaluation or admission in the previous year). 3. Severe confinement anxiety (e.g., patients who require anesthesia conscious sedation for MRI). 4. Other pre-TBI cerebral neurological diagnoses including stroke, dementia, degenerative diseases, multiple sclerosis, congenital neurological disorder. 5. Participation in another experimental trial with active intervention. 6. High probability of inability to complete the experimental protocol (e.g. terminal condition or inability to complete outcome instruments). 7. Previous HBOT. 8. History of hospitalization for past stroke, non-febrile seizures, or any seizure history other than seizure at the time of TBI. 9. Past or current history of mental retardation. 10. Pre-/post-TBI history of systemic illness with impact on central nervous system (P.I.'s decision). 11. Pre-injury psychiatric disorders for which the patient was on medication at the time of the brain injury responsible for the patient's diagnosis of TBI/PPCS. 12. Any concurrent systemic illness whose symptomatology confounds the diagnosis of PPCS (P.I.'s decision). 13. Active malignancy undergoing treatment. 14. Taking lithium.