Written by Dr. Ryan, DO · Updated September 28, 2026
Not sure if your TBI claim is worth pursuing?
Run a free case check and find out whether the connection is there before you spend a dollar. 5 minutes. No payment. No obligation.
Free Case Check →A TBI claim starts with proving the injury happened in service, and for combat veterans 38 USC 1154(b) lets your own account of the blast or blow prove it. Traumatic brain injury is one of the signature injuries of the post-9/11 wars, and one of the more frustrating claims to get right. The frustration usually isn't about whether the injury happened. It's that the blast or the blow got treated in the moment and never made it into the record in a way the VA will accept years later. The nexus, the documented link between that in-service event and the symptoms you carry now, is almost always the piece that's missing, and what makes a nexus letter strong comes down to how well that link is documented.
Here's how TBI actually gets service connected, why it matters far beyond the TBI rating itself, and what a rater needs to see.
Most TBI claims are direct service connection claims. The argument is simple in form: you sustained a head injury or blast exposure during active service, and you have residual symptoms today. Vehicle accidents, IED and blast exposure, falls, training injuries, and combat concussions are all common mechanisms. What the VA wants is a thread connecting the event to the present.
The catch is that mild TBI often looked minor at the time. You got checked, maybe got a day of rest, and went back to duty. So the trick is often reconstructing the event and its continuity: post-deployment health assessments, TBI screenings, buddy statements, and the pattern of symptoms that followed. Absence of a detailed contemporaneous note does not defeat these claims when the surrounding evidence is consistent with the service you actually had.
If your TBI came from combat, the law gives you real help proving the event happened. Under 38 USC 1154(b), lay testimony from a combat veteran about events consistent with the circumstances of that service is accepted as sufficient proof the event occurred, even without a paper trail from the moment it happened. A blast on patrol that never generated a formal medical entry can still be established through your own statement and corroborating buddy statements. This matters enormously for TBI, because the chaos of the event that caused the injury is exactly the situation where documentation tends to be thin.
Buddy statements carry weight here. A fellow service member who saw you get knocked out, saw the vehicle after the blast, or watched your memory and headaches change afterward is providing evidence a rater can use.
Here's the part a lot of veterans miss. TBI is frequently the primary condition that drives a whole cluster of secondary conditions. Post-traumatic headaches and migraines are among the most common, and there is well-established medical literature connecting brain injury to chronic headache disorders. TBI is also tied to sleep disturbance, cognitive and mood changes, tinnitus, and vestibular problems.
So once a TBI is service connected, it becomes an anchor. Migraines secondary to service-connected TBI under 38 CFR 3.310 is a strong and common secondary claim. The same goes for other residuals that flow from the injury. If you're only thinking about the TBI rating itself, you may be leaving connected conditions on the table.
The relationship also runs the other way in some records. Many veterans carry conditions that are secondary to a service-connected TBI, and building those out one at a time, each with its own clean nexus, is often where the real value of the claim lives.
Depression is one of the most prevalent and best-documented consequences of brain injury. The link isn't only the strain of living with an injury; TBI and depression share a common neurobiology, which is why a service-connected TBI is a solid foundation for a secondary depression claim.
TBI, particularly moderate and severe TBI, causes measurable changes to the prefrontal cortex, hippocampus, and limbic system, all of which are central to mood regulation. Diffuse axonal injury from blast exposure or blunt trauma disrupts white matter connectivity in ways that impair emotional processing, even when imaging appears normal.
TBI disrupts the serotonin, dopamine, and norepinephrine systems, the same pathways antidepressants act on. Post-TBI depression looks clinically and pharmacologically much like primary major depressive disorder, which is why the same medications are often used to treat it.
TBI can damage the pituitary gland, causing hypopituitarism and testosterone or cortisol dysregulation. Those hormonal changes can produce depressive symptoms on their own, often alongside fatigue and weight changes.
Depression doesn't always appear right after the injury. Published research documents onset months or even years later, as the neurological and psychosocial consequences of the injury accumulate. A nexus letter should address that gap in time directly rather than leave a rater to read it as a break in the chain.
TBI and PTSD often come out of the same blast or blow, and they tend to get treated as two conditions that simply happen to coexist. There is also a secondary pathway. When the TBI is already service connected and PTSD developed in its wake, the PTSD can be claimed as secondary to the TBI under 38 CFR 3.310, building on an injury the VA has already accepted.
TBI and PTSD affect overlapping brain structures:
Even mild TBI (concussion) alters the brain's stress-response systems in ways that increase vulnerability to PTSD. The same event that causes the TBI frequently causes the psychological trauma as well, but beyond that shared exposure, TBI-related changes can sensitize the stress-response system so that later stressors are more likely to produce PTSD.
Several TBI symptoms, including memory fragmentation, emotional lability, irritability, and sleep disruption, overlap substantially with PTSD. That overlap has historically led to PTSD being underdiagnosed in TBI patients, because providers attributed everything to the brain injury.
The strongest TBI-to-PTSD secondary cases have a TBI documented before the PTSD emerged, PTSD symptoms that developed in the months or years that followed, and records showing those symptoms tracking with the TBI-related changes rather than with a separately identified traumatic event. When both were diagnosed at the same time from the same event, the secondary theory is still available, but the opinion has to explain specifically why the PTSD is better understood as secondary to the TBI than as an independent condition that happened to co-occur.
This is where TBI mental health claims most often go sideways. The TBI rating itself (38 CFR Part 4, Diagnostic Code 8045) already accounts for certain psychiatric and cognitive symptoms. When depression is simply a direct manifestation of the TBI, it gets folded into the TBI rating rather than rated separately, to avoid pyramiding under 38 CFR 4.14.
When the depression or PTSD is a distinct disorder that developed secondary to the TBI, with its own diagnostic course and symptoms beyond what the TBI rating covers, it can be rated separately under its own diagnostic code (depression, for example, under DC 9434). That isn't double counting. It does mean the medical evidence has to say clearly which symptoms belong to which condition, and the nexus letter has to characterize the psychiatric disorder as a separate diagnosis, not a symptom cluster inside the TBI.
A single C&P examiner often evaluates both conditions at once. If that examiner absorbs the psychiatric symptoms into the TBI rating, the separate rating is lost. A well-reasoned independent medical opinion that draws the line between the two provides the counterweight.
Whatever the pathway, three things have to be in the file:
That third piece is what the VA found missing in most denials. Under Nieves-Rodriguez v. Peake, 22 Vet. App. 295 (2008), the value of a medical opinion comes from its reasoning, not from who signed it. A well-reasoned private nexus letter can outweigh a bare C&P conclusion.
For depression or PTSD secondary to TBI, the TBI can be service connected at any percentage, including 0%, and the psychiatric diagnosis should be a DSM-5 diagnosis from a licensed clinician. The opinion itself should:
A denial isn't the end. You generally have one year from the rating decision to file a Supplemental Claim with new and relevant evidence and keep your original effective date. For most TBI denials, that new evidence is a private nexus letter that speaks directly to the gap the VA named, whether that's establishing the in-service event or connecting a residual condition to the injury. Read the rating decision's rationale word for word first, then answer the specific reason they gave.
TBI claims are winnable when the event is established and the residuals are tied back to it properly. The most common fix is a physician-reviewed nexus letter built around your actual records, your lay and buddy statements, and the specific pathway that fits your case, whether that's the TBI itself or a condition secondary to it.
Flat Rate Nexus writes physician-signed independent medical opinions for TBI claims and their secondary conditions. A board-certified physician reviews your records for a fully refundable $149 deposit, refunded in full if we cannot support your case. You can also run a free case check first to see whether it's worth pursuing at all.
Thinking about your own claim? Every nexus letter we write goes through a full physician record review, cites peer-reviewed research, and is built around the actual evidence in your case.
Start My Nexus Letter See pricing