If you have a secondary VA disability claim in the pipeline — or you're about to file one — the rules just changed. On May 1, 2026, the Department of Veterans Affairs updated its internal adjudication manual, the M-21-1, to require a stricter causation standard for secondary service connection claims under 38 CFR §3.310. If your nexus letter still uses the old "proximately due to" or vague "caused and/or aggravated by" language, your claim may be at risk of denial under the new framework — even if the medicine is solid.
I'm Dr. Drew Brennes, DC, and I write nexus letters for veterans across the country. This is one of the most important changes to the VA claims process in recent memory, and most veterans — and frankly, most providers — don't know it happened.
Here's what changed, why it matters, and exactly what your nexus letter needs to say right now.
What Is Secondary Service Connection?
Secondary service connection allows veterans to receive VA disability compensation for a condition that wasn't directly caused by military service — but was caused or worsened by a condition that is service connected.
For example:
- A service-connected knee injury causes altered gait, which over time damages your hip.
- Service-connected allergic rhinitis causes chronic nasal obstruction that leads to obstructive sleep apnea.
- A service-connected cervical strain changes your neck position during sleep, aggravating an existing sleep disorder.
Secondary claims are governed by 38 CFR §3.310, which has two distinct pathways: causation (§3.310(a)) and aggravation (§3.310(b)). The May 2026 M-21-1 update changed how both pathways are evaluated.
What Changed: The May 1, 2026 M-21-1 Update
Following the federal court's decision in Spicer v. McDonough, the VA updated M-21-1 Section 5.ii.2.D.1.a — the section that tells VA raters exactly how to evaluate secondary service connection evidence.
The key change: the old "proximately due to" language was removed and replaced with a "but for" causation standard across both prongs of §3.310.
Here is the before-and-after in plain terms:
Before May 1, 2026
Raters were instructed to award secondary service connection for disabilities "proximately due to, or the result of" a service-connected condition. The "proximately due to" threshold was relatively flexible — it allowed for reasonable professional inference and benefited from the benefit-of-the-doubt rule.
After May 1, 2026
The manual now instructs raters to evaluate whether the secondary disability is "the result of, or would not have occurred but for" the service-connected condition (§3.310(a)), and whether the aggravation of a non-service-connected disability "would not have occurred but for" or "would have been less severe but for" the service-connected disability (§3.310(b)).
Three different phrases that gave raters analytical flexibility — "proximately due to," "attributable to aggravation by," and "not due to the natural progression" — were all removed. In their place: "but for" appears in three separate spots in the manual.
The Two-Prong "But For" Standard Explained
§3.310(a) — Direct Secondary Causation
This is the pathway used when a service-connected condition caused the secondary condition to exist in the first place.
The nexus opinion must now clearly state:
"The veteran's [secondary condition] would not have occurred but for his/her service-connected [primary condition]."
This is a direct causation standard. It means the secondary condition would not exist at all without the service-connected primary condition. Generic language like "there is a relationship between" or "may be related to" no longer clearly satisfies this threshold.
§3.310(b) — Secondary Aggravation
This is the pathway used when a secondary condition existed independently but was made worse by the service-connected condition — beyond what natural disease progression would explain.
The nexus opinion must now clearly state:
"The veteran's [secondary condition] would have been less severe but for his/her service-connected [primary condition]."
This is also where the Spicer v. McDonough ruling specifically applies: the updated manual now explicitly recognizes that a service-connected condition aggravates a secondary condition when it interferes with or impedes treatment for that secondary condition — even if the service-connected condition didn't directly worsen it medically.
What Is Spicer v. McDonough?
Spicer v. McDonough is a federal court case involving a veteran whose service-connected leukemia required medications that kept his blood counts too low to undergo knee replacement surgery. The service-connected illness didn't directly cause his knee condition — but it prevented the treatment that could have helped it.
The court ruled that this treatment-interference scenario qualified as secondary service connection. The VA then used this ruling as the vehicle to update the M-21-1 manual — and in doing so, not only addressed the treatment-interference scenario the case was about, but also rewrote the broader secondary causation standards for all §3.310 claims.
The result: a net tightening of the evidentiary standard for secondary claims across the board, not just treatment-interference cases.
Why This Matters for Your Nexus Letter
Here's the critical practical problem. VA raters are trained to evaluate evidence based on the M-21-1 manual. When a rater reads a nexus opinion that says a condition is "at least as likely as not proximately due to" the service-connected condition — which was perfectly acceptable language before May 2026 — they are now working under instructions that ask them to evaluate whether the evidence shows the condition "would not have occurred but for" the service-connected disability.
If your nexus opinion doesn't use the "but for" framing, the rater may:
- Request a new C&P exam or additional medical opinion
- Find the existing opinion insufficient under the updated standard
- Deny the claim and force you into the appeals process
The medical facts underlying the opinion haven't changed. What changed is the language the rater is looking for. And if that language isn't in your letter, you're leaving the door open for a denial that didn't need to happen.
What a Compliant Nexus Letter Now Needs to Say
The gold standard nexus opinion for a secondary claim in 2026 should explicitly address both prongs and use the following framework:
If the service-connected condition caused the secondary condition (§3.310(a)):
"It is my professional medical opinion, to a reasonable degree of medical certainty, that [Veteran's name]'s [secondary condition] would not have occurred but for his/her service-connected [primary condition], under 38 CFR §3.310(a)."
If the service-connected condition aggravated the secondary condition (§3.310(b)):
"It is my professional medical opinion, to a reasonable degree of medical certainty, that [Veteran's name]'s [secondary condition] would have been less severe but for his/her service-connected [primary condition], under 38 CFR §3.310(b)."
Many secondary claims involve both pathways simultaneously — for example, one service-connected condition that caused the secondary condition AND a second service-connected condition that has aggravated it. In those cases, both prongs should be explicitly addressed in the same opinion letter.
Beyond the "but for" language itself, a compliant nexus letter should still include:
- A review of the veteran's DD Form 214 and military service records
- Reference to current active diagnoses
- A clear pathophysiologic explanation of how the service-connected condition caused or aggravated the secondary condition
- A minimum of 5 peer-reviewed studies supporting the medical mechanism
- The "at least as likely as not (50 percent probability or greater)" probability standard
The "but for" language answers the legal causation question. The peer-reviewed literature answers the medical mechanism question. You need both.
Common Mistakes to Avoid Right Now
Using only "at least as likely as not" without "but for." The probability standard hasn't changed — you still need ALAN. But the M-21-1 now adds a causation framing requirement on top of the probability threshold. Both must be present.
Using vague association language. Phrases like "there appears to be a relationship," "may be related to," or "consistent with" do not satisfy "but for" causation. They never did, but under the new manual language, they're even more likely to trigger a denial or request for clarification.
Ignoring the two-prong split. §3.310(a) and §3.310(b) are different legal pathways with different language. If your claim involves one service-connected condition that caused the secondary condition and a second service-connected condition that aggravated it, both pathways should be explicitly addressed — "would not have occurred but for" for causation and "would have been less severe but for" for aggravation.
Relying on a doctor's note instead of a formal nexus letter. This was always a problem. Under the new standard, it's a guarantee of denial. A physician stating "patient has sleep apnea, which may be related to his nasal issues" is not a nexus letter. It does not satisfy "but for" causation, it does not cite peer-reviewed literature, and it does not address the regulatory framework.
The Bottom Line
The VA's May 1, 2026 M-21-1 update is real, it is in effect right now, and it changes what your nexus letter needs to say. Veterans with pending secondary claims, recently filed claims, or claims on appeal should review their existing medical opinions to determine whether the "but for" language is present and whether both the §3.310(a) and §3.310(b) pathways are properly addressed.
If your nexus letter was written before May 2026 — or if it was written by a provider who isn't aware of this update — it may not satisfy the current adjudication standard even if the underlying medicine is strong.
This is exactly the kind of regulatory change that separates claims that get granted on the first try from claims that spend years in the appeals process. Getting the language right in the initial letter matters more now than it ever has.
Need a Nexus Letter That Meets the 2026 Standard?
Every nexus letter I write is fully compliant with the updated M-21-1 "but for" causation standard, explicitly addresses both §3.310(a) and §3.310(b) where applicable, includes a minimum of 5 peer-reviewed studies, and is built on a complete review of your DD Form 214, VA records, and personal statement.
If your claim involves a secondary condition — whether it's sleep apnea secondary to rhinitis, a joint condition secondary to an existing service-connected disability, or anything in between — I can provide a letter that gives your claim the best possible chance of approval under the current standard.
Contact me at NexusLetterDoctor.com to get started.
