Nexus Letter for a Secondary Condition: Causation, Aggravation and What to Include
Quick answer: A secondary nexus letter is a medical opinion that a condition was caused or aggravated by a service-connected one. Under 38 CFR 3.310, causation and aggravation are separate routes, and aggravation needs a baseline. The opinion should state the connection is "at least as likely as not" and give reasoning, consistent with the benefit-of-the-doubt rule in 38 CFR 3.102.
Most secondary claims turn on one document: a medical opinion tying the new condition to the one VA already rates. Letters that are vague, cover only one legal route, or lack reasoning leave the rater little to work with. Here is how the regulation frames the question and what a complete letter addresses.
This is a spoke of our main VA nexus letter guide, which covers what a nexus letter is, who can write one and what makes any letter weak or strong. Here the focus is secondary service connection: how to frame the medical opinion when your new condition is linked to a disability VA has already granted. For the legal pathways themselves, see our secondary service connection guide.
How is a secondary nexus letter different?
A direct claim asks whether service caused a condition. A secondary claim asks whether a condition you already have service connection for caused, or worsened, a new condition. The chain has three links: a current diagnosis, an existing service-connected disability, and a medical opinion connecting the two. The letter supplies the third link.
Under 38 CFR 3.310(a), a disability that is proximately due to or the result of a service-connected disease or injury is service connected, and the secondary condition is then considered part of the original condition.
Causation vs aggravation: why the letter must address both
The regulation gives two separate routes, and a letter that covers only one can leave the other unanswered:
- Causation (3.310(a)). The service-connected condition proximately caused the new condition, for example through its symptoms, its effects on the body or its treatment.
- Aggravation (3.310(b)). The new condition existed independently but got worse because of the service-connected condition, beyond its natural progress. VA will not concede aggravation unless the baseline level of severity is established by medical evidence created before the aggravation began, or by the earliest medical evidence between the onset of aggravation and the evidence showing the current level.
Why both matter: In El-Amin v. Shinseki, 26 Vet. App. 136 (2013), the Court of Appeals for Veterans Claims treated a VA medical opinion that addressed only causation as inadequate because it did not address aggravation. A private letter has the same gap if it stops at "caused by." Ask your provider to address causation and aggravation as separate questions with separate reasoning.
Aggravation opinions are harder, because 3.310(b) calls for a baseline. If your provider thinks aggravation is the better theory, the records that show how the condition looked before it worsened are often the most useful thing you can hand them.
What does "at least as likely as not" mean?
The phrase is how VA medical opinions and private letters usually express the standard: the connection is 50 percent or more probable. Its legal footing is the benefit of the doubt. 38 CFR 3.102 says that when there is an approximate balance of positive and negative evidence on any point, the doubt is resolved in the claimant's favor. It defines reasonable doubt as one that is "within the range of probability as distinguished from pure speculation or remote possibility." The same rule appears in 38 U.S.C. 5107(b).
That is why wording matters. Phrases such as "possible," "could be" or "may be" describe a possibility, not a probability. Language such as "at least as likely as not (50 percent or greater probability)" tells the rater which side of the line the provider is on.
What should a secondary nexus letter contain?
- Provider details. Name, credentials, specialty, license number and contact information.
- What was reviewed. The records the provider actually looked at, such as VA rating decisions, treatment notes, imaging and lab results, and whether the claims file was available.
- The diagnosis. The current secondary condition, with the date and basis of diagnosis.
- The service-connected condition. Name it and note that VA has granted it, with the rating if you have it.
- The opinion on causation. Stated in probability terms, with the reasoning.
- The opinion on aggravation. Stated separately, with the baseline severity and the evidence for it.
- The medical rationale. The mechanism linking the two conditions, tied to your history and, where appropriate, to medical literature.
- Other possible causes. Obesity, smoking, age, family history or other conditions the provider considered, and why they do or do not change the opinion.
- Signature and date.
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The wording below is a template shape only. The opinion has to reflect your provider's own medical judgment and your actual records, and they should change or reject any part that does not fit.
Causation: "I reviewed [records]. Based on my examination, the records and my medical knowledge, it is at least as likely as not (50 percent or greater probability) that [secondary condition] was caused by [service-connected condition] because [mechanism and supporting facts]."
Aggravation: "Independently, it is at least as likely as not that [secondary condition] has been permanently worsened beyond its natural progress by [service-connected condition]. The baseline severity, shown by [earliest or pre-aggravation records], was [description]. The current severity is [description]."
What are common secondary condition pairs?
These pairings come up often, but every one still needs a diagnosis and a provider's opinion on your facts. Where the regulations say something specific, it is noted.
| Service-connected condition | Possible secondary condition | Notes |
|---|---|---|
| Diabetes mellitus | Peripheral neuropathy, kidney disease, eye disease | 38 CFR 4.119, DC 7913, Note 1: compensable complications of diabetes are evaluated separately. See neuropathy secondary to diabetes. |
| Traumatic brain injury | Depression, Parkinsonism, seizures, certain dementias, hormone deficiency | 38 CFR 3.310(d) lists conditions held to be proximately due to moderate or severe TBI, with time limits for depression (3 years after moderate or severe TBI, 12 months after mild TBI). See depression secondary to TBI. |
| Lower-limb amputation | Ischemic heart disease or other cardiovascular disease | 38 CFR 3.310(c) covers amputation of one leg at or above the knee, or both legs at or above the ankles. |
| PTSD | Sleep problems, hypertension, GERD | Medical-evidence dependent. See hypertension secondary to PTSD and GERD and PTSD medications. |
| Knee, hip or ankle condition | Back or opposite-leg problems through altered gait | Needs a provider's explanation of the mechanism. See knee secondary to back injury. |
| Treatment of a service-connected condition | Side effects of the medication or treatment | The letter should name the drug or treatment and the side effect. |
For a longer list of conditions and the claim steps, see our secondary conditions hub.
Common mistakes in secondary nexus letters
- Addressing causation but not aggravation, or the reverse.
- Using speculative language instead of probability language.
- Giving a conclusion with no rationale. VA weighs the reasoning, not just the opinion.
- Skipping the baseline when arguing aggravation.
- Ignoring other plausible causes, which makes the opinion easier to discount.
- Not stating which records were reviewed.
Where do you get a letter, and what comes next?
Options and costs are covered in the pillar guide. Two related questions are answered in can I use my VA doctor for a nexus letter and IMO vs nexus letter. Once you have the letter, you submit it as evidence with an original claim (Form 21-526EZ guide) or a supplemental claim (supplemental claim guide).
Frequently Asked Questions
It is a medical opinion from a qualified provider explaining whether a condition is connected to a disability VA has already service connected, either because the service-connected condition caused it or because it made the condition worse than its natural progression.
Section 3.310(a) covers a disability proximately due to or the result of a service-connected condition. Section 3.310(b) covers an increase in severity of a nonservice-connected condition that is proximately due to the service-connected condition and not due to natural progress. VA will not concede aggravation without a baseline level of severity established by medical evidence.
It describes a probability of 50 percent or greater. The legal basis is the benefit of the doubt: under 38 CFR 3.102 and 38 U.S.C. 5107(b), VA resolves doubt in the claimant's favor when the evidence is in approximate balance. It does not cover pure speculation or remote possibility.
It should. In El-Amin v. Shinseki, 26 Vet. App. 136 (2013), the Court of Appeals for Veterans Claims treated a VA opinion that did not address aggravation as inadequate. A letter that covers only causation can leave the aggravation question unanswered.
It should list what the provider reviewed, such as rating decisions, treatment notes, imaging and test results, and explain how those records support the opinion. For aggravation, it should identify the baseline severity and the evidence for it.
Examples include neuropathy and kidney disease with diabetes, depression with traumatic brain injury, cardiovascular disease with certain amputations, and conditions linked through gait changes or medication side effects. Each claim still needs a diagnosis and a provider's opinion on the individual facts.
No. A letter is evidence that VA weighs along with the rest of the record. Its value depends on the provider's credentials, the records reviewed and the quality of the reasoning, and VA makes the decision.
Official Sources
- 38 CFR 3.310: Disabilities proximately due to, or aggravated by, service-connected disease or injury (eCFR)
- 38 CFR 3.102: Reasonable doubt (eCFR)
- 38 U.S.C. 5107: Claimant responsibility; benefit of the doubt (Cornell LII)
- 38 CFR Part 4: Schedule for Rating Disabilities (eCFR), including DC 7913
- VA.gov: Evidence needed for your disability claim