A secondary nexus opinion should answer two separate questions when the evidence raises them: Did the service-connected condition cause the second condition, and did it aggravate a condition that already existed? A letter that blurs those questions can miss the actual theory.

38 C.F.R. § 3.310 is the current regulation to read alongside the facts of the claim.

Identify both conditions precisely

The packet should name:

  • the primary condition that VA has already recognized as service connected;
  • the claimed secondary condition and its current diagnosis;
  • whether the clinician is evaluating causation, aggravation, or both;
  • the date range and records reviewed.

Do not substitute a symptom for a diagnosis unless the medical question itself is about the symptom and the clinician explains its significance.

Causation asks how one condition produced another

The opinion should explain the proposed medical mechanism and apply it to the person's history. Timing can support the explanation, but “B happened after A” is not enough by itself.

Fictional example: A veteran has a service-connected knee condition, years of documented altered gait, and a later hip diagnosis. A useful causation opinion would address the gait history, examination findings, imaging, other hip risk factors, and why the knee condition is or is not a cause of the hip condition.

Aggravation asks what changed

If the secondary condition existed first, the clinician may need to compare an earlier baseline with later severity. Useful evidence can include symptom frequency, treatment intensity, testing, functional limits, medication changes, or other condition-specific measurements.

The record may not permit an exact numerical baseline. The opinion should say so instead of inventing one. It can still identify the supported before-and-after facts and explain the medical significance.

A six-part clinician worksheet

  1. Primary condition: diagnosis, service-connected status, course, and treatment.
  2. Secondary condition: diagnosis, onset, course, and treatment.
  3. Sequence: dated changes in both conditions.
  4. Mechanism: the medical explanation proposed for causation or worsening.
  5. Competing factors: other diagnoses, injuries, medications, exposures, or risk factors.
  6. Conclusion and limits: the clinician's independent opinion, reasoning, and missing information.

The worksheet is a review aid. It is not a fill-in medical conclusion.

Research is supporting material, not the opinion

A paper showing an association between two conditions may make a question worth examining. The clinician still has to explain whether and how the research fits the individual history. Note the study population, outcome measured, and limits. Avoid turning one abstract into a certainty claim.

Keep lay observations in their proper role

A spouse, coworker, or friend may describe changes they personally observed, such as a change in walking, sleep, or daily activity. Those details can help establish a timeline. They do not replace a qualified medical explanation of causation. 38 C.F.R. § 3.159 distinguishes competent lay and medical evidence.

Check the packet against the opinion

Before relying on the final letter, confirm that every important fact has a traceable source. Look for a direct answer to causation and a separate answer to aggravation when both were asked. Confirm that contrary facts are acknowledged and that the clinician signed and dated the final opinion.

Raven Nexus supports direct, secondary, and aggravation preparation. It can organize selected records and research into an editable draft and appendix for clinician review. A free account includes one starter assessment; full drafting and continued comparisons require Standard or Plus. The clinician provides the medical judgment and signature.

For the broader definition, see what a nexus letter is. For review questions, use how to review a nexus letter.

If VA has already denied the connection, read what evidence addresses a no-nexus denial.

Sources and scope

Reviewed September 20, 2026: VA evidence requirements; 38 C.F.R. § 3.159; 38 C.F.R. § 3.303; 38 C.F.R. § 3.310; Raven Nexus product page. This article does not say that any named condition causes another in an individual case.