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Page 3 of 4

Causation and Aggravation Are Two Questions

"Did this cause the condition?" and "did this make an existing condition worse?" are different medical questions, and VA treats them as different legal claims. Mixing them up is one of the most common reasons a nexus opinion gets sent back.

Direct Causation

  • The condition did not exist before service
  • Something in service directly caused it
  • Question: is it at least as likely as not that service caused this condition to exist?

Aggravation

  • The condition existed before service (or is now service-connected from another cause) and something — often a separate service-connected condition — made it permanently worse
  • Question: is it at least as likely as not that the aggravating factor caused a permanent worsening beyond the condition's natural progression?
  • Requires comparing severity before and after the aggravating event, not just noting that symptoms are now present
Why the distinction matters: A causation opinion ("service caused this") and an aggravation opinion ("a service-connected condition made this worse") answer different legal questions and are evaluated against different evidence. An opinion that says only "this condition is related to service" without specifying which theory it supports often gets returned for clarification, because the rater can't tell which standard to apply.

A common real-world example: a veteran's service-connected knee condition changes their gait, and years later they develop hip or back problems. That's not direct causation (the hip/back condition didn't originate in service) — it's a secondary/aggravation theory, and the medical question is whether the altered gait from the knee condition is at least as likely as not a contributing cause of the new or worsened condition.

Before You Write

Ask Which Question You're Answering

← The "At Least As Likely As Not" Standard

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