38 CFR 3.310 provides the legal framework for establishing service connection for disabilities caused or aggravated by an existing service-connected condition. Success requires a robust medical nexus demonstrating a direct physiological link or a clear path of aggravation supported by clinical evidence.
To prevail before the BVA under 38 CFR 3.310, practitioners must move beyond mere correlation and establish a 'proximate result' or 'aggravation' through a high-quality medical nexus letter. The Board frequently rejects claims that lack a detailed medical rationale; therefore, the expert opinion must cite specific clinical findings or peer-reviewed literature that bridges the gap between the primary and secondary disabilities. For aggravation claims specifically under 3.310(b), the evidence must clearly delineate the baseline severity of the secondary condition to prove that the primary service-connected disability caused a permanent increase in disability beyond the natural progression of the disease. Effective strategy involves leveraging the 'at least as likely as not' standard by providing a comprehensive longitudinal view of the veteran's medical history. This includes documenting how medications for a primary condition (like NSAIDs for chronic pain) led to a secondary condition (like GERD) or how altered gait mechanics from a service-connected lower extremity injury caused secondary spinal issues. By presenting a clear, evidence-based narrative that aligns with the 'benefit of the doubt' rule under 38 CFR 3.102, advocates can overcome the high evidentiary hurdles often set by C&P examiners.