Under 38 CFR § 3.310, a disability that is caused or aggravated by an already service-connected condition is itself service-connected. This is one of the most under-used pathways in the entire system, because the VA almost never volunteers the connection.

Common secondary chains

• Sleep apnea secondary to PTSD or the medications used to treat it
• Radiculopathy secondary to a service-connected back injury
• Depression secondary to chronic pain
• Diabetic peripheral neuropathy secondary to diabetes
• Hypertension secondary to PTSD

What proves it

Secondary claims live or die on the medical nexus opinion. The opinion must connect the secondary condition to the primary service-connected one, using the “at least as likely as not” standard and a clear rationale. Aggravation counts too, if service-connected condition made a non-service condition worse, you are owed for the worsening.

Secondary service connection under 38 CFR §3.310 is developed and adjudicated under M21-1, Part IV, Subpart ii, Chapter 2, Section B (Secondary Service Connection and Aggravation). The VA must obtain a medical opinion addressing whether the claimed condition is "at least as likely as not" caused or aggravated by an already service-connected disability. Under El-Amin v. Shinseki, an examiner who addresses only causation and ignores aggravation has provided an inadequate opinion, because §3.310(b) establishes aggravation as a separate and independent basis for service connection. Under Esteban v. Brown, each secondary condition rates independently and stacks on top of the primary through VA math.