There are two ways to establish service-connection for service-related disability(ies). The first, causal service-connection, is the most common and familiar. If some in-service incident (e.g. physical accident/injury, toxic exposure or psychological trauma) results in a permanent disability, the disability is deemed service-connected/service-related. It is important to keep in mind that the in-service incident need not immediately cause the disability; the disability can have a delayed onset many years, even decades, after the incident. Therefore, the disability itself need not show up during the period of service so long as the in-service incident is the cause of the disability. For example, many sailors were exposed to asbestos from the 1930s until the 1970s, but they did not manifest asbestosis or related respiratory disabilities until decades after exposure.
The other way to establish service-connection is called temporal service-connection. As its name suggests, temporal service-connection focuses on the timing of the disability, not its cause. If a disability first manifests or appears during the veteran’s period of service – though the cause is not service-related – the disability will nonetheless be considered service-connected. Temporal service-connection is often seen with hereditary diseases or with diseases having no known etiology/cause. For instance, an in-service heart attack or other cardiac disability due to a congenital heart disease will be considered service-connected. Certain psychiatric illnesses such as schizophrenia and bipolar disorder have no known causes. Psychiatrists speculate that they may have a genetic component, but this is uncertain. At any rate, if a veteran first manifests symptoms of schizophrenia or bipolar disorder during service, the condition will be deemed service-connected.