VA is authorized to compensate eligible individuals only for "service connected" conditions. A service-connected condition is a condition caused by, aggravated by, or the result of, an event during military service or a condition considered service-connected by law (such as Section 1151 claims). As such, "service connection" is a critical concept in VA benefits law. In practice, the determination of service connection can be difficult for VA and frustrating for the veteran. As a result, service connection is one of the most contested issues in the VA claims process.
Establishing service connection generally requires: (1) medical evidence of a current disability or condition; (2) evidence of an in-service occurrence or aggravation of a disease or injury; and (3) medical evidence of a nexus between the claimed in-service disease or injury and the current disease or injury. As a practical matter, establishing the existence of a current medical condition or disability is usually straightforward because the condition is often the motivation for filing a claim. A past condition that has been corrected or resolved or the anticipation of a future condition are not current conditions and do not provide a basis for service connection.
Next, the condition must have occurred in or resulted from the veteran's military service. In most cases, the evidence of the event (wounded by enemy action, training injury) can be found in service records, service medical records, or unit records. Under certain circumstances, a claimant may establish an in-service event by other evidence, such as "buddy statements" or testimony by other service members witnessing the event or private medical records. Whatever the case, VA will also review service medical records to determine if the claimed condition existed when the veteran entered service. If a condition is determined to be "pre-existing" and not aggravated in service, the claim will be denied.
There are also certain "presumptions" regarding specific conditions and in-service events (atomic test participation, agent orange exposure) that may apply. A presumption is when the law assumes an event occurs except when there is evidence that the event actually did not happen. So, for veterans who were exposed to radiation during atomic bomb tests, that radiation is assumed to cause certain diseases. If the veteran now suffers from one of those diseases, he or she does not have to prove the radiation actually caused the disease: VA must accept that the disease as service-connected.
Finally, VA must find a "nexus" (a "connection") between the current condition and the in-service disease, injury, or event. In practice, most service-connection issues boil down to whether a claimant can establish a nexus. For many medical conditions, such as cancer, it is extremely difficult to connect the current disease to specific events, even when occurrence of the event is not disputed. In such cases, it is especially important for the claimant to obtain strong medical evidence supporting nexus. This is not easy. Providing adequate nexus evidence becomes even more difficult as the time between service and the claim grows.
Although a condition must result from actions "in the line of duty," service-connected conditions are not limited to "battlefield" wounds or similar injuries. The "in the line of duty" requirement has been broadly interpreted to mean almost anything that occurs during service, including such things as car accidents, sports injuries, and illnesses unrelated to specific military activity. The condition generally need only have occurred or begun during service, including authorized leave periods.