If you had an in service injury/event and have waited x amount of years with no follow up treatment to file the claim, then please submit a statement describing how you've lived with the condition and any self treatment you've done for the condition when you submit your claim.

An examiner bases their opinion on the evidence at hand and if they don't see treatment for 20 years they'll most likely provide a negative opinion stating that there's been no chronicity of treatment therefore what you experienced in service and what you're experiencing now are unrelated.

A personal statement can be used to bridge that gap.