Hi Rachel,
I slightly disagree...does your office usually report external cause codes? Does your provider(s) collect and document the basics of how/where/why the accident/injury occurred to report them properly?
If your office usually reports external cause codes in your encounters (as it's not required to add them unless by local reg or carrier) and you have it doc'd how the inj/acc happened, then I would say you should add the appropriate V-W-Y code for the external cause of the injury/accident. Even for a prior case that started under ICD-9, we are adding the causal code to the encounters w the "D" extender to the f/ups in ICD-10.
Unlike ICD-9, where we added E codes only to the first encounter, ICD-10 guidelines say to continue carrying the cause code w the appropriate 7th character A/D/S for the length of the treatment (see Chap 20 a.2). Note: Only the main cause code (ie the V00-Y84 code range) is carried through. The Y90-99 for place of occurrence, activity and status are still coded only once, for the first encounter (Ch 20.b and 20.c second paragraph).
Especially if your case is a Work Comp, I would definitely add the external cause code! My experience w WC is that they want that info on the claims to help "tell the story" for their determination of liability and continued liability (I work for a large group, and used to do nothing but WC all day!).
HTH...best wishes!