I have not seen that. Who is the insurance carrier? I can research it and see if I can find out why they are requiring an external cause code.
There is nothing in ICD-10 indicating that an external cause code is required, but there are "use additional code" notes for tobacco use and exposure to smoke. I wonder if the insurance is trying to figure out if there was trauma that caused the vocal cord paralysis. Is there a cause for the VC paralysis listed in the medical records?
If there is no cause documented and the insurance absolutely requires an external cause code, perhaps Y99.9 - Unspecified external cause status would work for an "unknown cause".
Hope that helps!