He has said it is vaccine injury from a flu shot reaction
I never knew the causes of spasmodic dysphonia, what RFK, Jr. is diagnosed with, but I just looked it up:
What Is the Cause of Spasmodic Dysphonia?
The exact cause of spasmodic dysphonia is unknown, however, most cases result from a trigger in the brain and nervous system that can affect the voice. Sometimes it is caused by psychological stress.
For muscle tension dysphonia, there is usually an inciting event that leads it to develop, resulting in a strained vocal quality. These events may include:
Surgery
Virus
Inflammatory Illness
Lesions
Neurologic conditions such as multiple sclerosis or Parkinson’s disease