Although the exact cause of SD is unknown, evidence suggests that the problem starts at the base of the brain in the basal ganglia, which regulate involuntary muscle movement. To oversimplify, this nervous system regulator does not function properly and produces incorrect signals, which cause the muscles to contract or relax more than they should or at the wrong time.
What makes finding the cause even more difficult is that the spasms do not occur in all types of speech. Two studies conducted in 1968 by Aronson, et. al., and one conducted by Ludlow and Connor in 1987, have shown that the symptoms of SD improve or disappear during laughing, crying, yelling, throat clearing, coughing, whispering, and humming.\Generally, SD does not affect the emotional aspects of speech. As described earlier, SD is task-specific and tends to affect only normal conversational speech. Researchers consistently have identified abnormalities in brainstem reflexes and other aspects of disordered neurological function in people with spasmodic dysphonia.
Genetic factors may put some people at greater risk of developing spasmodic dysphonia, particularly those who have family members with any form of dystonia.
While anecdotal evidence may suggest that symptom onset follows illnesses such as viral infection, head trauma, bronchitis, surgery, or a stressful event, such linkages have not been scientifically proven and the medical community has not reached agreement on whether illness or stress plays any part in the onset of SD. Regardless of the cause, it is most important to make a correct diagnosis and then consider the available treatment options.