Spasmodic dysphonia (SD) is a very rare condition (dystonia), and there are several varieties. It is not much studied and often relegated to the status of a psychogenic anomaly. There is never a family history of the condition, so it seems to be polygenic, with uncommon interacting genes inherited from both the father and mother. One association is with the gene for familial tremor, which we have found to be unrelated to the A− trait. So, perhaps some affected individuals with SD have both familial tremor and A− trait.
As we have mentioned in the past, there may be several genes that determine the A− phenotype (genetic heterogeneity).
In 15 years of the NPA test (several hundred thousand tests), only two subjects selected SD from the dropdown list of “Conditions” (compared to hundreds in the categories of conditions like autism or ADD). One of these SD subjects tested NPA−/= with an S score of 79, while the other tested N/NA− with an S score of 37. So, I wouldn’t rule out the involvement of A− trait in SD.