A study published this week by HCM researchers in Canada found that double mutations in patients with hypertrophic cardiomyopathy are much less common than previously thought.  In particular, researchers found that except for those with double mutations in the gene MYBPC3, there is not much data to support the finding that there is a worse clinical course for those patients who have double HCM mutations.

Hence,  in the absence of extraordinary circumstances, such as two MYBPC3 mutations, the researchers caution that double mutations should not be the sole justification for the insertion of an implantable defibrillator. 

The study looked at patients >18 years of age who underwent genetic testing at the Toronto General Hospital between January 2005 and June 2016.  Out of a sample of 1411 patients, 9% of those who were gene-positive patients had 2 genes, but only in 1 case (0.4%) were both genes classified as those known to cause HCM.

In addition to looking at their own patients, the researchers also re-examined data from previously published studies. Similarly, they found when they re-analyzed the data that only 0.4% of the 8% of patients previously found to have double mutations in fact carried multiple pathogenic mutations.