The article abstracted below presents the case of child with a CAG repeat count of 75 who was misdiagnosed with attention deficit disorder because the family history of HD was not initially disclosed. His JHD symptoms were exacerbated when he was prescribed methylphenidate which is more widely known by the brand name of Ritalin.
It is not known how Ritalin actually helps with genuine attention deficit disorder but it is thought that the mechanism might be through the increase of dopamine. Methylphenidate is a dopamine reuptake inhibitor which means that the level of this neurotransmitter is increased at the snapses. There is evidence that people with HD are abnormally sensitive to dopamine so it makes sense that a dopamine agonist would be harmful.