'Huntington's Disease (Chorea)'

"Huntington's disease (also referred to in more formal medical research as Huntington Disease) is an hereditary neurological disorder of the central nervous system that causes progressive degeneration of cells in the brain, slowly impairing a person's ability to walk, think, talk and reason. "

Source
Huntingtons's Disease Association
                   Huntington's Disease association fact sheets page
Huntington's Disease Wiki
NHS Choices Huntington's Page.

(Video Source)

The above video shows an interview with a patient who has been diagnosed with Huntingtons after his mother died of the disease some time before, he goes on to talk about how he has become clumbsy, has a tendancy to mumble and can get highly stressed from the smallest of things. 

The doctor who is also interviewed in the video also expresses that many of the patients have a tendancy to become irritable and aggressive, presumably due to the increased amounts of stress related with completing simple every day tasks with the disease. This is further supported by the patient when he expresses that his mother could go from "being fine one minute to being in a complete rage the next". 

This would pose problems within a music therapy environment as it would mean that over complex decisions or instruments could confuse and agitate the user and end up with them becoming aggressive to the therapist, this is something that needs to be considered when designing instruments specficaly for the purpose of music therapy as there is potential for them to be used as weapons against the therapist or other members of the group.