GSD9 was first described in 1966. It is further categorized into four subtypes - 9a, 9b, 9c, 9d. Only GSD9d is a muscle GSD. It is an extremely rare form of the disorder and is characterized by phosphorylase kinase deficiency of the muscle.
Affected individuals may develop progressive muscle weakness, muscle degeneration, muscle cramps, and muscle pain following exercise. This can lead to dark coloured urine (myoglobinuria) due to the breakdown of myoglobin in the muscle.
Very little is know about GSD9d, but given that affected individuals experience exercise intolerance, it would be reasonable to follow the same advice about avoiding rhabdomyolysis as is given for McArdle disease (GSD5). In addition, the same advice applies for when to seek medical attention in the event of a strong cramp or contracture.
It is very likely that with the advent of Next Generation Sequencing and a gene panel for conditions which can be the cause of rhabdomyolysis, more cases of GSD9d will be identified. As understanding of the condition grows, IamGSD will be able to offer better support for this extremely rare muscle GSD.
We have only ever heard of a handful of cases of GSD9d worldwide, and we know it is really difficult for those affected to find each other and swap notes. If you have GSD9d please contact us so that we can try to put people in touch with each other.
IamGSD would welcome hearing from patients or doctors who could help us to improve and expand our information on this extremely rare Muscle GSD.