ABOUT MUSCLE GSD
There are over a dozen forms of glycogen storage disease (GSD), and a number of sub-forms, that can affect skeletal muscle, the liver, or the heart. We are concerned with ones that affect skeletal muscle.
Slow and steady all the way.
Taking a rest, even on the flat.
A level route on an old railway.
Scheme of glycogen metabolism and glycolysis.
Roman numerals denote muscle glycogenoses due to defects in the following enzymes:
II, acid α-glucosidase (AAG);
VI, liver phosphorylase;
VII, muscle phosphofructokinase (PFK);
VIII, phosphorylase b kinase (PHK);
IX, phosphoglycerate kinase (PGK);
X, phosphoglycerate mutase (PGAM);
XI, lactate dehydrogenase (LDH);
XIV, triosephosphate isomerase (TPI).
Symbols in regular typeface denote glycogenoses characterized by exercise intolerance, cramps, and myoglobinuria.
Symbols in italics denote glycogenoses characterized by fixed weakness.
Acta Myol. 2011 Oct;30(2):96-102.
Progress and problems in muscle glycogenoses.
DiMauro S, Spiegel R.
WHAT IS GLYCOGEN?
Our bodies require energy to perform day-to-day activities. To create this energy, our bodies use carbohydrates, fats, and protein. When we eat carbohydrates, our body breaks them down into glucose for energy. Any extra glucose is then converted to GLYCOGEN and stored in our muscles (80% of the body‘s reserve) and liver (20%). This stored fuel (glycogen) is then used for energy throughout our bodies, and in particular in our muscles.
WHAT IS THE PROBLEM?
GSDs are caused by a genetic defect that is inherited from both parents. Individuals with a GSD either have an absence or deficiency of one of the enzymes that are responsible for using or storing glycogen.
HOW DOES IT AFFECT PEOPLE?
Since glycogen is mainly stored in the muscles, liver, or both, GSDs usually affect functioning in these areas, with some GSDs also affecting the heart. In general, symptoms of GSD may include: enlarged liver, low blood sugar, muscle cramps, activity/exercise intolerance, and progressive weakness. Onset, symptoms and severity vary depending on the type of GSD.
HOW MANY GSDs ARE THERE?
At present there about a dozen distinct forms of GSD, split approximately half and half between liver and muscle GSDs. Some GSDs have several sub-forms.
WHAT CAN PATIENTS DO?
Early diagnosis of GSD is important to ensure proper clinical management. Unfortunately, some GSDs are considered ultra-rare with only a handful of diagnosed cases worldwide. We encourage patients with a GSD to get involved with national and international patient groups for support and information.
WHAT ABOUT THE FUTURE?
As more information about the natural history of muscle GSDs is gathered, future generations of people with these conditions will hopefully benefit from early diagnosis (especially with the help of genetic testing using Next Generation Sequencing), access to well-informed clinical management, and eventually an effective treatment or even a cure.