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Type II Glycogen Storage Disease (GSD Type II)
Synonyms:
Acid Maltase Deficiency - AMD; Pompe Disease
Type II Glycogen Storage Disease is an inborn error of metabolism that belongs to a group called lysosomal storage disorders. Every cell in our body contains vesicles called lysosomes that digest the waste products of the cell. Type II GSD is caused by a lack of function of the enzyme acid alpha-1,4-glucosidase, or acid maltase, which is present in lysosomes. Without the proper function of this enzyme, the glycogen that comes into the lysosomes is not broken down, but continuously accumulates and disrupts the normal functions of the cell. In muscle tissue, these enlarged lysosomes eventually cause the cells to become dysfunctional and die. When muscle cells are injured, their contents spill into the blood.
GSD II is a muscle disease that progressively affects skeletal muscle, primarily limb girdle muscles, and muscles involved in respiration. There are at least three forms of Type II Glycogen Storage Disease, with the most common being the infantile form.
Infantile-onset form, Type IIa: With this form, infants usually present during early infancy with weakness and floppiness, are unable to hold up their heads and cannot do other motor tasks common for their age. The muscles do not appear wasted. Limb girdle muscles and the muscles involved in respiration are affected. The heart muscle thickens and progressively fails in its pumping function. The patients usually die before 12 months of age due to heart failure and respiratory weakness.
Childhood-onset form, Type IIb: With this form, the disease has a later onset, in infancy or early child- hood, and progresses more slowly than the infantile form. Organ involvement varies among the individual patients but muscle weakness is generally seen. The life expectancy is better than for the infantile form.
Adult-onset form, Type IIc: Patients with this form of Type II GSD do not usually show signs of organ enlargement, but are marked by muscular weakness mimicking other chronic muscle diseases. Problems with walking are seen, due to weakness of the hip muscles. Some patients have presented with pulmonary (lung) insufficiency due to muscle weakness, and nighttime breathing must be evaluated. The involvement of the muscle weakness progresses slowly over the years. Heart involvement does not appear to be a significant feature.
Diagnosis of Type II Glycogen Storage Disease is done by determining the activity of the enzyme alpha glucosidase. This deficiency can be shown with a muscle biopsy or cultured cells from a skin biopsy. Biopsied tissues will show a great increase of glycogen of normal structure, and microscopic studies will show increased glycogen enclosed within the lysosomes.
Treatments for adults and children with Pompe disease is aimed at relieving stress on the muscles. A protein-rich diet is used, along with an intensive daily exercise program. Older patients, in particular, must have a well-established plan to follow in the event that pulmonary infections occur.
Links:
Pompe Disease Page from the AGSD-UK website (www.pompe.org.uk)
Acid Maltase Deficiency Association (www.amda-pompe.org)
United Pompe Foundation (http://www.unitedpompe.com)
International Pompe Association (www.worldpompe.org)
NORD information on Pompe Disease (www.rarediseases.org)
Type II info on Online Mendelian Inheritance in Man searchable database
(www.ncbi.nlm.nih.gov/entrez/dispomim.cgi?id=232300)
Type II info from eMedicine (www.emedicine.com/ped/topic1866.htm)
Muscular Dystrophy Association info on AMD (www.mdausa.org/disease/amd.aspx)
Glycogenosis Type II (http://moon.ouhsc.edu/kfung/JTY1/NeuroHelp/ZNN0IE23.htm)
Pompe Disease (www.madisonsfoundation.org/content/3/1/display.asp?did=199)
© October 2003, The Association for Glycogen Storage Disease back to AGSD website
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