Acid Maltase Deficiency (AMD), glycogen storage disease–type II, is genetically transmitted from carrier parents to their child. A baby receives a pair of autosomal (non–sex determining) genes, one from the mother and one from the father. If one gene in the pair is abnormal, the normal gene takes over. The person is unaffected by the disease but is a carrier.
Every person carries abnormal genes. The problem arises when both autosomal genes in the pair are abnormal. When both of the parents are carriers (i.e. have one abnormal and one normal gene), during each pregnancy there is a 25% chance that the child will have two abnormal genes and be affected.
Symptoms of Acid Maltase Deficiency
Acid Maltase Deficiency is caused by a build up of glycogen in the muscle cells. Glycogen is a thick, sticky substance that is converted from sugars and starches and is stored in the muscle cells for future use. Acid maltase enzyme breaks down the unused glycogen in the muscle cells. The person with AMD lacks or is deficient in this enzyme. Stored glycogen continues to build up in the muscle tissues and leads to progressive muscle weakness and degeneration.
Categories of Acid Maltase Deficiency
Acid Maltase Deficiency is categorized into three forms, depending on age of onset and progression of symptoms.
- Infantile form (type a) – Strikes early, usually between the ages of 2 and 5 months. Severe muscle weakness occurs. Enlargement of the heart, liver, and tongue also occurs. Mental ability is not affected. Respiratory and heart complications cause death by the age of 18 months.
- Childhood form (type b) – Usually begins in early childhood and progresses more slowly. Muscle weakness is progressive with the respiratory muscles most critically affected. The heart and liver are usually not involved. Life expectancy is to the second decade.
- Adult form (type c) – Produces progressive muscle weakness and again, can severely affect respiratory muscles. Onset of symptoms generally occurs between the third and fourth decades of life.
Acid Maltase Deficiency presently has no treatment or cure. Enzyme Replacement for AMD was tried in the 1970s but was unsuccessful. Since then, however, enzyme replacement for another genetic disease has been successful. Researchers believe that enzyme replacement and eventually gene therapy for Acid Maltase Deficiency is possible, but many obstacles remain. The biggest obstacle is the need for immediate funding.
Acid Maltase Deficiency (AMD) is a very rare disease. Only one infant in 100,000 is affected. Childhood and adult forms are often misdiagnosed or not diagnosed at all. Many pharmaceutical companies only fund research for diseases that affect the greatest numbers of people.