A group of genetically determined biochemical disorders of muscle characterised by myalgia, cramps, weakness and fatigue. These arc divided into conditions with reduced exercise tolerance and those of static weakness.
REDUCED EXERCISE TOLERANCE
McArdlc's disease - disorder of carbohydrate metabolism, - block in glycolytic pathway (phosphorylase deficiency).
Pain and -Muscles fail to relax hardening of muscles. and contractions occur
Glycogen -j-Glucose 6-phosphate
Absence of phosphorylase enzyme blocks conversion
Myoglobin appears in the urine
Diagnosis: Failure of serum lactate to rise following exercise.
Muscle biopsy - absence of phosphorylase activity with appropriate histochemical staining. Treatment with oral fructose may help.
Carnitine palmityl deficiency - disorder of fatly acid metabolism autosomal recessive localised to chromosome I.
Clinically, muscle weakness and contractures occur with exercise in children or adults. A failure to produce ketones following prolonged fast and a normal elevation in serum lactate following exercise differentiates this condition from McArdle's disease. Enzyme deficiency is shown in biochemical assay on muscle biopsy.
STATIC (FIXED) WEAKNESS Acid maltase deficiency
The development in adult life of limb girdle weakness characterises this disorder. In some, selective involvement of the respiratory muscles causes respiratory failure.
Acid maltase deficiency occasionally presents in infancy with a floppy hypotonic weakness associated with an enlarged tongue.
Diagnosis: Confirmed by muscle biopsy. Carnitine deficiency
Autosomal recessive lipid storage myopathy. In systemic deficiency onset is in childhood with cardiac and liver involvement. Untreated this is fatal. In muscle deficiency alone, proximal weakness and exercise induced pain are sole manifestations. Muscle biopsy shows lipid droplets. Plasma and muscle carnitine levels are low.
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