Glycogen storage disease type V - McArdle disease

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This page addresses the possible ophthalmologic manifestations of glycogen storage disease type V, a rare metabolic disorder.

Glycogen storage disease type V

Disease Entity

Glycogen storage disease type V (GSD5) , also known as McArdle's disease [1], is a metabolic disorder caused by a deficiency of myophosphorylase, the muscle enzyme of glycogen phosphorilase. Its incidence is reported as one in 100,000 [2].

Etiology

Two autosomal recessive forms of this disease occur, childhood-onset and adult-onset. The gene for myophosphorylase, PYGM (the muscle-type of the glycogen phosphorylase gene), is located on chromosome 11q13. Myophosphorylase is involved in the breakdown of glycogen to glucose. The enzyme removes 1,4 glycosyl residues from outer branches of glycogen and adds inorganic phosphate to form glucose-1-phosphate. Cells form glucose-1-phosphate instead of glucose during glycogen breakdown because the polar, phosphorylated glucose cannot leave the cell membrane and so is marked for intracellular catabolism.

Clinical manifestations

Clinically, it manifests as exercise intolerance and acute episodes of contractures, associated with rhabdomyolysis and myoglobinuria.

Diagnosis

- A muscle biopsy will note the absence of myophosphorylase in muscle fibers. In some cases, acid-Schiff stained glycogen can be seen with microscopy.

- Genetic sequencing of the PYGM gene

Ophthalmologic manisfestations

Several case-reports and case-series have associated GSD5 with a pattern retinal dystrophy.

References

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  1. McArdle B. Myopathy due to a defect in muscle glycogen breakdown. Clin Sci. 1951. doi:10.5694/j.1326-5377.1951.tb74724.x
  2. http://mcardlesdisease.org/