[Muscular phosphorylase deficiency in two siblings]

Area of Science:

• Biochemistry
• Genetics
• Pediatrics

Background:

• McArdle disease (glycogen storage disease type V) is an autosomal recessive metabolic disorder.
• It results from a deficiency in muscle glycogen phosphorylase, crucial for glycogenolysis in skeletal muscle.
• Clinical manifestations typically emerge in adulthood, making pediatric cases rare and diagnostically challenging.

Observation:

• This report presents a brother and sister from a consanguinous marriage, diagnosed with McArdle disease at ages 12 and 7.
• The 7-year-old girl exhibited exercise intolerance, while the 12-year-old brother was initially asymptomatic.
• Both patients displayed elevated serum creatine kinase (CK) levels and absent muscle phosphorylase activity, confirmed by biopsy showing subsarcolemmal glycogen accumulation.

Findings:

• Diagnostic challenges included a lack of lactic acidosis post-exercise and a normal lactate response to ischemic forearm testing.
• A significant rise in plasma ammonium levels was observed during ischemic forearm testing, a key diagnostic indicator.
• Electromyography in the affected girl revealed electrical silence post-ischemia, correlating with muscle biopsy findings.

Implications:

• This case underscores the importance of considering McArdle disease in pediatric patients with exercise intolerance, even with atypical presentations.
• Elevated ammonium levels during ischemic testing serve as a critical diagnostic marker in suspected pediatric McArdle disease.
• Further research into early diagnostic markers and management strategies for pediatric McArdle disease is warranted.