E74.04 McArdle disease

Name of the Condition

• McArdle disease (ICD-10 Code E74.04)

Summary

McArdle disease is a glycogen storage disease caused by a deficiency of the enzyme myophosphorylase, which is essential for breaking down glycogen in muscle tissue. This deficiency leads to abnormal glycogen accumulation in muscles, disrupting energy production during exercise and causing symptoms related to muscle fatigue and cramping. The condition primarily affects skeletal muscle function.

Causes

McArdle disease is caused by genetic mutations in the PYGM gene, which encodes myophosphorylase. These mutations are inherited in an autosomal recessive pattern, meaning both parents must carry a mutated gene for a child to be affected. The enzyme deficiency impairs glycogen breakdown in muscles, leading to energy deficits during physical activity.

Risk Factors

• Genetic predisposition or family history of McArdle disease.
• Consanguinity (closely related parents), increasing the likelihood of inheriting recessive mutations.
• Ethnic or population-specific prevalence for certain genetic variants.

Symptoms

Symptoms typically appear in childhood or adolescence and may include:

• Exercise intolerance, with rapid fatigue and muscle pain during activity.
• Muscle cramps or spasms, often triggered by exertion.
• Myoglobinuria (dark urine) after intense exercise, indicating muscle breakdown.
• Second-wind phenomenon, where symptoms improve after a brief rest during activity.
• Muscle stiffness or weakness, particularly in the limbs.

Diagnosis

Diagnosis involves clinical evaluation, family history, and laboratory tests. Blood tests may show elevated creatine kinase (CK) levels, especially after exercise. A muscle biopsy can confirm glycogen accumulation and enzyme deficiency. Genetic testing for PYGM mutations is also used to confirm the diagnosis.

Treatment Options

Treatment focuses on managing symptoms and preventing complications. Strategies may include:

• Avoiding strenuous exercise or pacing activity to reduce muscle stress.
• Consuming a high-protein diet to support alternative energy pathways.
• Oral sucrose supplementation before exercise to improve performance.
• Physical therapy to maintain muscle strength and flexibility.
• Monitoring for myoglobinuria and kidney function to prevent renal damage.

Prognosis and Follow-Up

Prognosis is generally good with proper management, though symptoms may persist lifelong. Regular follow-up with a neurologist or metabolic specialist is recommended to monitor muscle function and address complications. Most individuals can lead active lives with appropriate lifestyle adjustments.

Complications

Potential complications include:

• Rhabdomyolysis (severe muscle breakdown) leading to kidney injury.
• Chronic muscle weakness or atrophy over time.
• Increased risk of falls or injuries due to muscle fatigue.
• Psychological impact from exercise limitations.

Lifestyle & Prevention

• Engage in low-intensity exercise, such as walking or swimming, to maintain fitness without triggering symptoms.
• Stay hydrated and avoid extreme temperatures, which can exacerbate muscle stress.
• Carry a source of quick energy, like glucose tablets, during activity.
• Wear supportive footwear and use assistive devices if muscle weakness affects mobility.

When to Seek Professional Help

Seek medical attention if you experience:

• Severe muscle pain or cramping that does not improve with rest.
• Dark urine (myoglobinuria) after exercise.
• Signs of kidney problems, such as reduced urine output or swelling.
• Sudden worsening of muscle weakness or fatigue.

Tips for Medical Coders

Document the clinical confirmation of McArdle disease, including enzyme deficiency or genetic testing results, to support the E74.04 code. Ensure the diagnosis aligns with the specific enzyme deficiency (myophosphorylase) and muscle-related symptoms. Avoid using this code for other glycogen storage diseases without clear differentiation.