Introduction

Pompe disease, also known as glycogen storage disease type II (GSD-II), is a rare inherited disorder which causes a gradual weakness in the heart and skeletal muscles.¹ This condition is caused by a deficiency of the enzyme acid alpha-glucosidase (GAA) due to changes (mutations) in the GAA gene. The enzyme’s primary function involves the breakdown of glycogen, a complex sugar stored within the muscle cells. A shortage of the  GAA enzyme causes glycogen to build up in the cells throughout the body, particularly affecting the heart, skeletal muscles, and other tissues. 

The symptoms of Pompe disease vary from person to person due to several factors, however, the most common symptoms, alongside muscle weakness, may include eating issues, hearing impairment, and breathing problems.

The disease presents in two main forms:

• Infantile-onset Pompe disease: Infantile-onset begins within the first weeks of birth. This form is divided into two subtypes: classic and non-classic infantile onset. The classic type mainly affects the heart, causing serious heart problems  (cardiomyopathy), whereas the non-classic type has less severe symptoms and typically includes muscle weakness and delayed development of motor skills
• Late-onset (juvenile/adult) Pompe disease: This nonclassic form can develop at any point from late childhood to adulthood and usually presents with progressive muscle weakness and respiratory problems

Pompe Disease and Danon disease are unique among glycogen storage diseases as they are the only diseases to involve a lysosomal metabolism defect. Discovered in 1932 by Dutch pathologist Joannes Cassianus Pompe, Pompe Disease was the first glycogen storage disease to be discovered.²

The effects of Pompe disease also extend beyond the muscular system and have been found to impact bone health and overall skeletal integrity, which will be the main focus of this article. 

Pompe disease and its impact on bone health 

Bone health plays a key role in the overall well-being of the body, including providing structural support and protecting vital organs. However, in Pompe disease, bone health is compromised in the form of:³

• Reduced physical activity: The muscle weakness characteristic of Pompe disease often leads to decreased physical activity, which is essential for maintaining bone density
• Nutritional deficiencies: Patients may experience difficulties in nutrient absorption or dietary intake, potentially leading to deficiencies in bone-essential nutrients like calcium and vitamin D. These deficiencies can weaken bone development, leading to reduced bone density
• Hormonal imbalances: The disease can affect hormone production and regulation, including growth hormone and insulin-like growth factor 1 (IGF-1), which play crucial roles in bone metabolism
• Chronic inflammation: The ongoing inflammatory processes in Pompe disease may interfere with bone formation and resorption, leading to net bone loss over time

The effects of reduced bone density by pompe disease 

The factors mentioned above collectively contribute to reduced bone mineral density (BMD) in Pompe disease patients. Studies have consistently shown a higher prevalence of osteopenia and osteoporosis in this population compared to healthy individuals of the same age and sex.⁴

In Pompe disease, the reduction in BMD can be observed in various skeletal sites, including the spine, hip, and femur.⁵ The extent of bone loss can vary among individuals and may be influenced by factors such as disease severity, age of onset, and rate of disease progression. Additionally, the pattern of bone loss may differ between children and adults with Pompe disease, with children potentially experiencing more severe effects due to the impact on bone development during important growth periods.⁶

The decreased bone density also significantly increases the risk of fractures. Pompe disease patients are more susceptible to fractures, particularly in bones that bear the weight of the body.⁵ These fractures can severely impact quality of life and complicate disease management, potentially leading to further reductions in mobility and independence. It's worth noting that some treatments for Pompe disease, particularly long-term corticosteroid use, can also negatively impact bone density.⁷ While these medications are crucial for managing certain aspects of the disease, they can accelerate bone loss and further increase the risk of osteoporosis and fractures. 

Beyond the direct effects on bone density, Pompe disease can also impact bone structure and quality. Studies have shown alterations in bone microarchitecture, including thinner, weaker inner bone (trabecular thickness) and increased tiny holes in the outer bone (cortical porosity).⁸ These changes can further compromise bone strength, even in cases where BMD measurements may not fully reflect the extent of bone fragility. The impact of Pompe disease on bone density can also have secondary effects on overall health. For instance, reduced bone density in the spine can lead to vertebral compressions, potentially causing pain, postural changes, and respiratory complications.⁹ This can exacerbate the respiratory issues already present in many Pompe disease patients due to diaphragm weakness.

Diagnosis and monitoring of bone health in pompe disease 

Given the significant impact of Pompe disease on bone health, regular monitoring is crucial. Several methods are used to assess and monitor bone health in these patients:⁴

• Dual-energy X-ray Absorptiometry (DEXA): DEXA scans are the gold standard for measuring bone mineral density. This non-invasive test can accurately assess BMD at various skeletal sites, allowing for the diagnosis of osteopenia or osteoporosis
• Biochemical markers of bone turnover: Blood and urine tests can measure specific markers that indicate the rate of bone formation and resorption. These markers can provide insights into bone metabolism and help monitor response to treatments
• Imaging studies: X-rays, CT scans, or MRI scans may be used to assess bone structure and identify fractures or other skeletal abnormalities
• Regular physical examinations: Routine check-ups can help identify changes in posture, height loss, or other signs of skeletal complications

Management strategies for pompe disease

Addressing bone health is an essential component of comprehensive care for individuals with Pompe disease. While there is no cure for Pompe Disease, management strategies can be implemented to improve bone health.⁴ This may include: 

• Enzyme Replacement Therapy (ERT): Although ERT, such as myozyme, lumizyme, and nexviazyme, mainly targets muscle weakness, it has also been found to improve bone health in Pompe disease patients
• Physical therapy and exercise: Engaging in exercise programs that are tailored to suit the patient’s needs can help preserve muscle strength and promote bone health. Exercises that involve weight-bearing and resistance training can especially stimulate bone formation and improve overall skeletal health
• Nutritional support: Bone-essential nutrients such as calcium and vitamin D help manage bone health. Ensuring good intake of these nutrients may be done through dietary modifications or supplementation
• Medications: Certain bone-modifying medications may be prescribed to alleviate symptoms such as bisphosphonates, denosumab, or other similar agents

FAQs

How is pompe disease inherited? 

Pompe disease is passed through an autosomal recessive inheritance pattern. This means that a person must inherit two copies of the faulty gene, one from each parent, for the disease to develop and show symptoms. 

How often should bone density be monitored in pompe disease patients? 

Typically, a baseline DEXA scan is recommended upon diagnosis or at the start of treatment. Follow-up scans are often suggested annually or every two years, but this can be adjusted based on individual circumstances.

Are there any specific precautions pompe disease patients should take to prevent fractures? 

Some actions patients should try to focus on may include fall prevention, the use of mobility aids when necessary, and ensuring their living environment is safe and obstacle-free.

Where can I find more information about pompe disease? 

Information may be available from the following organisations and resources:

• Acid Maltase Deficiency Association (AMDA) 
• National Institutes of Health (NIH) Genetic and Rare Diseases Information Center      (GARD) 
• International Pompe Disease (IPA)
• National Organization for Rare Disorders (NORD)
• PubMed Central

Summary 

Pompe disease significantly impacts bone health, leading to decreased bone density and an increased risk of fractures. Understanding these effects is crucial for providing comprehensive care to Pompe disease patients. Regular monitoring of bone health, appropriate treatments, and lifestyle modifications can help mitigate the risks and improve the overall quality of life for individuals with Pompe disease. Ongoing research is exploring new therapies and interventions specifically targeting bone health in Pompe disease. These studies aim to develop more effective strategies to prevent bone loss and reduce fracture risk in this patient population. Research into new therapies for Pompe disease, such as gene therapy and next-generation enzyme replacement therapies, may offer hope for better preservation of bone health in the future. These treatments aim to address the underlying cause of the disease more effectively, potentially mitigating its wide-ranging effects, including those on bone density.