Progressive bone resorption after pathological fracture of the femoral neck in Hunter's syndrome

Mucopolysaccharidosis: What Pediatric Rheumatologists and Orthopedics Need to Know

Mucopolysaccharidosis (MPS) is a group of disorders caused by the reduced or absent activity of enzymes involved in the glycosaminoglycans (GAGs) degradation; the consequence is the progressive accumulation of the substrate (dermatan, heparan, keratan or chondroitin sulfate) in the lysosomes of cells belonging to several tissues. The rarity, the broad spectrum of manifestations, the lack of strict genotype-phenotype association, and the progressive nature of MPS make diagnosing this group of conditions challenging. Musculoskeletal involvement represents a common and prominent feature of MPS. Joint and bone abnormalities might be the main clue for diagnosing MPS, especially in attenuated phenotypes; therefore, it is essential to increase the awareness of these conditions among the pediatric rheumatology and orthopedic communities since early diagnosis and treatment are crucial to reduce the disease burden of these patients. Nowadays, enzyme replacement therapy (ERT) and hematopoietic stem cell transplantation (HSCT) are available for some MPS types. We describe the musculoskeletal characteristics of MPS patients through a literature review of MPS cases misdiagnosed as having rheumatologic or orthopedic conditions.

Evaluation of bone health in patients with mucopolysaccharidosis

INTRODUCTION

This study aimed to evaluate the relationship between clinical findings, height and weight standard deviation scores, 25-hydroxyvitamin D₃ (25(OH)D₃) level, and dual-energy X-ray absorptiometry (DXA) results in patients diagnosed with mucopolysaccharidosis (MPS), where effective current treatments such as enzyme replacement therapy (ERT) can be accessed.

MATERIALS AND METHODS

25(OH)D₃ level was measured in 126 patients with MPS (17 with MPS I, 14 with MPS II, 18 with MPS III, 33 with MPS IVA, and 44 with MPS VI; 24-524 months). DXA was performed in 45 of these patients (8 with MPS I, 4 with MPS II, 4 with MPS III, 12 with MPS IVA, and 17 with MPS VI; 62-197 months; all patients were under 18 when DXA was performed) to assess bone mineral density (BMD) of the lumbar spine.

RESULTS

In total, 67.5% patients had a short stature, and 50% of them were underweight for their age. Of the patients, 13.5% were immobile, 28.6% had 25(OH)D₃ deficiency, and 30.2% had an insufficient level of 25(OH)D₃. BMD z score of 45 patients was - 2.5 ± 1.7. In 40% patients, it was <  - 2. However, after correction for height-for-age z score (HAZ), HAZ-adjusted BMD z score was - 0.1 ± 0.9. In 2.2% patients, it was <  - 2.

CONCLUSION

The low BMD z score prevalence reported with DXA was misleadingly higher in children with MPS and short stature. To prevent exposure to unnecessary antiresorptive treatments in these children, the effect of severe short stature and bone geometry on DXA measurements should be considered; further studies on bone health are warranted.

Pathophysiology of Hip Disorders in Patients with Mucopolysaccharidosis IVA

Patients with mucopolysaccharidoses IVA (MPS IVA) have a progressive accumulation of the specific glycosaminoglycans (GAGs): chondroitin-6-sulfate (C6S) and keratan sulfate (KS), leading to the degeneration of the cartilage matrix and its connective tissue perturbing the regular microarchitecture of cartilage and successively distorting bone ossification and growth. Impaired cartilage quality and poor bone mineralization lead to serious hip disorders in MPS IVA patients. Although hip dysplasia is seen widely in musculoskeletal abnormality of this disorder, the pathophysiology of the hip bone and cartilage morphology in these patients remains unclear. Until now, no systemic study of the hip joints in MPS IVA has been reported by using the combined images of plain film radiographs (PFR) and Magnetic Resonance Imaging (MRI). This study aimed to assess the bony and cartilaginous features of hip joints and to explore the potentially related factors of femoral head osteonecrosis (FHN) and hip subluxation/dislocation in patients with MPS IVA. Hip joints in MPS IVA patients were retrospectively reviewed, based on the findings of PFR and MRI data from 2014 to 2019. Demographic information was also collected and analyzed with imaging measurements. A total of 19 patients (eight boys and 11 girls) were recruited, and 38 hip joints in these patients were examined. Eleven patients (57.9%) had FHN. FHN patients were statistically compared with those without FHN. Correlations between cartilaginous femoral head coverage (CFHC) and acetabular index (AI), cartilaginous AI (CAI), or neck-shaft angle (NSA) were investigated in patients with hip subluxation or dislocation. The greater cartilaginous coverage of the hips than their osseous inherency was observed. Significant correlation was observed between CFHC and AI (r =-0.351, p = 0.049) or CAI (r =-0.381, p = 0.032). Severe subluxations or dislocations were more likely to be present in those with more dysplastic bony and cartilaginous hips. In conclusion, our study provides the first systemic description of bony and cartilaginous characteristics in the hip morphology of MPS IVA patients. We have demonstrated that plain radiography alone leads to a misunderstanding of hip morphology and that MRI measurements with PFR are an essential tool to evaluate the 'true' characterization of hips for MPS IVA patients.

Mucopolysaccharidosis type II: skeletal-muscle system involvement

Mucopolysaccharidosis type II (MPS-II) is a rare lysosomal storage disorder caused by deficiency in the activity of the enzyme iduronate-2-sulphatase. This enzyme is responsible for the catabolism of two different glycosaminoglycans (GAGs), dermatan sulfate and heparan sulfate. Lysosomal accumulation of these GAG molecules results in cell, tissue, and organ dysfunction. The skeletal-muscle system involvement is because of essential accumulated GAGs in joints and connective tissue. MPS-II has many clinical features and includes two recognized clinical entities, mild and severe, that represent two ends of a wide spectrum of clinical severity. The aim of this study is to review the involvement of the skeletal-muscle system in MPS-II.

Magnetic resonance imaging findings in Hunter syndrome

Hunter syndrome is a rare genetic lysosomal storage disease that is caused by a deficiency, or absence, of iduronate-2-sulphatase, an enzyme needed to break down specific glycosaminoglycans (GAGs). As a result, GAGs build up in various tissues throughout the body leading to adverse neurological and non-neurological effects. This literature review focuses on the neurological findings. Although few magnetic resonance imaging studies have been conducted, those done have shown that patients with Hunter syndrome generally exhibit brain atrophy, enlarged periventricular spaces and ventriculomegaly. Similar findings have been reported in other mucopolysaccharide disorders. Enzyme replacement therapy is a novel treatment which has had success in treating peripheral disease in mice and humans.

CONCLUSION

Future studies should focus on how structural and chemical signatures in the brain of Hunter patients are altered before and after enzyme replacement therapy, and how those alterations correlate with clinical outcome.

[Mucopolysaccharidoses]

Mucopolysaccharidoses are a family of metabolic disorders characterized by a deficiency in the catabolic lysosomal pathways. They are rare, inherited diseases which lead to progressive cellular, tissue and organ damage across a broad spectrum of phenotypes. To prevent irreversible damage early diagnosis is essential. Typical signs and symptoms are the thoracolumbar gibbus, shortened and plumped metacarpal bones, hip dysplasia, deformed ribs and ovoid vertebral bodies. Due to the typical deformation of the pelvis hip dislocation occurs often in childhood. Bilateral carpal tunnel syndrome is frequent. Bone marrow transplantation and enzyme replacement therapy are available. Orthopaedic interventions are based on individual therapeutic decisions and indications.

Undisplaced femoral neck fractures in children have a high risk of secondary displacement

Femoral neck fractures in children are rare and known to have a high complication rate (e.g. femoral head necrosis, persistent deformities, and pseudarthrosis). While open reduction and internal fixation is the treatment of choice for displaced fractures, non-operative treatment methods have been proposed if the fracture is undisplaced. Three consecutive patients aged 11, 14 and 16 years with undisplaced femoral neck fractures were seen at our institutions and primarily treated with the recommendation of non-weightbearing and minimal flexion until consolidation. All three cases showed secondary displacement within the first 6 weeks. A second minor indirect trauma caused displacement in one case; in the other two cases, no further trauma had occurred. All three fractures healed uneventfully following reduction and osteosynthesis. Undisplaced femoral neck fractures treated non-operatively evidently bear the risk of secondary displacement. The observations in these patients suggest that primary internal stabilization, even of undisplaced femoral neck fractures in children, should be considered.