Ultrasonographic Features of Hip Joints in Mucopolysaccharidoses Type I and II

Diagnosis and Management of Mucopolysaccharidosis Type II (Hunter Syndrome) in Poland

Mucopolysaccharidosis type II (MPS II; also known as Hunter syndrome) is a rare, inherited lysosomal storage disease. The disease is caused by deficiency of the lysosomal enzyme iduronate-2-sulphatase (I2S) due to mutations in the IDS gene, which leads to accumulation of glycosaminoglycans (GAGs). Deficiency of I2S enzyme activity in patients with MPS II leads to progressive lysosomal storage of GAGs in the liver, spleen, heart, bones, joints, and respiratory tract. This process disturbs cellular functioning and leads to multisystemic disease manifestations. Symptoms and their time of onset differ among patients. Diagnosis of MPS II involves assessment of clinical features, biochemical parameters, and molecular characteristics. Life-long enzyme replacement therapy with idursulfase (recombinant human I2S) is the current standard of care. However, an interdisciplinary team of specialists is required to monitor and assess the patient's condition to ensure optimal care. An increasing number of patients with this rare disease reach adulthood and old age. The transition from pediatric care to the adult healthcare system should be planned and carried out according to guidelines to ensure maximum benefit for the patient.

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.

Misdiagnosis in mucopolysaccharidoses

Mucopolysaccharidosis (MPS) is a group of 13 hereditary metabolic diseases identified in humans (or 14 diseases if considering one MPS type described to date only in mice) in which an enzymatic defect results in the accumulation of glycosaminoglycans (GAG) in the lysosomes of cells. First of all, as a result of GAG storage, the proper functioning of the lysosome is disturbed; then, the cells, and finally, tissue, organs, and the whole organism malfunctions are observed. Due to the rarity, heterogeneity, and multi-systemic and progressive nature of MPS, they present a major diagnostic challenge. Due to the wide variation in symptoms and their similarity to other diseases, MPS is often misdiagnosed, usually as neurological diseases (like autism spectrum disorders, psychomotor hyperactivity, and intellectual disability) or rheumatology and orthopedic disorders (like juvenile idiopathic arthritis, Perthes disease, rickets, and muscular dystrophy). In this review article, we present the problems associated with the possibility of misdiagnosing MPS, discuss what diseases they can be confused with, and suggest ways to reduce these problems in the future.

A case of mucopolysaccharidosis type VI in a polish family. Importance of genetic testing and genotype-phenotype relationship in the diagnosis of mucopolysaccharidosis

Background and objectives

Mucopolysaccharidosis type VI (MPS VI) is a rare, autosomal recessive lysosomal storage disorder caused by deficient enzymatic activity of N-acetyl galactosamine-4-sulphatase, which is caused by mutations in the arylsulphatase B (ARSB) gene. To date, 163 different types of mutations in the ARSB have been reported. However, the full mutation spectrum in the MPS VI phenotype is still not known. The aim of this study was to perform molecular testing of the ARSB gene in the patient and his family members to confirm MPS VI.

Methods

Molecular characterisation of the ARSB gene was performed using Sanger sequencing. We studied a child suspected of having MPS VI and 16 other relatives.

Results

We identified a C-to-T transition resulting in an exchange of the Arg codon 160 for a premature stop codon (R160*, in exon 2). The transition was in CpG dinucleotides.

Interpretation and conclusions

The study provided some insights into the genotype-phenotype relationship in MPS VI and the importance of genetic testing when diagnosing MPS, which is not a mandatory test for the diagnosis and only very occasionally performed. Additionally, we present here the history of a family with confirmed MPS VI, which is extremely rare especially in south-eastern Poland. What is more, the position where the mutation is located is very interesting because it is the region of CpG, which is the site of the methylation process. Thus, this opens the possibility of a new approach indicating the involvement of an epigenetic mechanism that should be examined in the context of the pathomechanism of MPS.

The role of imaging in the skeletal involvement of mucopolysaccharidoses

This article discusses the role of imaging modalities including radiography, multi-detector computed tomography, magnetic resonance imaging, and ultrasound in diagnosing and monitoring skeletal abnormalities in mucopolysaccharidoses (MPS). The advantages and disadvantages of these different imaging tools will be discussed, along with their feasibility in this class of patients. As the musculoskeletal involvement is common to all MPS and is one of the main reasons for seeking medical attention, an increased awareness among paediatricians, rheumatologists, orthopaedists, radiologists, and other musculoskeletal specialists on the possible spectrum of abnormalities observed could facilitate a timely diagnosis, an appropriate severity evaluation, and better management.

Establishing percentile charts for hip joint capsule and synovial cavity thickness in apparently healthy children

BACKGROUND

The usefulness of musculoskeletal ultrasonography (MSUS) in paediatric population is limited by lack of reference values. One of such parameters is hip joint capsule thickness, postulated as an early measure for synovitis. However, the joint capsule is hardly a distinguished structure from slit synovial cavity in patients with little or no fluid collection. Therefore, in patients without effusion, it is more convenient to measure hip joint capsule thickness together with synovial cavity. The aim of the study was to establish percentile chart for hip joint capsule and synovial cavity thickness (HJC&SCT) in apparently healthy children.

MATERIAL AND METHODS

The analysis included 816 US of hip joint in 408 children without musculoskeletal disorders, distributed equally throughout the whole developmental period in 18 one-year subgroups. Hip joints US was performed according to standard protocol including measurement of HJC&SCT in a single rheumatology centre by three investigators.

RESULTS

The 3rd, 10th, 25th, 50th, 75th, 90th, and 97th HJC&SCT percentile curves were depicted in the age and height charts for the combined group of girls and boys. The median HJC&SCT values were increasing with age from 3.7 (C10 - C90: 3.3 - 4.2) mm in the first year of life up to 6.7 (5.8 - 7.3) in 16 years old, and above. In a similar way the increase was seen with height from 3.9 (3.5 - 4.7) mm in shorter than 95 cm to 6.9 (6.2 - 7.4) mm in taller than 169 cm subjects. Intra-observer and inter-observer mean precision was less than 1.8 and 12.5%, respectively.

CONCLUSION

The developed centile chart for hip joint capsule and synovial cavity thickness in the paediatric population is expected to improve detection of hip joint capsule disorders, including synovitis in juvenile idiopathic arthritis.