Growth impairment in mucopolysaccharidoses

Mucopolysaccharidoses (MPS) are a group of lysosomal storage disorders that affect regulation of glycosaminoglycan (GAG) processing. In MPS, the lysosomes cannot efficiently break down GAGs, and the specific GAGs accumulated depend on the type of MPS. The level of impairment of breakdown varies between patients, making this one of the many factors that lead to a range of clinical presentations even in the same type of MPS. These clinical presentations usually involve skeletal dysplasia, in which the most common feature is bone growth impairment and successive short stature. Growth impairment occurs due to the deposition and retention of GAGs in bone and cartilage. The accumulation of GAGs in these tissues leads to progressive damage in cartilage that in turn reduces bone growth by destruction of the growth plate, incomplete ossification, and imbalance of growth. Imbalance of growth leads to various skeletal abnormalities including disproportionate dwarfism with short neck and trunk, prominent forehead, rigidity of joints, tracheal obstruction, kyphoscoliosis, pectus carinatum, platyspondyly, round-shaped vertebral bodies or beaking sign, underdeveloped acetabula, wide flared iliac, coxa valgus, flattered capital femoral epiphyses, and genu valgum. If left untreated, skeletal abnormalities including growth impairment result in a significant impact on these patients' quality of life and activity of daily living, leading to high morbidity and severe handicap. This review focuses on growth impairment in untreated patients with MPS. We comprehensively describe the growth abnormalities through height, weight, growth velocity, and BMI in each type of MPS and compare the status of growth with healthy age-matched controls. The timing, the degree, and the difference in growth impairment of each MPS are highlighted to understand the natural course of growth and to evaluate future therapeutic efficacy.