The investigators propose a prospective clinical study focused on a unique large series of 9 adult patients (aged from 18 to 55 years) followed in a single expert center for metabolic disorders located at the university hospital of Bordeaux, France. Eight of these patients are affected with atypical MPS IVA characterized by less severe evolution of the disease and heights ranging from 135 to 176 cm (the last patient height is 102 cm). Investigators aim to increase knowledge on the natural history of the disease in adult patients with atypical MPS IVA, treated or not with ERT, and to develop new objective and robust clinical criteria to evaluate the efficiency of ERT over time, particularly in patients presenting an atypical phenotype. The entire cohort treated or not treated with ERT, will be evaluated at baseline and every year during a 5-years period. The complete evaluation at baseline will be our absolute priority as well as obtaining long-term and exhaustive follow up of the patients treated with ERT (two patients of the cohort already treated, and ERT expected in three additional patients in the next months).