Early Versus Late Enzyme Replacement Therapy in Siblings With Morquio A Syndrome: Insights Into Therapeutic Timing.

Enzyme replacement therapy (ERT) with elosulfase alfa is the only approved treatment for mucopolysaccharidosis type IVA. This case report delineates the 5-year outcomes of ERT in two Korean siblings with mucopolysaccharidosis type IVA, with the younger sibling initiating treatment at 0.8 years of age and the older one at 5.4 years. Both patients exhibited a progressive decline in their height standard deviation scores, with trajectories paralleling the natural history curves of the disease. At 5.2 years of age, persistent skeletal dysplasia was evident in both siblings. However, the younger sibling demonstrated attenuated disease severity, lacking cervical myelopathy or spinal stenosis requiring C1 laminoplasty. Cardiorespiratory assessment revealed normalized left ventricular mass index z-scores, stable ejection fractions, and the absence of valvular pathology. Overall, these findings suggest that early ERT attenuates severe spinal and upper body manifestations but does not prevent lower limb skeletal progression, highlighting the need for early therapeutic initiation along with orthopedic intervention to preserve cardiorespiratory parameters and functional independence.