Severe achondroplasia with developmental delay and acanthosis nigricans (SADDAN) is a bone dysplasia caused by a pathogenic variant of fibroblast growth factor receptor 3 (). Pathogenic variants in also cause thanatophoric dysplasia (TD) and achondroplasia. Although the findings of SADDAN and TD during the fetal and neonatal periods are similar, they differ in their long-term prognoses. We conducted analysis in one male patient because of the difficulty in differentiating SADDAN from TD during the neonatal period. We found that the patient had a pathogenic variant, p. Lys650Met, which was similar to that previously reported in patients with SADDAN. Reports on long-term survival in patient with SADDAN are scarce, and there have been no reports of treatment with GH. We administered GH therapy for a markedly short stature. After treatment, his height increased by 4 cm each year for 4 years, the frequency of hospitalizations due to respiratory failure decreased, and the health improved. analysis is useful for diagnosing SADDAN during the early neonatal period. GH therapy may have contributed to the patient's long-term survival.
