De Novo MYH9-Related Macrothrombocytopenia in a Toddler: Insights From Platelet Mass Index

Aims/Background: Myosin heavy chain 9-related disease (MYH9-RD) is a rare inherited disorder characterised by macrothrombocytopenia, often misdiagnosed as immune thrombocytopenia (ITP). Early identification is crucial to prevent unnecessary treatments and to ensure appropriate monitoring. The present case aims to highlight the diagnostic challenges and clinical management of MYH9-RD in a toddler, emphasising the importance of early genetic testing. Case Presentation: We discuss a 13.5-month-old girl with macrothrombocytopenia lacking Döhle bodies, who initially received intravenous immunoglobulin (IVIg) and corticosteroids without any response. Within two months, whole-exome sequencing identified a pathogenic MYH9 mutation (c.287C>T; p.Ser96Leu). Results: One year later, the patient remains clinically stable without significant bleeding. The occurrence of petechial rash exhibited a more pronounced correlation with platelet mass index (PMI) values compared to platelet count (PLT), underscoring its significance in clinical evaluation. Conclusion: MYH9-RD should be considered in cases of IVIg-resistant thrombocytopenia accompanied by macrothrombocytes. Timely genetic testing can facilitate accurate diagnosis and may help avoid unnecessary procedures, while routine renal and auditory monitoring is important for managing the S96L variant.