Primary intraosseous vascular malformation (VMPI, #606893) is an ultra-rare disorder caused by biallelic pathogenic variants in ELMO2. To date, only six families with pathogenic ELMO2 variants causing a VMPI phenotype have been described. VMPI is characterized by vascular malformations that compress the facial bones, often leading to life-threatening complications, such as massive bleeding and intracranial herniation. In VMPI, vascular malformations are progressive and there is no causal therapy available. We report on four unreported individuals with classical VMPI harbouring biallelic truncating variants in ELMO2, including a novel homozygous 25 bp duplication c.579_603dup
p.(Leu202Profs*47), detected by whole-exome sequencing. We present extensive clinical follow-up data, including a close monitoring of an individual from prenatal diagnosis onwards. Using computed tomography or magnetic resonance imaging angiography, we described the radiological characteristics of vascular malformations with fast-flow properties in the affected individuals. Additionally, we conducted a comprehensive histopathological evaluation of samples from one individual. This analysis revealed not only the similar morphological features described previously but also some atypical findings, such as increased de novo bone formation. Furthermore, we report for the first time the use of propranolol and sirolimus in VMPI. While we noted a reduction of bleeding episodes in one individual, no significant clinical improvement was observed overall in the other individuals treated with sirolimus. Moreover, sirolimus led to severe infectious complications with abscess formation in two individuals. Conversely, propranolol was relatively well tolerated, although it did not result in any notable clinical outcomes. During follow-up, one individual died due to severe bleeding.