Premature Fusion of the Posterior Intraoccipital Synchondrosis, A Minor Suture Mimic of Lambdoid Craniosynostosis.

The posterior intraoccipital synchondrosis (PIOS) is a cartilaginous division separating the exoccipital and supraoccipital bones, allowing for flexibility of the cranial base at birth and which later ossifies in adolescence. The authors report a case of right PIOS synostosis that resembled right lambdoid synostosis, with left occipital bossing in a healthy, six-month-old female with an unremarkable birth history and no antecedent trauma. An initial referral was made from the pediatrician to oncology because of a concern over the presence and growth of a possible mass at the left occiput. Over 8 months, this mass grew and became more firm, accompanied by left occipital bossing and right occipital flattening. A computed tomography was obtained, which demonstrated the fusion of the PIOS, prompting a referral to plastic surgery. Cranial vault remodeling with switch cranioplasty was performed at age 14 months, complicated only by a superficial infection along the suture line. There are exceedingly few reports of PIOS synostosis, with occipital osteodiastasis related to birth trauma as the only prior explanation for this condition. With no birth trauma and delayed onset, our case likely represents idiopathic PIOS synostosis. With the many similarities in presentation, the surgical approaches commonly used for lambdoid synostosis, particularly switch cranioplasty, are suitable solutions to PIO synchondrosis with optimal functional and aesthetic outcomes.