Hyperostosis Cranii Ex Vacuo in Shunted Children: A Proposed Fifth Subtype of CSF Overdrainage Syndrome

Background: Cerebrospinal fluid (CSF) shunting remains a crucial intervention in the treatment of paediatric hydrocephalus. Overdrainage syndrome is a well-recognised but potentially severe complication, in which hyperostosis cranii ex vacuo—diffuse thickening of the cranial bones—emerges as an adaptive response to chronic intracranial hypotension. Currently, no established diagnostic criteria exist to reliably identify and classify this phenomenon, nor are there defined strategies to prevent associated complications of reduced intracranial compliance. Objective: This study aimed to characterise the morphoradiological and clinical phenotype of hyperostosis cranii ex vacuo in paediatric patients with long-term shunt dependency and to propose its classification as a fifth subtype of CSF overdrainage syndrome with direct implications for long-term neurosurgical care. Methods: A retrospective observational study was conducted on nine paediatric patients with radiologically confirmed diffuse calvarial thickening secondary to surgical treatment of hydrocephalus. Quantitative morphometric analysis of frontal, parietal, and occipital bones, sella turcica dimensions, and dural enhancement was performed using high-resolution neuroimaging. Clinical records were reviewed for hydrocephalus aetiology, shunt revision history, and neurological impairment. Results: All patients exhibited a mean two-fold increase in age-adjusted calvarial thickness. Premature craniosynostosis was identified in 33.3% of cases. Diffuse pachymeningeal enhancement was noted in all patients with contrast-enhanced imaging. Neurological comorbidities included epilepsy, spastic paraparesis, and features of Chiari type I malformation. Conclusions: Hyperostosis cranii ex vacuo represents a distinct and underrecognised consequence of chronic CSF overdrainage. We propose preliminary diagnostic criteria and a structured management pathway—from radiological recognition through ICP assessment to tiered surgical intervention. Formal recognition of this entity as a fifth subtype of CSF overdrainage syndrome may enhance early diagnosis, improve risk stratification, and guide long-term surveillance of shunted children.