Incidence of Subdural Hygroma on Routine Early Postoperative Imaging Following Foramen Magnum Decompression for Chiari Malformation Type 1 in 156 Patients.

Objective: To determine the incidence of subdural hygroma (SDH) on routine early postoperative imaging following foramen magnum decompression (FMD) with dural opening in patients with Chiari 1 malformation (CM1).

Methods: Clinical and radiological data of 156 consecutive patients with CM1 who underwent FMD (2006-2023) were retrospectively analyzed. Computed tomography scans done on the seventh postoperative day or earlier were reviewed for the presence of SDH (infra-and supratentorial) and ventriculomegaly.

Results: There were 96 (61.5%) males. The median age was 29.1 years. Sixty four out of 156 (41%) patients had SDH on postoperative computed tomography done at median interval of 7 days (IQR, 5.8-7 days) after surgery. 22/64 (34.4%) patients were symptomatic for SDH, the main symptoms being holocranial headache (n = 16), CSF leak (n = 5) and acute respiratory distress (n = 3). There was significant association between development of ventriculomegaly or worsening of pre-existing ventriculomegaly and presence of SDH (P = 0.004). The associated ventriculomegaly, present in 7 of 22 patients with symptomatic SDH, was managed with external ventricular drainage (n = 2), ventriculoperitoneal shunt (n = 3) or widening of FMD and external ventricular drainage (1)/ventriculo peritoneal shunt (1) (n = 2). Two patients with SDH and ventriculomegaly died and 2 were moribund at 3 months after surgery. At median follow up of 12 months, the other 18 patients with symptomatic SDH had symptom resolution.

Conclusions: SDH is a common finding in the early postoperative scans of patients undergoing FMD and dural opening for CM1. While nearly two-thirds of these patients are asymptomatic, SDH with ventriculomegaly can be associated with mortality and significant morbidity and may require emergency treatment.