The Pathophysiology of Sinking Flap Syndrome Associated with Low-Pressure Hydrocephalus: A Case Study Suggests a New Hypothesis

Introduction: Decompressive craniectomy (DC) is often required to stabilize the intracranial pressure (ICP) in patients with traumatic brain injury (TBI). Both sinking flap syndrome (SFS) and hydrocephalus are known complications of DC. The pathophysiology of each is unknown. Case Report: We report on a patient who underwent DC for TBI who suffered both SFS and low-pressure hydrocephalus. We measured the changes in volumes of each hemisphere and the ventricles with CT and the cerebral blood flow (CBF) and aqueduct flow with phase-contrast MRI during different stages of the disease process. Discussion: The SFS in this patient was associated with a reduction in volume of both supratentorial cavities. There was a significant reduction in CBF bilaterally, which increased by an average of 26% following cranioplasty. During the low-pressure hydrocephalus phase of the patient’s illness, there was reversed CSF flow directed toward the ventricles. Once the ventricles returned to normal size, this reversed flow was lost. Conclusions: Lumped parameter modelling of the patients’ CSF and vascular systems suggested a new hypothesis, i.e., that the reduction in blood flow was due to reversible constriction of the arterioles secondary to a reset of the autoregulation rather than compression of the venous structures. We suggest there is an increase in CSF absorption efficiency despite the known CSF-absorption mechanisms being unlikely to function at such a low ICP. A hypothesis is put forward that CSF absorption occurs via the brain capillary bed in these diseases.