Conditions associated with mass effect, such as intracranial tumors and hemorrhagic stroke (intracerebral hemorrhage; ICH), disrupt intracranial pressure (ICP) regulation by exhausting the brain’s compliance reserves. Displacement of cerebrospinal fluid, blood, and brain tissue (“tissue compliance”) can buffer ICP elevations, but these reserves are limited. Brain tumor patients face elevated ICH risk, yet how pre-existing mass effect influences acute intracranial compliance remains unclear. This study examined how chronic intracranial mass effect alters tissue compliance following acute ICH.

In an aged spontaneously hypertensive rat (SHR) cohort, a high incidence of spontaneous intracranial tumors were discovered. This prompted an exploratory analysis in 18 SHRs (n = 6/group: ICH-24 h, ICH-72 h, sham) to evaluate the impact of chronic mass effect on post-ICH tissue compliance. Following collagenase striatal hemorrhage, brains were collected for macroscopic and microscopic volumetric morphological analysis.

Tissue compliance persisted in tumor-bearing animals, reflected by reduced contralateral hemisphere volume (~6% at 24; ~14 % at 72 h) and neuronal soma shrinkage in hippocampal and cortical regions. Despite smaller hematoma volumes, rats with tumors exhibited nearly twofold greater tissue compliance responses than non-tumor animals.

Pre-existing mass effect may modify the recruitment of intracranial compliance reserves following stroke in aged animals.