What causes dilated pupils in cerebral hemorrhage?

50% of patients with intracerebral hemorrhage develop pupillary dilation. The common cause is a rapid increase in intracranial pressure due to the hemorrhage, leading to displacement of brain tissue from areas of high pressure to areas of low pressure. This can occur through normal or pathological pathways such as the tentorial notch, foramen magnum, and the falx cerebri gap. Eventually, this displacement compresses critical structures including brain tissue, blood vessels, and nerves—especially the oculomotor nerve. As a result, the pupil on the same side as the hemorrhage dilates, indicating the development of brain herniation. Therefore, sudden pupillary enlargement signifies the formation of brain herniation.