What factors contribute to increased intracranial pressure (ICP)?

What factors contribute to increased intracranial pressure (ICP)?

The cranial vault contains brain tissue, blood, and CSF. It has a fixed volume with one exit, the foramen magnum. A small increase in its contents alters ICP minimally, because compensation occurs by decreasing CSF and blood volumes. Once compensatory mechanisms have been exhausted, however, intracranial compliance falls dramati-cally and ICP rises above its normal range of 5–13 mmHg. At this point, any increase in the volume of the three intracranial components has a profound effect on increasing ICP.

Brain tissue volume increases by two mechanisms. Tumors enlarge the parenchymal tissue volume, and sur-rounding edema increases brain water content. Both con-tribute to expanding total intracranial mass, which can be compounded by hemorrhage into or around the neoplasm.

CSF is contained in the subarachnoid space of the central nervous system and in the ventricles within the brain. In adults, total CSF volume approximates 150 ml, and daily production is approximately 600 ml. Enough CSF is synthesized to replace itself three to four times a day. CSF is formed in the choroid plexus and the ventricles within the brain. Production is reduced by decreased CPP and increased ICP, but the latter effect is minimal. Absorption of CSF occurs at the microscopic villi of the subarachnoid membrane. The rate of absorption is a balance between two interrelated factors: CSF pressure and resistance to absorption. Resistance to absorption decreases dramati-cally as CSF pressure rises above 30 mmHg. This appears to represent a closed loop venting mechanism to prevent excessive increases in ICP. Therefore, the amount of CSF present is determined by the balance between production and reabsorption.

Changes in cerebral blood volume usually have little effect on ICP. When intracranial compliance is low, however, and intracranial volume has increased to critical levels, then these changes may have dramatic consequences.