Increased Intracranial Pressure

After closure of the cranial fontanelles and sutures by 2 to 3 years of age, the skull is a rigid container of fixed volume. The intracranial contents constitute the brain bulk (80%), blood volume (10%), and cerebrospinal fluid (CSF) (10%). According to the Monro-Kellie doctrine, intracranial volume is equal to the volume of the brain bulk, blood, CSF, and other mass lesions. Therefore, an increase in the volume of one of these compartments can raise ICP and subsequently reduce cerebral perfusion pressure (which is defined as mean arterial pressure minus ICP) and cerebral blood flow (CBF). CSF volume appears to be the major compensatory mechanism, with the majority in the subarachnoid spaces of the spine and brain and only 10% in the intracranial ventricular system. This CSF volume compensatory mechanism can be expressed as the volume-pressure intracranial compliance (∆V/∆P = compliance) or pressure volume index (PVI). The PVI is the volume of fluid injected or withdrawn that would result in a 10-fold change in ICP and is calculated as PVI = ∆V / Log Pf / Po, where ∆V = volume of fluid injected or withdrawn, Pf = final ICP, and Po = initial ICP. The PVI varies in proportion to the estimated neural axis volume. The PVI is 8 mL in an infant and 25 mL in a 14-year-old; therefore, a 10 mL volume added to the neural axis of a 14-year-old may produce a modest elevation in ICP, whereas the same volume can be lethal in an infant (Figure 86-1). However, in infants and young children who have open cranial fontanelles and sutures, the Monro-Kellie doctrine does not apply, because the cranial vault will compensate by expanding.