The use of optic nerve sheath diameter (ONSD) measurements as a non invasive test for raised ICP has many potential benefits. This includes having assessments that can be made at the bedside instead of requiring patient transport to CT or MRI, which in itself can cause a rise in ICP. If ONSD represents an acute and sensitive measurement of elevated ICP, it may prove useful in the emergency department (ED) setting in the evaluation of shunts used for cerebrospinal fluid (CSF) diversion. The investigators hypothesize that ONSD will correlate with measured ICP and thus be a reliable tool for the bedside assessment of ICP. The investigators further hypothesize that ONSD changes will correlate with changes in ICP, making it a useful bedside tool for assessing ICP changes over time. This is a prospective, observational study using a convenience sample. Children who are 0 - 18 years old who have an invasive intracranial ICP monitoring device admitted to the hospital Pediatric Intensive Care Unit (PICU) are eligible for participation in the study. Children who are 0-18 years old who are determined to require a shunt revision following evaluation in the ED will also be included in the study. 10. After the invasive intracranial pressure monitoring device is placed, for every recruited patient, ONSD measurements will be taken and the ICP will be recorded 8 hours post placement and then repeated 3 times a day until the intracranial pressure monitoring device is removed.12. For ventriculoperitoneal (VP) shunt revision patients, the investigators will obtain a measurement just before surgery, immediately post-op, and then 12 hours, 24 hours, and 36 hours post-op. If the patient is otherwise ready for discharge prior to 36 hours, the measurement may be omitted or done early so as to not unnecessarily prolong the hospitalization