Imaging intracranial pressure: an introduction to ultrasonography of the optic nerve sheath.

I N clinical practice it is important to determine whether increased intracranial pressure is present. This allows one to alter hemodynamic management, predict the potential for neurologic deterioration, and guide therapeutic removal of cerebrospinal fluid or avoid such withdrawal. A spinal leak or drainage of cerebrospinal fluid can create an imbalance between supratentorial and infratentorial pressures that could lead to catastrophic herniation of the brainstem. Spinal anesthesia and unintended dural puncture with epidural placement have both been associated with acute herniation of the brainstem in patients with unexpectedly increased intracranial pressure. In obstetrical anesthesia, we commonly encounter patients who are at risk for increased intracranial pressure but for whom a spinal or epidural anesthetic is medically indicated. In this issue of ANESTHESIOLOGY, Dubost et al. introduce a noninvasive technology, ocular ultrasonography, that can be used to detect and monitor increased intracranial pressure in preeclampsia. Optic nerve sheath diameter has been shown in several clinical trials to be highly predictive of intracranial pressure in the settings of trauma, hemorrhage, and hydrocephalous. This technique has the potential to improve the clinical care of preeclamptic patients. Preeclampsia is a multisystem disease unique to human pregnancy that affects every organ system, including the brain. Although advances have been made in understanding disease pathophysiology, identification of preeclamptic women who will progress to severe disease and/or eclampsia remains a diagnostic challenge. In 10–15% of patients who progress to eclampsia, brain pathology is not mirrored by systemic signs. Hypertension is absent or modest and/or proteinuria not detected. Progression to eclampsia is associated with increased risk of cerebrovascular accident, aspiration, cardiac arrest, and death. In addition, eclampsia is thought to have longterm neurologic consequences, including the increased risk of subcortical white matter lesions and impaired neurocognitive functioning. Although the mechanism is poorly understood, increased intracranial pressure can be a complication of preeclampsia and likely is associated with progression to eclampsia. The rate of change in intracranial pressure in the setting of preeclampsia is not known, but eclampsia has been associated with increased perfusion pressure (as measured by transcranial Doppler), which resolved promptly after treatment with magnesium. Although we commonly worry about increased intracranial pressure, it can be difficult to detect. Symptoms such as hypertension and nausea associated with increased intracranial pressure can be caused by preeclampsia itself and thus are nonspecific. Yet knowledge of the presence of increased intracranial pressure could drastically modify a clinician’s approach by affecting choice of technique, need for additional monitoring, hemodynamic goals, the use of magnesium for seizure prophylaxis, and timing of delivery. The presence of an increased intracranial pressure would make dural puncture less desirable and epidural or general anesthesia preferable. To determine whether the intracranial pressure was increased, the authors used a 7.5-MHz linear ultrasound probe (available on many ultrasound machines used for vascular access and regional anesthesia) to measure the diameter of the optic nerve sheath 3 mm behind the globe. Five of 26 preeclamptic subjects had an increased optic nerve sheath diameter, with the mean measurement greater than 5.8 mm on the