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NCS Currents Sept 2016

TECH CORNER Tympanometry Tympanic membrane displacement (TMD) elicited by the acoustic reflex can be non-invasively measured quantitatively and has been shown to vary with ICP. The use of TMD is limited in the clinical setting, however, as it is useful in only a limited number of patients. The test relies on perilymphatic duct patency which is reduced with advancing age. Otoacoustic emission frequencies from cochlear cells similarly vary with changes in ICP, though this method is limited by wide patient variability. Near-Infrared Spectroscopy (NIRS) Transcranial near-infrared spectroscopy is a method of measuring cerebral oxygenation, an indirect measure of the metabolic state of cerebral tissue. Light from the near-infrared spectrum is transmitted transcranially at frequencies – typically in the 700-850 nm range – that are known to be absorbed specifically by oxygenated hemoglobin and not by deoxy-hemoglobin. The ratio of oxygenated to total hemoglobin – the Tissue Oxygenation Index (TOI) – is thought to correspond to cerebral blood flow. In patients with ICP elevation, cerebral blood flow becomes diminished due to loss of cerebral perfusion pressure and oxygen delivery. Advantages to this methodology are its ease of use and ability to monitor continuous intracranial perfusion, an important clinical endpoint following elevation in ICP. NIRS, however, does not allow actual measurement of ICP. As it is typically applied to the frontalis, monitoring is limited to frontal lobe perfusion, thus it is not a measure of global cerebral oxygenation. Difficulty is also encountered when dealing with artifactual spectroscopic absorption from clinically unimportant variable such as hair and skin pigment. Electroencephalography (EEG) EEG is useful for continuous monitoring of the adverse outcomes following ICP elevation, including seizures and status epilepticus, as well as identifying regions prone to ischemic disease. The ratio of preponderance of alpha activity to delta activity is thought to correlate with depressed cerebral perfusion. This is a quantifiable and continuous variable. Focal slowing may identify specific regions at risk for ischemic damage. Continuous EEG, however, has no specific ICP surrogate, requires specialized training for analysis, and has limited availability outside of large institutions. Visual Evoked Potentials (VEPs) Patients are shown a bright light and occipital EEG electrodes record characteristic electrical responses. Latency of the N2 and P3 waves can predict ICP elevation, though significant variability between patients limits the utility in the clinical setting. We are in the midst of a new era where neurointensivists possess previously unavailable tools that may allow them to avoid risks inherent with invasive monitoring. In cases where invasive intracranial monitoring is contraindicated or not feasible, as in cases of malignant hemispheric strokes or fulminant hepatic failure, the use of noninvasive surrogate markers of elevated ICP can be instrumental in guiding neurocritical care of these patients. The authors have no actual or potential conflict of interest in relation to the topics discussed in this section. We may discuss non-FDA approved devices and “off-label” uses. The NCS and Currents do not endorse any particular device. 21


NCS Currents Sept 2016
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