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NCS Currents Sept 2016

TECH CORNER Non-Invasive Intracranial Pressure Monitoring: Surrogate Markers of Elevated ICP By Fawaz Al-Mufti, MD and William Roth, MD Although invasive intracranial pressure (ICP) monitoring remains the gold standard in the acute diagnosis of ICP crisis, several strategies have been developed to avoid potential adverse effects of invasive monitoring. Utilizing surrogate markers of elevated ICP in conjunction with each other can provide rapid and accurate assessment of ICP beyond traditional bedside assessment. Optic Nerve Sheath Diameter (ONSD) The optic nerve sheath is an extension of the subarachnoid space, which expands and contracts with corresponding changes in ICP. Ultrasound evaluation of the nerve sheath diameter is a fast, accurate, low-cost, reliable, and reproducible method of monitoring changes to ICP from any cause. Though not continuous, it allows relatively rapid assessment and provides excellent sensitivity. Proper training is recommended for good inter-rater reliability. Fawaz Al-Mufti, MD ONSD that is under 5mm is consistent with normal ICP and over 6 mm implies elevated ICP. In our practice, ONSD assessment has become part of our routine initial evaluation with our Focused Assessment with Sonography in Trauma (FAST) and Focus Assessed Transthoracic Echo (FATE) assessments to obtain a baseline measurement and every 4-6 hours thereafter. A trend documenting an increase in ONSD is typically more helpful in clinical practice than a single measurement and should be used in conjunction with other surrogate markers of ICP. Pupillometry In a previous article, we discussed pupillometry as a valuable tool in the Neuro-ICU. The Frisen score has be used to assess the degree of papilledema and the presence of elevated ICP using an ophthalmoscope, however papilledema often lags behind acute increases in ICP. Loss of the pupillary reflex often occurs in the setting of elevated intracranial pressure due to compression of the oculomotor nerve from uncal herniation. Through automated pupillometry, pupillary reactivity can be quantified by the Neurological Pupil Index (NPI), which is a value obtained by an algorithm based on various calculated parameters (e.g., Constriction Velocity, Dilation velocity, latency of the pupillary reflex) graded on a scale of 0-5. In a study of 134 patients in eight Neuro ICUs, patients with abnormal pupillary light reactivity had an average peak ICP of 30.5 mmHg versus 19.6 mmHg for the normal pupil reactivity population (p=0.0014). Using NPI, abnormal pupillary reactivity was identified 15.9 hours prior to the time of the peak of ICP. Quantitative measurement and classification of pupillary reactivity using NPI may be useful in the early management of patients with increased ICP. Transcranial Doppler Ultrasonography (TCDs) Insonation of large intracranial vessels allows visualization of waveforms and flow velocities through which one can estimate both ICP and cerebral perfusion pressure (CPP). The Gosling Pulsatility Index (PI) – the difference between systolic and diastolic flow velocity/mean flow velocity – is thought to represent downstream resistance to intracranial blood flow. In one study of 762 TCD sessions from 290 patients, a PI of 0.74 corresponded to an ICP >35 mmHg and a PI of 0.81 corresponded to a CPP <50 mmHg, suggesting that extreme values of PI can be used to support a decision for invasive ICP monitoring. Insonation of large intracranial veins is less frequently used but provides similar information. Venous compression occurs prior to compression of arteries, thus hemodynamic changes may be reflected earlier in the venous system. Pulsatility index is similarly used to estimate resistance to venous flow. Due to technical difficulty of insonating the intracerebral veins in many patients with anatomic variants, the use of venous ultrasonography remains somewhat limited in the clinical setting. Simultaneous insonation of intra- and extracranial segments of the ophthalmic artery can also be used to estimate ICP. The intracranial segment is compressed with ICP elevation. By providing external pressure to the orbit, the extracranial segment also becomes compressed. When those flow velocities equalize, the applied pressure corresponds to ICP. William Roth, MD 20


NCS Currents Sept 2016
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