Advanced bedside neuromonitoring

Introduction

Little can be done to reverse the primary brain damage caused by an insult; however, one of the major factors influencing outcome in patients with acute brain injury is the additional brain damage that occurs from secondary brain injury processes. These secondary insult processes entail a combination of inflammatory, biochemical, and excitotoxic changes, ^, making it challenging for the clinician to detect which processes are most important to monitor in order to obtain the most favorable outcome. In addition, secondary ischemic insults of extracerebral origin (e.g., arterial hypotension, hypoxemia) can be prevented or treated before they become severe enough. The main purpose of advanced monitoring of the brain in the intensive care unit (ICU) is to detect these secondary insults, allowing for a more informed, individualized approach to treatment.

Monitoring neurologic status

The clinical approach to a patient with a neurologic problem requires the physician to have a specialized anatomic and physiologic knowledge of the nervous system. Daily evaluation of neurologic and mental status should be included in the neuromonitoring protocol. Neuromonitoring should include the Glasgow Coma Scale (GCS) score, function of pyramidal and extrapyramidal systems, status of cranial nerves, function of the cerebellum and spinal cord whenever possible, and any changing trend in the neurologic status. In critically ill patients, such a complete neurologic evaluation can sometimes be unreliable or impossible because of the use of sedatives and the need for intubation and ventilatory support as part of the medical treatment of the neurologic problem. However, in the nonsedated patient, the Full Outline of UnResponsiveness (FOUR) score, which measures ocular and limb responses to commands and pain, pupillary responses, and respiratory pattern, may provide a more complete assessment of brainstem function. The FOUR score has been shown to perform equally well as the GCS in critically ill patients after brain injury, possessing a similarly predictive power of outcome and the capability of being assessed in intubated patients. ^, Current evidence suggests that both the GCS and FOUR score provide useful and reproducible measures of neurologic state and can be routinely used to chart trends in clinical evaluation.

Pupillary evaluation is a strong predictor of outcome that must be integrated into the daily GCS evaluation. However, poor interrater reliability exists among practitioners when it comes to pupillary assessment. Devices like the handheld pupillometer provide objective measurement of pupillary response and diameter, but more clinical experience is needed to determine if they should be included as a standard of care.

Along with neurologic examination, information about vital signs and key laboratory values should be immediately available in a 24-hour record sheet or electronic medical record. Assessment of pain and sedation can be challenging in the context of brain injury. Evidence recommends the use of validated and reliable scales such as the Sedation-Agitation Scale (SAS) and Richmond Agitation-Sedation Scale (RASS), as these provide workable solutions in some patients.

“Wake-up tests” in patients with intracranial hypertension pose significant risks and show no proven benefits in terms of duration of mechanical ventilation, length of intensive care unit (ICU) and hospital stay, or mortality rate in patients with neurologic disorders.

The Glasgow Outcome Scale (GOS) has been the standard outcome tool for neurocritical care. However, other tools, such as the Neurological Outcome Scale for TBI (NOS-TBI), have been adapted for traumatic brain injury (TBI) patients from the National Institutes of Health Stroke Scale (NIHSS). This scale has demonstrated adequate predictive validity and sensitivity to change compared with gold-standard outcome measures and may enhance the prediction of outcome in clinical practice and research.

Intracranial pressure and cerebral perfusion pressure

The threshold that defines normal or raised intracranial pressure (ICP) is uncertain, but normal resting ICP in an adult is considered to be less than 15 mm Hg, and a sustained ICP >20 mm Hg is considered pathologic. ^, In the 2016 Guidelines for the Management of Severe TBI, an ICP threshold >22 mm Hg was adopted as a level IIb recommendation to initiate treatment to reduce ICP in order to reduce mortality. The necessity of ICP monitoring was questioned by the BEST-TRIP trial : the researchers found no significant difference in outcome when management of patients with severe TBI was guided by either (1) ICP monitoring or (2) neurologic examination and imaging. However, the results of this trial are research oriented and highlight the necessity for further studies and may not be generalizable to other populations. First, the study was conducted on two developing countries where prehospital resuscitation might be less developed, resulting in higher episodes of secondary insults such as hypotension and hypoxia, and severely injured patients may not survive long enough to reach the hospital. Thus the study population may have had less severe brain injury than comparable ICUs in developed countries. Finally, 6-month survival was confounded by increased mortality after ICU discharge, possibly related to limited resources to receive rehabilitation and prolonged medical care. In the most current Brain Trauma Foundation Guidelines (BTFG) for severe TBI, monitoring of ICP is recommended in the management of severe TBI patients to reduce in-hospital and 2-week postinjury mortality, and the previous 2007 BTFG recommendations are no longer valid because of lack of supporting evidence. Moreover, the Milan consensus does not recommend ICP monitoring in a comatose patient in the setting of normal imaging. Treatment of ICP is important, and in circumstances where adequate resources are available, such treatment is advised.

Cerebral perfusion pressure (CPP) is the difference between the mean arterial blood pressure (MAP) and ICP. Under normal physiologic conditions, a MAP of 80–100 mm Hg and an ICP of 5–10 mm Hg generate a CPP of 70–85 mm Hg. Cerebral blood flow (CBF) is determined by both CPP and cerebrovascular resistance (CVR) according to the formula CBF = CPP / CVR.

Under normal circumstances, the brain is able to maintain a relatively constant CBF of approximately 50 mL per 100 g/min over a wide range of CPP (60–150 mm Hg). After injury, the ability of the brain to pressure autoregulate can be impaired, and CBF is often dependent on CPP.

The recommendations for an adequate CPP have changed over time and may in part be associated with the variability in how MAP is measured and the autoregulatory function status. The 2007 BTFGs recommend targeting CPP values within the range of 60–70 mm Hg. CPP values <60 mm Hg increase the risk of cerebral ischemia and hypoperfusion, whereas therapies required to maintain CPP >70 mm Hg such as intravenous fluids administration have been associated with an increased risk of acute respiratory failure.

Intracranial pressure monitoring devices

The current gold standard for ICP monitoring is the ventriculostomy catheter, or external ventricular drain (EVD), which is a catheter inserted in a lateral ventricle, usually via a small right frontal burr hole. This ventricular catheter is connected to a standard pressure transducer that must be maintained at a specific level. The reference point for ICP is the foramen of Monro, although in practical terms, the external auditory meatus is often used as a landmark. Advantages of EVDs include the ability to measure global ICP and to perform periodic in vivo external calibration and therapeutic cerebrospinal fluid (CSF) drainage and sampling. However, intraventricular catheters are also associated with the highest rate of infection among the ICP monitors. Several microtransducer-tipped ICP monitors are available for clinical use (e.g., Camino ICP monitor, Codman microsensor, Hummingbird ICP, and Neurovent-P ICP monitor). These catheter-based transducers can measure pressure directly in the brain parenchyma. Although there are fewer risks of infection and intracranial hemorrhage with these catheters, the main disadvantage of these probes is that they cannot be calibrated in vivo. Also, after preinsertion calibration, they may exhibit zero drift (degree of difference relative to zero atmospheres) over time.

Noninvasive ICP monitoring devices have been developed to reduce the risk associated with invasive monitors. Such technologies include displacement of the tympanic membrane, optical detection of cerebral edema, transcranial Doppler pulsatility index, and magnetic resonance of the optic nerve sheath, among others. So far, none of these methods has provided sufficient accuracy to replace invasive monitors.

ICP waveforms

The normal ICP waveform consists of three arterial components superimposed on the respiratory rhythm. The first arterial wave is the percussion wave, which reflects the ejection of blood from the heart transmitted through the choroid plexus in the ventricles. The second wave is the tidal wave, which reflects brain compliance, and the third wave is the dicrotic wave that reflects aortic valve closure. Under physiologic conditions, the percussion wave is the tallest, with the tidal and dicrotic waves having progressively smaller amplitudes. When intracranial hypertension is present, cerebral compliance is diminished. This relationship is reflected by an increase in the peak of the tidal and dicrotic waves exceeding that of the percussion wave ( Fig. 42.1 ).

Complications

Among the complications related to ICP monitoring, intracranial hemorrhage and infections are the most common. ^, Less frequent complications are malfunction, malposition, and obstruction. Although these complications generally do not produce long-term morbidity in patients, they can cause inaccurate ICP readings and may increase hospitalization costs by requiring replacement of the monitor.

The incidence of infection is reported to range from 0% to 22%, depending on the type of device. Several other factors have been identified that may affect the risk of EVD infection: the use of prophylactic parenteral antibiotics, the presence of other concurrent systemic infections, the presence of intraventricular or subarachnoid hemorrhage (SAH), duration of monitoring, open skull fracture (including basilar skull fractures with CSF leak), leakage around the ventriculostomy catheter, and repeated flushing of the EVD. Prophylactic antibiotic-impregnated catheters may be used in order to reduce EVD-associated infections; however, more trials should be conducted to evaluate the beneficial effect on clinical outcome. Placement of ICP monitors should be done under the most sterile possible conditions, minimizing excessive manipulation and flushing. The second most common complication related to ICP monitoring is postprocedural intracerebral hemorrhage (<1%).