PART 3 IN A THREE-PART SERIES
The National Institutes of Health Stroke Scale is the most widely used deficit rating scale in modern neurology, but it was not meant to be a bedside rating tool. This critical piece of the neurologic exam for any suspected stroke patient, originally developed as a research tool to measure baseline data in acute stroke clinical trials, has considerable problems that may arise in clinical practice, however.
The NIHSS is used as a clinical assessment tool to evaluate the acuity of stroke patients, determine appropriate treatments, and predict patient outcomes. Several scale items require intact language, however, so it gives too much weight to deficits in patients with left versus right hemispheric strokes. A left hemispheric stroke can score up to four more points than a right hemispheric stroke of similar size.
Posterior circulation strokes also suffer under the NIHSS because they receive few to no points. Important elements such as diplopia, dysphagia, gait instability, nystagmus, and dizziness receive no score, and a patient with a life-changing stroke could be completely missed if the NIHSS were the only tool considered. A good correlation is seen between NIHSS and the likelihood of finding an eligible large vessel occlusion, but no specific number seems optimal for use. (Stroke 2017;48:513.)
Don't fall into some of these traps; the NIHSS must be used with caution in clinical practice for rating stroke severity. Most importantly, a complete and thorough neurologic examination should be second nature and should always include anterior and posterior components. Most electronic medical records will have an NIHSS template built in, and it is widely available online, but consider other online tools such as MDCalc to help calculate a score. (www.mdcalc.com.)
Neuroimaging of the patient with acute ischemic stroke is used to exclude hemorrhage, assess the degree of brain injury, and identify the vascular lesion responsible for the ischemic deficit. Noncontrast CT (NCCT) of the brain has the main advantage of widespread access and speed of acquisition. It is usually ordered to exclude or confirm hemorrhage and is highly sensitive for this indication, but it is not that sensitive for ischemia before 24 hours, and it has significantly poor sensitivity for the posterior fossa structures. (J Emerg Med 2012 May;42:559.)
CT angiography allows visualization of the extracranial and intracranial cerebral arteries, and CTA source images are more sensitive than NCCT scans for detecting early brain infarction. The value of CTA is to identify patients with occlusion of the major vessels within the circle of Willis or extracranial cerebral arteries, but it will still not be reliable for smaller to even medium areas of ischemia, especially in the posterior circulation territory. (Stroke 2009;40:3646.)
Recently, the Head Impulse, Nystagmus, Test of Skew (HINTS) exam gained attention as a series of three physical exam maneuvers to differentiate peripheral and central causes of acute vestibular syndrome. A positive HINTS exam can be 100% sensitive and 96% specific for the presence of a central lesion. It is also more sensitive than general neurologic signs: 100% vs 51%. Disclaimer ahead (and it is a big one!): The HINTS exam was tested in an outpatient setting by neuro-ophthalmologists, not emergency physicians, and may not be generalizable to an ED population. (Stroke 2009;40:3504.)
Use of brain MRI has significantly increased for evaluating patients with acute ischemic stroke. The rationale for MRI—diffusion-weighted imaging (DWI), in particular—is that this modality has substantially higher sensitivity (88-100%) than that of NCCT for detecting ischemia, with a specificity reported to be as high as 95%-100%. MRI also identifies acute intracranial hemorrhage with similar reliability to NCCT. As if our jobs were not hard enough, emerging evidence now shows that DWI fails to identify acute ischemic stroke in a substantial minority of patients, about seven percent. These DWI-negative stroke cases fall into three categories: posterior circulation ischemia, small strokes, particularly in the brainstem, and hyperacute ischemia (MRI done within six hours of onset). (Neurology 2017;89:256.)
Acute ischemic stroke remains one of the most challenging diagnoses in emergency medicine. We need to continue to rise to the challenge, using clinical judgment followed by a thorough and complete neurologic exam and appropriate imaging. Our patients can't afford for us to miss the diagnosis of acute ischemic stroke because early diagnosis can lead to improved outcome, and the neurologic exam and appropriate use of imaging are our best tools.