Role of Head Impulse, Nystagmus, Test of Skew Examination for Diagnosis of Acute Vestibular Syndrome: A Scoping Review

INTRODUCTION

Acute vestibular syndrome (AVS) is a medical condition marked by nystagmus, nausea/vomiting, head motion intolerance, and persistent acute-onset vertigo or dizziness.¹ An acute unilateral peripheral or central vestibular injury that results in an abrupt asymmetry of the usual neuronal firing rate of vestibular nuclei causes the AVS.¹ The head impulse, nystagmus, test of skew (HINTS) bedside examination is helpful for the diagnosis of AVS.² To distinguish between central and peripheral causes of vertigo, the HINTS exam comprises the head impulse, nystagmus, and the test of skew.² For patients experiencing isolated dizziness, it is a highly helpful tool in the emergency room to increase the diagnosis accuracy.³ Several investigations conducted by neurologists or otorhinolaryngologists in populations with a high frequency of stroke have documented the diagnostic accuracy of HINTS to date.⁴ Emergency clinicians now use the HINTS examination, a bedside diagnostic method that eliminates central causes of vertigo without the need for sophisticated imaging.⁵ Patients with AVS (acute onset of vertigo, ataxia, nystagmus, nausea and/or vomiting, and head motion intolerance) are the only ones who can use the HINTS examination for proper diagnosis. The goal of this review is to provide a better understanding of the intricate relationship between the brainstem, cerebellum, and main vestibular afferents by discussing the HINTS test for AVS diagnosis.

METHODS OF LITERATURE SEARCH

We conducted a search for research articles on the HINTS examination for the diagnosis of AVS by using different methods. This began with searching online databases such as Scopus, PubMed, Medline, and Google Scholar. A search strategy was created based on the Preferred Reporting Items for Systematic Reviews and Meta-Analysis (PRISMA) guidelines. The search approach found published article abstracts, and citations were used to manually find more research publications. The suitability of observational studies, comparative studies, case series, case reports, and randomized controlled trials for inclusion in this review was evaluated. A total of 61 articles (22 case reports, 18 case series, and 21 original articles) were found across various databases, with 42 being included in this review [Figure 1]. This review article discusses the role of the HINTS examination for the diagnosis of AVS to rule out central causes of vertigo from peripheral vertigo.

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Figure 1:

Methods of literature search.

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PREVALENCE

A common clinical presentation in emergency rooms and general practices is dizziness, which affects 15%–35% of people and has a 3% of 12-month incidence.⁶ The peripheral vestibular dysfunction is found in approximately 40% of dizzy persons.⁷ Patients those discharged from the emergency department with a diagnosis of dizziness or vertigo, only 1 in 500 was diagnosed with a stroke within the first month.⁸ The majority of stroke patients attend emergency departments, and about 25% of all strokes affect the posterior circulation.⁹ A report states that up to 25% of people attending emergency departments with AVS may have a cerebellar infarction.¹⁰ The prevalence of AVS among patients presenting to the emergency department with dizziness is unknown, although it has been documented as benign in 10% and 20%.¹¹ With an annual incidence of 3.5 per 100,000 people, vestibular neuritis is the most frequent cause of AVS.¹¹ The central causes of dizziness are more time sensitive and need hospitalization, serial neurological evaluations, and additional acute assessment and management, depending on specific etiology. A factor responsible for the lesser number of AVS patients is due to the absence of documented or observed nystagmus, which is an important feature of AVS that is often missing in the documentation of patients who received a HINTA exam.

PATHOPHYSIOLOGY OF HINTS EXAMINATION

Interpreting the results of the HINTS test requires a thorough grasp of the vestibulo-ocular reflex.¹² The semicircular canals, which provide the head velocity signal, are the first afferent pathways of the vestibulo-ocular reflex.¹³ The vestibular division of the vestibular nerve (eighth cranial nerve) relays this signal to the vestibular nuclei. To maintain the eyes still during head movements, the oculomotor neural integrator and vestibular nuclei on either side send out an equal and opposing eye velocity signal.¹⁴ This process allows people to keep their visual attention on interesting objects, like road signs, while engaging in activities like conversing or walking. Turning the head to the right stimulates the right peripheral vestibular circuit, which causes the eyes to move equally and oppositely to the left (typical vestibulo-ocular reflex gain).¹⁵ When peripheral vestibular dysfunction is present, the vestibulo-ocular reflex gain decreases, resulting in corrective eye movement (saccade), which is observed during the bedside examination.¹⁵ In cases of central vestibular lesions, the vestibulo-ocular reflex gain is normal. To distinguish between central and peripheral vertigo, a clinical examination is based on the vestibulo-ocular reflex test, or head thrust (also called the head impulse test).¹⁶

AVS

The sudden onset of continuous vertigo (lasting more than 24 hours) accompanied by nausea, vomiting, and sensitivity to head movements is known as AVS.¹⁷ Hotson and Baloh were the first to adopt the name AVS in 1998.¹⁸ AVS is characterized by an abrupt asymmetry in the usual neuronal firing rate of the vestibular nuclei and can be caused by either central or peripheral factors.¹⁹ Most people with AVS have a benign peripheral cause. The peripheral causes include peripheral neuronitis, neurolabyrinthitis, and Meniere’s syndrome.¹⁸ The central causes, such as cerebellar infarction and cerebellar haemorrhage, can mimic peripheral causes. Approximately 25% of people presenting to the emergency department with AVS may have a cerebellar infarction.²⁰ The central causes include ischemic stroke, intracerebral hemorrhage, intracranial mass, and bulbar ischemia.²⁰ Determining the causes is crucial because the intervention and results vary depending on whether the cause is central or peripheral vestibular. Using risk factors, the patient’s history, test results, or sophisticated imaging such as magnetic resonance imaging (MRI), clinicians can assess and determine the cause. Both static (head still) and dynamic (head moving) ocular motor findings are assessed by the examiners. These findings are linked to changes in the angular vestibulo-ocular reflex, such as nystagmus and head impulse test gain; other aspects include alterations in the vestibulo-spinal reflex, such as postural unsteadiness and gait imbalance.²¹ Clinicians employ a three-part test called the HINTS to determine if AVS is caused by a central or peripheral factor.¹⁷ HINTS include determining if the eyes are aligned or misaligned (test of skew) and evaluating how the eyes react to quickly turning the head (head impulse).¹⁷

HINTS TEST

A three-part bedside oculomotor test that is as accurate as an MRI in diagnosing central vertigo is the clinical HINTS examination.²² Any one of the three oculomotor signs, such as normal horizontal head impulse, gaze direction nystagmus, or skew deviation, suggests a central etiology, including stroke in AVS, and can be performed at the patient’s bedside using the HINTS.²³ The head impulse part examines the connections between the vestibular pathways and the brainstem.²⁴ Direction-changing nystagmus indicates injury to the brainstem and cerebellum’s gaze circuits.²⁵ Injury to the central otolithic connections to the brainstem is indicated by the eyes’ skew deviation.²⁶ It is often difficult to estimate the sensitivity of HINTS for each vascular territory; evidence suggests a better yield in strokes affecting the inferior cerebellar artery compared to the anterior inferior cerebellar artery (AICA).¹¹ The eye movement evaluation is the most important part of the HINTS exam. Doing a thorough examination in an emergency situation is difficult. Many doctors are unable to administer the test and accurately interpret the results. The emergency clinicians often feel uncomfortable while assessing the patient’s complaining of dizziness. The head impulse component examines the connection between the vestibular pathways and the brainstem.²⁷ Direction-changing nystagmus is a sign of impairment to the brainstem and cerebellum’s gaze circuits.²⁵ The damage to the central otolithic link to the brainstem is shown by the eyes’ skew deviation.²⁶ Although the sensitivity of HINTS cannot be estimated for every vascular region, there is evidence that strokes affecting the posterior inferior cerebellar artery (PICA) have a higher yield than those affecting the AICA.²⁸ Table 1 shows the difference between central and peripheral causes of vertigo by HINTS.

HOW TO PERFORM HINTS EXAMINATION

POSTERIOR CIRCULATION STROKE

Approximately 25% of all strokes affect the posterior circulation, and many of them attend the emergency department.⁹ These patients typically present with a range of symptoms and clinical indicators that challenge diagnosis.³² Posterior strokes usually present with dizziness that is misdiagnosed by emergency physicians in 35% of cases.³³ The clinical presentations of the posterior stroke can lead to severe debilitation and sometimes even death. Vertigo with no discernible neurologic symptoms can be mimicked by a posterior circulation stroke. It’s still difficult to distinguish between the central and peripheral causes of vertigo. The comparison of the diagnostic precision of the HINTS assessment and other bedside screening tools for cerebrovascular causes of vertigo is often challenging for an emergency physician dealing with posterior circulation stroke. If any of the three symptoms, such as normal head impulses, gaze direction nystagmus, or eye skew deviation, are observed, a positive HINTS test indicates a posterior circulation stroke in AVS. Since most of the strokes are ischemic, HINTS may be helpful in identifying individuals who may benefit from reperfusion therapy early on.

TRANSIENT ISCHEMIC ATTACK

The assessment of a patient with continuous vertigo leads to a proper diagnosis. The assessment of episodic vertigo is often challenging. According to the New England Posterior Circulation Registry, dizziness is the most prevalent sign of a compromised posterior circulation.³⁴ Patients of transient ischemic attack (TIA) may present with ataxia, oscillopsia, and diplopia. MRI confirms the diagnosis of pontine stroke by blockage of the lumen of a penetrator branch of the basilar artery.

HINTS PLUS

HINTS PLUS includes additional hearing loss as cochlear or brainstem ischemia is introduced as a new sign that can enhance the diagnostic accuracy.²⁸ The vascular supply to the labyrinth, the fourth nerve complex (cochlear, superior and inferior vestibular, and facial nerves) at the cochlear nucleus, the cerebellopontine angle, and the cochlear nerve root entrance are usually obtained from the basilar artery and AICA and infrequently from the PICA.³⁵ Acute strokes in AVS patients seldom result in deafness. Sensitivity to HINTS PLUS is demonstrated by severe truncal ataxia.²³

CLINICAL VALIDATION

The HINTS test has been developed and approved for use by neuro-ophthalmologists and neurophysicians. Emergency physicians conducted a validation study in which the sensitivity was insufficient to rule out a central etiology of AVS. Another study suggests that a combination of neurologic examination and HINTS examination may be useful to rule out stroke.³⁶ Due to its superior sensitivity (97%) and specificity (99%) compared to early diffusion-weighted MRI, this three-step bedside diagnostic has gained quick adoption.³⁷ HINTS analysis showed a 94.0% sensitivity in another study.³⁸ According to other reports, HINTS is more sensitive than diffusion-weighted imaging (DWI) MRI, especially in cases of lacunar stroke.³⁹

LIMITATIONS OF HINTS

HINTS has limits of its own. HINTS is challenging to conduct in individuals who are extremely ill, queasy, severely disabled, or under the influence of drugs or alcohol.^40,41 Positive HINTS may also suggest other central causes of AVS, even though this test is useful for identifying stroke.⁴²

CONCLUSION

A rapid, non-invasive test that can be used at the bedside is HINTS. Patients with acute-onset and chronic vertigo, gait instability, nausea/vomiting, nystagmus, and head motion intolerance are frequently evaluated with the HINTS examination. This is a useful clinical test to help rule out central nervous system conditions among patients with dizziness. The majority of strokes are ischemic, suggesting that HINTS can be helpful for early identification of potentially eligible patients for reperfusion therapy.