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Advances in Vestibular Function Testing

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Advances in Vestibular Function Testing ENT FEATURE Advances in vestibular function testing BY JAS SANDHU Vestibular function testing has historically been limited by difficulties in testing individual parts of the vestibular apparatus. Jas Sandhu describes new tests available to clinicians that address this problem. n 2008, ENT-UK published guidelines scientific underpinning and clinical primarily generated by the utricle and outlining ways in which balance services applicability. is mediated via the superior division of could be provided such that patients the vestibular nerve to the vestibular were diagnosed and managed in a Vestibular evoked myogenic nuclei which then connects to the inferior I oblique muscle via the vestibular-ocular timely manner [1]. As part of this initiative potentials reflex (VOR). it was suggested that all ENT departments It has been known for a long time now VEMP testing is well tolerated by have access to vestibular function that the balance organ responds to patients as it involves playing a series of testing – a role that is traditionally carried acoustic stimulation. This is thought to tone-pips into the ear whilst recording out by vestibular audiologists. The be a vestigial remnant as the vestibular myogenic activity from either the neck laboratory based vestibular test sequence apparatus outdates the hearing organ or from below the eye. Furthermore the usually comprises of looking for signs of from an evolutionary perspective. The vestibular evoked myogenic potential acquisition time for VEMP measurements peripheral and / or central nystagmus (VEMP) was brought to the attention is short, allowing the possibility of using it using videonystagmography (VNG) and of the wider scientific community by more regularly in patient monitoring. the assessment of low frequency function Colebatch and Halmagyi in 1992 [2]. It of the lateral semicircular canals using the has since provoked an exponential rise in Video head impulse testing bi-thermal caloric test. Advanced centres interest amongst scientists and clinicians The head impulse test (HIT) was first may have access to more complex tools seeking to understand its physiological described by Drs Halmagyi and Curthoys such as rotatory chair and computerised basis and to optimise the parameters almost 20 years ago and is based on postuography. required to elicit the response. For an the oculocephalic reflex. The examiner Recent advances in technology have excellent recent review see Curthoys et abruptly accelerates and then de- provided clinicians with a number of al. [3]. accelerates the head in the plane of the new methods for determining vestibular The VEMP response can be recorded canal-pair being tested whilst instructing in two testing paradigms. The first is function. These include the video head- the patient to maintain fixation upon a known as the cervical vestibular evoked impulse test (VHIT) which assesses the target. In an individual with no vestibular myogenic potential (c-VEMP). This is an function of all three semicircular canals deficit, fixation is retained and there are inhibitory response that can be measured and the vestibular evoked myogenic no aberrant eye movements. However if at the tonically contracted ipsilateral the head thrust is conducted in a subject potential (VEMP), which provides sternocleidomastoid muscle (SCM) in with a high frequency (2-7Hz) semicircular otolothic information. Combining these response to loud sounds, bone vibration canal lesion, then the eye is unable to new techniques with the established and galvanic stimuli. The response is allow for the rapid head movement and repertoire now allows clinicians to thought to be part of the vestibulo-collic in order to maintain fixation, a saccade establish sites of lesion more accurately, reflex (VCR) and starts in the saccule and is inserted involuntarily. If this saccade continues through the inferior vestibular which can influence subsequent occurs after the head has stopped moving nerve to the vestibular nuclei, the management. However despite the then this is termed an overt saccade as vestibulo-spinal tract and the ipsilateral advent of these new assessment methods, this is detectable with the human eye. SCM muscle. The second variant is the there remains an inertia associated with However if the saccadic correction occurs ocular vestibular evoked myogenic integrating them into clinical practice. during the head movement then this is not potential (o-VEMP), which is an excitatory There are many reasons for this, including response, recorded inferior to the discernable to the naked eye and is termed financial issues, training needs as well as contralateral eye with the subject looking a covert saccade. These compensatory scepticism of their clinical value. In this up. Although there remains some debate eye movements require high speed video article we discuss these new methods as to the precise pathways involved, the recordings of the ocular movements and the latest evidence relating to their general consensus is that the o-VEMP is in order to be seen. There are several technologies that enable video head impulse testing (VHIT) to be conducted, the most common of which utilise goggles with in-built high speed cameras and “Recent advances in technology have provided head position sensors. In addition to the clinicians with a number of new methods for information relating to the presence or absence of saccades the VHIT also determining vestibular function.” provides information relating to the gain of the VOR which should ideally be unity. ent and audiology news | NOVEMBER/DECEMBER 2015 | VOL 24 NO 5 | www.entandaudiologynews.com ENT FEATURE “For the first time we can now assess all five distinct parts of the vestibular apparatus and their respective sub-divisional innervations.” Figure 1: VHIT results in all six canals. Normal responses in all but left lateral canal which has reduced gain and overt saccades. case the role of preoperative counselling (a) (b) was important regarding the likelihood of postoperative vertigo as a result of residual function being sacrificed. Summary (c) (d) Over the past 20 years we have seen a paradigm shift in terms of technological advancement in vestibular testing. For the first time we can now assess all five Figure 2: (a) Normal right c-VEMP. (b) Normal left c-VEMP. (c) Normal right o-VEMP. (d) Absent left o-VEMP. distinct parts of the vestibular apparatus and their respective sub-divisional innervations. These tools have the The HIT is based on Ewald’s second strategies to be deployed. potential to help clinicians make more law which states that ampullopetal In addition to providing site of lesion accurate diagnoses and more importantly endolymph flow in the horizontal information the VHIT and VEMP also generate bespoke management plans, canal causes a greater response than have specific diagnostic benefits such which ultimately should result in better ampullofugal endolymph flow which as helping diagnose Ménière’s disease patient outcomes. is thought to be due to the inability of [4] and superior canal dehiscence [5]. inhibitory stimuli to decrease vestibular Furthermore both tests are well tolerated References by patients as unlike the bi-thermal caloric 1. ENT-UK. Balance 2008 – Excellence in practice nerve firing rates to less than zero. Thus – the provision of adult balance services by when the head is rapidly accelerated test they do not induce vertigo, thus otolaryngologists. London, UK; British Association of towards the weaker side the VOR is unable making them ideal for serial recording Otorhinolaryngologists - Head and Neck Surgeons; 2008. to monitor disease progression. These to maintain fixation upon a target and 2. Colebatch JG, Halmagyi GM. Vestibular evoked therefore a corrective saccade is inserted. attributes also make VHIT and VEMP potentials in human neck muscles before and after testing suitable for use in both the unilateral vestibular deafferentation. Neurology The head impulse can be done in the 1992;42(8):1635-6. horizontal plane (head tipped down 30°) paediatric and geriatric populations and 3. Curthoys IS. A critical review of the where the left and right lateral canals can given that the equipment needed for both neurophysiological evidence underlying clinical tests is entirely portable they are readily vestibular testing using sound, vibration and be assessed. Furthermore it can be used galvanic stimuli. Clin Neurophysiol 2010;121(2): to assess the vertical canals testing the available to clinicians working within 132-44. right-anterior / left-posterior (RALP) or the multidisciplinary environments. 4. Sandhu JS, Low R, Saunders NC, Rea PA. Altered frequency dynamics of cervical and ocular left-anterior / right-posterior (LARP) pairs vestibular evoked myogenic potentials in respectively. Case example patients with ménière’s disease. Otol Neurotol A 78-year-old gentleman with an 2012;33(3):444-9. 5. Welgampola MS, Myrie OA, Minor LB, Carey JP. Clinical applications MRI confirmed left-sided vestibular Vestibular-evoked myogenic potential thresholds schwannoma, complete left sided normalize on plugging superior canal dehiscence. The addition of VEMP and VHIT testing Neurology 2008;70(6):464-72. to the vestibular test sequence now sensorineural deafness and absent left provides those managing patients with sided caloric response was presumed to dizziness and imbalance with a more have a ‘dead left ear’ and was listed for comprehensive picture of vestibular resection of the tumour. VHIT testing function. Building on the low frequency
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