Current concepts and future approaches to vestibul

Authors: Fredrik Tjernström Oz Zur Klaus Jahn

Publish Date: 2016/04/15

Volume: 263, Issue: 1, Pages: 65-70

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Abstract

Over the last decades methods of vestibular rehabilitation to enhance adaptation to vestibular loss habituation to changing sensory conditions and sensory reweighting in the compensation process have been developed However the use of these techniques still depends to a large part on the educational background of the therapist Individualized assessment of deficits and specific therapeutic programs for different disorders are sparse Currently vestibular rehabilitation is often used in an unspecific way in dizzy patients irrespective of the clinical findings When predicting the future of vestibular rehabilitation it is tempting to foretell advances in technology for assessment and treatment only but the current intense exchange between clinicians and basic scientists also predicts advances in truly understanding the complex interactions between the peripheral senses and central adaptation mechanisms More research is needed to develop reliable techniques to measure sensory dependence and to learn how this knowledge can be best used—by playing off the patient’s sensory strength or working on the weakness To be able using the emerging concepts the neurootological community must strive to educate physicians physiotherapists and nurses to perform the correct examinations for assessment of individual deficits and to look for factors that might impede rehabilitationVestibular rehabilitation is a broad concept that not only implies compensation training after a vestibular lesion or disease but also postural training and compensation in other causes of vertigo dizziness or general unsteadiness It covers a wide clinical area in which central nervous adaptation mechanisms to a sensory loss or mismatch are vital To guide training it is equally important using the right tools for assessments of sensory function sensory weighting and identification of factors that might protract compensation The need for knowledge of methods and concepts for vestibular and postural rehabilitation should not be underestimated In developed countries the cost associated with falls is high and with an aging population it is a mounting problem that will demand huge resources from hospitals as well as from the community In Sweden 10 M inhabitants the annual cost from falls 2009 amounted to 14 billion euro of which almost 500 million were direct costs and 900 million were related to deterioration in quality of life These costs are expected to increase to approximately 22 billion euro by 2050 if the situation continues to develop at its present rate 1 Abnormal performance on balance tests is the second most important intrinsic predictor for falls in elderly 2 and 35  of people above 40 years of age have vestibular dysfunction 3Acute vestibular loss is the most studied condition and compensation follows certain welldefined steps The first compensation process consists of central vestibular adaptation in which the symptoms of acute vestibular loss spontaneous nystagmus head and ocular tilt postural disequilibrium altered vestibuloocular and spinal reflexes 4 are diminished within the first week due to cerebellar modulation inhibition 5 of the initial asymmetric activity of the vestibular nuclei 6 7 The symptoms gradually resolve and the spinal imbalance normalizes and behavioral recovery is initiated 4 8 in a process where plastic changes of the activity of the vestibulocerebellum are essential 7 9 However the dynamic loss of vestibular reflexes persists and remains functionally inadequate and asymmetric 10 It should be mentioned that the rebalancing of the vestibular nuclei is not only a consequence of cerebellar inhibition but also of direct cellular and synaptic adaptations within the vestibular nuclei This is important as must drugs used in the early phase of an acute vestibular syndrome do interact directly with binding site in the vestibular nuclei eg histaminergic GABAergic glutamatergic receptors The second stage consists of sensory reweighting in which the importance of each sensory system vision somatosensation and vestibular for maintaining postural control is reevaluated and changed 11 12 The sensory systems overlap in terms of detection of motion frequency and share some properties of the feedforward mechanisms involved in maintaining postural control In that sense the normal postural control system is redundant and sensory systems are able to replace each other The last stage is continuous sensory calibration and formation of internal feedforward models generated by everyday activities and from specific postural trainingPostural learning or rehabilitation follows the same processes as general memory formation ie from shortterm to longterm learning through the process of consolidation in which the training experiences are reprocessed during inactivity 14 The process is affected by the cognitive state of mind eg anxiety 17 and by central nervous plasticity 59 60 and impeded by reduced alertness from sedatives 61 or sleep deprivation 62 When designing rehabilitation exercises it is important to take into account that one exercise might affect succeeding exercises 63 and that the exercises themselves must be sufficiently challenging to promote learning 16 63 Learning might be through sensory reweighting or formation of internal models ie motor programs whose output consists of preformed neuromuscular strategies activated automatically or voluntarily in given situations anticipated movements 64To plan the right rehabilitation strategy it is crucially important to be able to correctly assess sensory function and weighting of the dizzy patient and thus the sensory strategy of an individual All rehabilitation programs aim at training the remaining senses to strengthen their interactions as well as their integration in the brain balance network For most patients reweighting is beneficial but some become overreliant on a certain sensory system and thus suffer from sensory mismatch 18 The most flagrant example of this maladaptation is the concept of “visual vertigo” or secondary “phobic postural vertigo” 19 ie that visual stimuli induce illusions of selfmotion or an erroneous spatial orientation 20 21 22 Accordingly in a demanding environment it is significantly more difficult for these patients to ignore visual stimulations even when they are asked to focus on a stationary target 23 Although the concept of sensory reweighting is widely accepted it is difficult to measure or even assess reliably The most widely used clinical test for sensory dependence is the Romberg test comparing postural sway when the eyes are opened vs closed In posturography a ratio can be calculated between the tests which has been labeled both as an index of visual dependence 12 and as somatosensory dependence 24 One problem with calculating ratios is that variations or changes could be artifacts from small variations in either the denominator or the numerator Because of the sensory overlap the innate ability to change postural strategy at least in easy postural tasks and adaptation to difficult postural tasks 14 15 the subjects could have more or less postural sway in any condition yields low consistency in the sensory profile of an individual when tested repeatedly 25The sensory organization test SOT in Equitest posturography is often used to appreciate individual sensory weighting in the postural control system 24 The SOT consists of a series of postural challenges of increasing complexity which have been shown to correspond to sensory deficits Ratios from the different conditions are often labeled as indices of vestibular visual and somatosensory weighting Measuring different conditions during stance and gait often helps to identify factors contributing to unsteadiness 26 27 However these measures have not been validated between labs and are therefore not available everywhere This makes the results difficult to interpret when it comes to sensory weighting The tests also induce central adaptation which stresses the need for additional methods to assess sensory reweighting 15 Other evaluations for appreciating visual dependency are the rod and frame and rod and disk tests 28 29 The results from these tests have rarely been compared to posturography measurements but there are definite correlations between the tests in patients suffering from visual vertigo that validates the rod and disk test at least in the subgroup of dizzy patients 30Factors that protract compensation and prolong subjective symptoms have to be recognized early to prevent development of chronic unsteadiness It is important to screen for risk factors for a prolonged course of compensation after acute vestibular lesions as for example advanced age medication microvascular brain lesions and preexisting sensory deficits Concepts like “Functional dizziness” persistent postural–perceptual dizziness PPPD 31 phobic postural vertigo PPV and chronic subjective dizziness CSD have helped to advance knowledge of the relationship between anxiety/depression and dizziness Patients suffering from one of these overlapping syndromes have been shown to interpret sensory cues incorrectly 32 By doing so the postural control system in unchallenged conditions is already extended to the point that they do not learn or adapt to postural training 17 33 These factors compromise rehabilitation It has also been shown that if the diagnosis is delayed the condition will be harder to cure 34 Various treatments have been tried eg regular vestibular exercises combined with cognitive behavioral therapy Although the shortterm results are promising the longterm result is less convincing so far 35 Retrospective studies on anxiolytic drugs and antidepressants have shown beneficial effects especially if the original triggering disease was vestibular 36 Considering the incidence of the condition and the ensuing functional disability randomized controlled studies on treatment of phobic dizziness and vertigo are urgently needed Muscular pain and tension are important conditions that coexist with dizziness and result in protracted disability as well as distorted spatial orientation and dizziness 37 38 39 Chronic pain also results in central sensitization 40 which is very much in line with motion sensitivity observed in dizzy patients

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