Fredrik Tjernström

Adaptation of postural control to perturbations--a process that initiates long-term motor memory.

The objective was to investigate postural control adaptation during daily repeated posturography with vibratory calf stimulation. The posturography was performed with eyes open and closed daily for 5 days and after 90 days on 12 healthy subjects. The postural control adaptation could be described as two separate processes, a rapid adaptation during the test progress and a long-term habituation between consecutive test days. The adaptive improvements gained during the 5 days consecutive testing, largely remained 90 days later but seemed restricted to the same test situation. The findings suggest that balance rehabilitation should include a variety of repeated exercises, which are sufficiently long to induce habituation.

Vestibular PREHAB and gentamicin before schwannoma surgery may improve long-term postural function

Background: Unilateral vestibular deafferentation (uVD), as performed in vestibular schwannoma surgery, results in a chronic vestibular deficit, though most of the insufficiency can be compensated by other sensory input. By vestibular training (prehabituation) performed before surgery, motor adaptation processes can be instigated before the actual lesion. The adaptation processes of the altered sensory input could be affected if the vestibular ablation and surgery were separated in time, by pretreating patients who have remaining vestibular function with gentamicin. Objective: To determine whether presurgical deafferentation would affect postsurgery postural control also in a long-term perspective (6 months). Method: 41 patients subjected to trans-labyrinthine schwannoma surgery were divided into four groups depending on the vestibular activity before surgery (with no clinical significant remaining function n = 17; with remaining function n = 8), whether signs of central lesions were present (n = 10), and if patients with remaining vestibular activity were treated with gentamicin with the aim to produce uVD before surgery (n = 6). The vibratory posturography recordings before surgery and at the follow-up 6 months after surgery were compared. Results: The subjects pretreated with gentamicin had significantly less postural sway at the follow-up, both compared with the preoperative recordings and compared with the other groups. Conclusion: The results indicate that by both careful sensory training and separating the surgical trauma and the effects of uVD in time, adaptive processes can develop more efficiently to resolve sensory conflicts, resulting in a reduction of symptoms not only directly after surgery but also perhaps up to 6 months afterwards.

Analysis of short- and long-term effects of adaptation in human postural control.

Abstract. The short-term (i.e., days) and long-term (i.e., months) effects of adaptation to posturography examinations were investigated in 12 normal subjects who were repeatedly examined for five consecutive days and again after 90 days. The examinations were conducted both with eyes open and closed, and the perturbations were evoked by a pseudorandomly applied vibration stimulation to the calf muscles. The evoked anteroposterior responses were analyzed with a method considering adaptation in the slow changes in posture and in the stimulus–response relationship. Repetition of examinations on a daily basis revealed a gradual improvement of postural-control performance. The body sway induced by the stimulation was significantly reduced and the dynamical properties changed. Most of the improvements remained after 90 days, but some parameters such as the complexity of the control system used were increased to the initial level. The results confirm previous observations that postural control contains several partially independent adaptive processes, observed in terms of alteration of posture and as a progressive reduction of body sway induced by stimulation. The method used for the adaptation analysis in this study could be applied to analyze biological systems with multiple individual adaptive processes with different time courses or characteristics, or where the adaptation processes are related to multiple internal or external factors.

How to uncover the covert saccade during the head impulse test.

Objective The appearance of “covert” saccades in the head impulse test in patients with vestibular loss may lead to diagnostic misinterpretations. Here, we demonstrate a procedure that can convert covert eye saccades to overt when performing the head impulse test. Patients Patients with known vestibular deficits that have covert saccades during head impulse test. Intervention Diagnostic: random and sudden changes of the amplitude (degree of head turn) when performing the head impulse test. Main Outcome Measure Evaluation of bedside test with video recording and registration from video head impulse test (v-HIT). Results A video recording and registration from v-HIT that demonstrate the covert saccades of a subject and how the overt saccade is uncovered by changing the amplitude of the head impulse. Five cases are briefly presented. Conclusion By performing the head impulse test with random amplitudes, overt saccades may become detectable and advance accuracy in bedside diagnosis of vestibular deficits.

Adaptation to vibratory perturbations in postural control

There are several modes of adaptation to postural perturbations used to withstand balance disturbances and reduce the effects of muscle fatigue. We summarize experimental results, which demonstrate that there are several modes of adaptation to postural perturbations, and we discuss the properties of these adaptive processes. The presented findings suggest that patients with balance disorders would benefit from rehabilitation programs that include a mixture of exercises aimed at training specific motor tasks or motor programs.

Romberg ratio in quiet stance posturography—Test to retest reliability

UNLABELLED We investigated test to retest reliability and intraindividual variability of Romberg ratios in quiet stance posturography. Thirty-six healthy young adults (17 males, 19 females aged 15-38 years) were divided into 3 groups with different time-intervals between consecutive trials (20 min, 3h and 24h respectively). Each group performed 5 posturography recordings in a randomized order of eyes open (EO) or closed (EC)+once after 3 months. We measured the torque variance in posturography and calculated Romberg ratios. Total postural sway as well as sway above and below 0.1Hz was analyzed. RESULTS Test to retest reliability was found to be poor for Romberg ratios (intraclass correlation coefficients (ICC) <0.4) despite that the individual EO and EC posturography recordings were consistent. For sway >0.1Hz the Romberg ratios were found to be more consistent (fair to good, ICC 0.49-0.71). The variation between two consecutive tests (absolute difference (%)) was high when using the traditional Romberg ratio (EC/EO), but became less varied if an alternate formula that includes the total postural sway was used ((EC-EO)/(EC+EO)×100). CONCLUSION In healthy young adults the evaluation of ratios from repeated quiet stance posturography show great intraindividual inconsistency. This questions the Romberg ratio as being a reliable tool for evaluation of postural performance and determination of sensory preference in postural control, at least in healthy controls. Whether test-retest reliability is acceptable in patient cohorts needs to be evaluated for proper validity of intervention and outcome studies and for detection of clinical relevance.

Postural control and adaptation are influenced by preceding postural challenges

We investigated the possible consequences of two consecutive postural tasks on adaptation. Four groups (total number of 46 healthy subjects) were perturbed on two consecutive days with vibration stimulus to tibialis anterior or posterior calf muscles, or both in different orders. Postural movements were recorded with a force platform. There were three major results: (1) tibialis anterior vibration instigated postural adaptation during exposure to the vibration, but did not induce long-term adaptation from day to day, contrary to posterior calf vibration. (2) The long-term postural adaptation from day to day when the posterior calf was vibrated was not affected by prior or subsequent tibialis anterior vibration, which contrasts to other studies on motor learning. (3) Exposure to posterior calf vibration prior tibialis anterior vibration, led to changes of postural strategies and larger amount of torque variance, implying that postural strategies initiated by the gastrocnemius vibration were re-employed during the subsequent tibialis anterior stimulation. This may represent the formation of an internal model, used as feed-forward control of posture, possibly consisting of sensory reweighting. Postural perturbations need to be sufficiently difficult to withstand, in order to induce long-term learning, and postural strategies may be transferred between different postural challenges if they post different demands. Clinically, this suggests that exercises designed to rehabilitate patients should be sufficiently challenging to instigate learning processes, and spaced in order to avoid development of inappropriate postural strategies.

Current concepts and future approaches to vestibular rehabilitation.

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 neuro-otological 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 rehabilitation.

Short and long-term postural learning to withstand galvanic vestibular perturbations

We investigated changes of postural responses to repeated bipolar galvanic vestibular stimulation on 5 consecutive days and once again after 3 months. Subjects consisted of 21 healthy volunteers. Except for the first day did the induced torque variance in response to galvanic vestibular stimulation not decrease within each test session, but there was a major reduction from day to day (p< 0.001) reflecting a continued processing of the postural experience gained during the stimulation. The decreased end level magnitude of postural responses after 5 days was retained after 3 months. The galvanic stimulation failed to induce larger torque variance compared to quiet stance toward the end of the 5 days as well as after 3 months, indicating a down-regulation of a repeated erroneous vestibular stimulation by the postural control system - i.e. sensory reweighting. This argues that a major adaptation effect to galvanic vestibular perturbation takes place after the exposure to the stimulation - similar to the concept of the consolidation process involved in motor learning. This should be considered when repeatedly assessing vestibular function both clinically and in studies. It implies that sensory training involved in rehabilitation from vestibular diseases/deficiencies should be executed with spaced intervals in order to procure more efficient learning processes and in the end, a better function.

Outward Versus Inward Head Thrusts With Video-Head Impulse Testing in Normal Subjects: Does it Matter?

Objective To investigate the effect of outward and inward head thrusts on the efficacy of compensatory eye movements generated during the video-Head Impulse Test (v-HIT) in healthy subjects. Intervention Diagnostic. Rapid movements of the head were performed either from the midline towards the side (outward thrusts) or from the side towards the midline (inward thrusts). Main Outcome Measure Gain of the vestibulo-ocular reflex (VOR) as reported by two different v-HIT devices. Results A small but statistically significant increase of the VOR gain was found for outward impulses. This small difference is likely to be attributed to increased neck strain during outward impulses, and/or hypothetically, that an effect of Alexander’s law during fast, high-frequency VOR could play a role. However, the differences are minute. Conclusion We conclude that because the difference in the VOR gain between outward and inward thrusts was slight, both methods are acceptable for clinical use.