Caloric stimulation during short episodes of microgravity

Caloric testing was performed during parabolic flight at the NASA Reduced Gravity Facility in Houston, Texas. Six test subjects were stimulated with continuous unilateral air insufflation (25 degrees R), in a manner similar to the experiments performed in the extended weightlessness of orbital flight during the SL1 and D1 Spacelab missions. Nystagmus response was recorded by electro-oculography and eye video image. It was the purpose of the experiments to re-examine the apparent discrepancy between the disappearance of caloric nystagmus during short episodes of weightlessness and the finding that caloric responses can be elicited during periods of extended weightlessness. The present results agree with those of earlier experiments in that a prompt reduction of caloric nystagmus occurs on transition from hypergravity (1.8 G) to weightlessness. The time constant of nystagmus decay was estimated to be approximately 2-3s, a value which cannot be explained by cupular mechanics. A central gating mechanism involving the labyrinthine canal and otolithic afferents is proposed for the observed modulation of caloric nystagmus.     

Caloric vestibular test in the weightless phase of parabolic flight

Twenty-four caloric vestibular tests were conducted in 20 test subjects during parabolic flight, in which weightless periods of about 10 seconds were elicited. The caloric nystagmus disappeared completely in all experiments in weightlessness, whereas in the higher G-periods the speed of the nystagmus increased. With regard to the modes of response during the periods with increased G-values prior to and following the weightlessness, four different types could be distinguished. In mode I the speed of the slow nystagmus phase increased in the higher G-periods, the nystagmus showed a prolonged duration, while in weightlessness it faded out completely. A reversed nystagmus appeared when the primary caloric nystagmus had disappeared. In modes II, III and IV the direction of the nystagmus reversed during weightlessness. In modes II and III this reversed nystagmus appeared even when the primary caloric nystagmus had disappeared. In mode III the nystagmus resembled the type of mode II, with the difference that it faded away much earlier than could be expected. In mode IV the primary nystagmus lasted rather briefly, while the reversed nystagmus during weightlessness was also much shorter. The experiments produced two remarkable facts: the appearance of a nystagmus reversal, a 'secondary nystagmus', in all experiments in weightlessness, and furthermore great differences in findings between the individuals as well as between experiments in the same person. The only explanation for the inconsistency in the findings is the assumption that effects on other canals, otoliths and vestibular nuclei are continuously changing and therefore interfere with the only endproduct measured, the horizontal nystagmus.(ABSTRACT TRUNCATED AT 250 WORDS)     

The caloric vestibular test in weightlessness

Summary Although the theoretical background for the caloric vestibular test was assumed to be known, recent studies in weightlessness have thrown doubts on the original theory of its mechanism. It is most likely that several mechanisms are involved in the generation of caloric nystagmus in addition to the convection current theory. Experiments with the caloric test in humans conducted during parabolic flight are described. These results are compared with the results of similar experiments performed in orbital missions.Part of the Round Table Conference presented at the Collegium ORLAS, Munich, Federal Republic of Germany, 7–10 September 1986     

The caloric vestibular test in weightlessness

Although the theoretical background for the caloric vestibular test was assumed to be known, recent studies in weightlessness have thrown doubts on the original theory of its mechanism. It is most likely that several mechanisms are involved in the generation of caloric nystagmus in addition to the convection current theory. Experiments with the caloric test in humans conducted during parabolic flight are described. These results are compared with the results of similar experiments performed in orbital missions.     

Thermal stimulation of the vestibular labyrinth during orbital flight

Summary During the European Spacelab mission (SL1) in 1983, caloric testing was performed for the first time in long-term weightlessness. After 2 days into orbital flight an unequivocal caloric nystagmus was observed in both subjects tested which corresponded in both quality and intensity with that measured in one-g conditions on Earth. The subsequent D1 mission enabled the experiment to be repeated on further subjects and with improved measurement procedures. As with the SL1 findings, the observed caloric nystagmus response proved to be equivalent to that measured during baseline testing on Earth. Renewed consideration of peripheral and central mechanisms, which might be involved in the elicitation of the caloric response — both in one-g and zero-g environments — has led to the reopening of a number of associated issues. One important observation which has been addressed by various research groups concerns the influence of the labyrinth's orientation to the gravity vector on the caloric response. The present authors have examined a group of healthy subjects in various body positions in the sagittal plane. The interindividual variability in the response behavior was found to be high; indeed single cases were observed in which the nystagmus response did not invert from the supine to the prone positions. These findings are discussed together with earlier reports in the literature.Part of the Round Table Conference presented at the Collegium ORLAS, Munich, Federal Republic of Germany, 7–10 September 1986     

Thermal stimulation of the vestibular labyrinth during orbital flight

During the European Spacelab mission (SL1) in 1983, caloric testing was performed for the first time in long-term weightlessness. After 2 days into orbital flight an unequivocal caloric nystagmus was observed in both subjects tested which corresponded in both quality and intensity with that measured in one-g conditions on Earth. The subsequent D1 mission enabled the experiment to be repeated on further subjects and with improved measurement procedures. As with the SL1 findings, the observed caloric nystagmus response proved to be equivalent to that measured during baseline testing on Earth. Renewed consideration of peripheral and central mechanisms, which might be involved in the elicitation of the caloric response--both in one-g and zero-g environments--has led to the reopening of a number of associated issues. One important observation which has been addressed by various research groups concerns the influence of the labyrinth's orientation to the gravity vector on the caloric response. The present authors have examined a group of healthy subjects in various body positions in the sagittal plane. The interindividual variability in the response behavior was found to be high; indeed single cases were observed in which the nystagmus response did not invert from the supine to the prone positions. These findings are discussed together with earlier reports in the literature.     

Caloric nystagmus decay during parabolic flight

In human subjects the caloric test was conducted during parabolic flight. The slow-phase velocity (SPV) of the caloric nystagmus increased proportionally to the value of the g-force. The nystagmus disappeared in microgravity, after an exponential SPV decay that showed a specific time constant (Tc). The average Tc value of this SPV decay is on a lower level than the one found after a sudden stop-test in the laboratory in a 1-g condition. Because of the exponential characteristics of the SPV decays and their Tc values, evoked in both conditions, a common working mechanism of cupular stimulation is likely. Most probably a fluid movement, caused either by a sudden stop in a 1-g condition or by weightlessness after a calorization, provokes a cupula flexion followed by a reflexion. The results support the Bárány convection theory with regard to the endolymph movement following the caloric test.     

Periodic alternating nystagmus during caloric stimulation

Periodic alternating nystagmus (PAN) is a form of horizontal jerk nystagmus characterized by periodic reversals in direction. We report a case who exhibited transient PAN induced by caloric stimulation. The patient was a 75-year-old male. He had experienced floating sensation in January 2010. Eight months later, he was referred to our university hospital. Gaze nystagmus and positional tests revealed no nystagmus. Only weak right-beating horizontal nystagmus was observed during left Dix–Hallpike maneuver. Electronystagmography showed normal saccadic and smooth pursuit eye movements. The optokinetic nystagmus pattern test was also bilaterally normal. However, during the caloric stimulation to the right ear, at 166 s from the start of irrigation, the direction of nystagmus alternated from leftward to rightward, and thereafter this reversal of direction repeated 15 times. Magnetic resonance imaging showed no significant lesion except for chronic ischemia in the brain. The patient probably had some kind of latent lesion of impaired velocity storage and exhibited transient PAN induced by caloric stimulation. Caloric stimulation is useful and simple examination to disclose latent eye movement disorders of which velocity storage mechanism is impaired.     

Flow kinetics of the endolymph in caloric stimulation

From our experiment in fluid mechanics there is strong evidence that caloric nystagmus is caused by streaming endolymph in weightlessness as well as by gravity. Two different mechanisms are postulated: thermoconvection (dependent on gravity and head position) and thermo-induced change in volume (independent of those two factors). Our results indicate that tangential endolymph flow in the ampulla directed from crista to cupula leads to a nystagmus towards the examined ear, whereas the adverse flow leads to a nystagmus to the other side. Depending on the head position the flow due to thermoconvection is increased or decreased by the flow due to extension. In mathematical approaches we found the flow due to the thermoconvection to be somewhat higher than that due to fluid extension, but still in the range of micron/s.     

Functional significance of peripheral head-shaking nystagmus

OBJECTIVES/HYPOTHESIS: The objective was to determine the characteristics of horizontal head-shaking nystagmus of peripheral origin and its relationship to vestibular dysfunction. STUDY DESIGN: Retrospective case series. METHODS: Eighty-three patients met the inclusion criteria of having peripheral and unilateral vestibular disease. Patients were tested with video nystagmography. Head-shaking nystagmus was performed in the horizontal plane during 30 cycles at a frequency of approximately 3 Hz. Head-shaking nystagmus was classified as monophasic or biphasic and, based on the pathological ear, as ipsilateral or contralateral related to nystagmus fast phases. The two-tailed t test, ANOVA, Mann-Whitney and chi2 tests, and lineal and polynomial regression tests were used for statistical analysis. RESULTS: Twenty-three patients showed a positive head-shaking nystagmus. All cases of head-shaking nystagmus observed were horizontal. There were four biphasic and 19 monophasic cases of head-shaking nystagmus. First phases of biphasic head-shaking nystagmus beat toward the normal ear. Eleven of the monophasic cases of head-shaking nystagmus were ipsilateral, and nine were contralateral. There was a statistically significant correlation between caloric weakness and head-shaking nystagmus. Ipsilateral head-shaking nystagmus corresponded to lower caloric asymmetries, and contralateral and biphasic head-shaking nystagmus corresponded to greater caloric weakness (P <.001). As the caloric asymmetry increased, the maximal slow-phase eye velocity of head-shaking nystagmus was greater (P =.01) and its duration shortened (P =.008). Ipsilateral responses could be distinguished from contralateral responses based on their latency (P =.03), maximal slow-phase eye velocity (P <.05), and duration (P =.02). The frequency of head-shaking nystagmus was significantly higher among older patients. There was no correlation between head-shaking nystagmus and clinical patterns. CONCLUSION: Head-shaking nystagmus of peripheral vestibular origin is a response both qualitatively and quantitatively associated with the degree of the vestibular loss.     

Neuro-otological examination in patients with unilateral cerebral lesion--fixation suppression

Fixation suppression of caloric nystagmus was examined in 33 cases with unilateral cerebral vascular lesion. Only Lesion included the unilateral internal capsule was certified by CT scan in all of the subjects. The rate of fixation suppression (FS%): FS% = (1 - F/D) x 100 was calculated by the mean slow-phase velocity (F) of caloric nystagmus during fixation and the maximum slow-phase velocity (D) during eyes open in darkness. The normal FS% of nystagmus to both sides was observed in 12 cases, the reduction of FS% of nystagmus to the ipsilateral side of the lesion was observed in 11 cases, that of nystagmus to the contralateral side in one, and that of nystagmus to the bilateral side in eight. In 30 out of 32 cases which were able to measure the slow-phase velocity (SPV) of caloric nystagmus, FS% of nystagmus to the ipsilateral side of the lesion was lower than that to the contralateral side, and there was statistically a significant difference between them (p less than 0.05). Furthermore, the difference between the SPV of nystagmus to the ipsilateral side of the lesion and that to contralateral side was examined during fixation and during eyes open in darkness separately. The SPV of nystagmus to the ipsilateral side of the lesion was significantly (p less than 0.01) higher than that to the contralateral side during fixation. There was no significant difference between them during eyes open in darkness. In 15 cases, however, tonic eye deviation over 20 degrees to the slow-phase side was only observed during nystagmus to the ipsilateral side.(ABSTRACT TRUNCATED AT 250 WORDS)     

Inverted caloric nystagmus of perforated ears upon air caloric stimulation

It is well known that inverted caloric nystagmus is seen during air caloric testing in cases of chronic otitis media. The mechanism of inversion and its clinical significance are discussed here. Temperature changes in the tympanic cavity and external ear canal were measured with a microthermister and a digital tester in seventeen ears with tympanic membrane perforation, during bithermal air caloric testing. The tympanic cavity mucosa was cooled by hot stimulation because of the evaporation of heat. When the perforation was closed or humidified air was used, the tympanic cavity mucosa was not cooled by hot stimulation and the inverted caloric nystagmus changed to a normal response. Inverted caloric nystagmus occurred in 30.4% of 335 ears affected by chronic otitis media with perforation. Inverted caloric nystagmus occurred in 90 ears with hot stimulation and in 12 ears with cold stimulation. Inverted caloric nystagmus turned to normal response after myringoplasty in all of 10 ears. The cooling effect caused by evaporation of water from the moist middle ear mucosa during dry air blowing and direct thermal conduction to the vestibulum through a perforation of the ear drum and inversion of the endolymphatic convection seemed to cause the inversion.     

The vertical component in a caloric nystagmus and the existence of a second phase of the nystagmus--the possibility of canal otolithic interaction in normal subjects]

To clarify the existence of the vertical component during a caloric nystagmus and the existence of a second phase of the nystagmus, 194 induced incidents of a caloric nystagmus in 29 normal subjects have been analyzed. Each nystagmus episode was recorded by using ENG and an infra-red video camera. The caloric stimuli were given by pouring 5 ml of water at 20 degrees C into the ear at an ear-up position. After irrigation, each subject then assumed a supine or a prone position, with the head bent 30 degrees forward in either position. All recordings contained vertical components that depended on the supine or prone head position and not on the side of the stimulated ear, i.e., an up-beating nystagmus resulted in the supine position and a down-beating nystagmus in the prone position. Further, the vertical component was far stronger in the prone position. In contrast, the horizontal component had larger velocities and was of longer duration in the supine position than in the prone position. When the first phase of the caloric nystagmus ended, the body position was changed 90 degrees, i.e., to a sitting position or a right-ear-down or left-ear-down position. All trials showed a horizontal component during the secondary phase when the head assumed the sitting position. As for the ear-down positions, only when the irrigated ear was moved upwards from the prone position during the secondary phase, an up-beating vertical nystagmus resulted in almost all the trials. These findings suggest that a caloric nystagmus may originate not only from the lateral semicircular canal but also from the vertical canals, and the second phase of a caloric nystagmus may be strongly influenced by the otolithic organs.     

Vestibulo-ocular reflex (VOR) preserved in bilateral severe vestibular malformations with internal auditory canal stenosis

A 60-year-old woman, who has suffered from bilateral deafness throughout her life, visited our outpatient clinic. Computed tomography (CT) revealed inner ear malformations, which comprise cochlear aplasia with hypoplastic vestibule in the right ear and a common cavity in the left ear, and narrow internal auditory canals. We performed electronystagmography with caloric stimulation and stimulation of earth-vertical axis rotation (EVAR) or off-vertical axis rotation (OVAR), and studied vestibular evoked myogenic potentials (VEMPs) and vestibular ocular reflex (VOR). Slight horizontal nystagmus was induced by the stimulation of EVAR but not by caloric stimulation. Slight vertical nystagmus was observed during OVAR, whereas the VEMP test elicited no response. The result of horizontal or vertical VOR performed in the dark was almost normal. These findings suggest that VOR can be acquired even with severe malformation of the inner ear whose labyrinthine functions markedly reduce bilaterally until nystagmus is slightly induced by rotation stimulation.     

鼓膜穿孔眩晕者的冷热气试验结果分析

目的：分析冷热气刺激下鼓膜穿孔患者的试验结果特点，探讨鼓膜穿孔者冷热气试验的可行性。方法：慢性中耳炎鼓膜穿孔伴眩晕的患者43例（49耳），行双耳冷热气试验，观测眼震特点、慢相角速度（SPV）和眼震不对称比（UW）。结果：穿孔耳冷热气刺激诱发眼震呈现4种类型：①增强型：穿孔耳诱发性眼震SPV增强并超过正常范围，或强于非穿孔耳、UW＞15％者15例（17耳）；②“正常型”：穿孔、非穿孔耳眼震SPV均在正常范围，UW〈15％者7例；③减弱型：穿孔耳眼震SPV弱于非穿孔耳，UW〉15％者9例；④反向型：非穿孔耳冷热气刺激眼震正常，穿孔耳热气刺激眼震反向者12例（14耳）。反向型眼震均出现在近期慢性中耳炎复发、鼓膜穿孔且有渗出或潮湿者；其余类型均为慢性中耳炎静止期患者。结论：对于穿孔耳，冷热气成为强刺激，半规管功能正常时诱发的眼震强于非穿孔耳；穿孔耳半规管功能不同程度减弱时，诱发眼震可为正常或减弱反应；活动期的中耳炎患者，穿孔耳热气刺激可转化为冷刺激效应，诱发出反向眼震；冷热气试验可用于鼓膜穿孔伴眩晕患者的前庭功能评定。     

Effect of microgravity on spatial orientation and posture regulation during coriolis stimulation

Objective To elucidate spatial orientation and posture regulation under conditions of microgravity.

Material and Methods Coriolis stimulation was done with five normal subjects on the ground (1g) and onboard an aircraft (under conditions of microgravity during parabolic flight). Subjects were asked to tilt their heads forward during rotation at speeds of 0, 50, 100 and 150°/s on the ground and 100°/s during flight. Body sway was recorded using a 3D linear accelerometer and eye movements using an infrared charge-coupled device video camera. Flight experiments were performed on 5 consecutive days, and 11–16 parabolic maneuvers were done during each flight. Two subjects boarded each flight and were examined alternately at least five times.

Results Coriolis stimulation at 1g caused body sway, nystagmus and a movement sensation in accordance with inertial inputs at 1g. Neither body sway, excepting a minute sway due to the Coriolis force, nor a movement sensation occurred in microgravity, but nystagmus was recorded.

Conclusions Posture, eye movement and sensation at 1g are controlled with reference to spatial coordinates that represent the external world in the brain. Normal spatial coordinates are not relevant in microgravity because there is no Z-axis, and the posture regulation and sensation that depend on them collapse. The discrepancy in responses between posture and eye movement under conditions of microgravity may be caused by a different constitution of the effectors which adjust posture and gaze.     

Vestibulo-ocular reflex (VOR) preserved in bilateral severe vestibular malformations with internal auditory canal stenosis

A 60-year-old woman, who has suffered from bilateral deafness throughout her life, visited our outpatient clinic. Computed tomography (CT) revealed inner ear malformations, which comprise cochlear aplasia with hypoplastic vestibule in the right ear and a common cavity in the left ear, and narrow internal auditory canals. We performed electronystagmography with caloric stimulation and stimulation of earth-vertical axis rotation (EVAR) or off-vertical axis rotation (OVAR), and studied vestibular evoked myogenic potentials (VEMPs) and vestibular ocular reflex (VOR). Slight horizontal nystagmus was induced by the stimulation of EVAR but not by caloric stimulation. Slight vertical nystagmus was observed during OVAR, whereas the VEMP test elicited no response. The result of horizontal or vertical VOR performed in the dark was almost normal. These findings suggest that VOR can be acquired even with severe malformation of the inner ear whose labyrinthine functions markedly reduce bilaterally until nystagmus is slightly induced by rotation stimulation.     

The caloric vestibular reaction in space. Physiological considerations

Caloric stimulus testing was performed as part of the vestibular research program during the European Spacelab 1 mission in Nov/Dec 1983. Contrary to prediction according to the classical endolymph flow theory originally forwarded by Bárány, caloric nystagmus was elicited in both tested astronauts. The intensity of the response was found comparable to that measured on earth. The theoretical consequences of these findings are discussed and possible mechanisms are considered. The direct volume displacement hypothesis is favoured as the primary effect responsible for the observed vestibulo-ocular nystagmus. Estimated differential pressure conditions resulting in the endolymph canal support this hypothesis and are in agreement with the observed response intensity. It is further speculated that interaction in the central vestibular system between canal and otolith signals be responsible for the well-known body position modulation of the observed nystagmus.     

Effect of inner ear immune response on vestibular function in guinea pigs]

We examined vestibular dysfunction and histological damage caused by direct antigen challenge to the endolymphatic sac in guinea pigs. We observed spontaneous nystagmus every eight hours and performed caloric testing every week following endolymphatic sac secondary KLH challenge. Spontaneous nystagmus was seen in 12 of 18 animals, and nystagmus in all directed toward the unchallenged ear (paralytic). The caloric response time courses were classified into two types, which were irreversible type and reversible type after endolymphatic sac KLH challenge. The immune injury of animals with irreversible type was thought to be stronger than that of these with reversible type. The spontaneous nystagmus of irreversible type animals was longer than that of reversible type animals. The temporary vestibular dysfunction was thought to be similar to that observed in Meniere's disease.