Temperature changes in the perilymph space in response to caloric stimulation in man

Experimental Validation of a Three-Dimensional Heat Transfer Model Within the Scala Tympani With Application to Magnetic Cochlear Implant Surgery

Magnetic guidance of cochlear implants is a promising technique to reduce the risk of physical trauma during surgery. In this approach, a magnet attached to the tip of the implant electrode array is guided within the scala tympani using a magnetic field. After surgery, the magnet must be detached from the implant electrode array via localized heating, which may cause thermal trauma, and removed from the scala tympani.

OBJECTIVES

The objective of this work is to experimentally validate a three-dimensional (3D) heat transfer model of the scala tympani which will enable accurate predictions of the maximum safe input power to avoid localized hyperthermia when detaching the magnet from the implant electrode array.

METHODS

Experiments are designed using a rigorous scale analysis and performed by measuring transient temperatures in a 3D-printed scala tympani phantom subjected to a sudden change in its isothermal environment and localized heating via a small heat source.

RESULTS

The measured and predicted temperatures are in good agreement with an error less than 6 % ( p= 0.84). For the most conservative case where all boundaries of the model except the insertion opening are adiabatic, the power required to release the magnet attached to the implant electrode array by 1 mm 3 of paraffin is approximately half of the predicted maximum safe input power.

CONCLUSIONS

A 3D heat transfer model of the scala tympani is successfully validated and enables predicting the maximum safe input power required to detach the magnet from the implant electrode array.

SIGNIFICANCE

This work will enable the design of a thermally safe magnetic cochlear implant surgery procedure.

Exploring the biomechanical responses of human cupula by numerical analysis of temperature experiments

The vestibular receptor of cupula acts an important role in maintaining body balance. However, the cupula buried in the semicircular canals (SCCs) will be destroyed if it is detached from the relevant environment. The mechanical properties of human cupula still remain ambiguous. In this paper, we explored the cupula responses changing with temperature by experiments and numerical simulation of SCCs model. We obtained 3 volunteers’ nystagmus induced by constant angular acceleration when the temperature of volunteers’ SCCs was 36 °C and 37 °C respectively. The slow-phase velocity of 3 volunteers decreased by approximately 3°/s when the temperature of SCCs reduced by 1 °C, which corresponded to the reduction of cupula deformation by 0.3–0.8 μm in the numerical model. Furthermore, we investigated the effects of the variation of endolymphatic properties induced by temperature reduction on cupula deformation through numerical simulation. We found that the decrease of cupula deformation was not caused by the change of endolymphatic properties, but probably by the increase of cupula’s elastic modulus. With the temperature reducing by 1 °C, the cupula’s elastic modulus may increase by 6–20%, suggesting that the stiffness of cupula is enhanced. This exploration of temperature characteristic of human cupula promotes the research of alleviating vestibular diseases.

Heat transfer analysis in an uncoiled model of the cochlea during magnetic cochlear implant surgery

Magnetic cochlear implant surgery requires removal of a magnet via a heating process after implant insertion, which may cause thermal trauma within the ear. Intra-cochlear heat transfer analysis is required to ensure that the magnet removal phase is thermally safe. The objective of this work is to determine the safe range of input power density to detach the magnet without causing thermal trauma in the ear, and to analyze the effectiveness of natural convection with respect to conduction for removing the excess heat. A finite element model of an uncoiled cochlea, which is verified and validated, is applied to determine the range of maximum safe input power density to detach a 1-mm-long, 0.5-mm-diameter cylindrical magnet from the cochlear implant electrode array tip. It is shown that heat dissipation in the cochlea is primarily mediated by conduction through the electrode array. The electrode array simultaneously reduces natural convection due to the no-slip boundary condition on its surface and increases axial conduction in the cochlea. It is concluded that natural convection heat transfer in a cochlea during robotic cochlear implant surgery can be neglected. It is found that thermal trauma is avoided by applying a power density from 2.265 × 10⁷ W/m³ for 114 s to 6.6×10⁷ W/m³ for 9 s resulting in a maximum temperature increase of 6°C on the magnet boundary.

Computing simulated endolymphatic flow thermodynamics during the caloric test using normal and hydropic duct models

CONCLUSION

The obtained simulations support the underlying hypothesis that the hydrostatic caloric drive is dissipated by local convective flow in a hydropic duct.

OBJECTIVE

To develop a computerized model to simulate and predict the internal fluid thermodynamic behavior within both normal and hydropic horizontal ducts.

METHODS

This study used a computational fluid dynamics software to simulate the effects of cooling and warming of two geometrical models representing normal and hydropic ducts of one semicircular horizontal canal during 120 s.

RESULTS

Temperature maps, vorticity, and velocity fields were successfully obtained to characterize the endolymphatic flow during the caloric test in the developed models. In the normal semicircular canal, a well-defined endolymphatic linear flow was obtained, this flow has an opposite direction depending only on the cooling or warming condition of the simulation. For the hydropic model a non-effective endolymphatic flow was predicted; in this model the velocity and vorticity fields show a non-linear flow, with some vortices formed inside the hydropic duct.

A biomechanical model of the inner ear: numerical simulation of the caloric test

Whether two vertical semicircular canals can receive thermal stimuli remains controversial. This study examined the caloric response in the three semicircular canals to the clinical hot caloric test using the finite element method. The results of the developed model showed the horizontal canal (HC) cupula maximally deflected to the utricle side by approximately 3 μm during the hot supine test. The anterior canal cupula began to receive the caloric stimuli about 20 s after the HC cupula, and it maximally deflected to the canal side by 0.55 μm. The posterior canal cupula did not receive caloric stimuli until approximately 40 s after the HC cupula, and it maximally deflected to the canal side by 0.34 μm. Although the endolymph flow and the cupular deformation change with respect to the head position during the test, the supine test ensures the maximal caloric response in the HC, but no substantial improvement for the responses of the two vertical canals was observed. In conclusion, while the usual supine test is the optimum test for evaluating the functions of the inner ear, more irrigation time is needed in order to effectively clinically examine the vertical canals.

Effect of Gravity on the Caloric Stimulation of the Inner Ear

Robert Barany won the 1914 Nobel Prize in medicine for his convection hypothesis for caloric stimulation. Microgravity caloric tests aboard the 1983 SpaceLab 1 mission produced nystagmus results that contradicted the basic premise of Barany's convection theory. In this paper, we present a fluid structural analysis of the caloric stimulation of the lateral semicircular canal. Direct numerical simulations indicate that on earth, natural convection is the dominant mechanism for endolymphatic flow. However, in the microgravity environment of orbiting spacecraft, where buoyancy effects are mitigated, an expansive convection becomes the sole mechanism for producing endolymph motion and cupular displacement. Transient 1 g and microgravity case studies are presented to delineate the different dynamic behaviors of the 1 g and microgravity endolymphatic flows. The associated fluid-structural interactions are also analyzed based on the time evolution of cupular displacements.

Observations on conduction of caloric stimulation to the middle ear cavity by thermoscanning

OBJECTIVE

To elucidate the ways in which caloric stimulation conducts and the temperature recovers on the temporal bone by the caloric stimulation test.

METHODS

Four cases examined temperature changes of the bony lateral semicircular canal eminence with thermal images by caloric stimulation tests. Changes in the heat distribution were observed by means of a Thermoscan (Nihondenki Sanei Thermotracer 6T67R, Tokyo, Japan).

RESULTS

The temperature began to decrease from the external canal and the decrease reached the posterior cranial fossa plate through the middle ear cavity while the stimulation continued. After the end of stimulation, the stimulation advanced in the direction opposite to the order in which it was transmitted and recovered in a manner resembling a mirror image despite the difference in speed.

CONCLUSIONS

The way in which the caloric stimulation was conducted and the way in which the temperature recovered were in the opposite direction.

Effects of caloric stimuli on frog ampullar receptors

The observation that caloric nystagmus can be evoked even in microgravity conditions argues against Barany's convective theory. To justify this result, gravity-independent mechanisms (mainly endolymphatic volume changes and direct action of the temperature on vestibular sensors) are believed to contribute to caloric-induced activation of vestibular receptors. To define the importance of both gravity-dependent and gravity-independent mechanisms, the posterior semicircular canal of the frog was thermally stimulated by a microthermistor positioned close to the sensory organ. The stimulus produced a gravity-dependent transcupular pressure difference that, depending on the position of the heater, could result in either excitation or inhibition of ampullar receptor sensory discharge. When the heater was positioned on the ampulla, or when the canal rested on the horizontal plane, no responses could be evoked by thermal stimuli. These results suggest that, in our experimental conditions (DeltaT up to 1.5 degrees C), neither a thermally induced expansion of the endolymph nor a direct action of the temperature on vestibular sensors play any major role.

In vitro and in vivo determination of the thermal effect of middle ear endoscopy

During middle ear endoscopy, patients frequently experience vertigo. We theorized that this effect is secondary to heat produced by the scope. We evaluated two Hopkins rods and two fiberoptic scopes to assess the temperature elevation at the lateral semicircular canal (LSCC) of temporal bones and in a live canine model. These results were compared to the results of a standard 44 degrees C water caloric. We demonstrated in both models that the temperature rise at the LSCC increased relative to the scope diameter and that the Hopkins rods produced the same or greater heating effect than a warm caloric. Direct exposure of the thermocouple to the light produced a greater temperature elevation. We conclude that endoscopes produce sufficient heat to induce caloric stimulation of the LSCC. Care should be exercised to prevent the possibility of thermal injury.

Adaptation in the oculomotor response to caloric irrigation and the merits of bithermal stimulation

Caloric irrigation of the ear is a familiar clinical technique for the investigation of vestibular function that has the advantage of allowing each inner ear to be examined separately. However, it also has the disadvantage that the heating effect lasts for a period of 10-20 minutes, with the result that it is normally necessary to leave a period of at least 10 minutes between successive irrigations. This prolonged heating effect is not immediately apparent from the induced nystagmus, which normally decays within a period of 3 minutes, even in darkness. In the first part of this article, evidence is presented to show that the premature decay of eye movement can be attributed to the effect of adaptation, which is known to operate during other forms of vestibular stimulation. Experiments in which the subject was repeatedly re-orientated with respect to gravity counteracted this adaptation effect and thus revealed the continuation of the underlying thermal stimulus. In the second part of the article, an irrigation technique is described in which an attempt is made to avoid the prolonged heating effect by administering a bithermal caloric stimulus. The effectiveness of this technique is reviewed on the basis of data from previously published trials.

Minimum amount of calories needed to elicit the vestibulo-ocular reflex in normal human subjects

Calorie as a unit of heat quantity is considered more appropriate than temperatures and seconds as in conventional caloric testing. In an attempt to determine the minimum amount of calories needed to elicit the vestibulo-ocular reflex, we modified free-flowing irrigation into a closed irrigation of the external meatus. Using this method, we studied 61 ears in 34 normal healthy subjects, revealing the threshold to be 310.26 +/- 151.19 calories. Concern about the excessive heat used in conventional stimuli, which is believed to be the cause of unwanted autonomic responses, has been answered by this finding. Conventional irrigation with water at 44 degrees C is about 5-fold stronger than that minimum. With the closed irrigation technique, the autonomic responses have been reduced relatively.

Motile responses of vestibular hair cells following caloric, electrical or chemical stimuli

Isolated, living vestibular hair cells (VHCs) from the guinea pig are capable of producing self-movements. Current injections and extracellular alternating current (a.c.) fields evoked mechanical responses of cell body or sensory hairs. Furthermore, superfusion in the presence of kainic acid or slow depolarizations by K-/gluconate evoked reversible slow motile responses of solitary vestibular sensory cells. If present in vivo, active VHC mechanics will influence the mechano-sensitive stereocilia and modulate stiffness and compliance of the receptor structure and its cupular or macular relationship. A tonic VHC motility might directly influence the displacement configuration in the cupula and macula organs and thus the micromechanical operating conditions of these sensory organs. Active mechanical events could contribute to adaptation processes (automatic gain control) and micromechanical non-linearities of stereociliary displacements. In addition, we demonstrate that direct caloric stimulation of isolated, living type I VHCs from guinea pig elicited mechanical responses of the sensory cells. This mechanism could contribute to the caloric induction of a nystagmus both under terrestrial and microgravitational conditions.

Modelling the action of caloric stimulation of the vestibule. I. The hydrostatic model

In this first article of four the problem is displayed using experiments on humans for two kinds of gravity related positions. In each case the nystagmus depends sinusoidally on a particular orientation of the canal to the vertical. In each case, too, a dissymmetry of the response occurs. This kind of behavior confirms that the caloric stimulation induces both a gravity dependent and a gravity independent effect. A simple mechanical model gives account of the gravity dependent effect. It does not imply the otolithic system. Since the cupula adheres to the ampullar wall and to the crista, this is a hydrostatic model in contrast to Bárány's model, which is of hydrodynamic type.

Fluid kinetics of endolymph during calorization

Various theories on the mechanism of caloric vestibular excitability are discussed and compared with our model, which involves two factors of endolymph movement: thermoconvection and fluid expansion. With this theory, caloric nystagmus in microgravity can be explained, as well as certain phenomena on earth, such as different vestibular responses depending on body position.

Conduction of thermal stimuli in the human temporal bone

Temperature changes at different locations in the labyrinth were measured in human temporal bone preparations after syringing with water. In order to simulate physiological conditions, the preparations were placed in a water bath at 37 degrees C. The maximum temperature changes in the horizontal semicircular canal after syringing with temperatures symmetrical to body temperature (44 degrees or 30 degrees C) were found to be clearly asymmetrical (with mean values of 0.6 and -0.3K). From measurements in the external auditory meatus, findings showed that the reference temperature was 34 degrees C in front of the tympanic membrane, which explains the asymmetry recorded. Measurements at different locations showed that the temperature first changes in the regions of the ampullae of the horizontal and the superior semicircular canals. In the vestibule the onset and decay of the temperature change is delayed. The time courses of the temperature difference between locations demonstrate that the temperature difference across the horizontal semicircular canal, which would be responsible for any convective effect in the endolymph, is of shorter duration than the absolute temperature change, which would be responsible for any temperature-mediated volume changes.

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.

Caloric responses after horizontal canal inactivation

Horizontal eye movements in squirrel monkeys were recorded in response to ice-water caloric stimuli before and after horizontal canal (HC) inactivation, achieved by a plugging procedure. Calorics were presented with the subjects supine and prone, and the maximum slow phase eye velocity (SPEV) of the induced nystagmus was assessed. SPEV responses from normal ears were always directed toward the stimulated side when the monkeys were supine, and toward the opposite side when they were prone. Supine responses were always greater than prone ones. After HC-plug, which abolishes the canal's mechanical response to rotatory or convection current stimulation, small SPEV responses were routinely observed. However, they were always directed toward the stimulated side, regardless of head orientation. The results are consistent with the notion that the normal caloric response is composed of the sum of a convention current component which is dependent upon head orientation, a smaller position-dependent component of unclear origin, and a direct temperature effect on the canal's sensory apparatus which is independent of head orientation.

The effect of variations in temporal bone structure upon the caloric response

A high degree of inter-subject variability reduces the clinical usefulness of the caloric test. Both anatomical and physiological factors are responsible for this problem, but the proportion of variability caused by either factor has not been determined. This study sets a quantitative estimate on the amount of caloric test variability which can be ascribed to anatomical variations in the study population. Polytome radiographs of 48 "normal' subjects were studied do determine how individual variations in the intensity of caloric-induced nystagmus might be related to anatomical features of the temporal bone. A statistical analysis showed that pneumatization in the petrous and "buttress' areas, and the dimensions of the internal auditory canal were significantly correlated with caloric response intensity. Furthermore, about 23% of caloric response variability in this population could be accounted for by a combination of anatomical characteristics. However, neither the extent of mastoid pneumatization nor the dimensions of the external auditory canal were found to be significantly related to caloric responsiveness.

Alexander's law: a model and resulting study

Recently we developed an analog model to simulate Alexander's law in nystagmus secondary to dysfunction of a semicircular canal. Alexander's law is based on the observation that the amplitude of the nystagmus grows with increasing gaze in the direction of the fast phase and diminishes with gaze in the opposite direction. To investigate the assumptions made in the model, we conducted quantitative experimental studies on the effect of gaze on caloric-induced nystagmus in human subjects. A weak stimulus (water at 26.5 degrees C and 240 ml/min) was administered for several minutes which caused the development of jerk nystagmus. Both the average slow phase velocity and frequency reached a steady state at about three minutes after the start of irrigation and remained stable until the flow of water was stopped. To investigate the effect of gaze, each subject was asked to hold gaze at various positions from center, to the right, to the left, and to repeat the cycle. Results indicated that the slow phase velocity of the nystagmus was greatest in the direction of the fast phase and decreased approximately linearly with gaze in the other direction in accordance with Alexander's law. Frequency was not a function of gaze. We speculate as to the biological advantages of the brainstem neural circuitry responsible for Alexander's law.

Temperature characteristics of squirrel monkey horizontal semicircular canals during caloric irrigation

Current assumptions concerning body temperature of experimental animals, particularly as it relates to the selection of caloric test stimuli, are likely to be inaccurate guesses. Although the temporal bone of squirrel monkeys attenuates irrigation temperature by a factor of nearly 10, there is a high correlation between thermal changes in inner ear fluid and irrigation values. In this study, nystagmus (defined by electronystagmographic thresholds) occurred when horizontal canal temperatures deviated from resting temperature by +/- 0.14 C.

Results of new air caloric testing method among normal subjects. I. Biphasic testing

A new air caloric testing method is described in which the temperature of a continuous aural irrigation is switched hot and cold values at times calculated to control the intensity of the resulting vestibular stimulation. Applications of low or high caloric stimulus intensities to normal subjects were well tolerated and reliably produced appropriate low or high intensity nystagmic responses. Nystagmus intensity values obtained from this study were compared with predicted intensity values from a computerized simulation of the actual test conditions, and also with values obtained when using biphasic water irrigations. As a result, further improvements in our methodology have been effected.

New approach to caloric stimulation of the vestibular receptor

A new caloric testing method is described. During continuous aural irrigation, fluid is switched between hot and cold values at times computed according to a mathematical model of heat conduction in the labyrinth area. As a result, the induced temperature difference across the lateral semicircular canal describes an approximately sinusoidal time course, reaching peak values of equal magnitude but opposite sign. The magnitude of the caloric stimulus may be selected by choosing appropriate irrigation durations from a graph or table. Application of the test to clinical subjects demonstrated that the heat conduction model and analysis used in timing the sequence of thermal pulses was accurate. The new procedure causes less patient discomfort and requires less time to complete than does the conventional Fitzgerald-Hallpike test.

Stimulation of the vestibular receptor by means of step temperature changes during continuous aural irrigation

A technique for rapid, balanced hot/cold stimulation of the vestibular receptor is presented. During continuous aural irrigation the temperature of the irrigation fluid is switched between hot and cold values at times computed according to a mathematical model of heat conduction in the labyrinth area. As a result, the induced temperature difference across the lateral semicircular canal describes an approximately sinusoidal time course, reaching peak values of equal magnitude but opposite sign. Application of the test to 32 clinical subjects demonstrated that the heat conduction model and the analysis used in timing the sequence of thermal pulses was accurate. We expect that, with further refinements, the new technique will prove superior to conventional caloric test methods in the detection and measurement of subtle as well as gross abnormalities of the vestibular system.