Impedance audiometry: Stapedial reflex eliciting conditions

The work is an attempt to create a complete system of conditions which infl uence genesis, existence, and response characteristics of stapedial refl ex in impedance audiometry. The author divides the conditions into the internal-refl ex arc integrity, temporal acoustic summation, mixed-external auditory meatus and middle ear, and internal-side of stimulation and energy content of the stimulus. The system of conditions that infl uence stapedial refl ex is based on a criterion, that stapedial refl ex depends on energy, which is percepted by the inner ear as subjective loudness. The system of conditions stated in this work is based mainly on the author’s own experiments and measurements, which are herein also documented. At the same time, these results are in accordance with data in literature as quoted. This system is not closed-it potentially may be completed using the basic criterion and further knowledge. Thesis


Introduction
Impedance audiometry involves the measurements of tympanometric curve and the measurements of stapedial re lex. Concerning the area covered by Impedance Audiometry, stapedial re lex measurements are the most signi icant. It brings the most information. It enables the testing of the middle ear function, inner ear status, and the stem part of the auditory path. It also provides illumination on the whole range of causes of disorders and hearing impairments and evaluates physiological characteristics of hearing.

Work objective
The main work objective is to describe and de ine conditions which in luence the initiation, existence, and character of a stapedial re lex and to create their systematic layout.
Objective: To create a systematic layout of conditions eliciting a stapedial re lex.

Theoretical background
Should the measuring tone of stapedial muscle re lex be constant and has its usual value 220 Hz (or 226 Hz), then the quality of that evoked re lex depends on the conditions which are determined by an organic state and the function of the hearing organ (inner), and on conditions which in luence acoustic provoking stimulus (outer), as well as on conditions which contain both elements (mixed). In order to create the system, a crucial and important fact is that a stapedial re lex is dependent on energy, which is perceived in an inner ear and is subjectively sensed as loudness.

A. Internal conditions
• re lex arc integrity

Internal conditions
Re lex Arc Integrity: Bioelectric impulse is created during ipsilateral acoustic stimulation in hearing cells of an inner ear. The bioelectrical impulse is led with the help of an acoustic neuron to the ventral cochlearis nuclei [1][2][3]. The central axon of this neuron goes within the VIII head nerve to the brain stem mainly to the ventral cochlearis nuclei [4], where the second acoustic neuron lies. Most axons of ventral cochlearis nuclei go through corpus trapezoideum [5] to the medial part of a motor nucleus of a facial nerve. From that nucleus bioelectrical impulses are transmitted via the facial nerve into a stapedial muscle on the same side. The re lex arc of ipsilateral stapedius re lex is mainly trineural.
For contralateral acoustic summation, the irst and second acoustic neurons are in concord with the ipsilateral stimulation arc. The second neuron leads a bioelectrical impulse to the area of nucleus olivaris medialis superior. The third neuron connects the nucleus olivaris medialis superior with the motor nucleus of the facial nerve of the contralateral side. The fourth neuron leads than the impulse to the opposite stapedial muscle. The contralateral re lex arc is tetraneuronal [1][2][3].
Temporal Acoustic Summation: Jerger, et al. [3], claim that the stapedial re lex depends mainly on the loudness of an initiating stimulus. The ability of an organism to transfer simultaneously a range of physical parameters on subjective loudness is called "Temporal acoustic summation".
Exact anatomical localisation of this ability is unknown. Nevertheless, many experiments show that the function is central. Re lex contraction of the stapedial muscle is controlled by neural activity of the medial superior olive complex. The temporal summation centre is located in that area [3]. Temporal acoustic summation is an internal bodily ability to differentiate into mixed acoustic signals. The external equivalent of this ability-the physical characteristics of stimulation (intensity, frequency, length, character of stimulation) is called "criterion response" by Jerger, et al. [3] and better yet as "energy content of the stimulus" by Rossi et al, [6].

Mixed conditions
External Auditory Meatus: An acoustic signal is physically modulated in a narrow closed space of the external auditory meatus-re lection, interference or diffraction. These energetic changes depend on the auditory meatus and the elasticity of the ear drum, as well as on the atmospheric conditions in the external auditory meatus.
Volume and shape of the auditory meatus: Crucial change of energy in the external auditory meatus is the genesis of acoustic resonance [7][8][9][10][11][12][13]. By the resonance, a certain frequency area of acoustic spectrum is increased, and therefore the resonance of external auditory meatus can improve or complicate simulative relations of stapedial re lex.
The bigger the volume of the measured system is, the lower the frequency resonance peak is. At the same time the intensity of the peak is constant. The change of resonance value is dependent on the elasticity of the drum igure 2. The resonance peak position is variable. Westwood, et al. [14], de ine 2950±404 Hz as an average of mature auditory meatus. Novak [10] places the resonance peak at about 2700 Hz and Syka,et al [15] at 2700-3600 Hz. The increase of intensity reaches up to 17 dB [10] or 13-20 dB [8]-a difference of just 6 dB represents double the acoustic pressure. According to the logarithmic scale, the resonance difference of 13-20 dB is then about triple the acoustic pressure. Figure 1,2 The auditory meatus resonance changes in dependence on the auditory meatus volume and the elasticity of the drum.

Middle ear
Acoustic pressure difference: The dependence between the middle ear and hearing is generally known. The more the pressure difference is between the middle ear and atmospheric pressure, which is in external auditory meatus, the lower the transport drum capacity is, Furthermore, the higher the increase of impedance of the transferring system, which in luences even the stimulating threshold of existence of the stapedius re lex. Pospíšil, et al. [16] and Lejska, et al. [17] measured the threshold of stapedius re lex during a controlled change of pressure in the external auditory meatus. They con irmed that the pressure change signi icantly in luences the system impedance and the threshold of stapedius re lex. Table 1 shows the change of threshold values in dB.

External conditions
The side of stimulation (ipsi x contra): Initiation of the stapedial re lex on the side where simultaneously a measurement is made=the ipsilateral uncrossed stimulation is simpler, and is not in luenced by pathological changes of the other ear, which happens when the contralateral crossed re lex is initiated [25,29,30]. The record of uncrossed initiated re lex is the same in shape, to an uncrossed initiated one of intact persons [25,29,31]. Thresholds of ipsilateral re lex are signi icantly lower than ones of contralateral re lex [25,29,[32][33][34][35]. Ipsilateral stimulated re lex has in most cases higher amplitude than collateral re lex [25,29,34]. Ipsilateral stimulated re lex is more sensitive for both intact hearing and hearing disorders [36].
Energy content of the stimulus: De ined as a complex of acoustic stimulation which is called "criterion response," by Jerger, et al. [3] and "energy content of the stimulus," by Rossi, et al. [6].

Stimulation intensity:
Stimulation intensity is certainly the most signi icant physical parameter of acoustic stimulus. All characteristics of re lex response are most in luenced by stimulation intensity. Therefore the re lex existence, re lex threshold, re lex shape or shapes of re lex parts are functions of intensity [6,41].

Stimulation frequency:
The in luence of acoustic stimulus on the inal stapedial re lex has been known for a long time [2,3,40,[42][43][44]. Reactivity and quality of re lex  drum elasticity (a, b, c). [8]. relate to the sensitivity of the frequency part of the acoustic spectrum. Simultaneously, the frequency is in luenced by the auditory meatus resonance. Pospíšil, et al. [16], Lejska [51], published their results and compared them with the results of Jerger et al. [2,3], Table 3.

Stimulation length (time):
The stimulation time is the period when an initiative acoustic stimulus in luences the acoustic analyzer. Butenko [42], states that the stapedial re lex threshold increases the more and the shorter the stimulation time is, from 1s to 20 ms. He also states that the most suitable time is from 400 ms to about a double of the standard stimulation, i.e. 2 s [45,46].
To evaluate the in luence of stimulation time on the re lex response Rossi et al. [6], recommend "rise time" as the most sensitive parameter. Lejska et al. [47]. compared parameters of the re lex response at stimulation from 1s to 10 seconds. They have found statistically signi icant differences in the re lex parameter-"rise time." They have not proved the difference in the parameter-"amplitude." Table 4.
The results clearly con irm that the character of the re lex response is in luenced by frequency and intensity, and even the stimulation time. In luence can be very much seen in the parameter "rise time." Rossi, et al. [6], researched the relation of the length, intensity and re lex rise time. They established the expression "the energy content per time unit" and supposed that the rate of intensity is as follows: the shorter the stimulation, the higher energy content per time unit. With reference to that, they present different levels of reactivity of motor units of stapedial muscle and its fatigue [47].   Band width: According to researches of Lehnhardt and Margolis,et al. and others [24,[48][49][50][51] it is possible to ind a different stapedial re lex result during stimulation with the stimulus of the narrow band extension (mostly one clear tone or narrow band noise), in addition to sound stimulus consisting of several tones or wider band noise. If the extension of the band width is wider, there is no change up to the critical point. If the stimulating band width is continuously increased, there is a linear decrease of the re lex threshold [2,3,52], igure 3, he critical bandwidth is the width of the sound band in which suddenly there is changed some subjective hearing constants, mainly loudness, while the overall sound intensity stays constant.
Stimulation tonality: As in acoustic stimulus in impedance audiometry, it is possible to use sound with characteristics of tone or sound without tonal characteristics-often various types of noises. Wide band noise initiates stapedial re lex at a lower initiating level of intensity [2,3,24,28,35,51,53]. Wurzer, et al. [28], states that the noise threshold can be lower than the threshold with tonal stimulus by 20-25 dB. Lejska [51], measured the intensity of initiating acoustic stimulus with the help of tonal stimulus of various frequencies with audiologically controlled noises (WN, LP, HP). The results con irm, that the threshold values of stapedial re lex at stimulation by tonal stimuli, are higher than at the stimulation by noise stimuli Table 5.

Conclusion
There are systematized conditions which in luence genesis, existence, and character stapedial re lex response at acoustic stimulation. The basic criterion is parameter "loudness" which perceived in an inner ear.
The current system is not closed. It is possible to develop it using the same principle, and according to new knowledge about problems in the ield of stapedial re lex.  Threshold values at stimulation by non-tonic stimuli are lower and their reactivity is more consistent [51].