The stapedius muscle--the present opinions about anatomy and physiology

The authors present current opinions about anatomy and physiology of the stapedius muscle and its role of the hearing process. The stapedius muscle is the smallest striped muscle of the human body and contracts reflexive in response to acoustic stimulation. The stapedius muscle puls the neck of the stapes in the direction of the stapedius tendon. This movement causes stiffening of the incus and the malleus and also changes the pressure of the perilymph in the inner ear. This is the protective inner ear action of the stapedius reflex against hearing damage by noise. The stapedius reflex shows bilateral interactions and its center is located in the brainstem. The binaural interaction of the stapedius reflex plays an important role in the maintaining of the sound direction. The stapedius tendon also plays role in the vascularization of the long process of the incus.     

Physiological activation of the stapedius muscle in Gallus gallus.

The function of the avian middle ear muscle was investigated in the chicken, Gallus gallus (domesticus). The avian species offers excellent conditions for study of middle ear muscle function since it possesses a single middle ear muscle, the stapedius, which is located extracranially. Electromyograms (EMG), measurements of impedance change, and volume change in the middle ear cavity were used to assess the muscle's activity. The results showed that the middle ear muscle of Gallus does not exhibit an acoustic reflex. However, the stapedius is regularly activated during the animal's own vocalization. Measurements of the EMG and volume change showed the stapedial activity to increase systematically with increases in the vocal sound level. The use of volume change as a measure of stapedius function was found to be highly suitable in the present experiments in that it allows for measurements of the magnitude of the stapedius contraction without altering the intact physiological state of the middle ear, and is insensitive to the ambient noise and vocal sounds, that hamper the impedance technique.     

The role of the stapedius reflex in poststimulatory auditory fatigue.

The human ear is very resistant against noise-induced damage in the low frequency range. The aim of the present study was to investigate whether or not the stapedium reflex is of any importance for this resistance. Subjects with peripheral facial palsy (Bell's palsy) including unilateral stapedius muscle paralysis were exposed to several different levels of narrow band noise centered at 0.5 and 2.0 kHz. Temporary threshold shift (TTS) at 0.75 kHz was significantly higher in the affected than in the nonaffected ear after 0.5 kHz noise at and above 110 dB SPL. After the exposure with 2.0 kHz narrow band noise there was no difference in TTS at 3.0 kHz between the affected and nonaffected ear. It is concluded that the attenuation provided by the stapedius reflex reduces TTS after low frequency noise exposure. An implication is that the stapedius reflex also may have the function of protecting the ear from hearing damage caused by low frequency noise exposure.     

Automatic Tympanometry in Clinical Practice

The probe tone of an impedance bridge has a direct effect on the results of tympanometry. The frequency of the probe tone should be close to the resonant frequency of the middle ear. Of three electro-acoustic bridges using three different probe tones, i.e. 800, 625 and 220 Hz, the one using 800 Hz provided the most indentifiable tympanographic pattern incases with ossicular discontinuity. Five different types of tympanograms could be indentified when an 800 Hz tone is used

The stapedius reflex thresholds, on the other hand, show only minor differences with different probe tone frequencies

Important clinical information can be obtained by combining the type of tympanogram with the results of stapedius reflex measurements     

Effect of the acoustic reflex on inner ear damage induced by industrial noise

Shipyard noise is a variable noise which induces permanent threshold shift (PTS) in exposed workers. In humans, the stapedius reflex has been found to be very stable in this type of exposure. Temporary threshold shift (TTS) in the absence of stapedius reflex has been found to extend downwards through the speech-frequencies instead of showing a high frequency dip as when the stapedius reflex is normal. The features of PTS produced by the same type of noise was investigated in rabbits with and without functioning middle ear muscles. The auditory sensitivity was measured by auditory brain stem response (ABR) and by the stapedius reflex response. Middle ear muscle function was blocked by denervation of the stapedius muscle or by general anesthesia. With normal middle ear reflex very little PTS was found. When the muscles were inactivated during the noise exposure the PTS was very extensive and covered the mid frequency range. On the basis of previous findings in humans and the present animal study it is suggested that the features of the stapedius reflex should be considered both in assessment of individual susceptibility and design of optimal acoustic environments.     

The clinical utility of ipsilateral stapedius reflex tests.

Ipsilateral stapedius reflex testing has been evaluated in 166 patients. It proves of value in defining the state of an ear (1) opposite to a unilateral conductive loss; (2) the less hard-of-hearing ear in the presence of bilateral asymmetrical sensorineural hearing losses where the worse ear has sufficiently severe loss to prevent a contralateral reflex from being elicited; (3) in patients with absent contralateral acoustic reflexes at 500, 1000, and 2000 Hz; and (4) in suspected central lesions. The test is still bedevilled by artifacts which are produced in the ear rather than the machine; atypical results must be evaluated with extreme caution.     

Detection of the acoustic stapedius reflex in infants using wideband energy reflectance and admittance

This study examined the measurement of the contralateral acoustic stapedius reflex in six-week-old infants and adults using wideband shifts in admittance and energy reflectance (YR). The reflex activator was bandpass noise from 2,500 to 11,000 Hz presented at a maximum spectrum level of 51 dB SPL measured in the ear canal. Reflexes were detected by calculating a cross-correlation between one-twelfth-octave measurements of YR for the highest activator level and responses to lower levels. The reflex-induced shifts in YR for the infant ears were similar in pattern to adult responses but were noisy at frequencies below 1000 Hz. Infant reflexes were more successfully detected when the cross-correlation was calculated from 1000 to 8000 Hz, whereas adult reflexes were more successfully detected for a cross-correlation from 250 to 2000 Hz. This method may be useful in capturing the most robust frequency region for acoustic reflex detection across postnatal middle ear development.     

Normal values of the ipsilateral acoustic stapedius reflex threshold

It is frequently taken for granted that the acoustically evoked stapedius reflex is bilaterally symmetrical. Contrary to this, M?ller described an asymmetry of the acoustic stapedius reflex with an ipsilaterally 2--14 dB lower threshold. The determination of normal values of the ipsilateral acoustic stapedius reflex threshold with a large number of patients is difficult as the intensity of the stimulus depends considerably on the position of the probe in the acoustic meatus and is therefore not defined with sufficient accuracy. For this reason we determined the values of the ipsilateral threshold by applying a stimulus sound of high intensity to the deaf ear of unilaterally completely deaf patients with a normal headphone, which can be calibrated much more accurately. After subtraction of the individual cross hearing loss, the exact ipsilateral intensity was obtained. By this method a stapedius reflex could be evoked with 49 of the 62 patients. By mathematical consideration of the data of the positive cases, as well as the maximum available intensities with the negative cases, determination of the median value of the ipsilateral threshold was possible: at 0.5 kHz 59 dB; at 1kHz 62.5 dB; at 2 kHz 67 dB; at 4 kHz approx. 67 dB. The difference between ipsilateral and contralateral stapedius reflex threshold was in the range of 15 dB. A new definition of the normal value for the ipsilateral measured Metz recruitment appears necessary.     

Auditory fatigue in patients with stapedius muscle paralysis.

Experiments were performed in 18 patients with peripheral facial palsy (Bell's palsy) including unilateral stapedius muscle paralysis. After exposure to narrow band noise centered at 0.5 kHz temporary threshold shift at 0.75 kHz was significantly higher in the ear with paralysis than in the normal ear. After exposure to 2.0 kHz narrow band noise there was no difference in TTS at 3.0 kHz between affected and non-affected ear. It was concluded that the stapedius reflex has a protective function against low frequency sound exposure and suggested that this protection might be extended to higher frequencies only when high frequency noise also contains low frequency components.     

Stapedius reflexes to electrical stimulation in the rabbit for the assessment of hearing

Summary The possibility of using electrically induced stapedius reflexes as a means of objective hearing evaluation was investigated in the rabbit as an animal model. The contralateral stapedius reflex to acoustic and electrical stimulation was measured in anesthetized rabbits. For electrical stimulation, the middle ear was opened surgically and stimulation was applied by a monopolar electrode placed at or into the round window. Contraction of the middle ear muscle was monitored by digital sampling and averaging of the impedance changes in the contralateral ear. Acoustically evoked reflexes were recorded within expected limits despite anesthesia. The level of the electrical stimulus was raised until mass reflexes of the neck muscles were observed. A contralateral stapedius reflex to electrical stimulation could not be demonstrated. In our experiment, monopolar electrical stimulation at the round window could not elicit contralateral stapedius reflexes.     

Normal values of the ipsilateral acoustic stapedius reflex threshold

Summary It is frequently taken for granted that the acoustically evoked stapedius reflex is bilaterally symmetrical. Contrary to this, Møller described an asymmetry of the acoustic stapedius reflex with an ipsilaterally 2–14 dB lower threshold. The determination of normal values of the ipsilateral acoustic stapedius reflex threshold with a large number of patients is difficult as the intensity of the stimulus depends considerably on the position of the probe in the acoustic meatus and is therefore not defined with sufficient accuracy.For this reason we determined the values of the ipsilateral threshold by applying a stimulus sound of high intensity to the deaf ear of unilaterally completely deaf patients with a normal headphone, which can be calibrated much more accurately. After subtraction of the individual cross hearing loss, the exact ipsilateral intensity was obtained. By this method a stapedius reflex could be evoked with 49 of the 62 patients. By mathematical consideration of the data of the positive cases, as well as the maximum available intensities with the negative cases, determination of the median value of the ipsilateral threshold was possible: at 0.5 kHz 59 dB; at 1 kHz 62.5 dB; at 2 kHz 67 dB; at 4 kHz approx. 67 dB. The difference between ipsilateral and contralateral stapedius reflex threshold was in the range of 15 dB. A new definition of the normal value for the ipsilateral measured Metz recruitment appears necessary.     

Eardrum displacement following stapedius muscle contraction.

Simultaneous monitoring in human subjects on the same ear of eardrum displacement by tympanomanometry, and impedance with the electroacoustic bridge, provided information concerning contraction of the stapedius muscle and its effect on eardrum displacement. Extensive control procedures were employed to elicit only the stapedius reflex; lower intensity auditory stimulation, electrocutaneous stimulation of the homolateral external ear canal, and anesthetization of nerves leading to the tensor tympani. Following these procedures the following results were obtained: (1) Extremely small biphasic and monophasic eardrum movements were seen in the stapedius--only ear to auditory and electrocutaneous stimulation; the form of the response was much less predictable to auditory stimulation. (2) At high sound intensities relatively large inward and biphasic movements of the eardrum occurred in the normal ear, unquestionably due to contraction of the tensor tympani. These results were further validated in a group of stapedectomized ears, without the stapedius but with normal tensor tympani. (3) Biphasic responses did not occur in the tensor tympani--only ear, only monophasic inward responses. (4) Upon air-jet stimulation to the orbit of the eye, these subjects had an accentuated tensor response in that large inward movements of the eardrum occurred as compared with those in normal ears, suggesting that there is an alteration of the tensor response by the presence of the stapedius muscle. Estimates of the actual eardrum displacement were calculated based on a model of the external ear canal and eardrum.     

Die Auslösung des Stapediusreflexes durch Clicks und die dadurch bewirkte Änderung der schnellen Hirnstammpotentiale

Summary The behavior of the acoustic reflex induced by 1/4 ms clicks in rates of 10, 20, 50, and 100 Hz (3 s duration, 2 s interstimulus interval) was examined. We found that 1. only rates of > 20 Hz are able to produce a contraction of the middle ear muscle, 2. the reflex threshold for clicks with respect to the peak sound-pressure is below the reflex threshold of pure tone stimuli, and 3. full contraction of the middle ear muscle can be reached comparatively quicker by high stimuli rates.In a different test arrangement we have recorded auditory brainstem potentials (P₆) in normal hearing people with and without elicited stapedius reflex and found a considerable delay of the latency of P₆ while the middle ear muscle was contracted.In order to prove that this delay was produced only by damping of the middle ear we recorded P₆ from patients with otosclerosis. Here, too, we found a delay of the brainstem response but comparatively slighter. This proves that contraction of middle ear muscle causes a delay in recording of P₆.     

声场下噪声刺激对镫骨肌声反射幅值的影响

目的研究声场下不同强度的白噪声刺激对镫骨肌声反射幅值的影响。方法观察80名听力正常受试者在声场方式下给予不同强度（30、40、50和60dBSL）的白噪声刺激时,1、2和4kHz频率的声反射幅值的变化。结果受试者在给予白噪声刺激时声反射幅值较未给予白噪声刺激时明显降低（P〈0．01）,40dBSL强度的白噪声刺激时,声反射幅值降低最明显,2、4kHz的声反射幅值降低幅度较1kHz处明显（P〈0．01）,而2、4kHz处之间无明显差异（P〉0．05）。结论声场下白噪声刺激可致镫骨肌声反射幅值降低,临床上可以通过检测声场下白噪声刺激后镫骨肌声反射幅值的变化评估听觉传出神经的功能。     

On the influence of the middle ear muscles upon changes in sound transmission

Summary The influence of artificially induced impedance changes on the frequency response of the middle ear has been investigated in three experienced listeners. The tensor-reflex has been simulated by application of positive and negative air-pressure to the outer ear canal. In this test-situation, the thresholdshift (L) obtained is greatest in the surrounding of the first resonance of the middle ear (L = 8 dB at 500 Hz). For bone-conduction, the effect is weaker by about 2 dB than for air-conduction. The change in sound transmission is constant for all SPL's and is mainly due to the impedance change of the middle ear.In contrast the influence of the stapedial reflex is strongly dependent on SPL. In the range between hearing-threshold and 70 dB (HL) the attenuation is only 2 dB below 1 kHz. At higher levels the attenuation may amount to 30 dB. Thus, excitation of the cochlea remains nearly constant. We suppose that the intensitydependent influence of the stapedius reflex on sound transmission is due to a change of the stapes motion. The regulating device works without considerable distortion but with an attack-time of about 100 ms. The phenomenon of conductive recruitment in otosclerosis is probably due to the lack of this regulating effect by the fixed stapes. One function of the stapedial reflex could be the protection of the ear from high amplitudes during phonation and shouting. Furthermore, because frequencies above 1 kHz are also attenuated, a protective function of the stapedial reflex in industrial noise exposure can be assumed.     

Speech discrimination in patients with Bell's palsy and a paralysed stapedius muscle

The contraction of the stapedius muscle results in a frequency dependant attenuation of sound through the middle ear. Idiopathic facial nerve paralysis (Bell's palsy) usually results in a paralysis of the stapedius muscle. This prospective study included 119 patients presenting with Bell's palsy over an 18-month period. After applying exclusion criteria, 80 patients with normal pure-tone audiograms underwent speech audiometry. This was performed on presentation and after recovery of the facial nerve palsy. Fifty-six patients (70%) with an absent stapedius reflex showed marked‘roll-over’from means of 98–49% on their speech audiogram. This resolved completely with recovery of the facial nerve palsy and return of the stapedial reflex. To determine whether this effect was due primarily to the paralysis of the stapedius muscle or to an associated polyneuropathy, a second study on six normal patients was done. These patients had both ears (12 in total) tested with speech filtered to simulate a paralysed stapedius muscle. This resulted in a mean‘roll-over’from 90.9 to 59.9%. The magnitude of this roll-over (31%) was only two-thirds of that seen in Bell's palsy patients (49%) with the difference between these means statistically significant (P < 0.05). This suggests that Bell's palsy, usually considerd a mononeuropathy, involves certain of the auditory fibres of the eighth nerve and is a polyneuropathy. Stapedius function is important in speech discrimination at higher levels of sound intensity such as speech in noise and severing the stapedius tendon in stapes surgery may affect speech discrimination after successful surgery.     

Stapedius Reflex Abnormalities in Multiple Sclerosis

Thirteen patients suffering from multiple sclerosis were analyzed by a Madsen ZO 70 electroacoustic bridge connected to an electronystagmograph through an impedance adaptor. Threshold, amplitude, decay and difference limen of intensity (DLI) of the stapedius reflex were examined for the frequencies 500, 1 000, 2 000 and 4 000 Hz. The difference between standard audiometric results and stapedius reflex data stresses the value of reflex measurements in assessing brain stem pathology     

Stapedius Reflex and Auditory Fatigue

Subjects with unliateral Bell's palsy and stapedius muscle paralysis in one ear and normal stapedius reflex in the other were exposed to narrow band noise with the center frequencies 0.5 and 2.0 kHz at several intensities. There was significantly greater temporary threshold shift (TTS) in the affected ear than in the unaffected ear after the 0.5 kHz noise at and above 110 dB SPL. A difference was already present after 100 dB SPL noise exposure. After the 2.0 kHz noise there was no such difference. We conclude that the stapedius reflex in man may protect the ear aginst auditory fatigue, probably also against permanent injury within the low frequency region     

Stapedius reflex abnormalities in multiple sclerosis.

Thirteen patients suffering from multiple sclerosis were analyzed by a Madsen ZO 70 electroacoustic bridge connected to an electronystagmograph through an impedance adaptor. Threshold, amplitude, decay and difference limen of intensity (DLI) of the stapedius reflex were examined for the frequencies 500, 1 000, 2 000 and 4 000 Hz. The difference between standard audiometric results and stapedius reflex data stresses the value of reflex measurements in assessing brain stem pathology.     

Clinical observations on the on-off effect of the acoustic stapedius reflex

The on-off effect of the acoustic stapedius reflex was found in association with several kinds of ear disease: otosclerotic stapes fixation, incudostapedial dislocation and sensorineural hearing loss. One fifth (8/40) of the patient group with an on-off effect and sensorineural hearing loss showed the typical symptoms of Ménière's syndrome. Keywords on-off effect stapedius reflex hearing loss