ACOUSTIC (STAPEDIUS) REFLEXES Jackie L

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ACOUSTIC (STAPEDIUS) REFLEXES Jackie L. Clark

The acoustic reflex, also known as the The premise behind the immittance stapedius reflex refers to an involuntary instrumentation used to measure the muscle contraction of the stapedius muscle acoustic reflex is the ability to accurately in response to a high-intensity sound sti- measure changes in reflected energy mulus. Due to ease of administration and occurring from stiffening of the tympanic information yielded, the acoustic reflex is membrane as a result of contraction of the considered one of the most powerful stapedius muscle (For the premise behind differential diagnostic audiological proce- tympanometry see tympanometry chapter). dures. The amount of reflected and absorbed energy varies depending on the admit- The acoustic reflex and the tympanogram tance/impedance (flow/resistance) of ener- (see tympanometry chapter) are commonly gy within the system and the complex used to assess middle ear function and are interaction between the ear structures. considered best practice. Each test yields invaluable information based on the The presence of reflexes within normal delivery of acoustic energy (sound) to the intensity limits is consistent with normal ear canal. Thanks to microprocessors, the middle ear and brainstem function and instruments used are now capable of rapid suggests that auditory sensitivity is not middle ear function assessment (typically significantly impaired. An elevated or ab- <1min/ear). sent acoustic reflex threshold is consistent with a middle ear disorder, hearing loss in Acoustic reflexes do not measure hearing the stimulated ear, and/or interruption of threshold. Rather, they measure reflected neural innervation of the stapedius muscle. energy which is a function of stapedius muscle contraction; it allows one to indi- rectly assess the middle ear, cochlea and Anatomy & Physiology neural innervation of the stapedius muscle. Neural mechanisms mediate the acoustic As the acoustic reflex is involuntary and reflex that results in involuntary stapedius bilateral, it is replicable and provides muscle contraction stiffening the stapes valuable diagnostic information when within the middle ear. It is presumed that comparing the amount of reflected energy the physiologic reason for the reflex is that according to signal intensity, as well as the it serves as an inhibitory response to re- presence of ipsilateral and bilateral acous- duce the sound intensities reaching the in- tic reflexes. ner ear by as much as 20 dB evoked when individuals vocalise (Møller, 2000).

Premise behind instrumentation As is illustrated in Figure 1, once a high intensity auditory stimulus is initiated and An acoustic stimulus is presented to the reaches the cochleae, neural impulses from external auditory canal; energy is transfer- the auditory nerves (CN VIII) ascend from red from the ear canal through the middle both cochleae to each ipsilateral ventral ear, some of which is reflected and some cochlear nucleus (VCN). From VCN the absorbed by the tympanic membrane and reflex has two main neural pathways: one external ear structures. passes from the VCN directly to the ipsilateral facial motor nucleus (CN VII) that directly innervates the stapedius muscle is elicited at each frequency for each ear. via the facial nerve and its stapedius branch; the other passes from the VCN to Stapedius muscle contraction increases as the superior olivary complex (SOC) before the stimulus intensity is increased; it is the impulses cross at the brainstem to therefore possible to determine acoustic innervate both ipsilateral and contralateral reflex thresholds by incrementally increas- facial motor nuclei. ing the stimulus by 5 dB steps, beginning at 80 dB HL, until a contraction occurs (Figure 2). With a normally functioning middle ear and cochlea, an acoustic reflex is elicited contralaterally at four frequencies (500, 1000, 2000, 4000 Hz), and ipsilaterally at two frequencies (1000, 2000 Hz) when stimulus intensities of 85-90 dB HL (hearing level) are delivered to the ear canal.

Figure 1: Schematic of the acoustic reflex neural pathways. CNVII (facial/seventh cranial nerve); CNVIII (auditory/eighth cranial nerve); MNVII (motor nucleus of seventh cranial nerve); SOC (superior olivary complex); VCN (ventral cochlear nucleus). Dashed lines represent the contralateral pathway and solid lines represent ipsilateral pathway of acoustic reflex

Contraction of the stapedius muscle stiffens the middle ear ossicles and tilts the stapes in the oval window of the cochlea; this effectively decreases the vibrational energy transmitted to the cochlea. Stape- dius muscle contraction is clinically appa- Figure 2: Example (right ear) of acoustic reflex rent by a marked change in the impedance threshold obtained from an immittance instrument. properties of the middle ear. Results shown would indicate 85 & 90 dB HL thresholds for 1000 and 2000 Hz respectively, It is important to realize that a bilateral which would be expected for a normal ear acoustic reflex occurs i.e. stimulating one ear (ipsilateral ear) elicits an acoustic re- In a normal ear, after an initial contraction flex in both the ipsilateral and contralateral the strength increases with each sequential ears. increase of signal intensity until a plateau is reached. Threshold is defined as the lowest acoustic stimulus intensity elicited Acoustic Threshold with a deflection magnitude of at least 0.03ml. As seen in the magnitude of An acoustic reflex threshold is the lowest deflections in Figure 2, thresholds for both intensity level at which an acoustic reflex frequencies assessed clearly fall within the 2 criteria of at least 0.03ml deflection. To bone with otosclerosis (and sometimes identify a threshold it is advisable to with tympanosclerosis). This may in- confirm the magnitude of the deflection terfere with the acoustic reflex in two with a repeat presentation. Once a thres- ways: it causes a conductive hearing hold has been established at one frequency, loss; and reduced mobility of the stapes the intensity is recorded, and the other prevents stiffening of the tympanic frequencies are then assessed. During membrane when the stapedius muscle quick screening it is not unusual to assess contracts an acoustic reflex threshold at only one Ossicular disarticulation: As with frequency and a single set intensity (~95 or otosclerosis, it causes a conductive 100 dB HL. An ipsilateral acoustic reflex hearing loss, and if located lateral to threshold tends to be attained at a slightly the stapes, prevents stiffening of the lower intensity stimulus than an acoustic ossicles and tympanic membrane when reflex threshold elicited by a contralateral the stapedius muscle contracts stimulus. Middle ear effusion: This causes a conductive hearing loss, and reduces compliance of the tympanic membrane Factors affecting acoustic reflexes and middle ear structures and may mask the presence of an acoustic reflex Ipsilateral conductive hearing loss: Tympanic membrane perforation: Anything that obstructs transmission of This causes a conductive hearing loss. a sound signal to the cochlea in the Furthermore, due to the perforation, stimulated ear results in reduced or changes in compliance caused by con- absent contraction of the stapedius traction of the stapedius muscle cannot muscles. Middle ear disorders therefore be measured easily mask an acoustic reflex. Forty Negative/abnormal middle ear pres- percent of patients with conductive sure: To maximize the likelihood that a hearing loss of >20 dB HL do not have reflex will be detected, the pressure on an acoustic reflex. Once conductive either side of the tympanic membrane hearing loss reaches 40 dB HL, about has to be equal for the tympanic mem- 80% of patients do not have a reflex brane to be at or near the point of (Jerger, Anthony, Jerger and Mauldin, maximum compliance (see tympano- 1974) metry chapter for further explanation). Ipsilateral sensorineural hearing loss: This necessitates one to match the peak This may obstruct transmission of a pressure of the tympanogram to the signal beyond the cochlea of the stimu- actual middle ear pressure so that valid lated ear and usually causes reduced or acoustic reflex threshold values may be absent contraction of the stapedius obtained muscles. However, because of recruit- Facial nerve dysfunction: The stape- ment of loudness in cochlear sensori- dius muscle is innervated by the facial neural hearing loss, acoustic reflexes nerve. Contraction of the stapedius may occur even within the expected muscle has to occur for both ipsi- and normal or partially elevated intensity contralateral acoustic reflexes. range in the presence of mild or moderate-to-severe sensorineural hearing loss. Stapes fixation: The stapes footplate is more-or-less fixed to the surrounding

3 Interpretation of acoustic reflexes transduction of neural impulses resulting in an absence or reduction of acoustic reflex- Acoustic reflexes may be reported as: es elicited with ipsi- and/or contralateral Ipsilateral: Reflex recorded in ear to stimulation. which auditory stimulus is presented Reflex Probe Probe Reflex Description Interpretation Contralateral: Reflex recorded in ear Pattern Right Left contralateral to which auditory stimu- Contra lus is presented Normal Normal Normal Partially present: Reflex present at Ipsi Mild middle ear some frequencies and absent at others Contra Abnormal disorder (left ear) whenever Elevated threshold: Reflex thresholds Vertical probe in CN VII (facial elicited >100 dB HL Ipsi affected ear nerve) disorder Absent reflex: No reflex elicited (left side) Right nerve Contra Abnormal disorder (left side) with sound Partial or elevated reflex thresholds may Diagonal to the Severe cochlear indicate the presence of hearing loss at the Ipsi affected ear frequencies where they are specifically loss (left ear) absent. Crossed Unilateral middle Contra stimulation ear disorder (left abnormal in ear) Absent reflexes have also been observed in Inverted both ears; L-shape uncrossed Intra-axial stimulation brainstem disorder individuals with normal or near-normal Ipsi hearing, which may then indicate middle abnormal in eccentric to one affected ear side (left side) ear disease or neurological involvement of Abnormal th Contra to crossed Extra-axial and/or the 8 cranial nerve, such as in pontine Horizon- stimulation intra-axial tal angle tumours and auditory neuropathy on both brainstem disorder Ipsi (see below), or neurological involvement ears th Abnormal of the 7 cranial nerve. With facial nerve with sound Contra Extra-axial and/or to affected paralysis, the absence of acoustic reflexes Unibox intra-axial ear on cros- brainstem disorder in the presence of normal middle ear Ipsi sed stimu- function suggests a lesion in the neural lation only pathway proximal to the stapedius nerve, Red box = Absent reflex whereas the presence of an acoustic reflex Table 1: Six patterns of acoustic reflex thresholds in patients with facial nerve paralysis sug- based on presence or absence of each ipsilateral gests that the lesion is distal to the origin and contralateral reflexes, based on Jerger & of the nerve. A very small percentage of Jerger (1977). This example depicts left sided people will have normal auditory sensiti- involvement. vity and absent acoustic reflexes across all frequencies without other identifiable pa- Table 1 is a system to classify acoustic thology. reflex thresholds according to the presence or absence of ipsilateral and contralateral reflexes; it has great diagnostic value Interruption of Neural Transmission (Jerger and Jerger, 1977). The system categorises acoustic reflex thresholds into As is illustrated in Figure 1, both ipsi- and six distinct patterns based on presence or contralateral acoustic reflex pathways pass absence of ipsilateral and contralateral through the SOC of the lower brainstem reflexes. Cochlear, CN VII, CN VIII, or before proceeding to the facial nerve. Con- brainstem sites of lesion are represented by sequently, brainstem lesions can interrupt vertical, diagonal, inverted L-shape, hori- zontal, and unibox patterns. 4 Wideband reflectance technique

This is a newer measuring technique (Fee- ney, Douglas and Sanford, 2004) to assess acoustic stapedius reflex thresholds by using complex wideband (125 - 10,000 Hz range) reflectance with a stimulus A. resembling “chirplike” sounds instead of a single probe tone frequency. The wideband reflectance technique reportedly yields more reliable results than the single probe tone technique, and appears to hold B. promise as a clinical procedure for measuring acoustic reflexes for normal- hearing subjects who fail to demonstrate reflexes with the standard clinical procedure. Figure 4: (A) Normal and (B) abnormal findings on acoustic reflex decay test

Acoustic Reflex Decay Concerns have recently been raised about This is another powerful differential diag- causing temporary or permanent auditory nostic test and has a high degree of sensi- threshold shifts as a consequence of tivity to identify retrocochlear pathology presenting acoustic stimuli for extended due to e.g. tumours of the cerebellopontine periods of time at exceptionally high angle. However magnetic resonance intensities. For this reason, clinicians are imaging is considered the examination of cautioned to administer acoustic reflex choice to identify retrocochlear pathology. decay tests judiciously and never to exceed In order to examine acoustic reflex decay, 105 dB HL presentation levels at any an extended duration signal (>10 seconds) frequency. is presented, using low-frequency stimuli (500, 1000 and 2000 Hz) contralateral to the probe (test) ear. Muscle contraction of Caveats with acoustic reflex measures the acoustic reflex is sustained in the normal ear at maximum or near-maximum It is important to recognize that there are amplitude throughout the entire duration of some limitations with acoustic reflex the stimulus. measures. Correct probe placement may become The standard procedure is to present the challenging for those patients with stimulus contralaterally at 10 dB SL external auditory meatal openings (threshold of acoustic reflex of test ear), or being larger than the available probe between 90 and 105 dB HL for 10 seconds. tips (as seen in the elderly) Abnormal reflex decay is evidenced by an Conversely, the very small canal inability of the stapedius muscle to openings (as seen in infants) may not maintain full contraction (amplitude) for be compatible with any of the available the duration of the stimulus signal. When probes reflex decay occurs within 5 seconds or at In addition, care must be taken that the frequencies less than 1000 Hz, it is patient is capable of sitting quietly. suggestive of 8th nerve dysfunction. Any jaw movements that occur during 5 coughing, crying, talking, swallowing, Author or jaw clenching will result in artifact and provide fallacious results. Jackie L. Clark, PhD/CCC-A; F-AAA Reportedly, collapsed canals due to Clinical Associate Professor placement of supra-aural headphones School of Behavioral & Brain Sciences, on the pinna of the contralateral ear UT Dallas/Callier Center may also result in fallacious findings Research Scholar; Univ Witwatersrand, Johannesburg, South Africa Managing Editor International Journal of Concluding remarks Audiology [email protected] Despite the few limitations with acoustic http://www.utdallas.edu/~jclark reflex threshold testing, the advantages far overshadow those limitations. Although few new discoveries have been made with Editors acoustic reflex threshold testing, it is still considered one of the most powerful Claude Laurent, MD, PhD differential diagnostic tools that should be Professor in ENT within the standard battery of tests ENT Unit, Department of Clinical Science attempted with all patients. Within a very University of Umeå brief time period it allows one to identify Umeå, Sweden middle ear pathology; cochlear or retro- [email protected] cochlear pathology; evaluate neural transmission efficiency; while providing a De Wet Swanepoel PhD general indicator of magnitude of hearing Associate Professor loss. Department of Communication Pathology University of Pretoria Pretoria, South Africa References [email protected]

Feeney MP, Douglas HK, & Sanford Johan Fagan MBChB, FCORL, MMed CA (2004). Wideband reflectance Professor and Chairman measures of the ipsilateral acoustic Division of Otolaryngology stapedium reflex threshold. Ear University of Cape Town Hearing, 25: 421-30 Cape Town, South Africa Jerger S & Jerger J (1977). Diagnostic [email protected] value of cross vs. uncrossed acoustic reflexes: Eighth nerve and brain stem OPEN ACCESS GUIDE TO disorders. Arch Otolaryngol, 103: 445- AUDIOLOGY & HEARING AIDS 53 FOR OTOLARYNGOLOGISTS Jerger J, Anthony L, Jerger S, and http://www.entdev.uct.ac.za Mauldin L. (1974). Studies in impedence audiome-try. III. Middle ear disorders. Arch Otolaryngol, 99: 165-

71 Open Access Guide to Audiology & Hearing Aids for Møller A. (2000). Hearing: Its physio- Otolaryngologists by Johan Fagan (Editor) logy and pathophysiology. Academic [email protected] is licensed under a Creative Commons Attribution - Non-Commercial 3.0 Unported Press.181-90 License