The diagnostically powerful acoustic stapedial reflex
The acoustic stapedial reflex is one of several muscle responses to sound. It falls in the same general category as the post-auricular muscle response, the eye blink reflex, and the startle response. Careful measurement of stapedial acoustic reflexes yields considerable information on the anatomical status of the auditory system, especially when recorded in four conditions, that is, measurement of ipsilateral and contralateral acoustic reflex activity with right and left ear stimulation.
Much of the auditory system is involved in the acoustic reflex measurement including the middle ear, the cochlea, the 8th (auditory) cranial nerve on the side of the stimulus, brainstem neurons within the cochlear nuclei and, for contralateral acoustic reflex measurement, the trapezoid body and medial superior olivary complex plus polysynaptic pathways including neurons within the reticular activating system, an efferent pathway involving motor fibers within the 7th cranial nerve on the side of the probe, plus the stapedius muscle and middle ear on the side of the probe. The presence of acoustic reflexes is highly dependent on normal middle ear function. Most middle ear abnormalities obscure confident detection of acoustic reflexes, even relatively subtle disorders that are not associated with markedly abnormal tympanograms or a significant (>10 dB) gap between air- and bone conduction pure tone thresholds.
Tympanometry has unquestionable value as a screening tool for the detection of middle ear abnormalities. However, hearing requires integrity of much more than the middle ear. The application of hearing loss estimation with acoustic reflex thresholds was first reported in the early 1970s [
9]. Presuming normal middle ear function, acoustic reflexes permit quick, ear specific objective differentiation of normal versus abnormal cochlear function. The acoustic reflex threshold for broadband noise (BBN) increases rather systematically with worsening pure tone thresholds for sensory hearing loss [
20]. In contrast, acoustic reflexes elicited with pure tone signals showed little change in threshold from normal hearing sensitivity through 50 or even 60 dB HL, a reflection of the loudness recruitment phenomenon.
Fig. 1 illustrates the differential effect of sensory hearing loss on acoustic reflex thresholds elicited with tonal versus BBN signals. Patients with hearing loss rarely have acoustic reflex thresholds of less than 85 dB for the BBN stimulus. Conversely, BBN acoustic reflex thresholds greater than 90 dB are invariably associated with sensory hearing loss. Results from a study of acoustic reflex thresholds in neonates provides further support for the use of a BBN stimulus in objectively differentiating between normal hearing sensitivity versus sensory hearing loss. Kei [
21] reported acoustic reflex threshold data collected with pure tone and BBN stimuli and a 1,000 Hz probe tone in a group of 66 health newborn infants who had passed hearing screening. Acoustic reflexes were recorded in all stimulus conditions from all of the infants. The median acoustic reflex threshold in the normal hearing infant group was 55 dB HL for the BBN stimulus with a range of 50 to 75 dB HL. These findings confirm that an acoustic reflex threshold of 75 dB HL or better is consistent with normal hearing sensitivity.
The value of acoustic reflexes in quickly and objectively differentiating among sites of auditory dysfunction is also impressive. There are four possible distinct and different measurement conditions in acoustic reflex measurement: 1) right ear ipsilateral, 2) left ear ipsilateral, 3) contralateral reflexes with the probe in the right ear and sound in the left ear, and 4) contralateral reflexes with the probe in the left ear and sound in the right ear. These four measurement conditions and normal findings for each condition can be displayed graphically in a diagram like the one shown in
Fig. 2. An open box in the figure indicates the presence of normal acoustic reflexes with thresholds of <90 dB HL. A shaded box indicates abnormally elevated acoustic reflex thresholds, whereas a filled in black box indicates that no acoustic reflex activity was detected in the test condition.
Combinations or patterns of findings for pure tone audiometry, tympanometry, and acoustic reflex recordings are generally related to likely clinical etiologies or diagnoses. In analysis of acoustic reflex patterns, it's useful to first examine the findings for tympanometry to confirm or rule out middle ear disorder followed by inspection of the acoustic reflex pattern for the four measurement conditions. An audiogram if available may offer additional evidence of conductive hearing loss. Several common acoustic reflex patterns are cited here.
A vertical pattern is often encountered clinically, particularly in pediatric populations where middle ear disorders are commonplace. Typically, acoustic reflexes are absent whenever the probe is on the side with middle ear dysfunction. Greater conductive hearing loss for the ear would result in elevation of the contralateral acoustic reflex measured with stimulation of that ear and the probe in the opposite normal ear. A conductive hearing loss essentially reduces the effectiveness of the acoustic reflex stimulation by the magnitude of air bone gap. Since the acoustic reflex is normally activated with an intensity level of 85 dB HL, a conductive loss of 25 to 30 dB raises the contralateral acoustic reflex threshold for stimulation of the ear with conductive hearing loss to about 110 to 115 dB HL. Facial nerve disorder is another explanation for the vertical pattern of acoustic reflex abnormality. Two factors clearly distinguish this vertical pattern from the acoustic reflex pattern typical of mild conductive hearing loss. The most obvious factor is normal tympanometry in facial nerve disorder, consistent with normal middle ear function. A normal audiogram or at least no difference between air and bone conduction pure tone thresholds also argues against middle ear disorder.
When acoustic reflexes are abnormally elevated in threshold or absent with stimulation of one ear, the most likely explanation is a sensory hearing loss. The chances of detecting acoustic reflex activity decline as the degree of sensory hearing loss increases. Normal acoustic reflex findings are anticipated in mild and even moderate sensory hearing loss, reflecting the loudness recruitment phenomenon. The acoustic reflex pattern in patients with unilateral neural dysfunction is similar, but the degree of hearing distinguishes it from sensory hearing loss. With neural auditory dysfunction secondary to an acoustic tumor such as a vestibular schwannoma, the diagonal acoustic reflex abnormality is often associated with only mild hearing loss. The neural pattern may also be suspected due to acoustic reflex decay.
Finally, a horizontal abnormal acoustic reflex pattern is sometimes encountered in patients with brainstem auditory dysfunction. The presence of normal ipsilateral acoustic reflexes and normal tympanometry unequivocally rule out conductive hearing loss, sensory hearing loss, neural auditory dysfunction, and facial nerve disorder. The horizontal acoustic reflex pattern is a strong sign of brainstem auditory dysfunction in patients at risk for central auditory nervous system dysfunction, including those with head injury and suspected APD.
Jerger, et al. [
22] first described rare version of the horizontal pattern for acoustic reflex findings. The pattern is characterized by an abnormality is only one contralateral acoustic reflex condition. All pathologic explanations other than an isolated unilateral brainstem auditory abnormality are convincingly ruled out due to the presence of normal acoustic reflexes in the other three acoustic reflex conditions, plus normal tympanograms and typically normal hearing sensitivity bilaterally. Observation of the uni-box acoustic reflex abnormality prompts a comprehensive assessment of central auditory function and, in many cases, otolaryngology and/or neurology referral.