Acoustic Reflex Testing

The acoustic reflex is the contraction of the stapedius muscle in the middle ear in response to a loud sound. The contraction stiffens the ossicular chain and reduces the sound reaching the inner ear, which protects it to some degree from loud noise.

Acoustic reflex testing measures this reflex. It tests the middle ear, the cochlea, the auditory nerve, the lower brainstem, and the facial nerve in one test.

The reflex arc starts in the cochlea, passes along the auditory nerve to the cochlear nucleus in the brainstem, crosses to the facial nerve nuclei on both sides, and returns along the facial nerve (which supplies the stapedius muscle) to the middle ear.

A loud tone is presented to one ear. The stiffening of the eardrum caused by stapedius contraction is detected by the same probe used for tympanometry, as a small decrease in compliance.

The sound is presented to the same ear as the probe (ipsilateral) and to the opposite ear (contralateral). Frequencies are 500, 1000, 2000 and 4000 Hz, and broadband noise.

The test takes 5 to 10 minutes, and follows tympanometry, since the middle ear must be normal for a reliable test. In normal hearing, the reflex threshold is about 70 to 100 dB HL, which is about 85 dB above the hearing threshold. Reflex decay testing measures whether the reflex is sustained, by presenting a tone at 10 dB above the threshold for 10 seconds, at 500 and 1000 Hz.

Indications.

• Differentiating the site of the lesion in hearing loss: middle ear, cochlea, auditory nerve, or brainstem.
• Suspected retrocochlear lesion (vestibular schwannoma and other cerebellopontine angle lesions): reflex decay and absent reflexes.
• Facial nerve palsy, to locate the lesion in relation to the branch to the stapedius muscle and to give an idea of the outlook.
• Confirmation of the middle ear status, together with tympanometry, in conductive hearing loss and otosclerosis.
• Estimation of hearing in children and in patients who cannot cooperate with behavioural tests (acoustic reflex threshold is used as a cross-check).
• Recruitment (loudness growth) assessment in cochlear loss, and detection of non-organic hearing loss, when reflex thresholds are better than the reported hearing thresholds.
• Brainstem and neurological disease (multiple sclerosis, brainstem stroke, tumours), where reflexes help to locate lesions.
• Myasthenia gravis (fatigable reflex), and other neuromuscular disorders in selected cases. Not used alone for diagnosis, and not used for screening in infants under about 4 to 6 months, in whom the reflex may be unreliable.

Contraindications and precautions.

• Loud sounds up to 100 to 110 dB HL are presented, so the test is avoided or limited in patients with hyperacusis, painful loudness intolerance, or very low discomfort levels, and in those with noise-induced tinnitus that is aggravated by loud sounds.
• Recent ear surgery (stapes surgery), and ear discharge, perforations, or blocked ear canal: no valid test.
• Patients who cannot keep still or tolerate the probe.
• Very recent ear infection or acute otitis media: the test is delayed.
• A normal tympanogram (type A) is needed. A middle ear abnormality that prevents the probe from detecting the contraction (flat curve) makes the reflex test invalid.
• Test time and sound levels are kept to the minimum necessary to avoid damage, and equipment is calibrated.
• Neonates and young infants have a high-frequency probe tone with careful interpretation.

Preparation and technique.

Otoscopy and tympanometry are done first. The probe is sealed in the ear canal, and the pressure is set to the peak of the tympanogram, where the eardrum is most mobile.

Tones are presented at 500, 1000, 2000 and 4000 Hz, starting at about 70 to 80 dB HL, and raised in 5 dB steps. The response is a small, repeatable decrease in compliance (a deflection of at least 0.02 mL), time-locked to the sound.

The lowest level at which the response occurs is the reflex threshold. Ipsilateral and contralateral reflexes are measured for each ear. For reflex decay, a tone at 500 and 1000 Hz at 10 dB above the threshold is presented for 10 seconds, and the amplitude is observed to check whether it falls to half or less. The test is stopped at 110 dB HL if there is no response, or if the patient is uncomfortable.

Normal appearances.

A reflex is present at 70 to 100 dB HL (about 85 dB above the pure tone threshold) at 500 to 2000 Hz, for both ipsilateral and contralateral stimulation, with a slightly higher threshold at 4000 Hz. The reflex is sustained during the 10-second tone, with a decay under 50% at 500 and 1000 Hz. The reflex thresholds are symmetrical between ears.

Interpretation.

• Conductive hearing loss (middle ear disease or otosclerosis): the reflex is absent when the probe ear is affected, since the contraction cannot be detected, or the stimulus is reduced by the loss.
• In otosclerosis, an on-off pattern is seen in early disease, and the reflex is later absent.
• Cochlear (sensorineural) hearing loss: the reflex is present at a reduced sensation level (under about 60 dB above the threshold), a finding called recruitment. It is absent if the loss is severe (over about 70 to 80 dB HL), since the sound does not reach the loudness needed.
• Retrocochlear lesion (vestibular schwannoma, auditory nerve disease): absent or elevated reflexes, or an abnormal reflex decay (a drop of over 50% within 10 seconds at 500 or 1000 Hz), out of proportion to the hearing loss.
• Facial nerve lesion proximal to the nerve to the stapedius: the reflex is absent on the affected side when the probe is in that ear (ipsilateral and contralateral stimulation of the other ear).
• If the facial nerve lesion is distal, the reflex is present. This helps locate the lesion in Bell palsy and in temporal bone trauma.
• Brainstem lesion: absent crossed (contralateral) reflexes with present ipsilateral reflexes, or the reverse, depending on the level of the lesion.
• Non-organic hearing loss: a reflex threshold that is at the same level as, or lower than, the reported hearing threshold, which shows that hearing is better than claimed.
• Interpretation needs the audiogram and the tympanogram.

Critical values and findings needing immediate action.

• Absent reflexes or abnormal reflex decay with asymmetric sensorineural hearing loss: MRI of the internal auditory canals for a retrocochlear lesion.
• Absent reflex with facial palsy after head injury, or with progressive palsy: urgent assessment and imaging of the temporal bone and facial nerve.
• Abnormal reflexes with new neurological symptoms (imbalance, headache, other cranial nerve signs): urgent neurological evaluation and imaging.
• Absent reflexes in a child with suspected hearing loss: urgent audiological assessment.
• Reflex decay in a patient with fatigable weakness: assessment for myasthenia gravis.

Limitations and artefacts.

• The test needs a normal middle ear on the probe side. Any middle ear disease gives an absent reflex, and so the test cannot give information about the inner ear or brainstem in such a patient.
• Loud sounds and test time need to be kept as brief as possible, with caution in patients at risk of discomfort.
• A poor seal, movement, swallowing, and external noise cause artefacts. Heartbeat and breathing in the canal give small changes that look like reflexes.
• Reflexes may be absent in normal people (about 5 to 10%), and in older adults, so absent reflexes alone are not diagnostic.
• Reflex decay is a non-specific sign, and false positives occur in cochlear loss, and in older adults.
• The test has limited sensitivity for small retrocochlear lesions (smaller than about 1 cm), and is not used to exclude them. MRI and auditory brainstem response have higher sensitivity.
• In infants and in some adults, probe tone frequency and sound levels need to be adjusted.
• The test does not measure hearing thresholds directly.

Special groups.

• Children: reflex thresholds help to confirm hearing and exclude non-organic loss. Infants under 6 months need 1000 Hz probe tones.
• Patients with facial palsy: reflex testing helps to locate the site of the lesion, but modern imaging and electrophysiology have taken over some of this role.
• Patients with suspected retrocochlear disease: reflex decay is one of the older screening tests, now replaced by MRI as the first-line test in most places.
• Patients with non-organic hearing loss: reflex thresholds provide an objective check.
• Patients with otosclerosis: absent reflexes, with a conductive hearing loss, support the diagnosis.
• Regions: the test needs a tympanometer with reflex capability, available in some centres.
• Where MRI is not available, reflex decay, auditory brainstem response, and clinical features (asymmetric loss, tinnitus, imbalance) are used to select patients for referral for tumour exclusion.

Follow-up tests.

• Pure tone and speech audiometry.
• Tympanometry, with repeat when the first test is abnormal.
• Auditory brainstem response.
• MRI of the internal auditory canals and brain.
• CT of the temporal bone for conductive loss and facial nerve injury.
• Facial nerve electrophysiology (nerve excitability test, electroneurography) in facial palsy.
• Neurological and neuro-otological assessment.