The Acoustic Reflex Test (ART) is a non-invasive, objective measurement used in audiology to assess the involuntary response of the middle ear to loud sound. This test provides valuable insight into the functional integrity of the middle ear, the inner ear, and the auditory nerve pathways connecting the ear to the brainstem. By measuring a specific muscle contraction, the ART helps localize potential problems within the auditory system and is used for diagnosing various hearing and neurological conditions.
The Physiology of the Acoustic Reflex
The acoustic reflex is a rapid, bilateral muscle contraction triggered by an intense acoustic stimulus. This action is executed primarily by the stapedius muscle, a tiny muscle located within the middle ear space. When activated, the stapedius muscle pulls on the stapes bone, one of the three small bones responsible for transmitting sound vibrations to the inner ear.
This contraction stiffens the middle ear system, resulting in a measurable decrease in sound energy transmitted to the cochlea, the organ of hearing. The stiffening acts as a protective mechanism, dampening the intensity of loud sounds to prevent overstimulation of inner ear structures. The reflex pathway involves the auditory nerve, which carries the sound signal to the brainstem.
Within the brainstem, the signal travels through several nuclei before reaching the facial nerve nucleus (Cranial Nerve VII). The facial nerve then sends a motor signal back to the stapedius muscle, causing contraction. Because the neural pathways cross and connect within the brainstem, stimulating one ear causes the stapedius muscles in both ears to contract simultaneously.
Performing the Acoustic Reflex Test
The test uses an immittance bridge or tympanometer, which measures the mobility or compliance of the middle ear system. A small, soft-tipped probe is placed snugly into the ear canal, creating an airtight seal. A constant, low-level probe tone (typically 226 Hz) is emitted, allowing the instrument to continuously monitor middle ear compliance.
The test then introduces a loud, brief stimulus tone, usually a pure tone at frequencies like 500 Hz, 1000 Hz, 2000 Hz, and 4000 Hz. If the stimulus is sufficiently loud, the stapedius muscle contracts, stiffening the middle ear and causing a temporary decrease in compliance. The immittance bridge records this change, confirming the reflex has been elicited.
The procedure is performed under four conditions to map the entire reflex arc.
Ipsilateral Measurement
In an ipsilateral measurement, the stimulus tone is presented through the same probe that is measuring the compliance change in that ear.
Contralateral Measurement
For a contralateral measurement, the stimulus is presented to the opposite ear, usually through an earphone, while the probe records the reflex contraction in the first ear. Testing all four pathways establishes a detailed picture of the auditory system’s integrity.
Reading the Thresholds: Normal vs. Absent Reflexes
The primary result is the Acoustic Reflex Threshold (ART), defined as the lowest intensity level (measured in decibels Hearing Level, or dB HL) required to elicit a measurable reflex. For individuals with normal hearing, the ART typically falls within 70 to 100 dB HL. This threshold confirms that the auditory system is functionally intact from the outer ear through the brainstem.
Ipsilateral thresholds are often measured at a slightly lower intensity (5 to 10 dB) than contralateral thresholds. An abnormally elevated ART means the reflex is present but requires a sound louder than 100 dB HL to trigger contraction. This elevation suggests the sound signal is being attenuated or weakened before it reaches the brainstem.
The most telling result is an absent acoustic reflex, meaning no muscle contraction could be measured even at the maximum stimulus intensity. An absent reflex indicates a breakdown somewhere along the reflex pathway. This lack of response requires careful interpretation to determine if the problem lies in the middle ear’s ability to transmit sound, the inner ear’s ability to detect the signal, or the nervous system’s ability to carry the signal or execute the motor command.
Diagnosing Specific Conditions Using Reflex Data
The pattern of present, elevated, or absent reflexes across the four test conditions provides a powerful tool for localizing hearing or neurological problems.
Conductive Pathology
A condition affecting the middle ear, known as a conductive pathology, typically causes the reflex to be absent regardless of where the stimulus is presented. This absence occurs because the middle ear is too stiff for the measurement probe to detect the minute change in compliance caused by the muscle contraction.
Cochlear Hearing Loss (Recruitment)
In cases of inner ear or cochlear hearing loss, the ART often exhibits “recruitment.” The reflex may be present at near-normal intensity levels, even with mild to moderate hearing loss. This means the reflex is triggered at an abnormally low sensation level (loudness above the hearing threshold). The inner ear is overly sensitive to loudness, causing the reflex to kick in earlier than expected relative to the hearing loss.
Retrocochlear Pathology
Retrocochlear pathologies, which involve the auditory nerve or the brainstem, often produce specific asymmetrical patterns of reflex absence. A lesion on the auditory nerve, for example, typically causes the reflex to be absent when the stimulus is presented to the affected ear, regardless of where the measurement is taken. Conversely, a lesion in the brainstem might result in normal ipsilateral reflexes but absent contralateral reflexes in both ears, suggesting a disruption where the neural pathways cross the midline. This differential diagnostic capability makes the acoustic reflex test an indispensable part of a comprehensive audiological evaluation.