Acoustic reflex thresholds and the four-box patterns

A loud sound contracts the stapedius muscle in both ears, stiffening the ossicular chain and producing a small, measurable dip in middle ear admittance. The pathway runs from the cochlea up the eighth nerve to the ventral cochlear nucleus on the side of the stimulated ear, then splits: one branch stays on that side and drives the ipsilateral stapedius, and the other crosses the midline at the superior olivary complex to drive the contralateral stapedius as well. Because the reflex is bilateral no matter which ear is stimulated, and because it can be measured with the probe in either ear, a single stimulus produces two measurements: one where the probe ear equals the stimulus ear (ipsilateral), and one where they differ (contralateral).

For tonal stimuli at 500 to 2000 Hz, normal contralateral acoustic reflex thresholds run roughly 75 to 95 dB HL. Ipsilateral thresholds for the same stimuli tend to run about 2 to 5 dB lower than contralateral. Broadband noise reflexes are elicited at much lower levels than tones, roughly 20 dB lower, so a normal broadband threshold is closer to 60 to 75 dB HL.

A useful rule of thumb is that the reflex is typically elicited about 70 to 80 dB above the patient's own audiometric threshold at that frequency, in an ear with normal or only mildly to moderately impaired hearing. A small fraction of normal adults, on the order of a few percent, have no measurable reflex in an otherwise normal ear, so a single absent reflex should never be treated as proof of pathology by itself.

A full reflex battery reports four measurements: probe right with right stimulation, probe right with left stimulation, and the mirror image for the left probe. Arranged as a two-by-two grid, a lesion at a particular point in the pathway abolishes a recognizable subset of the four boxes rather than a random one, which is what makes the pattern useful for localization.

A vertical pattern, both reflexes measured with the probe in one ear absent regardless of which ear was stimulated, points to a problem on the probe side that prevents the admittance change from being detected there: a conductive loss or a facial nerve lesion affecting that stapedius. A diagonal pattern, both reflexes elicited by stimulating one ear absent regardless of which ear the probe was in, points to a problem getting the stimulus in on that side: a significant cochlear loss or an eighth nerve lesion on the stimulus side. A horizontal pattern, both crossed contralateral reflexes absent while both uncrossed ipsilateral reflexes remain present, is the signature of a brainstem lesion interrupting the crossing fibers, and is the one pattern that only immittance testing, not audiometry, can reveal.

A unilateral cochlear loss does not abolish reflexes right away. Reflexes generally remain present, though elevated, for cochlear losses up to roughly 50 to 60 dB HL, become inconsistent between about 60 and 80 dB HL, and are usually absent above about 80 dB HL. That a reflex is present at a reduced sensation level in a moderate cochlear loss is itself a useful finding: it is the physiologic signature of recruitment, the same phenomenon that makes loudness growth abnormally steep in cochlear hearing loss. An eighth nerve lesion behaves differently: reflex abnormality, and a positive reflex decay result, can appear before the audiogram shows anything unusual at all, which is why the reflex battery is part of a retrocochlear workup rather than only a routine screening tool.