Reversing click polarity may uncover auditory neuropathy in infants

To identify patients with primary auditory neuropathies whose cochlear potentials to a 100 microsec click persist after click cessation and simulate synchronous auditory brain stem responses (ABRs) at high intensities.

ABRs to condensation and rarefaction clicks, as well as Maximum Length Sequence ABRs and one transtympanic electrocochleogram (ECochG), were collected from five infants with absent middle ear muscle reflexes and normal or near normal otoacoustic emissions. These infants failed ALGO screens, which used alternating polarity clicks, and/or failed full ABRs done elsewhere with alternating polarity clicks.

When ABRs were collected in response to a single polarity pulse, they revealed robust and reproducible wave forms over 4 to 6 msec that initially were mistaken for a normal ABR by the referring agents. However, when condensation and rarefaction click data are compared, the waveforms change polarity when the stimulus is inverted. Furthermore, the waveforms fail to shift in latency as the intensity of the stimulation is reduced. Transtympanic ECochG on one of the children revealed the same polarity reversal and fixed latency functions, confirming that they were cochlear rather than neural responses.

Comparing responses with positive versus negative polarity clicks may help separate ABRs from cochlear potentials and alert clinicians to the possibility of an auditory neuropathy. Therefore, absent or abnormal ABRs in the presence of normal otoacoustic emissions need not always implicate a purely "central disorder," but might be consistent with dysfunction between outer hair cells and primary afferent fibers.