Cochlear microphonics in sensorineural hearing loss: lesson from newborn hearing screening

The diagnostic dilemma surrounding the presence of cochlear microphonics (CM) coupled with significantly elevated auditory brainstem response (ABR) thresholds in babies failing the newborn hearing screening is highlighted. A case report is presented where initial electo-diagnostic assessment could not help in differentiating between Auditory Neuropathy/Auditory Dys-synchrony (AN/AD) and sensorineural hearing loss (SNHL). In line with the protocol and guidelines provided by the national Newborn Hearing Screening Programme in the UK (NHSP) AN/AD was suspected in a baby due to the presence of CM at 85 dBnHL along with click evoked ABR thresholds of 95 dBnHL in one ear and 100 dBnHL in the other ear. Significantly elevated thresholds for 0.5 and 1kHz tone pip ABR fulfilled the audiological diagnostic criteria for AN/AD. However, the possibility of a SNHL could not be ruled out as the 85 dBnHL stimuli presented through inserts for the CM would have been significantly enhanced in the ear canals of the young baby to exceed the threshold level of the ABR that was carried out using headphones. SNHL was eventually diagnosed through clinical and family history, physical examination and imaging that showed enlarged vestibular aqueducts. Presence of CM in the presence of very high click ABR thresholds only suggests a pattern of test results and in such cases measuring thresholds for 0.5 and 1 kHz tone pip ABR may not be adequate to differentiate between SNHL and other conditions associated with AN/AD. There is a need for reviewing the existing AN/AD protocol from NHSP in the UK and new research to establish parameters for CM to assist in the differential diagnosis. A holistic audiological and medical approach is essential to manage babies who fail the newborn hearing screening.