Contralateral masking in air-conduction Auditory Brainstem Response recordings: A literature review

Abstract

INTRODUCTION: Auditory Brainstem Response (ABR) is a clinical test for evaluating the functional status of the auditory system. Electric potentials of the auditory nerve and brainstem are measured with stimulus sounds. An ABR waveform includes five waves; wave V is the largest in most cases. Contralateral masking with noise is used to prevent the cross-hearing phenomenon in hearing tests. This occurs in patients with unilateral or asymmetric hearing loss when the test tone in the test ear is so loud that it crosses over to the non-test ear, which causes the hearing in the test ear to appear better than it is. Thus, even in the early years of application of ABR recording, researchers suggested employing contralateral masking in patients with a significant difference in hearing between ears. Some studies investigated whether contralateral masking is necessary and effective to eliminate false ABR waves due to crossover, while others investigated if the masking sound in the non-test ear affects true ABR waves recorded in the test ear. The outcomes of these studies have been controversial. Literature reviews are needed to summarize and evaluate previous work and inspire further clinical research on this topic. PURPOSE: To determine (1) the necessity of contralateral masking in ABR tests of individuals with unilateral hearing loss; (2) the effectuality of this procedure in eliminating crossover; and (3) the validity of the masking procedure in ABR recordings. METHODS: The search engines Google Scholar and WSU Library SmartSearch were used. The keywords used to select articles were: "auditory brain-stem response," "contralateral masking," "auditory brainstem response," and "brain stem auditory evoked response." Selection criteria were set up concerning type of study, hearing status, and objective of study. RESULTS: Thirteen studies were collected. Three studies included participants with normal hearing only, five with hearing loss only, and five with both conditions. Of a total of 68 patients with unilateral hearing loss in ten studies, 73.5% had a wave V recorded in the deaf ears, for 88% of which, wave V disappeared with white noise introduced to the normal-hearing ear. Results suggested that crossover occurred in most patients and contralateral masking eliminated the false wave V. Seven of the eight studies of normal-hearing individuals found that wave V was not significantly altered by contralateral noise. Results indicated that certain factors, such as central masking and the acoustic stapedial reflex, could be ruled out. CONCLUSION: (1) Contralateral masking is needed to prevent crossover in patients when unilateral or asymmetric hearing loss is suspected. (2) Contralateral masking is an effective and valid procedure to prevent crossover in ABR recordings. (3) Limitations of previous studies on unilateral hearing loss are: (A) Almost all studies reported only a few cases; (B) Almost all studies were conducted over two to four decades ago, which used headphones instead of commonly applied insert earphones today. Further well-designed research is warranted.