Contralateral Distortion Product Otoacoustic Emission Suppression in the Evaluation of the Efferent Auditory System: A Systematic Review

Objective: The efferent auditory system contributes to speech perception in noise and auditory filtering processes by regulating cochlear amplification through the medial olivocochlear (MOC) reflex.However, the clinical applicability and reliability of the contralateral distortion product otoacoustic emission (DPOAE) suppression test, which is used for the objective assessment of this reflex, remain controversial in the literature.The aim of this systematic review is to examine the clinical use of the DPOAE suppression test in the evaluation of the efferent auditory system and to determine its diagnostic value in light of current evidence.Methods: Studies published between 2020 and 2025 in PubMed, Web of Science and Scopus databases were systematically searched.Following the removal of duplicates and application of inclusion criteria using the keywords "contralateral suppression" and "distortion product otoacoustic emission," a total of 17 original research articles were included in the review.The included studies were analyzed in terms of clinical application areas, methodological characteristics, reliability and findings reflecting efferent system function.Results: Most of the 17 included studies were conducted in young adult populations.Eight studies reported significantly reduced suppression values compared with control groups in conditions such as tinnitus, multiple sclerosis, COVID-19, glutaric aciduria and noise exposure.Two studies demonstrated that suppression in Parkinson's disease may vary depending on disease characteristics.Three studies emphasized that cognitive tasks and attentional processes can influence suppression magnitude, suggesting that the MOC reflex may be modulated by top-down cognitive mechanisms.Four studies evaluated the reliability and temporal stability of the test; while some reported high test-retest reliability, others highlighted interindividual variability and measurement inconsistency.Across all studies, stimulus frequency, intensity and contralateral noise parameters varied and no standardized protocol was identified.Clinically, the test appears to be a complementary objective method for the early detection of subclinical efferent dysfunction, monitoring of noise exposure and assessment of auditory function in neurological disorders. Conclusion:The contralateral DPOAE suppression test is a promising and noninvasive method for evaluating the efferent auditory system.However, interindividual variability, methodological differences and lack of standardization limit its clinical diagnostic validity.Current findings suggest that the test should be considered a complementary assessment tool rather than a standalone diagnostic method.Further studies with larger samples and standardized measurement protocols are needed to strengthen its clinical utility.