Cochlear Implantation for Patients With Single-Sided Deafness or Asymmetrical Hearing Loss: A Systematic Review of the Evidence *†Alice van Zon, *†Jeroen P. M. Peters, *†Inge Stegeman, *Adriana L. Smit, and *†Wilko Grolman *Department of OtorhinolaryngologyYHead and Neck Surgery, University Medical Center Utrecht, Utrecht, The Netherlands; and ÞBrain Center Rudolf Magnus, University Medical Center Utrecht, Utrecht, The Netherlands Objective: A systematic review of the literature to evaluate the clinical outcome of cochlear implantation for patients with single- sided deafness (SSD) or asymmetrical hearing loss (AHL). Data Sources: We searched the PubMed, Embase, Cochrane Library, and CINAHL databases from their inception up to December 10, 2013 for SSD or AHL and cochlear implantation or their synonyms. Study Selection: In total, 781 articles were retrieved, of which 15 satisfied the eligibility criteria. Our outcomes of interest were speech perception in noise, sound localization, quality of life (QoL), and tinnitus. Data Extraction: Critical appraisal showed that six studies reported on less than five patients or that they carried a low direct- ness of evidence or a high risk of bias. Therefore, we extracted the data of nine studies (n = 112). Patient numbers, age, duration of deafness, classification of deafness, pure tone audiometry, follow-up duration, and outcome measurements were extracted from all nine articles. Data Synthesis: Because of large heterogeneity between studies, we were not able to pool data in a meta-analysis. We therefore summarized the results of the studies specified per outcome. Conclusion: There are no high-level-of-evidence studies con- cerning cochlear implantation in patients with SSD or AHL. Current literature suggests important benefits of cochlear implantation re- garding sound localization, QoL, and tinnitus. Varying results were reported for speech perception in noise, possibly caused by the large clinical heterogeneity between studies. Larger and high- quality studies are certainly warranted. Key Words: Asymmet- rical hearing lossVCochlear implantationVDeafnessVHearing disordersVHearing loss VSingle-sided deafness VSystematic review. Otol Neurotol 36:209Y219, 2015. BACKGROUND Single-sided deafness (SSD) is defined as a condition in which an individual has non-functional hearing on one side and normal hearing on the contralateral side. Patients who develop SSD become aware of the importance of bin- aural hearing in their daily life in terms of social interaction and communication (1). Binaural hearing has been proven to be superior to uni- lateral hearing with regard to speech perception in noise and sound localization (2Y5). The advantages that normal- hearing listeners gain from binaural hearing are based on three principles: 1) the squelch effect (ability of the brain to separate sound and noise signals from spatially separated sources) (4,6), 2) the binaural summation effect (redun- dancy of auditory input) (7), and 3) the head shadow effect (better signal-to-noise ratio) (8). Current clinical practice for patients with SSD consists of optimizing hearing with either a contralateral routing of signal (CROS) or a bone conduction device (BCD). Both devices are effective in addressing the head shadow effect and thus restoring sound awareness to the deaf side, but they do not provide bilateral auditory input, which is needed for actual binaural hearing. The limitations of CROS or BCD may be overcome by providing a cochlear implant (CI). During the last decades, cochlear implantation has become a widely accepted in- tervention for patients with bilateral sensorineural hearing loss (HL) and the selection criteria for implantation have been broadened. Recently, Van Schoonhoven et al. reviewed current literature on bilateral cochlear implantation and con- cluded that patients with bilateral sensorineural HL perform Address correspondence and reprint requests to Alice van Zon, M.D., Department of OtorhinolaryngologyYHead and Neck Surgery, University Medical Center Utrecht, House Postal Number: G05.129, Heidelberglaan 100, 3584 CX Utrecht, The Netherlands; E-mail: ENT-research@umcutrecht.nl W.G. received unrestricted research grants from Cochlear Ltd., Med-El GmbH, and Advanced Bionics. The authors have no other funding, fi- nancial relationships, or conflicts of interest to declare. Otology & Neurotology 36:209Y219 Ó 2015, Otology & Neurotology, Inc. 209 Copyright © 2015 Otology & Neurotology, Inc. Unauthorized reproduction of this article is prohibited.