Abstract: We included four cross-over RCTs with a total of 209 participants, ranging in age from 23 to 85 and with a preponderance of men. All the studies allowed the use of hearing aids for a total period of at least eight weeks before questions on preference were asked. All studies recruited patients with bilateral hearing loss but there was considerable variation in the types and degree of sensorineural hearing loss that the participants were experiencing. Three of the studies were published before the mid-1990s whereas the fourth study was published in 2011. Therefore, only the most recent study used hearing aids incorporating technology comparable to that currently readily available in high-income settings. Of the four studies, two were conducted in the UK in National Health Service (NHS – public sector) patients: one recruited patients from primary care with hearing loss detected by a screening programme whereas the other recruited patients who had been referred by their primary care practitioner to an otolaryngology department for hearing aids. The other two studies were conducted in the United States: one study recruited only military personnel or veterans with noise-induced hearing loss whereas about half of the participants in the other study were veterans. Only one primary outcome (patient preference) was reported in all studies. The percentage of patients who preferred bilateral hearing aids varied between studies: this was 54% (51 out of 94 participants), 39% (22 out of 56), 55% (16 out of 29) and 77% (23 out of 30), respectively. We have not combined the data from these four studies. The evidence for this outcome is of very low quality. The other outcomes of interest were not reported in the included studies. This review identified only four studies comparing the use of one hearing aid with two. The studies were small and included participants of widely varying ages. There was also considerable variation in the types and degree of sensorineural hearing loss that the participants were experiencing. For the most part, the types of hearing aid evaluated would now be regarded, in high-income settings, as 'old technology', with only one study looking at 'modern' digital aids. However, the relevance of this is uncertain, as this review did not evaluate the differences in outcomes between the different types of technology. We were unable to pool data from the four studies and the very low quality of the evidence leads us to conclude that we do not know if people with hearing loss have a preference for one aid or two. Similarly, we do not know if hearing-specific health-related quality of life, or any of our other outcomes, are better with bilateral or unilateral aids.

Plain-language summary: We included four studies with a total of 209 patients, ranging in age from 23 to 85 and with more men than women. All the studies allowed the use of hearing aids for a total period of at least eight weeks before questions were asked about their preference for one or two aids. In all the studies the patients had bilateral hearing loss but there was considerable variation in what type of hearing loss they suffered from and how bad their hearing was. Three of the studies were published before the mid-1990s and the fourth study was published in 2011. Therefore, only the most recent study used 'modern' hearing aids similar to those that are widely available in high-income countries. Of the four studies, two were conducted in the UK in National Health Service (NHS – public sector) patients. One of these looked at patients from primary care whose hearing loss had been picked up by a screening programme. The other looked at patients whose primary care practitioner thought they might benefit from hearing aids so had referred them to the local ENT department to get them. The other two studies were conducted in the United States: one study recruited only people on active military duty, or who had served in the military and had hearing loss due to being exposed to loud noises. About half of the people in the other study were ex-military. Only one of the outcomes we thought was most important - patient preference - was reported in all studies. The percentage of patients who preferred two hearing aids to one varied between studies: this was 54% (51 out of 94), 39% (22 out of 56), 55% (16 out of 29) and 77% (23 out of 30), respectively. We did not combine the numbers from these four studies because it would not have been right to do so. We graded the quality of evidence for this outcome as very low on a scale that goes high – medium – low – very low. There was no information in the four studies on the other outcomes we were interested in. This review identified only four studies comparing the use of one hearing aid with two. The studies were small and included people of widely varying ages. There was also considerable variation in the types of their deafness and in how deaf they were. For the most part, the types of hearing aid evaluated would now be regarded, in high-income countries, as 'old technology', with only one study looking at 'modern' digital aids. However, we do not know if this is relevant or not. This review did not look at the differences between other 'old' and 'new' types of hearing aid. We could not combine the numbers from the four studies. Overall, this fact and the very low quality of the evidence leads us to conclude that we do not know if patients have a preference for one aid or two. Similarly, we do not know if a patient's quality of life is better with one or two aids.