We identified 1256 records through database searching and 356 through forward-backward citation analysis for a total of 1206 records to screen after deduplication. We excluded 1181 records after screening titles and abstracts and obtained 25 records for full-text screening. We excluded 20 full-text articles with reasons for exclusion recorded. (Fig. 1) We also screened 36 clinical trial registries and found one relevant trial that we already identified in database searches. We included five studies in the narrative synthesis.
Characteristics of included studies
Of all the five included randomised controlled trials, two were conducted in the United Kingdom [25, 26], with the others in the United States , Taiwan , and Columbia  (Table 1). Three studies were published in the 1970s and 1980s [25,26,27], with the two most recent published in 2019 and 2020 [28, 29]. All studies were small, with total number of participants ranging from 14 to 71, and were of short-term duration, with the longest intervention being for 6 weeks [26, 29]. Four studies were cross-over trials [25, 27,28,29] and one was a factorial RCT . All trials had participants with gastroesophageal reflux symptoms, with one trial conducted among participants who had previously had oesophageal cancer and undergone esophagectomy and gastric tube reconstruction . All included studies were reported in English except for one reported in Spanish, which was also reported in English in clinicaltrials.gov (NCT02706938) .
In terms of intervention details (i.e. how elevation was achieved), two trials used ‘bed blocks’ (i.e. 20 cm blocks under the legs of the head of the bed) [26, 29], one trial used ‘sleeping on a wedge’ (i.e. sleeping on a 20 cm wedge-shaped pillow) , and two trials used both interventions i.e. 20-28 cm ‘bed blocks’ and ‘sleeping on a wedge’ (as two separate intervention arms) [25, 27]. None of the included studies evaluated the left lateral sleep position as an intervention.
Risk of bias assessment
All five included studies were judged to be at high risk in two or more of the domains of risk of bias. All five studies were judged to be at high risk of performance bias (either blinding of patients and personnel or outcome assessors), and four at high or unclear risk for selection bias (either random sequence generation or allocation concealment) (Fig. 2).
Heterogeneity, especially of outcome measures and the reporting of outcome data, precluded meta-analyses of the primary and secondary outcomes. Therefore, we reported the results narratively grouped for each outcome (Table 2).
Gastroesophageal reflux symptoms
All the four (of the five included) studies that we identified evaluating the impact of head of bed elevation on patient-reported gastroesophageal reflux symptoms found an improvement among participants in the head-of-bed elevation intervention arm [25, 26, 28, 29].
A crossover randomized trial analysed the change in GORD symptoms among 39 participants (out of the 65 enrolled participants) at 6 weeks using a 6-point scale, the Reflux Disease Questionnaire Score, with a higher score meaning a worse outcome and a change of ≥0.6 points considered clinically meaningful . The trial found that participants in the head-of-bed elevation group were more likely to report a clinically meaningful change of ≥10% in their symptom scores at 6 weeks compared to the participants in the control group (27 [69%] versus 13 [33%], risk ratio of 2.1; 95% CI 1.2 to 3.6); with a mean difference in change scores of − 1.3, 95% CI − 2.0 to − 0.6).
A crossover trial of 63 participants , evaluating the effect of head-of-bed elevation using ‘bed blocks’ or ‘sleeping on a wedge’, found a reduction in the number of reflux symptoms (i.e. heartburn or acid regurgitation) reported during the intervention (i.e. half-night) in the head-of-bed elevation arm (mean difference in ‘bed blocks’ -1, 95% CI − 2.2 to − 0.1; and ‘sleeping on a wedge’ -1, 95% CI − 1.9 to − 0.01) compared to the control group.
A factorial RCT , evaluating the effect of ranitidine and head-of-bed elevation among 71 participants, found that participants in the head-of-bed elevation group were more likely to report an overall improvement in their symptoms at 6 weeks compared to the control group (72% [23 of 32] vs. 55% [17 of 31]; unadjusted odds ratio 2.1, 95% CI 0.7 to 6.0; odds ratio adjusted for ranitidine use 3.1, p < 0.005). It also found a reduction in the reported scores, measured on a 4-point-scale, of gastroesophageal reflux and retrosternal pain symptom in the head-of-bed elevation group compared to the control group. There was no statistically significant difference in overall improvement of symptoms reported by adults receiving the head-of-bed elevation only compared to adults receiving ranitidine only (59% [10 of 17] head-of-bed elevation only vs. 77% [13 of 17] ranitidine only; RR 0.8, 95% CI 0.5 to 1.2).
Another crossover trial measured the change in GORD symptoms among 14 post-esophagectomy participants at 2 weeks on a 30-point score, the modified Dysfunction after Upper Gastrointestinal Surgery scale, with a higher score meaning a worse outcome . It found a statistically significant improvement in GORD symptoms in the head-of-bed elevation group compared to the control group (mean difference in change scores − 5.1, 95% CI − 7.6 to − 2.6).
Intra-oesophageal pH measurement
We identified two trials evaluating the impact of head-of-bed elevation using ‘bed blocks’ or ‘sleeping on a wedge’ on the intra-oesophageal pH measurement (i.e. reported as the number of reflux episodes; percent of total time pH remained < 4; and acid clearance time) [25, 27].
A cross-over RCT of 15 participants , found a statistically significant reduction in acid exposure (defined as percent of total time intra-oesophageal pH remained ≤4) in both groups (mean % of total time: ‘sleeping on a wedge’ 15, 95% CI 3 to 26; ‘bed blocks’ 15, 95% CI 0 to 31; control 21, 95% CI 10 to 33); but not in the number of reflux episodes (defined as the total number of occasions of intra-oesophageal drop to < 4) and acid clearance time (defined as the total time that intra-oesophageal pH remained < 4 by the number of reflux episodes) in any of the two groups compared to the control group.
In addition to measuring GORD symptoms , participants in the ‘bed blocks’ arm in a cross-over study had statistically significant reductions in acid exposure (mean difference in % of total time that intra-oesophageal pH remained < 4, − 7.3, 95% CI − 13.9 to − 0.7) and reflux episodes (mean difference in number of drops in intra-oesophageal pH by two per participants, − 2.5, 95% CI − 4.2 to − 0.8) compared to participants in the control arm. But there were no statistically significant reductions neither in acid exposure (mean difference in % of total time, 1.2, 95% CI − 3.9 to 6.3) nor in reflux episodes (mean difference in number of episodes per participants, − 0.6, 95% CI − 2.3 to 1.1) among participants in the ‘sleeping on a wedge’ compared to the control.
We identified two studies that measured patient preferences for head-of-bed elevation using ‘bed blocks’ or ‘sleeping on a wedge’. [27, 29] In a crossover trial , 36 of the 57 participants (63, 95% CI 51 to 76%) preferred the head-of-bed elevation to control intervention. Of the 15 participants included in another trial , 9 (60%) preferred ‘bed blocks’; 5 (33%) preferred ‘sleeping on a wedge’; and one (7%) preferred neither.
We identified two trials reporting adverse events associated with the intervention [26, 29]. A 2020 crossover RCT found that 39 of 63 participants (62%) in head-of-bed elevation group reported largely minor adverse events at 14 weeks compared to no one in the control group (24 bed-related e.g. bed slipping and unsteadiness; and 15 others e.g. varicose vain pain, musculoskeletal pain, and sexual activity interference) . A factorial RCT found that two of 32 participants in the head-of-bed elevation group reported adverse events (one bed slipping and one sexual activity interference); both resolved and neither discontinued the intervention .