Inconclusive evidence that breathing shapes pupil dynamics in humans: a systematic review

Abstract

More than 50 years ago, it was proposed that breathing shapes pupil dynamics. This widespread idea is also the general understanding currently. However, there has been no attempt at synthesizing the progress on this topic since. We therefore conducted a systematic review of the literature on how breathing affects pupil dynamics in humans. We assessed the effect of breathing phase, depth, rate, and route (nose/mouth). We followed the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) guidelines, and conducted a systematic search of the scientific literature databases MEDLINE, Web of Science, and PsycInfo in November 2021. Thirty-one studies were included in the final analyses, and their quality was assessed with QualSyst. The study findings were summarized in a descriptive manner, and the strength of the evidence for each parameter was estimated following the Grading of Recommendations Assessment, Development, and Evaluation (GRADE) approach. The effect of breathing phase on pupil dynamics was rated as "low" (6 studies). The effect of breathing depth and breathing rate (6 and 20 studies respectively) were rated as "very low". Breathing route was not investigated by any of the included studies. Overall, we show that there is, at best, inconclusive evidence for an effect of breathing on pupil dynamics in humans. Finally, we suggest some possible confounders to be considered, and outstanding questions that need to be addressed, to answer this fundamental question. Trial registration: This systematic review has been registered in the international prospective register of systematic reviews (PROSPERO) under the registration number: CRD42022285044.

Keywords: Breathing; Hippus; Phase; Pupil dynamics; Pupil size; Respiration.

Fig. 1

Flow diagram of study selection process. The included studies refer to journal articles, whereas the reports refer to academic theses

Fig. 2

Breathing phase study overview. The x-axis depicts the quality assessment score each individual study received (1 being the highest and 0 the lowest possible score), and the y-axis depicts the number of participants included in each study for the breathing phase outcome. Each individual shape represents one study. The color of the shape indicates whether the study outcomes align with there being an effect of breathing phase on pupil dynamics. The shape fill indicates whether the main outcome of the study is breathing and pupil dynamics interactions or not. Finally, the form of the shape indicates whether the breathing and pupil related outcomes of the study have been analyzed statistically, and if so, whether changes in breathing phase and pupil dynamics were assessed separately ( +), or whether statistics were used to look at an interaction (x)

Fig. 3

Breathing depth study overview. The x-axis depicts the quality assessment score each individual study received (1 being the highest and 0 the lowest possible score), and the y-axis depicts the number of participants included in each study for the breathing depth outcome. Each individual shape represents one study. The color of the shape indicates whether the study outcomes align with there being an effect of breathing depth on pupil dynamics. The shape fill indicates whether the main outcome of the study is breathing and pupil dynamics interactions or not. Finally, the form of the shape indicates whether the breathing and pupil related outcomes of the study have been analyzed statistically, and if so, whether changes in breathing depth and pupil dynamics were assessed separately ( +), or whether statistics were used to look at an interaction (x)

Fig. 4

Breathing rate direct assessment study overview. The x-axis depicts the quality assessment score each individual study received (1 being the highest and 0 the lowest possible score), and the y-axis depicts the number of participants included in each study for the breathing rate outcome. Each individual shape represents one study. The color of the shape indicates whether the study outcomes align with there being an effect of breathing rate on pupil dynamics. The shape fill indicates whether the main outcome of the study is breathing and pupil dynamics interactions or not. Finally, the form of the shape indicates whether the breathing and pupil related outcomes of the study have been analyzed statistically, and if so, whether changes in breathing rate and pupil dynamics were assessed separately ( +), or whether statistics were used to look at an interaction (x)

Fig. 5

Breathing rate indirect assessment study overview. The x-axis depicts the quality assessment score each individual study received (1 being the highest and 0 the lowest possible score), and the y-axis depicts the number of participants included in each study for the breathing rate outcome. Each individual shape represents one study. The color of the shape indicates whether the study outcomes align with there being an effect of breathing rate on pupil dynamics. The shape fill indicates whether the main outcome of the study is breathing and pupil dynamics interactions or not. Finally, the form of the shape indicates whether the breathing and pupil related outcomes of the study have been analyzed statistically, and if so, whether changes in breathing rate and pupil dynamics were assessed separately ( +), or whether statistics were used to look at an interaction (x)