Olfactory Function in Autism Spectrum Disorder Across the Lifespan: A Systematic Review and Meta-Analysis.

Abstract

Altered sensory processing is recognized as a core diagnostic feature of autism spectrum disorder (ASD). Olfactory perception in autism remains poorly characterized, despite its important role in safety, nutrition, and socialization. We conducted a preregistered systematic review and meta-analysis to quantify differences and identify moderators in olfactory function between autistic and non-autistic individuals. Following the Preferred Reporting Items for Systematic reviews and Meta-Analyses (PRISMA) guidelines, we searched Web of Science, SCOPUS, PubMed, EMBASE, and CINAHL databases and identified 46 eligible studies with 87 effect sizes from 1,580 autistic and 7,698 non-autistic individuals. Moderator analyses examined olfactory assessment type, olfactory function tested, age, sex, and intelligence quotient (IQ). Autistic individuals showed significantly poorer olfactory performance than non-autistic (g = -0.64), with reduced performance in psychophysical testing (g = -0.58) and informant reports (g = -1.31), but not in self-report or odor rating measures. In autism, significant differences were observed for psychophysically measured odor identification (g = -0.66), threshold (g = -0.47), and taste-smell measures from informant reports (g = -1.33). Autistic children exhibited moderate deficits (g = -0.88; odor identification: g = -0.80; threshold: g = -0.65). Olfactory alterations in autism appear most pronounced in childhood and vary by assessment method and olfactory function measured. Our results highlight the need for age-appropriate standardized measures to accurately evaluate sensory function in autism across the lifespan.Lay AbstractMany autistic individuals experience sensory sensitivities that can make everyday activities more difficult. While researchers have studied how autism affects senses like sight and sound, the sense of smell has received much less attention, even though it plays important roles in helping us detect danger, judge whether food is safe to eat, and connect with others socially. Past studies have suggested that autistic individuals may experience differences in their sense of smell, including how well they can detect, distinguish, or identify odors by name. However, the results of smell studies in autism have been mixed because studies often use different testing methods, have small numbers of participants, and do not always account for demographic factors (e.g., age or sex). In this study, we reviewed the published research literature to better understand how the sense of smell differs based on: (1) whether someone is autistic or not, (2) the type of smell test used, (3) age (children vs. adults), and (4) the specific smell ability being measured. Overall, we found that autistic individuals tend to have poorer smell abilities than non-autistic individuals. These differences between autistic and non-autistic individuals were most noticeable when smell was measured by caregiver reports or standardized laboratory tests, but not when people rated their own sense of smell or described how odors smelled to them. These differences in smell were mainly seen in autistic children. The strongest differences were found in odor identification, sensitivity to smells, and caregiver-reported taste and smell experiences.