The Epidemiology and Clinical Presentation of the Acute Imbalance Syndrome ( AIS )—A Systematic Review and Meta‐Analysis

OBJECTIVES: The acute imbalance syndrome (AIS) refers to acute-onset and persistent vertigo, dizziness and/or imbalance without nystagmus, reflecting a subset of the acute vestibular syndrome (AVS) with or without nystagmus. While AVS with nystagmus is well characterized and the approach to these patients is validated, much less is known about patients presenting with AIS. This systematic review synthesizes current epidemiologic and clinical evidence on AIS. METHODS: MEDLINE and Embase (2000-2024) were searched for English-language original articles addressing acute imbalance, vertigo/dizziness and reporting on nystagmus and/or AVS/AIS. Two reviewers independently screened articles and extracted predefined parameters. The review was registered with PROSPERO (CRD42024623068). RESULTS: Forty studies (n = 11,731 patients) were included, with 2730 patients being classified as AVS. Prevalence of AVS (with/without nystagmus) amongst all acutely dizzy patients was 16.4% and 50.5%, respectively, in two studies. 49.4% of central AVS cases were classified as AIS. Whereas AIS presentation was rare in AICA strokes (9%), fractions reached 40% (PICA), 57% (SCA), and 62% (PCA) for other vascular territories. Data on underlying causes in noncentral AIS was limited, with dysautonomia, drug intoxication, and heart disease being most frequently identified. CONCLUSIONS: With almost 50% of central AVS cases presenting as AIS, this emphasizes the importance of selecting bedside testing appropriately, focusing on algorithms such as STANDING or graded truncal instability. Patients presenting with AIS have a distinct differential diagnosis than those with AVS with nystagmus, mostly due to the distribution of noncentral causes. Promoting awareness of AIS and its diagnostic approach should be prioritized in emergency and acute neurological settings.