Normative cerebellar tonsil position in healthy individuals: a systematic review and meta-analysis

The cerebellar tonsils are critical anatomical landmarks in craniospinal imaging, yet normative data describing their position in healthy populations are sparse and inconsistent. Defining statistically grounded reference values may support anatomical interpretation in conditions such as Chiari malformation type I, while recognising that clinical diagnosis remains symptom-driven.

To determine the average cerebellar tonsil position relative to the McRae line using MRI, data from asymptomatic individuals with normal craniospinal MRI were synthesised to provide a standardised normative benchmark.

A systematic review and meta-analysis were conducted in accordance with PRISMA 2020 guidelines. Studies were identified through PubMed, Embase, Scopus, and Web of Science searches up to 14 May 2025. Eligible studies were MRI-based cross-sectional designs reporting mean cerebellar tonsil position, standard deviation, and sample size in healthy populations. Risk of bias (ROB) was evaluated using the Adaptation of the Newcastle-Ottawa Scale for Cross-Sectional Studies (NOS-xs). A Der Simonian-Laird random-effects model was used to calculate pooled means, and heterogeneity was assessed via I², Tau², and Cochran's Q. Sensitivity analyses were conducted to test robustness. Certainty of evidence was appraised using the GRADE approach.

Fifteen studies encompassing 3,940 healthy individuals were included. The pooled mean cerebellar tonsil position was + 1.13 mm above the McRae line with reference interval (- 4.67 to + 6.93 mm). However, existing studies show substantial heterogeneity (I² = 98%, p < 0.01), underscoring the need for cautious application and further standardisation. Sensitivity analyses confirmed the consistency of findings across multiple exclusion strategies. Certainty of evidence was rated as moderate due to variation in study quality and reporting.

The pooled estimate provides a normative reference for cerebellar tonsil position in healthy individuals. However, high heterogeneity (I² = 98%), moderate ROB, and geographic clustering limit generalizability. These findings may aid anatomical interpretation, but clinical decisions should remain symptom-guided.

The pooled estimate establishes a normative reference for the position of the cerebellar tonsils in asymptomatic individuals with normal craniospinal MRI findings. This information can assist in the anatomical interpretation of craniospinal imaging. However, considerable variability and geographic clustering can limit its general applicability. Clinical decisions should continue to be guided by symptoms, and future studies should use standardised imaging techniques and broader sampling methods to enhance diagnostic relevance.