Neck flexion to 60° maximizes cisternal subarachnoid space for cerebrospinal fluid collection in dogs

To quantify ultrasonographic changes in cisternal subarachnoid space dimensions across graded neck-flexion angles and to identify the angle providing adequate expansion for CSF collection in dogs.

The study included 6 healthy adult male Beagles at a single veterinary teaching hospital (July 14 through August 14, 2025). Under general anesthesia in right lateral recumbency, the neck was flexed to 10°, 15°, 20°, 30°, 60°, and 90°. Transverse-plane images were used to measure subarachnoid space depth and area, and sagittal-plane images were used to measure atlanto-occipital membrane length. Three measurements/angle/variable were obtained. Data were analyzed using repeated-measures ANOVA and linear mixed models.

All dogs completed the study without adverse events. Subarachnoid space depth increased from 1.31 ± 0.13 mm at 10° to 2.26 ± 0.51 mm at 60° and 2.19 ± 0.33 mm at 90° (60° vs 90°). Subarachnoid space area reached 29.9 ± 14.8 mm2 at 60° and then declined to 26.8 ± 9.9 mm2 at 90°. Membrane length rose from 7.6 ± 1.3 mm at 30° to 10.2 ± 1.0 mm at 60° and 11.4 ± 1.1 mm at 90°.

Neck flexion up to 60° achieved sufficient expansion of the cisternal subarachnoid space and atlanto-occipital membrane, with no meaningful gain beyond this angle.

Routine 90° flexion provides no benefit over 60° and may increase risk; practitioners who flex beyond 60° may compromise patient safety without enhancing access, supporting moderate positioning as effective and safe.