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

Sixty degrees of neck flexion optimally expands the cisternal subarachnoid space; routine 90° flexion offers no benefit and may risk patient safety.

This prospective experimental study evaluated the optimal neck flexion angle for maximizing cisternal subarachnoid space dimensions during cerebrospinal fluid (CSF) collection in dogs. Six healthy adult male Beagles underwent ultrasonographic measurement of subarachnoid space depth, subarachnoid space area, and atlanto-occipital membrane length at graded neck flexion angles (10°, 15°, 20°, 30°, 60°, and 90°) under general anesthesia in right lateral recumbency. Repeated-measures ANOVA and linear mixed models were used for statistical analysis. Results demonstrated that subarachnoid space depth increased progressively from 1.31 ± 0.13 mm at 10° to 2.26 ± 0.51 mm at 60°, with no significant further increase at 90° (2.19 ± 0.33 mm). Similarly, subarachnoid space area peaked at 29.9 ± 14.8 mm² at 60° before declining to 26.8 ± 9.9 mm² at 90°. Atlanto-occipital membrane length continued to increase slightly beyond 60°, but the clinical benefit was deemed negligible. These findings challenge the conventional practice of positioning dogs at 90° neck flexion for cisternal CSF collection, demonstrating that 60° flexion achieves equivalent or superior space expansion. Excessive flexion beyond 60° may pose unnecessary risks, including potential compromise of venous drainage, respiratory function, and spinal cord perfusion, particularly in neurologically compromised patients. The study supports adopting 60° neck flexion as a safer, evidence-based standard for cisternal CSF collection in canine patients, balancing procedural access with patient safety.

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