Ultrasonographic Versus Anthropometric Measurement of Subarachnoid Space Depth in Adults Undergoing Spinal Anaesthesia: A Prospective Comparative Study.

BACKGROUND: Accurate identification of subarachnoid space depth (SSD) is crucial for successful spinal anesthesia. While traditional anthropometric methods are commonly used, their reliability varies, particularly in obese patients and those with spinal deformities. Ultrasonography offers real-time visualization but requires validation against established techniques. METHODS: A prospective observational study was conducted on 56 adults (18-65 years) scheduled for elective surgeries under spinal anesthesia. Ultrasonographic SSD (USG-SSD) was measured preoperatively at the L3-L4 level using a curvilinear probe, while anthropometric SSD (ANTHRO-SSD) was calculated using Bonadio's formula. The actual SSD was recorded during needle insertion. Statistical analysis included Bland-Altman agreement, Pearson correlation, and receiver-operating characteristic (ROC) curves. RESULTS: USG-SSD (4.5 ± 0.6 cm) closely matched actual SSD (4.6 ± 0.5 cm; with p value = 0.12), whereas ANTHRO-SSD overestimated depth (5.1 ± 0.7 cm; p< 0.001). Ultrasonography showed superior agreement (limits: -0.6 to +0.4 cm), lower errors (MAE: 0.3 cm vs. 0.7 cm), and stronger correlation (r = 0.92 vs. 0.75). Accuracy was consistent across BMI categories, unlike anthropometry, which overestimated SSD in obese patients (error: 1.1 ± 0.5 cm). Ultrasonography achieved 87.5% first-attempt success and discriminated difficult cases (SSD >5 cm) effectively (area under the ROC curve (AUC): 0.93 vs. 0.69 for anthropometry). CONCLUSION: Ultrasound-guided SSD estimation is more accurate and reliable than anthropometric methods, particularly in high-BMI populations. Its integration may improve procedural success rates and reduce complications.