Comparative Analysis of Actual Skin-to-Subarachnoid Space Depth Versus Depth Predicted by Ultrasonographic and Anthropometric Measurements

Spinal anaesthesia, also known as subarachnoid block, is one of the most frequently employed regional anaesthetic techniques for surgeries involving the lower abdomen, pelvis, and lower limbs. The success of spinal anaesthesia relies on accurate identification of the subarachnoid space and precise deposition of local anaesthetic agents. One of the critical determinants of procedural success is the skin-to-subarachnoid space depth (SSD), defined as the distance between the skin surface and the subarachnoid space¹.

Inaccurate estimation of SSD may result in repeated needle insertions, traumatic punctures, bloody cerebrospinal fluid (CSF) taps, post-dural puncture headache, and increased patient anxiety². These complications not only affect patient comfort but also prolong procedure time and increase the risk of failure. Traditionally, anaesthesiologists rely on tactile feedback and clinical experience, which may be unreliable, particularly in obese patients or those with altered spinal anatomy³.

Anthropometric-based prediction formulas, such as Stocker’s and Chong’s modified formulas, have been proposed to estimate SSD using parameters like weight, height, and body mass index (BMI). While these formulas offer simplicity and ease of use, they provide indirect estimations and may lack accuracy across different populations⁴⁻⁶.

The advent of ultrasonography has revolutionized regional anaesthesia by allowing direct visualization of spinal structures, including the ligamentum flavum–dura mater complex and posterior longitudinal ligament⁷. Pre-procedural ultrasound enables accurate measurement of SSD, identification of optimal intervertebral space, and determination of needle trajectory, thereby improving success rates and reducing complications⁸. Several studies have demonstrated superior correlation between ultrasonographically measured SSD and actual needle depth compared to anthropometric formulas⁹⁻¹¹. However, variability exists based on patient demographics, ethnicity, and body habitus. Moreover, limited studies have directly compared ultrasound-derived SSD, anthropometric predictions, and actual needle insertion depth in the adult Indian population.

Therefore, the present study was designed to evaluate the correlation between actual SSD observed during spinal anaesthesia and SSD predicted using ultrasonographic measurements and anthropometric formulas. Identifying the most accurate and reliable method will help optimize spinal anaesthesia practice and improve patient safety.

This prospective cross-sectional observational study was conducted at The Oxford Medical College, Hospital and Research Centre, Bengaluru, after obtaining institutional ethical committee approval. Written informed consent was obtained from all participants.

Study Population

A total of 180 adult patients scheduled for elective surgeries under spinal anaesthesia were enrolled.

Inclusion Criteria

·          Age between 18 and 60 years

·          Eiter sex

·          ASA physical status I or II

·          Patients undergoing elective surgeries under spinal anaesthesia

Exclusion Criteria

·          Previous spinal surgery

·          Spinal deformities or anatomical abnormalities

·          Local site infection

·          Paramedian spinal approach

·          Refusal to consent

Preoperative Assessment

All patients underwent routine pre-anaesthetic evaluation. Anthropometric measurements including height and weight were recorded.

Prediction of SSD

Predicted SSD was calculated using:

·          Stocker’s Formula: SSD (mm) = 0.5 × weight (kg) + 18

·          Chong’s Modified Formula: SSD (cm) = 10 × [weight (kg) / height (cm)] + 1

Ultrasonographic Measurement

Patients were positioned sitting with flexed spine. Using a low-frequency (2–5 MHz) curved-array probe, ultrasonography was performed in the midline transverse view at the L3–L4 interspace. The distance from the skin to the anterior aspect of the ligamentum flavum–dura mater complex was measured using electronic calipers.

Actual SSD Measurement

Under strict aseptic precautions, spinal anaesthesia was performed at the L3–L4 interspace using a 25G Quincke needle via the midline approach. Upon free flow of CSF, the needle was marked at skin level, withdrawn, and the depth of insertion measured using a sterile ruler.

Statistical Analysis

Data were analysed using SPSS software. Continuous variables were expressed as mean ± SD. Pearson’s correlation coefficient assessed relationships between predicted and actual SSD. Bland–Altman analysis evaluated agreement. A p-value <0.05 was considered statistically significantfor the categorical data analysis. P < 0.05 was considered statistically significant.

Table 1. Demographic Characteristics of Study Population

The study population consisted predominantly of middle-aged adults with a wide BMI distribution.

Table 2. Mean SSD Measurements by Different Methods

 Ultrasonography closely approximated actual SSD compared to anthropometric formulas.

Table 3. Correlation Between Predicted and Actual SSD

 Ultrasonography demonstrated a strong positive correlation with actual SSD.

Table 4. Bland–Altman Analysis (Mean Bias)

 Ultrasonography showed minimal bias and best agreement.

Table 5. Gender-wise Comparison of Actual SSD

Males demonstrated slightly higher SSD.

Table 6. SSD According to BMI Categories

 SSD increased significantly with BMI.

Accurate prediction of skin-to-subarachnoid space depth is fundamental for successful spinal anaesthesia. In this study, ultrasonographic measurement demonstrated superior accuracy and correlation with actual needle insertion depth compared to anthropometric formulas. These findings reinforce the growing role of ultrasound in regional anaesthesia.

The strong correlation observed between ultrasonographic SSD and actual depth (r = 0.88) aligns with findings reported by Girimurugan et al. and Sutagatti et al., who showed ultrasound to be a reliable predictor across different patient populations⁴,⁷. Ultrasonography allows direct visualization of spinal structures, eliminating variability introduced by soft tissue thickness and body habitus.

Anthropometric formulas, though convenient, demonstrated only moderate correlation. Stocker’s formula tended to overestimate SSD, particularly in overweight and obese patients. Similar trends were reported by Tyagi et al. and Khandelwal et al., suggesting that formula-based predictions may lack generalizability across populations⁶,¹².

BMI emerged as a significant determinant of SSD, corroborating earlier studies that identified BMI as the strongest predictor of increased SSD²,⁸. Male patients showed slightly greater SSD, likely due to higher muscle mass and body habitus differences.

The minimal bias observed with ultrasonographic measurement on Bland–Altman analysis highlights its reliability and clinical utility. Incorporation of ultrasound may reduce multiple puncture attempts, procedure time, and complications, especially in technically difficult cases.

Limitations of this study include single-centre design and exclusion of patients with spinal deformities. However, the large sample size and standardized methodology strengthen the validity of findings.

Pre-procedural ultrasonographic measurement of skin-to-subarachnoid space depth provides the most accurate estimation compared to anthropometric formulas. Routine use of ultrasound-guided assessment can enhance success rates, minimize complications, and improve patient comfort during spinal anaesthesia. infection as well as preventing the development of resistant strains.

1.        Girimurugan N, Siddharth M, Pavithira S, Lakshmi R, Srinivasan V. Actual depth of subarachnoid space: Correlation between Stocker’s formula, Craig’s formula and ultrasound-guided spinal anaesthesia in lumbar region. Indian J Clin Anaesth. 2022;9(1):4–7.

2.        Pattanayak T, Parmar V, Vaghela K, George T, Moosani F, Mehta R. Prospective observational study of skin to subarachnoid space depth in various conditions at a tertiary hospital. Eur J Cardiovasc Med. 2024;14(1):1134–1139.

3.        Sutagatti JG, Kurdi MS, Bilung PA. Ultrasonographic estimation of skin to subarachnoid space depth in pre-eclamptic Indian parturients and comparison with physical index-based formula. Indian J Appl Radiol. 2019;5(1):135–140.

4.        Nair N, Gnanasekar R, Joshi R, Lakshmi R. Correlation between depth of subarachnoid space estimated by Stocker’s formula, ultrasound and actual needle insertion depth. J Anaesthesiol Clin Pharmacol. 2022;38(4):548–552. doi:10.4103/joacp.joacp_363_21

5.        Khandelwal M, Tiwari E, Jain P, Singh C, Bharadwaj N. Prediction of depth from skin to subarachnoid space based on preoperative anthropometric assessment. J Appl Pharm Res. 2023;11(3):24–28.

6.        Tyagi V, Jain V, Agrawal B, Jain M, Rastogi B. Comparison of subarachnoid space depth using anthropometric, ultrasonographic and actual needle depth measurements. Ann Clin Lab Res. 2019;7(1):284.

7.        Hazarika R, Choudhury D, Nath S, Parua S. Estimation of skin to subarachnoid space depth: An observational study. J Clin Diagn Res. 2016;10(10):UC06–UC09. doi:10.7860/JCDR/2016/21054.8664

8.        Taman HI, Farid AM, Abdelghaffar WM. Measuring skin to subarachnoid space depth in Egyptian population: A prospective cohort study. Anesth Essays Res. 2016;10(3):468–472. doi:10.4103/0259-1162.177187

9.        Chin KJ, Perlas A, Chan V. The ultrasound-assisted approach to neuraxial blockade. Anesthesiology. 2016;124(4):965–975. doi:10.1097/ALN.0000000000001025

10.     Grau T, Leipold RW, Conradi R, Martin E. Ultrasound control for presumed difficult epidural puncture. Acta Anaesthesiol Scand. 2015;59(5):613–620.

11.     Helayel PE, Silva RC, Ceccon MS, et al. Skin-to-epidural space distance: Correlation with body mass index. Reg Anesth Pain Med. 2017;42(2):215–220. doi:10.1097/AAP.0000000000000538

12.     Perlas A, Chaparro LE, Chin KJ. Lumbar neuraxial ultrasound for spinal and epidural anesthesia. Reg Anesth Pain Med. 2016;41(2):251–260.

13.     Sahin T, Balaban O, Sahin L, et al. Ultrasound-guided spinal anesthesia in obese patients. J Anesth. 2017;31(2):333–338. doi:10.1007/s00540-016-2300-9

14.     Balki M, Lee Y, Halpern S, Carvalho JCA. Ultrasound imaging of the lumbar spine in the transverse plane: The correlation between estimated and actual depth to the epidural space. Anesth Analg. 2017;124(4):1351–1357.

15.     Shaikh F, Brzezinski J, Alexander S, et al. Ultrasound imaging for lumbar punctures and epidural catheterisations: Systematic review and meta-analysis. BMJ. 2018;361:k1635. doi:10.1136/bmj.k1635

16.     Ramasamy D, Govindarajan M, Raman R. Correlation of anthropometric parameters with skin to subarachnoid space depth. Indian J Anaesth. 2016;60(10):752–756. doi:10.4103/0019-5049.191667

17.     Kim JH, Song SY, Kim BJ. Prediction of spinal needle depth using ultrasound and anthropometric variables. Korean J Anesthesiol. 2018;71(6):457–463. doi:10.4097/kja.d.18.00030

18.     Vallejo MC, Phelps AL, Singh S, et al. Ultrasound decreases the failed spinal anesthesia rate in obese patients. J Clin Anesth. 2017;39:99–102.

19.     Elsharkawy H, Maheshwari K, Farag E. Development of ultrasound-guided neuraxial anesthesia. Anesthesiology. 2019;130(2):323–335.

20.     Chin KJ, Macfarlane AJR, Chan V, Brull R. The analgesic efficacy of ultrasound-guided neuraxial techniques. Anesth Analg. 2017;124(4):1237–1248.

21.     Bhatia K, Arora S, Dwivedi D. Ultrasound-guided spinal anesthesia: A review of current evidence. Saudi J Anaesth. 2018;12(2):264–272.

22.     Grau T. Ultrasonography in the identification of landmarks for neuraxial anesthesia. Best Pract Res Clin Anaesthesiol. 2019;33(2):129–140.

23.     Nomura JT, Leech SJ, Shenbagamurthi S, et al. A randomized controlled trial of ultrasound-assisted lumbar puncture. Acad Emerg Med. 2016;23(2):149–156.

24.     Sahota JS, Carvalho JCA. Ultrasound-guided neuraxial techniques in obstetric anesthesia. Curr Opin Anaesthesiol. 2019;32(3):292–297.

25.     Karmakar MK, Li X, Kwok WH, et al. Sonoanatomy relevant for lumbar neuraxial block. Br J Anaesth. 2017;119(2):305–314.