European Journal of Cardiovascular Medicine

Lumber puncture is routinely performed by anaesthesiologists for administering spinal anaesthesia. An accurate placement of spinal needle is crucial while injecting drugs. Apart from knowledge of anatomy and technical skill, a pre-puncture estimate of skin to subarachnoid space depth (SSD) may guide spinal needle placement.¹ A failure in obtaining cerebrospinal fluid despite spinal needle being inserted further than the estimated depth suggests it is not in subarachnoid space and needs to be withdrawn and redirected.

Knowledge of SSD would also aid in selecting a spinal needle of an appropriate length. A conventional spinal needle. may be too long for a lean patient while it may fall short of length in the obese patient resulting in multiple punctures, unsuccessful attempts and increased patient discomfort.²

While there are several studies on distance from skin to epidural space studies SSD are relatively few. Of these, most focused on SSD in the paediatric population, considering spinal anaesthesia to be more challenging in children.^1.3 Present study was aimed to study skin to subarachnoid space depth in various conditions (between males, females, full term parturient, bed ridden for more than 7 days) at a tertiary hospital.

Present study was single-center, prospective, observational study, conducted in department of anaesthesiology, at P.D.U. Medical College, Civil Hospital, Rajkot, India. Study duration was of 2 years (January 2021 to December 2022).

After obtaining approval from Institute Ethics Committee (IEC) and informed consent 200 adult patient of either gender we will take 50 cases in each group. (50 male,50 female,50 parturient,50 bedridden for more than 7 days) randomly. Randomisation was done as per computer generated numbers to prevent bias for selection of cases.

We recruited patients of age group (15-65 year) of ASA I, II, III. Demographic and anthropometric data included age, gender, height, weight BSA (body surface area) and body mass index. The study population was divided into four groups Group M (Male), Group F(Female), Group PF (Parturient), Group (Bed ridden Male, Female more than 7 days).

All the patients were assessed pre operatively before surgery and all the required investigations like hematological (CBC, RFT, RBS, and other routine investigation) Chest X-RAY was done and documented. Anaesthesia consent with ASA grading was taken. Patient was kept nil per oral before surgery at least 8 hours. Prior to the surgery after randomisation patient explained about the procedure. Standard monitors like ECG, NIBP and pulse oximeter applied and monitor the same. All patients were premedicated with injection ondansetron 0.08mg/kg.

Selected case considered with the formula for SSD to be applied. Before starting the procedure, the predicted value of SSD was measured and after selection of patient observed SSD measured.

Depending on patient’s requirement for the surgical procedure, the patients were placed either in lateral recumbent or in sitting position with their back fully flexed. Under aseptic precaution the L3 L4 intervertebral space identified, guided by the Tuffiers’ line. With Quincke’s (3.5 inch/8.9cm) 23/25-gauge midline approach selected and Dural puncture was performed. The spinal needle was inserted perpendicular to the skin. The needle was advanced until loss of resistance was obtained, signifying entry into the subarachnoid space and confirmed by free flow of cerebrospinal fluid. Following intrathecal injection, the point was marked with permanent marker, the spinal needle was grasped firmly between the thumb and index finger abutting the patient’s back and then needle removed. The depth of insertion was then measured using standard scale and noted.

Abe’s⁴, Bonadio’s⁵, Craig’s⁶, Stocker’s⁷ and Chong’s⁸ modified formulae were applied individually to all the patients to determine predicted SSD in the overall population. For the purpose of comparison, the value obtained in millimeters by stocker’s formula was converted to centimeters unit. The formulae by previous investigator’s as follows

Data was collected and compiled using Microsoft Excel, analysed using SPSS 23.0 version. Frequency, percentage, means and standard deviations (SD) was calculated for the continuous variables, while ratios and proportions were calculated for the categorical variables. We had done one-way ANOVA test between four GROUP M, GROUP F, GROUP PF, GROUP B and paired T test between GROUP M and GROUP F, GROUP PF and GROUP F, one-way ANOVA between group M, GROUP F, GROUP B. P value less than 0.5 was considered as statistically significant.

There was no statistically significant (p value>0.05) correlation between age, Height, BMI, BSA, Weight between group M, group F, group PF & group B.

There was statistically significant difference (p value<0.001) between GROUP M (4.95 ± 0.98 cm) and GROUP F (4.61 ± 1.1 cm) in observed SSD (p value <0.001). There is no statistical significance in predicted SSD in between GROUP M and GROUP F using ABE, BONADIO, CRAIG, MODIFIED CHONG’S FORMULA (p value>0.05) Predicted SSD by using MODIFIED CHONG’S formula showed nearer value (4.96 ± 0.88 cm) to GROUP M (4.95 ± 0.98 cm) and by using CRAIG’S formula (4.92 ± 0.37 cm) showed nearer value to GROUP F (4.61 ± 1.1 cm).

We had used paired t test between GROUP F and GROUP PF there was statistically significant (p value<0.001) difference present of SSD between GROUP F (4.61 ± 1.1 cm) and GROUP PF (4.96 ± 0.98 cm). There was no significant difference (p value >0.05) for predicted SSD between GROUP F and GROUP PF using ABE, BONADIO, CRAIG, STOCKER, MODIFIED CHONG’S FORMULA. Predicted SSD by using CRAIG’S formula showed nearer value (4.89 ± 0.38 cm) to GROUP PF (4.95 ± 0.98 cm) and by using CRAIG’S formula showed nearer value to GROUP F (4.61 ± 1.1 cm).

Mean value for GROUP B is higher 5.01 ± 0.93 cm and it was statistically significant (p value <0.05) when comparing with GROUP F and GROUP M. Predicted SSD did not show any statistically significant (p value>0.05) using ABE’s formula between GROUP M (7.78 ± .56 cm), GROUP F (7.72 ± 1.56 cm), GROUP B (7.74 ± 1.54 cm).

We had used ANOVA one-way testing showed Predicted SSD did not show any statistically significant using BONADIO’S formula between GROUP M (5.14 ± 0.66 cm), GROUP F (5.12 ± 0.68 cm), GROUP B (5.15 ± 0.66 cm) (p value>0.05) GROUP B observed SSD (5.01 ± 0.92 cm) was nearer to predicted SSD using BONADIO’S formula (5.15 ± 0.66 cm) among 3 groups.

Predicted SSD did not show any statistically significant using CRAIG’S formula between GROUP M (4.91 ± 0.36), GROUP F (4.92 ± 0.37 cm), GROUP B (4.9 ± 0.20 cm) (p value>.05). Among three groups predicted SSD shows no strategically significant correlation (p value>0.05). GROUP M observed SSD (4.95 ± 0.98 cm) was nearer to predicted SSD value using CRAIG’S FORMULA (4.91 ± 0.36 cm) among 3 groups.

We used ANOVA ONE-way testing showed Predicted SSD did not show any statistically significant using STOCKER’S formula between GROUP M (5.07 ± 0.86 cm), GROUP F (5.05 ± 0.88 cm), GROUP B (5.03 ± 0.88 cm) (p value>.05). Among three groups predicted SSD showed no strategically significant correlation (p value>0.05).

GROUP B observed SSD (5.01 ± 0.92 cm) was nearer to predicted SSD value using STOCKER’S formula (5.03 ± 0.66 cm) among 3 groups. We had used one-way ANOVA one-way test showed Predicted SSD did not show any statistically significant using MODIFIED CHONG’S formula between GROUP M (4.96 ± 0.88 cm), GROUP F (4.93 ± 0.92 cm), GROUP B (4.94 ± 0.88 cm) (p value>.05). GROUP M observed SSD (4.95 ± 0.98 cm) was nearer to predicted SSD value using MODIFIED CHONG’S formula (4.96 ± 0.88 cm) among 3 groups.

Lumber puncture is routinely performed by anaesthesiologists for administering spinal anaesthesia. An accurate placement of spinal needle is crucial while injecting drugs. A conventional spinal needle may be too long for a lean patient and too short for obese patient and may end up in multiple punctures, unsuccessful attempts and increase patient discomfort. There are many studies available regarding skin to epidural space but study on SSD is relatively few of this mostly focused in paediatric population considering spinal anaesthesia to be more challenging in them.

In present study, we observed that SSD in group M was 4.95 ± 0.98 cm in Group F is 4.61 ± 1.1 cm and GROUP PF was 4.96 ± 0.92 cm and in GROUP B was 5.01 ± 0.92 cm there was statistically significant difference between SSD in GROUP M and GROUP F, GROUP F and GROUP PF and between GROUP M, GROUP F and GROUP B.

Prakash S et al.,¹ noted that mean SSD was 4.71 ± 0.70 cm in the overall population. SSD in adult males (4.81 ± 0.68 cm) was significantly longer than that observed in females (4.55 ± 0.66 cm) but was comparable with SSD in parturient (4.73 ± 0.73 cm). Stocker’s formula best correlated with the observed SSD.

In our study SSD of GROUP M (male population) was 4.95 ± 0.98 cm which was longer than GROUP F (female population) is 4.61 ± 1.1 cm and comparable to parturient female group 4.96 ± 0.92 cm. GROUP M observed SSD value showed nearer value to predicted SSD (4.96 ± 0.88 cm) using MODIFIED CHONG’s formula.

Hazarika R et al.,² noted that mean SSD was 4.37 ± 0.31 cm in the overall population. SSD in adult males was 4.49 ± 0.19cm which was significantly longer than that observed in female’s 4.18 ± 0.39cm which was comparable with SSD in parturient 4.43 ± 0.19 cm. In our study SSD of GROUP M (male population) was 4.95 ± 0.98 cm which was longer than GROUP F (female population) is 4.61 ± 1.1 cm and comparable to parturient female group 4.96 ± 0.92 cm

Taman H I et al.,⁹ noted that mean SSD was 4.99 ± 0.48 cm in the overall population. SSD in adult males (4.93 ± 0.47 cm) was significantly longer than that observed in females (4.22 ± 0.49 cm) but was comparable with SSD in parturient (4.32 ± 0.47 cm). Craig’s formula when applied correlated best with the observed SSD.

E Bassiakou et al.,¹⁰ measured the skin to epidural space distance (SED), the skin to subarachnoid space distance (SSD) and the epidural to subarachnoid space distance (ESD) at the L3-4 interspace in parturient scheduled for caesarean section (CS). Mean values for SED, SSD and ESD were 5.6 ± 1.6 cm, 6.5 ± 1.2 cm and 0.9 ± 0.5 cm, respectively. Statistically significant correlations were observed between SED, SSD and ESD with body mass index and body weight of the parturient, as well as between the SED and the parturient height. Furthermore, a significant negative correlation was observed between the ESD and gestational age. Finally, a significant correlation existed between the SSD and ESD. In our study observed SSD in parturient was 4.96 ± 0.92 cm which was comparable to male population 4.95 ± 0.98 cm.

Basgul et al.,¹¹ reported mean SSD of 5.40 ± 0.66 cm. Vassiliadis et al.,¹² showed SSD of 5.4 ± 0.7 cm in male patient which is longer than our male group 4.95 ± 0.98 Likewise, Bassiakou et al.,¹⁰ found SSD in parturient to be 6.5 ± 1.2 cm which is 1.54 cm longer than that observed in our parturient. The shorter SSD in our population is possibly because of anthropometric differences between the study subjects, our patients being shorter and less heavy compared with the Western population.

Our study SSD in parturient population SSD longer (4.96 ± 0.92 cm) compared with the GROUP F (4.61 ± 1.1 cm) could be attributed due to hormonal effects during pregnancy, weight gain, softening of tissues and ligaments, and collection of fat in subcutaneous tissue.¹

Abe et al.,⁴ reported that use of their formula resulted in needle selection that was too short in 6% population and too long in 31% population, in our study in 20 patients out of which 16(approximately 80%) patients’ using ABE’s formula in GROUP M overpredicted SSD by 2.83 cm, CHONG’s et al. also found that ABE’s formula overpredicted SSD 1.2 cm.

Therefore, SSD predicting using ABE’s formula could result in selecting longer spinal needle. Excess projection of spinal needle beyond the skin may cause difficulty in controlling the needle while injecting the drug, thus increasing the technical difficulty. It could also prompt the clinician to insert the needle too far anteriorly, increasing the risk of traumatic tap or nerve injury.^13,14

Our study has limitations. The applicability of formula derived from our study is pertinent only when midline approach is used with spinal needle insertion perpendicular to the skin, in patients with no spinal anomaly. We relied on visual impression regarding needle being perpendicular to the skin. If needle insertion is 30° to the skin surface, skin to epidural distance may be increased up to 1.5 mm for every 10 mm perpendicular distance. A paramedian approach would also increase the needle insertion distance by varying degrees depending on the inclination of the needle.

Skin to subarachnoid distance in adult males was significantly greater than that in females who were not pregnant but was comparable with skin to subarachnoid distance in parturient, bed ridden patient had longer SSD than other groups. In the overall population skin to subarachnoid space distance depended on BMI as the only variable. Amongst the various formula such as Abe’s, Bonadio’s, Craig’s, Stocker’s and Chong’s modified formulae, Modified Chong’s formula most accurately predicted the SSD when applied to our population.

Conflict of Interest: None to declare

Source of funding: Nil

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