Dura-to-Spinal Cord Distance at Different Vertebral Levels in Indian Children: A Retrospective Computerized Tomography Scan-Based Study.

Background: Neuraxial techniques provide good postoperative analgesia for painful procedures in the pediatric population. However, any injury to the spinal cord can lead to irreversible long-term effects. Aims: We aimed to evaluate dura-to-cord (DTC) distance from computerized tomography (CT) images of thoracolumbar spine in pediatric age group (1-16 years) at T8-9, T9-10, and L1-2 interspaces to identify the safe space for epidural insertion in Asian children. Settings and Design: It was a retrospective study including 141 children aged 1-16 years who underwent routine CT scan of the thoracolumbar region for unrelated diagnostic indications. Materials and
Methods: Patients with spinal abnormalities were excluded. Sagittal CT images of the thoracolumbar spine were obtained to calculate the DTC at T8-9, T9-10, and L1-2 interspaces. The measurements at all levels were obtained perpendicular to the long axis of the vertebral body. Statistical Analysis Used: Continuous data were depicted as mean with standard deviations. The categorical data were presented as counts with percentages.
Results: The mean DTC distance at T8-9, T9-10, and L1-2 interspaces was 3.51 ± 0.98 mm (95% confidence interval [CI]: 3.35-3.67), 2.73 ± 0.94 mm (95% CI: 2.57-2.89), and 2.83 ± 1.08 mm (95% CI: 2.66-3.02), respectively. A significant difference was found between the genders at T9-10 (P = 0.02) and L1-2 levels (P = 0.04). No difference in DTC was found in toddlers, preschool children, school-going children, and adolescents. DTC at T8-9 showed a significant correlation with age (R 2 = 0.0479; P = 0.04), weight (R 2 = 0.038; P = 0.02), and height (R 2 = 0.037; P = 0.03).
Conclusion: Thoracic epidural space can be used in children and adolescents for epidural catheter placement. T8-9 level showed maximum DTC distance and significant correlation with age, height, and weight in CT imaging in the present study. Copyright:

INTRODUCTION

Postoperative analgesia in pediatric population can be supplemented with the use of neuraxial techniques.[1] However, any injury to the spinal cord may lead to irreversible long-term effects.[2] Accidental dural puncture has been reported during epidural needle insertion in pediatric population. The Pediatric Regional Anesthetic Network reported around 0.9% incidence of dural puncture with 14 lumbar punctures and 8 thoracic punctures.[3] The use of neuraxial anesthesia has seen an exponential rise in children.[4] The epidural placement is done primarily under general anesthesia (GA) in pediatric age group when the ligaments become lax and chances of dural puncture are higher. The spinal cord has no sensory fibers which leads to no pain sensation if the spinal cord is damaged with the Tuohy needle.[5] Estimation of the dura-to-cord (DTC) distance may help us know about the safe epidural space for optimal catheter placements without causing any neural injury.

Wani et al. measured the DTC at T6–7, T9–10, and L1–2 interspaces using magnetic resonance imaging (MRI) images in 88 children <8 years of age and found the maximum DTC at the T6–7 level and least at the L1–2 level.[6]

There has been no literature evidence of DTC evaluation in the Indian population using computerized tomography (CT) scan. Since CT is readily available and more commonly done modality than MRI in children, we evaluated if the DTC distance at thoracic and lumbar levels could be obtained using CT images in pediatric population to identify the optimal level for placement of epidural catheters. The present study retrospectively evaluated the DTC from the CT images of the thoracolumbar spine in children aged 1–16 years at T8–T9, T9–10, and L1–2 interspaces in Indian (Asian) population.

MATERIALS AND METHODS

The study design was a retrospective observational study done in a tertiary care center after obtaining institutional ethics approval (IEC-568/02.11.2018). The database was searched for the CT scan examinations of the thoracic and lumbar spines performed in the radiology suite of the institute for various diagnostic indications in the pediatric age group of 1–16 years. The informed consent was waived off for the study as it was done retrospectively and de-identified radiological data were collected (CTRI/2021/07/034982). The procedure follows the guidelines laid down in the Declaration of Helsinki (1964).

Chest and abdominal CT scan examinations of children aged 1–16 years which were performed in the hospital for other indications in which the thoracolumbar spine was included were retrieved over 15 days in August 2021 from the hospital Picture Archiving and Communication System (PACS)© database. The exclusion criterion for the study was any CT image showing any deformities such as scoliosis, growth, or spina bifida. Images with poor quality were also excluded. The data retrieved didnot include the name and unique identification number of the patient. The researcher retrieving the data was not involved in the analysis and measurements. The measurements were taken by two independent investigators and were calculated as the mean of the two.

Radiologically, the DTC was marked as the measurement from the ligamentum flavum toward the dura till the spinal cord posterior margin [Figure 1].[6] All the measurements were taken using the electronic caliper available in our PACS workstation. Sagittal reformatted images in the midline section were used with optimal magnification for accurate measurement. Appropriate windowing parameters were applied to the images for better visualization of the structures of interest. An internal measurement computerized scale was used.

Figure 1

Measurement of dura-to-cord distance at three interspaces

Sagittal CT images of the thoracic spine at T8–9 and T9–10 interspaces and lumbar spine at L1–2 interspace were used to measure the DTC distance. The measurements were obtained at a perpendicular level to the long axis of the vertebral body. The three measurements were labeled as DTC1, DTC2, and DTC3, respectively. The age groups were divided into toddler (group T – 1–2 years), preschool (group P – >2–5 years), school going (group S >5–13 years), and adolescent (group A >13–16 years).

The primary objective of the study was to ascertain the DTC distance at each level and identify the level where it was maximum and if there was any difference in different age groups. The secondary objective was to correlate the DTC distance at the three levels with gender, age, height, and weight.

Statistical analysis

The data were analyzed using IBM Corp. Released 2017. IBM SPSS Statistics for Windows, Version 25.0. Armonk, NY: IBM Corp.© and Microsoft© Excel 365 version 16.61. Continuous data in the study were depicted as mean with standard deviations. The categorical data were presented as counts with percentages. P < 0.05 was considered to be statistically significant.

RESULTS

A total of 141 patients were recruited in the present study, of which 92 were males and 49 were females. The mean age, height, weight, and body mass index were 9.06 ± 5.31 years, 32.23 ± 16.48 kg, 125.54 ± 30.98 cm, and 19.10 ± 3.36, respectively. The demographic data are presented in Table 1.

Table 1

Demographic data and dura-to-cord distance of pediatric study population

	Male (n=92)	Females (n=49)	Total (n=141)	P
Age (years)	8.56±0.55	10.02±0.74	9.06±5.31	0.001
Weight (kg)	32±17.28	32.66±15.02	32.23±16.48	0.59
Height (cm)	125.34±31.68	125.92±29.92	125.54±30.98	0.54
BMI	18.76±2.94	19.74±3.98	19.10±3.36	0.95
DTC1 (mm)	3.58±0.98	3.37±0.97	3.51±0.98	0.11
DTC2 (mm)	2.85±1.01	2.51±0.76	2.73±0.94	0.02
DTC3 (mm)	2.95±1.08	2.61±1.06	2.83±1.08	0.04

P<0.05 is significant. Values presented as mean±SD. SD=Standard deviation, BMI=Body mass index, DTC1=Dura-to-cord distance at T8–9 interspace, DTC2=Dura-to-cord distance at T9–10 interspace, DTC3=Dura-to-cord distance at L1–2 interspace

The mean DTC distance at T8–9, T9–10, and L1–2 interspaces was 3.51 ± 0.98 mm, 95% confidence interval (CI): 3.35–3.67 (DTC1); 2.73 ± 0.94 mm, 95% CI: 2.57–2.89 (DTC2); and 2.83 ± 1.08 mm, 95% CI: 2.66–3.02 (DTC3), respectively. The DTC1 was 3.58 ± 0.98 mm (95% CI: 3.38–3.79) in males and 3.37 ± 0.97 mm (95% CI: 3.10–3.66) in females (P = 0.11). The DTC2 was 2.85 ± 1.01 mm (95% CI: 2.64–3.06) and 2.51 ± 0.76 mm (95% CI: 2.30–2.74) in males and females, respectively (P = 0.02). The DTC3 in males was 2.95 ± 1.08 mm (95% CI: 2.73–3.18) and in females was 2.61 ± 1.06 mm (95% CI: 2.31–2.92) (P = 0.04) [Table 1].

The total toddlers (group T) were 22 in number and preschool children (group P) were 27 in number. There were a total of 48 children in the school-going age group (group S) and 44 adolescents (group A) in the study data. The demographic data and the DTC distance in different age groups are represented in Table 2. The comparison of DTC in different age groups showed a P value of 0.20, 0.14, and 0.11 for DTC1, DTC2, and DTC3, respectively.

Table 2

Demographic data and dura-to-cord distance of different age groups

	Group T (n=22)	Group P (n=27)	Group S (n=48)	Group A (n=44)	P
Weight (kg)	11.9±2.4	17.62±2.87	32.88±10.70	50.65±7.63	0.001
Height (cm)	80.08±6.5	96.03±10.87	132.61±13.25	158.6±7.61	0.001
BMI	18.52±2.95	19.57±4.38	18.21±3.35	20.08±2.53	0.01
DTC1 (mm)	3.54±0.76	3.99±0.80	3.41±1.02	3.30±1.05	0.20
DTC2 (mm)	2.72±0.91	2.76±0.71	2.84±1.09	2.60±0.93	0.14
DTC3 (mm)	3.13±1.32	2.76±0.79	2.90±1.10	2.67±1.10	0.11

P<0.05 is significant. Values presented as mean±SD. Group T=Toddlers, Group P=Preschool, Group S=School-going, Group A=Adolescents, BMI=Body mass index, DTC1=Dura-to-cord distance at T8–9 interspace, DTC2=Dura-to-cord distance at T9–10 interspace, DTC3=Dura-to-cord distance at L1–2 interspace, SD=Standard deviation

The DTC-age correlation [Figure 2] showed the highest correlation of DTC1 with age (R2 = 0.0479; P = 0.04). DTC2 and DTC3 had a R2 value of 0.008 (P = 0.35) and 0.004 (P = 0.18), respectively. The DTC-weight correlation [Figure 3] showed the highest correlation of DTC1 with weight (R2 = 0.038; P = 0.02). DTC2 and DTC3 had a R2 value of 0.008 (P = 0.74) and 0.017 (P = 0.34), respectively. The DTC-height correlation [Figure 4] showed the highest correlation of DTC1 (R2 = 0.037; P = 0.03). DTC2 and DTC3 had a R2 value of 0.006 (P = 0.29) and 0.0008 (P = 0.09), respectively.

Figure 2

Correlation of DTC distance with age. DTC=Dura-to-cord

Figure 3

Correlation of DTC distance with weight. DTC=Dura-to-cord

Figure 4

Correlation of DTC distance with height. DTC=Dura-to-cord

DISCUSSION

The present study evaluated the DTC distance in children aged 1–16 years for estimation of the widest depth between dura and cord and evaluated the safe space for epidural needle and catheter insertion. We found a higher value of DTC distance at T8–9 interspace in all age groups. The mean DTC at T8–9 (DTC1) was found to be 3.51 ± 0.98 mm, at T9–10 interspace (DTC2) was 2.73 ± 0.94 mm, and at L1–2 interspace (DTC3) was 2.83 ± 1.08 mm. A marginal increase was found on progressing from T9–10 toward the lumbar region.

A CT scan was chosen to assess the feasibility of using it for calculating the DTC dimensions as it is easily available and is the more commonly performed modality for obtaining images in children, especially in resource-limited settings.[7] CT scan was found to be a satisfactory assessment method for dural sac and epidural fat measurements in a study comparing CT and MRI for assessment of anteroposterior and transverse diameters and the cross-sectional area of the dural sac with a statistically significant correlation between the two imaging modalities.[8]

Park et al. evaluated the MRI scans of the thoracic vertebral segments in 346 adult patients to assess the margin of safety for thoracic epidural block. They found the vertical DTC to be longest at T5–6 intervertebral level (4.22 ± 1.43 mm) and shortest at T11–12 intervertebral level (2.51 ± 0.87 mm). The DTC distance was found to be 3.35 ± 1.17 mm at T8–9 and 2.96 ± 1.05 mm at T9–10.[9]

A retrospective study investigated the DTC distance at T2, T5, and T10 segments using MRI images of 50 adult patients. The results showed that the posterior DTC distance was higher at the T5 level (5.8 ± 0.8 mm) than at T2 (3.9 ± 0.8 mm) and T10 (4.1 ± 1.0 mm) (P = 0.015). However, the entry angle of the Tuohy needle at T5 was 45° ± 7.4° and 9.5° ± 4.2° at T10. The needle path traversed to reach the epidural space was 8.2 ± 2.1 mm at T5 level and 4.2 ± 0.6 mm at T10 level.[10]

Wani et al. found the mean DTC distance using MRI to be 5.0 ± 1.6 mm at T9–10 (range of 1.2–8.1 mm) and 3.6 ± 1.2 mm at L1–2 (range of 1.2–6.8 mm) in children aged <8 years.[6] The values obtained in our study were lower. This could be due to different ethnicity and also difference in the mean height and weight of Asian children when compared to the Western pediatric population.

Although the DTC distance has been found to be widest at mid-thoracic level by few studies,[6910] the increased angulation and longer needle path can prove to be an uphill task for accurate placement of epidural needle in a single attempt in children, more so under general anesthesia (GA). The ease of epidural insertion with the midline approach in low-thoracic region is akin to the lumbar region as the angulation of the spinous processes is same at both the levels.[11] Since the T8–9 lower thoracic vertebra have a much higher DTC distance than T9–10 level as well as the lumbar level, it can be safely used for epidural insertion with a higher margin of safety against inadvertent neural injury.[9] This is also validated by our study.

On subgroup analysis, no difference was found in the DTC at T8–9 between males and females (3.58 ± 0.98 mm, 3.37 ± 0.97; P = 0.11). However, a statistically significant difference was found between the genders at T9–10 (P = 0.02) and L1–2 levels (P = 0.04). This is in contrast to the study by Wani et al. who did not find any statistical difference in the DTC at T9–10 and L1–2 levels between males and females.[6] This could again be due to different ethnicity and inclusion of older children in our study population.

When the DTC was correlated at each level with age, height, and weight, it was found that DTC1 showed a significant correlation with age (R2 = 0.0479; P = 0.04), height (R2 = 0.038; P = 0.02), and weight (R2 = 0.037; P = 0.03). Imbelloni et al. did not find any correlation between DTC and age at T10 level in adults.[10] No significant difference was found in correlation of DTC distance with age, height, and weight at T9–10 and L1–2 levels by Wani et al.[6] No correlation was found between DTC and age, height, and weight at T9–10 and L1–2 levels in our study as well (P > 0.05).

Age group analysis did not show any statistically significant difference in DTC distance among different age groups at all levels (P = 0.20, 0.14, and 0.11, respectively). The maximum DTC1 was found in the preschool age group (3.99 ± 0.80 mm) which decreased progressively with age. The DTC2 was found to be narrowest at T9–10 level across all age groups. However, the maximum DTC2 was found in the school-going age group (2.84 ± 1.09 mm). The maximum DTC3 was found in the toddler age group (3.13 ± 1.32 mm). We did not find any previous study to correlate the incidence.

Forestier et al. evaluated the volume of spinal canal from MRI in 20 children aged 0–3 years at T6, T10, and L1. They found the median epidural volume at T6 to be 0.204 mL.cm-1 and 1.78 mL.kg-1. The T10 values were 0.172 mL.cm-1 and 1.57 mL.kg-1, respectively. The L1 values were 0.146 mL.cm-1 and 1.30 mL.kg-1, respectively.[12] The presence of higher epidural volume in upper thoracic segments also provides a wider margin for dural puncture and associated neurological damage.

Wong and Lim estimated the skin-to-epidural distance (SED) in the thoracolumbar region. They correlated the age, weight, and ethnicity to the SED in Southeast Asian children aged 0–19 years in 616 midline epidurals (225 thoracic and 363 lumbar epidurals). The mean depth of SED for thoracic region was 21.7 (range of 8.0–65.0) mm and for lumbar region was 21.5 (range 3.0–70.0) mm which revealed a correlation with the body weight. They also found clinically significant differences between Southeast Asian and other pediatric population groups.[13] The study did not mention the level of thoracic and lumbar epidural insertions. However, the findings can also be applied to the DTC distance which has been found to be lower in our study population as compared to the Western population. Estimation of DTC distance at the level of thoracic spine may help in providing a higher safety margin with thoracic epidural placement.

Our study had few limitations. First, it was a retrospective study. Second, the study population is limited to Indian children. Further studies are needed to extrapolate our findings to the general population. Furthermore, we did not calculate the SED and DTC distance in upper and mid-thoracic spine in our study.

CONCLUSION

Thoracic epidural space can be used safely in children and adolescents for catheter placement with the T8–9 level showing the maximum DTC distance and significant correlation with age, height, and weight in CT scan imaging in the present study. No significant difference was found among different age groups. Estimation of the DTC distance may be helpful in pediatric population to choose a safe space for epidural insertion and avoid neurological injury as epidural placements are mostly conducted under GA in this age group.

Financial support and sponsorship

Nil.

Conflicts of interest

There are no conflicts of interest.