Evaluation of the skin to epidural and subarachnoid space distance in young children using magnetic resonance imaging

[Correlation of the epidural space measured intraoperatively and estimated by MRI or US: an observational study]

BACKGROUND AND OBJECTIVES

To assess the agreement between the epidural depth measured from the surgical site with the epidural depths estimated with magnetic resonance imaging (MRI) and ultrasound scanning.

METHODS

Fifty patients of either sex, scheduled for L4-5 lumbar disc surgery under general anesthesia were enrolled in this prospective observational study, and the results of 49 patients were analyzed. The actual epidural depth was measured from the surgical site with a sterile surgical scale. The MRI-derived epidural depth was measured from the MRI scan. The ultrasound estimated epidural depth was measured from the ultrasound image obtained just before surgery.

RESULTS

The mean epidural depth measured from the surgical site was 53.80 ± 7.67mm, the mean MRI-derived epidural depth was 54.06 ± 7.36mm, and the ultrasound-estimated epidural depth was 53.77 ± 7.94mm. The correlation between the epidural depth measured from the surgical site and MRI-derived epidural depth was 0.989 (r² = 0.979, p < 0.001), and the corresponding correlation with the ultrasound-estimated epidural depth was 0.990 (r² = 0.980, p < 0.001).

CONCLUSIONS

Both ultrasound-estimated epidural depth and MRI-derived epidural depth have a strong correlation with the epidural depth measured from the surgical site. Preprocedural MRI-derived estimates of epidural depth are slightly deeper than the epidural depth measured from the surgical site, and the ultrasound estimated epidural depths are somewhat shallower. Although both radiologic imaging techniques provided reliable preprocedural estimates of the actual epidural depth, the loss of resistance technique cannot be discarded while inserting epidural needles.

Correlation of the epidural space measured intraoperatively and estimated by MRI or US: an observational study

Background and objectives

To assess the agreement between the epidural depth measured from the surgical site with the epidural depths estimated with magnetic resonance imaging (MRI) and ultrasound scanning.

Methods

Fifty patients of either sex, scheduled for L4‒5 lumbar disc surgery under general anesthesia were enrolled in this prospective observational study, and the results of 49 patients were analyzed. The actual epidural depth was measured from the surgical site with a sterile surgical scale. The MRI-derived epidural depth was measured from the MRI scan. The ultrasound estimated epidural depth was measured from the ultrasound image obtained just before surgery.

Results

The mean epidural depth measured from the surgical site was 53.80 ± 7.67 mm, the mean MRI-derived epidural depth was 54.06 ± 7.36 mm, and the ultrasound-estimated epidural depth was 53.77 ± 7.94 mm. The correlation between the epidural depth measured from the surgical site and MRI-derived epidural depth was 0.989 (r² = 0.979, p < 0.001), and the corresponding correlation with the ultrasound-estimated epidural depth was 0.990 (r² = 0.980, p < 0.001).

Conclusions

Both ultrasound-estimated epidural depth and MRI-derived epidural depth have a strong correlation with the epidural depth measured from the surgical site. Preprocedural MRI-derived estimates of epidural depth are slightly deeper than the epidural depth measured from the surgical site, and the ultrasound estimated epidural depths are somewhat shallower. Although both radiologic imaging techniques provided reliable preprocedural estimates of the actual epidural depth, the loss of resistance technique cannot be discarded while inserting epidural needles.

Comparison of the skin-to-epidural space distance at the thoracic and lumbar levels in children using magnetic resonance imaging

Background:

Several studies have attempted to estimate the approximate distance from the skin-to-epidural space using different imaging modalities (computed tomography [CT], ultrasound, and magnetic resonance imaging [MRI]) and direct needle measurements. The objective of our study was to compare the distance from the skin to the epidural space (SED) at multiple levels, focusing on T_6-7, T_9-10, and L_2-3 using MRI.

Methods:

After institutional review board (IRB) approval, sagittal T2-weighted MRI images of the spine of 108 children in the age group ranging from 3 months to 8 years undergoing radiological evaluation in the supine position at our institution were analyzed. The SED at T_6-7 and T_9-10 levels (straight and inclined) and SED at L_2-3 (straight) were determined and compared using repeated-measures ANOVA and paired t-tests with a Bonferroni correction for 10 pairwise comparisons (P < 0.005 was considered statistically significant).

Results:

The average SED (measured straight and inclined) was 18.2 mm and 21.6 mm at T_6-7; 18.3 mm and 20.5 mm at T_9-10; and 21.8 mm (straight) at L_2-3. The repeated-measures ANOVA F-test indicated significant variability in SED (P < 0.001) among the 5 measurements obtained. At the P < 0.005 significance level, corrected for multiple comparisons, the SED (straight) at T_9-10 straight was shorter than the other measured distances.

Conclusion:

The distance from the skin to the epidural space is not constant at various vertebral levels. At the levels measured, it was greatest at the lumbar level and at least at the thoracic level of T_9-10. A single predictive formula was not applicable for calculating the approximate SED at all vertebral levels.

Estimation of the depth of the thoracic epidural space in children using magnetic resonance imaging

BACKGROUND

The estimation of the distance from the skin to the thoracic epidural space or skin to epidural depth (SED) may increase the success rate and decrease the incidence of complications during placement of a thoracic epidural catheter. Magnetic resonance imaging (MRI) is the most comprehensive imaging modality of the spine, allowing for the accurate determination of tissue spaces and distances. The present study uses MRI-derived measurements to measure the SED and define the ratio between the straight and inclined SEDs at two thoracic levels (T_6-7 and T_9-10) in children.

METHODS

The T₂-weighed sagittal MRI images of 109 children, ranging in age from 1 month to 8 years, undergoing radiological evaluation unrelated to spine pathology were assessed. The SEDs (inclined and straight) were determined, and a comparison between the SEDs at two thoracic levels (T_6-7 and T_9-10) was made. Univariate and multivariate linear regression models were used to assess the relationship of the inclined thoracic T_6-7 and T_9-10 SED measurements with age, height, and weight.

RESULTS

Body weight demonstrated a stronger association with the SED than did the age or height with R² values of 0.6 for T_6-7 and 0.5 for T_9-10. The formulae describing the relationship between the weight and the inclined SED were T_6-7 inclined (mm) = 7 + 0.9 × kg and T_9-10 inclined (mm) = 7 + 0.8 × kg.

CONCLUSION

The depth of the pediatric thoracic epidural space shows a stronger correlation with weight than with age or height. Based on the MRI data, the predictive weight-based formulas can serve as guide to clinicians for placement of thoracic epidural catheters.

Assessing the Agreement Between Radiologic and Clinical Measurements of Lumbar and Cervical Epidural Depths in Patients Undergoing Prone Interlaminar Epidural Steroid Injection

BACKGROUND

Fluoroscopy-guided epidural steroid injection (ESI) commonly is performed to treat radicular pain yet can lead to adverse events if the needle is not advanced with precision. Accurate preoperative assessment of the distance from the skin to the epidural space holds the potential for reducing the risks of adverse effects from ESI. It was hypothesized that the distance from the skin to the epidural space as measured on preoperative magnetic resonance imaging (MRI) would agree with the distance traveled by a Tuohy needle to reach the epidural space during midline, interlaminar ESI. This study compared the final needle depth measurement at the point of loss of resistance (LOR) from cervical or lumbar ESI to the distance from the skin to the anterior and posterior borders of the epidural space on the associated cervical and lumbar preoperative MRI.

METHODS

This retrospective chart review analyzed the procedure notes, MRI, and demographic data of patients who received a prone, interlaminar ESI at an outpatient chronic pain clinic between June 1, 2013, and June 1, 2015. The following data were collected: body mass index (BMI), age, sex, intervertebral level of the ESI, and LOR depth. We then measured the distance from the skin surface to the anterior border of the ligamentum flavum (ligamentum flavum depth [LFD]) and dura (dura depth [DD]) on MRI. A total of 335 patients were categorized into the following patient subgroups: age ≥65 years, age <65 years, BMI ≥30 kg/m (obese), BMI <30 kg/m (nonobese), male, and female. Secondary analyses were then performed to compare the agreement between LOR depth and DD with that between LOR depth and LFD within each patient subgroup. Intraclass correlation coefficient (ICC) and Bland-Altman plot were used to assess the agreement between DD or LFD and LOR depth.

RESULTS

Data from 335 ESIs were analyzed, including 147 cervical ESIs and 188 lumbar ESIs. Estimated ICC values for the agreement between LOR depth and LFD for all lumbar and cervical measurements were 0.88 (95% confidence interval [CI], 0.85-0.91) and 0.72 (95% CI, 0.64-0.79), respectively. Estimated ICC values for the agreement between LOR depth and DD for all lumbar and cervical measurements were 0.86 (95% CI, 0.82-0.89) and 0.69 (95% CI, 0.60-0.77), respectively.

CONCLUSIONS

This study assessed the agreement between MRI-derived measurements of epidural depth and those determined clinically. MRI-derived measurements from the skin to the anterior border of the ligamentum flavum, which represents the most posterior aspect of the epidural space, revealed stronger agreement with LOR depths than did measurements to the dura or the most anterior aspect of the epidural space. These results require further analysis and refinement before supporting clinical application.