Prediction of the distance from skin to epidural space for low-thoracic epidural catheter insertion by computed tomography

Mathematical Models for the Skin to Lumbosacral Epidural Distance in Dogs: A Cadaveric Computed-Tomography Study

This study aimed to validate previously published computed tomography (CT) derived mathematical equations with the true skin to lumbosacral epidural distance (SLED) in dog cadavers. Phase 1: The lumbar region of 11 dog cadavers were scanned in sternal recumbency to determine the effect of cranial, neutral, and caudal pelvic limb positioning on the CT derived lumbosacral epidural distance (CLED). Phase 2: The epidural space was determined using contrast epidurography, and the SLED was analysed against the mathematical equations using a body condition score (BCS) and either the cadaveric occipital-coccygeal length (OCL) (Equation (1): = 7.3 + 0.05*OCL + 16.45*BCS) or the ilium wing distance (IWD) (Equation (2): = 3.5 + 0.56*IWD + 16.6*BCS). There were no differences detected between the pelvic limb positions and the CLED. Both equations demonstrated strong correlations (Equation (1): r = 0.7196; Equation (2): r = 0.7590) with the SLED. The level of agreement was greater for Equation (1) than with Equation (2) (concordance coefficient 0.6061 and 0.3752, respectively). Equation (1) also demonstrated a closer fit to the concordance line compared with Equation (2) (bias correction factor 0.8422 and 0.4960, respectively). Further studies in live anaesthetised dogs will help to determine the usefulness of the pre-procedural knowledge when performing lumbosacral epidurals.

Computed Tomography-Derived Occipital-Coccygeal Length and Ilium Wing Distance Correlates with Skin to Epidural and Intrathecal Depths in Dogs

The current clinical techniques for neuraxial needle placement in dogs are predominantly blind without prior knowledge of the depth required to reach the desired space. This study investigated the correlation and defined the relationship between easily obtainable external landmark variables in the dog; occipital-coccygeal length (OCL) and ilium wings distance (IWD), with the skin to epidural and intrathecal space distances using computed tomography (CT). The CT images of 86 dogs of different breeds were examined in this retrospective observational study. Images of dogs in sternal recumbency were optimized to the sagittal view. The distances between the skin and lumbosacral epidural space (LSE) and skin to sacrococcygeal space (SCE) were measured to the ligamentum flavum surrogate (LFS) line. The distance between the skin and the intrathecal space (ITS) was measured from the skin to the vertebral canal at the interlumbar (L5-L6) space. Measurements of the IWD and OCL were performed on dorsal and scout views, respectively. Linear regression equations and Pearson's correlation coefficients were calculated between variables. Data were reported as mean (standard deviation). Significance was set as alpha < 0.05. After exclusion of four dogs, 82 CT scans were included. The depths were LSE 45 (15) mm, SCE 23 (10) mm, and ITS 50 (15) mm. There was a moderate correlation between OCL with LSE (=14.2 + OCL * 0.05 (r = 0.59, p < 0.0001)), and a strong correlation with ITS (=11.4 + OCL * 0.07 (r = 0.76, p < 0.0001)), while a very weak correlation was found with SCE (=14.0 + OCL * 0.02 (r = 0.27, p < 0.0584)). Similarly, with IWD, there was a moderate correlation with LSE (=10.8 + IWD * 0.56 (r = 0.61, p < 0.0001)), and strong correlation with ITS (=9.2 + IWD * 0.67 (r = 0.75, p < 0.0001)), while a weak correlation was found with SCE (=11.2 + IWD * 0.2 (r = 0.32, p < 0.0033)). Mathematical formulae derived from the multiple regression showed that the body condition score (BCS) improved the relationship between IWD and OCL and the LSE, SCE and ITS, while the addition of body weight was associated with multicollinearity. Further studies are required to determine the accuracy of the algorithms to demonstrate their ability for prediction in a clinical setting.

Anatomical and technical factors affecting the neural response to epidural spinal cord stimulation

OBJECTIVE

Spinal cord stimulation (SCS) is a common neurostimulation therapy to treat chronic pain. Computational models represent a valuable tool to study the potential mechanisms of action of SCS and to optimize the design and implementation of SCS technologies. However, it is imperative that these computational models include the appropriate level of detail to accurately predict the neural response to SCS and to correlate model predictions with clinical outcomes. Therefore, the goal of this study was to investigate several anatomic and technical factors that may affect model-based predictions of neural activation during thoracic SCS.

APPROACH

We developed computational models that consisted of detailed finite element models of the lower thoracic spinal cord, surrounding tissues, and implanted SCS electrode arrays. We positioned multicompartment models of sensory axons within the spinal cord to calculate the activation threshold for each sensory axon. We then investigated how activation thresholds changed as a function of several anatomical variables (e.g. spine geometry, dorsal rootlet anatomy), stimulation type (i.e. voltage-controlled vs. current-controlled), electrode impedance, lead position, lead type, and electrical properties of surrounding tissues (e.g. dura conductivity, frequency-dependent conductivity).

MAIN RESULTS

Several anatomic and modeling factors produced significant percent differences or errors in activation thresholds. Rostrocaudal positioning of the cathode with respect to the vertebrae had a large effect (up to 32%) on activation thresholds. Variability in electrode impedance produced significant changes in activation thresholds for voltage-controlled stimulation (38% to 51%), but had little effect on activation thresholds for current-controlled stimulation (less than 13%). Changing the dura conductivity also produced significant differences in activation thresholds.

SIGNIFICANCE

This study demonstrates several anatomic and technical factors that can affect the neural response to SCS. These factors should be considered in clinical implementation and in future computational modeling studies of thoracic SCS.

Cervical Epidural Depth: Correlation Between Cervical MRI Measurements of the Skin-to-Cervical Epidural Space and the Actual Needle Depth During Interlaminar Cervical Epidural Injections

Objective

The purpose of this study was to assess the correlation between skin-to-epidural space depth, as measured on cervical magnetic resonance imaging (MRI), and actual needle depth, as measured by Tuohy needle markings during cervical epidural steroid injections.

Methods

We conducted a retrospective review of cervical MRI images to determine estimated depth from skin to epidural space. Of the 121 reviewed patients who underwent cervical epidural steroid injections, 81 met inclusion criteria and were retained for data analysis.

Results

At the C6-C7 level, the estimated needle depth according to MRI images was 6.03 ± 1.15 cm (mean ± SD) and the actual needle depth was 5.62 ± 0.77 cm. At the C7-T1 level, the estimated needle depth based on MRI images was 5.90 ± 1.05 cm and the actual needle depth was 5.73 ± 0.98 cm. At both C6-C7 and C7-T1, MRI depth (P < 0.009, P < 0.001) and body mass index (P < 0.001, P < 0.002) were significantly associated with actual depth.

Conclusions

Estimates of needle depth made with MRI were consistently slightly deeper than the actual loss-of-resistance needle depth, indicating that the provider should employ caution when using MRI predictive depths. Information garnered from preprocedure MRIs can be used to improve the safety of cervical epidural steroid injection procedures.

Increasing body mass index and abdominal subcutaneous fat thickness are associated with increased skin-to-epidural space distance in pregnant women

BACKGROUND

Body mass index does not indicate the distribution of adipose tissue. Central adiposity may be measured using ultrasound measurement of subcutaneous fat thickness. This study determined if the abdominal subcutaneous fat thickness measured correlated with skin-to-epidural space distance at delivery, and compared this with the booking body mass index.

METHODS

We analysed a sub-set of participants from a single-centre, prospective cohort study that assessed the relationship between subcutaneous fat thickness and maternity outcomes. Abdominal subcutaneous fat thickness measurements were obtained during the routine fetal anomaly scan. The skin-to-epidural space distance was obtained in those having epidural or combined spinal-epidural analgesia. Linear regression was used to test for strength of association and adjusted R² values calculated to determine if subcutaneous fat thickness or body mass index was more strongly associated with skin-to-epidural space distance.

RESULTS

The 463 women had a median (IQR) booking body mass index of 25.0 kg/m² (21.8-29.3) and subcutaneous fat thickness of 16.2 mm (13.0-21.0). The median (IQR) skin-to-epidural space distance was 5.0 cm (4.5-6.0). Both parameters significantly correlated with skin-to-epidural space distance (r=0.53 and 0.68 respectively, P <0.001). Adjusted linear regression coefficient (95% CI) for subcutaneous fat thickness was 0.09 (0.08 to 0.11), R²=0.30 and for body mass index 0.12 (0.11 to 0.13), R²=0.47.

CONCLUSIONS

Booking body mass index had a stronger relationship with skin-to- epidural space distance at delivery than subcutaneous fat thickness, explaining 47% of the variation in the skin-to-epidural distance.

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

BACKGROUND AND OBJECTIVES

Epidural catheters placed for perioperative analgesia in young children confer clinical benefits but are technically challenging to insert. Approximations of the skin to epidural space depth in this population are limited to direct needle measurement and ultrasonography. Magnetic resonance imaging (MRI) is the most comprehensive imaging modality of the spine. This study aims to produce a more clinically useful formula from MRI data to estimate pediatric epidural depth.

METHODS

Seventy children with normal lumbar spine MR images were enrolled. After determination of epidural depth, linear regression was used to estimate a weight-based formula. Analysis of variance and bootstrap methods were used to evaluate this formula against 4 commonly cited formulae. The quality of predictions was evaluated using the mean absolute prediction error.

RESULTS

The estimated weight-based formula as derived by MRI data is given by: skin to epidural depth (mm) = 9.00 + 0.62 * weight in kilograms. The mean absolute prediction error was 2.56 mm (95% confidence interval [95% CI], 2.12-3.04) for the new formula. Additional derived formulae are skin to dorsal dura depth (mm) = 13.52 + 0.71 * weight in kilograms (mean absolute prediction error, 2.48 mm; 95% CI, 2.00-3.03) and skin to ventral dural depth (mm) = 23.08 + 0.86 * weight in kilograms (mean absolute prediction error, 2.50 mm; 95% CI, 2.04-3.06).

CONCLUSIONS

We provide the first predictive formulae, based on MRI data, for pediatric epidural depth estimation.

Using computed tomography scans and patient demographic data to estimate thoracic epidural space depth

Background and Objectives. Previous studies have used varying methods to estimate the depth of the epidural space prior to placement of an epidural catheter. We aim to use computed tomography scans, patient demographics, and vertebral level to estimate the depth of the loss of resistance for placement of thoracic epidural catheters. Methods. The records of consecutive patients who received a thoracic epidural catheter were reviewed. Patient demographics, epidural placement site, and technique were collected. Preoperative computed tomography scans were reviewed to measure the skin to epidural space distance. Linear regression was used for a multivariate analysis. Results. The records of 218 patients were reviewed. The mean loss of resistance measurement was significantly larger than the mean computed tomography epidural space depth measurement by 0.79 cm (p < 0.001). Our final multivariate model, adjusted for demographic and epidural technique, showed a positive correlation between the loss of resistance and the computed tomography epidural space depth measurement (R (2) = 0.5692, p < 0.0001). Conclusions. The measured loss of resistance is positively correlated with the computed tomography epidural space depth measurement and patient demographics. For patients undergoing thoracic or abdominal surgery, estimating the loss of resistance can be a valuable tool.

Measurements of Epidural Space Depth Using Preexisting CT Scans Correlate with Loss of Resistance Depth during Thoracic Epidural Catheter Placement

Background. Thoracic epidural catheters provide the best quality postoperative pain relief for major abdominal and thoracic surgical procedures, but placement is one of the most challenging procedures in the repertoire of an anesthesiologist. Most patients presenting for a procedure that would benefit from a thoracic epidural catheter have already had high resolution imaging that may be useful to assist placement of a catheter. Methods. This retrospective study used data from 168 patients to examine the association and predictive power of epidural-skin distance (ESD) on computed tomography (CT) to determine loss of resistance depth acquired during epidural placement. Additionally, the ability of anesthesiologists to measure this distance was compared to a radiologist, who specializes in spine imaging. Results. There was a strong association between CT measurement and loss of resistance depth (P < 0.0001); the presence of morbid obesity (BMI > 35) changed this relationship (P = 0.007). The ability of anesthesiologists to make CT measurements was similar to a gold standard radiologist (all individual ICCs > 0.9). Conclusions. Overall, this study supports the examination of a recent CT scan to aid in the placement of a thoracic epidural catheter. Making use of these scans may lead to faster epidural placements, fewer accidental dural punctures, and better epidural blockade.

Does obesity preclude lumbar puncture with a standard spinal needle? The use of computed tomography to measure the skin to lumbar subarachnoid space distance in the general hospital population

OBJECTIVES

Failed lumbar puncture (LP) is a common indication for referral for radiologically guided LP. This study aims to evaluate what percentage of the hospital population would fail an LP using a standard 9-cm needle because of obesity and a skin to subarachnoid space distance greater than 9 cm.

METHODS

Images of 402 consecutive patients undergoing computed tomography of the abdomen and pelvis were reviewed. Skin to subarachnoid space distance was calculated using sagittal images. A survey was conducted among junior hospital doctors to assess their experience of performing lumbar puncture in obese patients.

RESULTS

Four hundred patients were included. Fifty-five patients (13.8 %) had a skin to subarachnoid space distance greater than 9 cm. Intra-abdominal fat, subcutaneous fat and abdominal girth correlated with distance between the skin and subarachnoid space. Among junior doctors, 68.3 % (n = 41) reported LP failure on an obese patient; 78.4 % (n = 47) were unaware of the existence of a longer needle and 13.3 % (n = 8) had experience using a longer needle.

CONCLUSIONS

A significant proportion of the hospital population will fail LP with a standard length spinal needle. Selecting a longer needle may be sufficient to successfully complete LP in obese patients.

KEY POINTS

• Lumbar puncture failure commonly leads to referral for an image-guided procedure • Standard lumbar puncture may fail in 13.8 % of patients due to obesity • 78.4 % of trainee doctors are unaware of the existence of longer spinal-needles • Using longer spinal needles may allow successful LP in obese patients.

Fluoroscope guided epidural needle insertioin in midthoracic region: clinical evaluation of Nagaro's method

Background

In the midthoracic region, a fluroscope guided epidural block has been proposed by using a pedicle as a landmark to show the height of the interlaminar space (Nagaro's method). However, clinical implication of this method was not fully evaluated. We studied the clinical usefulness of a fluoroscope guided thoracic epidural block in the midthoracic region.

Methods

Twenty four patients were scheduled to receive an epidural block at the T6-7 intervertebral space. The patients were placed in the prone position. The needle entry point was located at the junction between midline of the pedicle paralleled to the midline of the T7 vertebral body (VB) and the lower border of T7 VB on anteroposterior view of the fluoroscope. The needle touched and walked up the lamina, and the interlaminar space (ILS) was sought near the midline of the VB at the height of the pedicle.

Results

The authors could not insert an epidural needle at T6-7 ILS in two patients and it was instead inserted at T5-6 ILS. However, other patients showed easy insertion at T6-7 ILS. The mean inward and upward angulations were 25° and 55° respectively. The mean actual depth and calculated depth from skin to thoracic epidural space were 5.1 cm and 6.1 cm respectively. Significant correlation between actual needle depth and body weight, podendal index (kg/m) or calculated needle depth was noted.

Conclusions

The fluorposcope guided epidural block by Nagaro's method was useful in the midthoracic region. However, further study for the caudal shift of needle entry point may be needed.

Sudden transient paraplegia shortly after preoperative thoracic epidural catheterization--a case report

We report a case of sudden onset of paraplegia shortly after thoracic epidural catheterization for postoperative analgesia and discuss the possible causes of this event. A 38-year-old woman was scheduled to receive right lobectomy of liver because of hepatocellular carcinoma. Thoracic epidural catheterization for postoperative analgesia was performed before the induction of anesthesia. After skin disinfection and local anesthetic skin infiltration with lidocaine, epidural catheterization through T(10-11) interspace was performed. Dural puncture without any neurological symptoms was noticed in the attempt and the epidural space was successfully identified through T(9-10) interspace in the second attempt. However, acute motor weakness and sensory impairment were met as the epidural catheter was being threaded into the epidural space. Magnetic resonance imaging (MRI) revealed no abnormal findings and the neurological deficits resolved spontaneously within 2h without any sequela. Finally, it was supposed that the transient neurological deficits were resultant from accidental subarachnoid injection of the local anesthetics used for skin infiltration. Preoperative image studies of the spine revealed a relatively short skin-to-dura distance either from median or paramedian approach, which might be the cause of the inadvertent intrathecal injection of local anesthetic during skin infiltration.

The effect of flexion on the level of termination of the dural sac in paediatric patients

SUMMARY

Although the anatomy of the spinal cord and its associated structures have been well defined, the effects of body position relevant to neuraxial blockade have not been elucidated. This study was designed to determine the effect of body position on the end of the dural sac in children. After induction of anaesthesia, ultrasound examination was performed to evaluate the location of the dural sac end in the lateral position with a straight back and knee, and in the lateral position with the knees, legs, and neck flexed. The level of the end of the dural sac was determined in relation to the vertebrae. Our data demonstrate that the dural sac shifts significantly cephalad in the lateral flexed position used for neuraxial blockade (p < 0.001). These results suggest that the safety margin to avoid dural puncture during hiatal or S2-3 approach for caudal block can be increased in younger children.

A prospective, observational study of the relationship between body mass index and depth of the epidural space during lumbar transforaminal epidural steroid injection

BACKGROUND AND OBJECTIVES

Previous studies have concluded that transforaminal epidural steroid injections (ESIs) are more effective than interlaminar injections in the treatment of radiculopathies due to lumbar intervertebral disk herniation. There are no published studies examining the depth of epidural space using a transforaminal approach. We investigated the relationship between body mass index (BMI) and the depth of the epidural space during lumbar transforaminal ESIs.

METHODS

Eighty-six consecutive patients undergoing lumbar transforaminal ESI at the L3-L4, L4-L5, and L5-S1 levels were studied. Using standard protocol, the foraminal epidural space was attained using fluoroscopic guidance. The measured distance from needle tip to skin was recorded (depth to foraminal epidural space). The differences in the needle depth and BMI were analyzed using regression analysis.

RESULTS

Needle depth was positively associated with BMI (regression coefficient [RC], 1.13; P < 0.001). The median depths (in centimeters) to the epidural space were 6.3, 7.5, 8.4, 10.0, 10.4, and 12.2 for underweight, normal, preobese, obese I, obese II, and obese III classifications, respectively. Sex (RC, 1.3; P = 0.02) and race (RC, 0.8; P = 0.04) were also significantly associated with needle depth; however, neither factor remained significant when BMI was accounted as a covariate in the regression model. Age, intervertebral level treated, and oblique angle had no predictive value on foraminal depth (P > 0.2).

CONCLUSION

There is a positive association between BMI and transforaminal epidural depth, but not with age, sex, race, oblique angle, or intervertebral level.

The epidural space is deeper in elderly and obese patients in the Japanese population

BACKGROUND

Epidural anaesthesia is an efficient method of providing both regional anaesthesia and post-operative pain relief. Detection of the epidural space is critical, but it is difficult to predict the depth of the epidural space. Published results are inconsistent. We retrospectively investigated the differences in the depth of the epidural space depending on the puncture site, approach type and physical findings of patients.

METHODS

All surgical patients from 1994 to 2005 were included in the study and 4964 cases were analysed. The recorded distance from the needle tip to the skin surface was defined as the depth of the epidural. The differences in the depth were compared according to the puncture site, level and approach. We then searched for the factors determining the depth using multivariate regression analysis.

RESULTS

The depth in upper thoracic sites (T1/2-T9/10) was 5.0 (1.0) cm [mean (SD), n= 465] via the midline approach and 5.2 (1.0) cm (n= 1226) via the paramedian approach. For lumbar sites, the depth was 4.1 (0.9) cm (n= 1835) via the midline approach and 4.6 (1.0) cm (n= 298) via the paramedian approach. In the multivariate regression analysis, patient age, body weight and more cephalad puncture were significantly and positively correlated with the depth at thoracic sites.

CONCLUSIONS

The epidural space was deeper at upper thoracic sites than lower thoracic or lumbar sites. The depth with the paramedian approach was greater than with the midline approach at both sites. Patient age and weight were positive factors for depth.