Is spinal anaesthesia at L2 L3 interspace safe in disorders of the vertebral column? A magnetic resonance imaging study

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1 British Journal of Anaesthesia 105 (6): (2010) Advance Access publication 15 September doi: /bja/aeq246 Is spinal anaesthesia at L2 L3 interspace safe in disorders of the vertebral column? A magnetic resonance imaging study N. Lin 1, J. F. Bebawy 2,L.Hua 3 and B. G. Wang 4 * 1 Department of Anaesthesiology, Beijing Tiantan Hospital, Capital Medical University, Beijing , P.R. China 2 Northwestern University Feinberg School of Medicine, 251 E. Huron St., Suite F5-704, Chicago, IL 60611, USA 3 Biomedical Engineering Institute of Capital Medical University, Beijing , China 4 Department of Anaesthesiology, Beijing Sanbo Brain Hospital, Capital Medical University, Beijing , China * Corresponding author. wbgttyy@sina.com Key points The authors have described the impact of spinal disorders on the positioning of conus medullaris terminus (CMT). Magnetic resonance image scans of 1047 Chinese patients were reviewed retrospectively. A lower than normal location of CMT was seen in females, and in patients with thoracic vertebral compression. The risk of spinal cord injury during spinal anaesthesia at L2 L3 interspace is increased in female patients with thoracic vertebral compression. Background. The varying point at which the spinal cord terminates in the lumbar spinal canal may affect the incidence of spinal cord injuries associated with needle insertion for spinal anaesthesia, especially in patients with vertebral body or intervertebral disc disease. This is a complication which has been frequently reported when spinal needle insertion was performed at higher lumbar spinal levels. Methods. We retrospectively reviewed magnetic resonance images of the spine in 1047 Chinese patients to determine the conus medullaris terminus (CMT) in patients with and without vertebral disorders. Patients with tumours in and around the spine and those with congenital spinal anomalies were excluded from the study. Patients with mixed vertebral disorders were also excluded. Results. Our data demonstrate that patients with thoracic vertebral compression fractures had lower ending points of the CMT than those without (P,0.05), while patients with lumbar compression fractures did not demonstrate such a correlation. With regard to this difference, females were significantly at higher risk for a lower CMT than males. Conversely, lumbar disc disorders such as intervertebral disc extrusion, herniation, or bulging did not have any significant influence on the level of CMT. Moreover, patients with spondylolisthesis or scoliosis did not demonstrate an abnormal CMT location. Conclusions. When performing spinal anaesthesia, anaesthesiologists should be aware of potential differences of the CMT location, particularly in female patients with thoracic vertebral compression fractures, who may have a lower CMT than normal, extending to the level of L2. Performing spinal anaesthesia at the L2 L3 interspace would seem to be ill-advised in this patient population. Keywords: conus medullaris; intervertebral disc disorder; spinal cord injury; subarachnoid block; vertebral disease Accepted for publication: 9 July 2010 One of the major concerns during needle insertion for spinal anaesthesia is the location of the conus medullaris terminus (CMT). Anaesthesiologists remain aware that any manoeuvre which places the spinal needle in contact with the spinal cord may lead to serious neurological injury. 1 6 This risk may be increased when the CMT is at a lower than expected level and the L2 L3 interspace is chosen as the needle entry site. The CMT may be lower in disorders of the vertebral column. The position of the CMT has been studied previously in cadavers, 7 but a possibly better evaluation of its true position, and one which might translate into clinical practice more readily, might be magnetic resonance imaging (MRI). It is widely known that MRI is an extremely accurate and practical way to evaluate the spinal cord, and a more accurate method of determining the location of the CMT than cadaveric examination. 8 Several retrospective studies 9 12 have investigated the influences of age, gender, and position of the body on the CMT previously; however, the influence of disorders of the vertebral bodies or intervertebral discs has not been well investigated. Our aim in this study is to investigate the effects of these disorders on the CMT location, especially with regard to the customary sites of needle insertion during spinal anaesthesia. Methods With institutional Ethics Board approval, we retrospectively studied MRIs of the spine of 1047 Chinese patients (444 males and 603 females), of age ranging from 10 to 79 yr, in order to determine the location of CMT. These patients & The Author [2010]. Published by Oxford University Press on behalf of the British Journal of Anaesthesia. All rights reserved. For Permissions, please journals.permissions@oxfordjournal.org

2 BJA Lin et al. L4 L5 L3 L1-U1/3 L1-M1/3 L1-L1/3 L2 CMT Table 1 Given values of CMT levels and age decades. T12-U1/3, upper 1/3 of T12; T12-M1/3, middle 1/3 of T12; T12-L1/3, lower 1/3 of T12; T12-L1 disc, intervetebral disc between T12 and L1; L1-U1/ 3, upper 1/3 of L1; L1-M1/3, middle 1/3 of L1; L1-L1/3, lower 1/3 of L1; L1-L2 disc, intervertebral disc between L1 and L2; L2-U1/3, upper 1/3 of L2; L2-M1/3, middle 1/3 of L2; L2-L1/3, lower 1/3 of L2; L2 L3 disc, intervetebral disc between L2 and L3. The youngest patient studied was 10 years old, corresponding to a value of 1 for age decade (10 19 yr); the oldest patient studied was 79 yr old, corresponding to a value of 7 for age decade (70 79 yr) Value CMT level Age decade (yr) 1 T12-U1/ T12-M1/ T12-L1/ T12-L1 disc L1-U1/ L1-M1/ L1-L1/ L1-L2 disc 9 L2-U1/3 10 L2-M1/3 11 L2-L1/3 12 L2 L3 disc Fig 1 T2-weighted sagittal MRI of the lumbar spine, demonstrating the method used of dividing the vertebral bodies into three equal parts for the purpose of classification. The CMT is indicated by the arrow; the dashed line corresponds to its assigned spinal level (in this case, the L1 L2 intervertebral disc). were outpatients who presented for diagnosis and/or treatment of low back pain, hip pain, and/or lumbar radiculopathic pain. Patients with diseases such as tumour, infection, ischaemia, or haemorrhage of the spine or spinal cord, patients with congenital spinal anomalies, and patients with the spine or spinal cord diseases that made CMT observations difficult were excluded. MRI examinations were performed as T2-weighted images on 1.5 T Visart (Toshiba, Tokyo, Japan) or 3.0 T Magnetom Trio (Siemens, Erlangen, Germany) systems; other technical specifications of the images obtained included: matrix , 4 mm slice thickness, and 0.8 mm interslice gap. All scans were obtained in the supine position. In each case which was used, the CMT was clearly visualized on the sagittal sequences. For the purposes of quantification, we drew a perpendicular line from the ending point of the CMT to the long axis of the spine and determined the level of the surrounding vertebrae and intervertebral discs. Each vertebral body was divided into three equal parts (upper 1/3, middle 1/3, and lower 1/3, abbreviated as U1/3, M1/3, and L1/3, respectively); the entire intervertebral disc was considered an additional separate part (Fig. 1). CMT levels were then assigned a value ranging from 1 (U1/ 3 of T12) to 12 (L2 L3 intervertebral disc) for the purpose of statistical analysis, which were the highest and lowest CMT levels observed in this population, respectively. This system of CMT level correlation to adjacent vertebrae or intervertebral discs as a method of measurement has been reported previously. On the basis of age, patients were stratified into seven groups, such that ages yr corresponded to a value of 1, ages yr corresponded to a value of 2, and every decade thereafter corresponded to the next value number, with value 7 corresponding to patients aged yr (Table 1). We performed all statistical analyses utilizing Statistical Package for the Social Sciences (SPSS) version 11.5 (Chicago, IL, USA). The Kolmogorov Smirnov tests or the Skewness Kurtosis tests were used to describe the distribution; two-sample t-tests were used for analysing variance. Statistical significance was defined as a P-value of,0.05. Results The location of the CMT in patients with and without various vertebral and intervertebral disc diseases demonstrated a normal age distribution among every disease cohort (Table 2). The mean position of the CMT for each disease cohort, as designated by its assigned value, is also shown in Table

3 Spinal anaesthesia in spinal disorders BJA Table 2 Frequency distribution and mean CMT of normal, vertebral disease, and disc disease cohorts. The Kolmogorov Smirnov testing was used for evaluating the age distribution; P.0.05 for the Kolmogorov Smirnov test indicates a normal distribution. The t-test was used for comparing each cohort with the normal population; P,0.05 indicates a significant difference Number Frequency (%) Age CMT Mean (SD) Kolmogorov Smirnov P-value Mean (SD) t P-value Normal (9.7) (1.85) Intervertebral disc extrusion (14.4) (2.01) Intervertebral disc herniation (13.9) (1.96) Intervetebral disc bulging (12.6) (2.04) Scoliosis (15.2) (2.37) Spondylolisthesis (11.4) (1.77) Thoracic vertebral compression fracture (16.0) (2.56) Lumbar vertebral compression fracture (15.9) (2.20) Total Beij WUPEIZHEM L1 T12 T11 T12 L2 L1 L3 L2 L4 L3 L5 L4 L5 Fig 2 T2-weighted sagittal MRI demonstrating a T12 vertebral compression fracture (left) and a T11 vertebral compression fracture (right), as shown by the arrows. Compared with the disease-free cohort [mean CMT¼6.50 (1.85), corresponding to the M1/3 and L1/3 of L1], the position of the CMT in lumbar disc disorders such as lumbar intervertebral disc extrusion, herniation, and bulging was not shown to be significantly different. Likewise, for patients with spondylolisthesis, acquired degenerative scoliosis, and lumbar vertebral compression fractures, no significant difference compared with the normal population was found. The mean CMT position value in thoracic vertebral compression fracture patients [7.5 (2.56), corresponding to the L1/3 of L1 and the L1 L2 intervetebral disc], however, was significantly different (t¼22.068, P,0.05) from that in the disease-free cohort (Table 2 and Fig. 2). In anatomical terms, thoracic vertebral compression fracture patients presented with lower positions of the CMT than normal patients by approximately one-third of a vertebral body length. On the basis of gender, some significant differences in the location of the CMT also appeared for certain diseases. Female patients with thoracic vertebral compression fractures, lumbar intervertebral disc herniation, and disc bulging had 859

4 BJA Lin et al. Table 3 Diseases of vertebral bodies and discs compared with normal cohort in number and mean position of CMT between males and females. P,0.05 indicates a significant difference Male Female Statistic result Number Mean (SD) CMT Number Mean (SD) CMT t P-value Normal (2.09) (1.67) Thoracic vertebral compression fracture (2.33) (2.12) Lumbar vertebral compression fracture (1.57) (2.61) Intervertebral disc extrusion (1.83) (2.16) Intervertebral disc herniation (1.77) (2.05) Intervertebral disc bulging (2.08) (1.98) significantly lower CMT values than their male counterparts (Table 3). Among all patients with thoracic vertebral compression fractures, females (mean CMT corresponding to the U1/3 of L2) had a CMT which was one-third of a vertebral body plus a whole disc width lower than that in males (mean CMT corresponding to the M1/3 of L1). Those females with lumbar intervertebral disc herniation or bulging (mean CMT corresponding to the L1/3 of L1) had a CMT which was one-third of a vertebral body lower than the males with the same diseases (mean CMT corresponding to the M1/3 of L1). Discussion The administration of anaesthetic agents by a needle into the thecal sac surrounding the spinal cord is a routine and commonplace technique used by anaesthesiologists. The intervertebral needle insertion site that is usually targeted is the L3 L4 interspace, or less commonly, the L2 L3 interspace. Spinal anaesthesia is generally considered to be a very safe technique, with the complication of spinal cord injury being rarely reported, 16 but several recent reports 1 6 highlight cases of permanent damage to the spinal cord after spinal anaesthesia. Fettes and Wildsmith 17 report and hypothesize as to the possible causes and mechanisms of direct spinal cord injury, including patient factors, equipment factors, and technique. Vertebral body and intervertebral disc disorders, such as vertebral compression fractures and disc herniations, are found commonly in general population, and MRI is not routinely performed on these patients, as it is often unnecessary and expensive. As this study shows, however, some of these patients, namely those with thoracic vertebral compression fractures (especially females), may be at risk for having a CMT which is low enough within the spinal canal to be traumatized directly or indirectly by a spinal needle inserted at the L2 L3 interspace. The results of this retrospective analysis, which focused on the anatomical inconsistency of the CMT in various spinal disease states in 1047 Chinese patients who underwent spinal MRI, may serve as an alert to those anaesthesiologists who perform subarachnoid block at the L2 L3 interspace in the susceptible population. To date, this study is the largest analysis of the variation of the CMT location in the vertebral body and intervertebral disc disease population, and the first to describe and discuss the possible effects of such diseases on the CMT position. We did not identify a statistically significant difference in the CMT level between healthy people and those with lumbar spinal disorders such as lumbar vertebral compression fractures, spondylolisthesis, scoliosis, or intervertebral disc diseases. The CMT position in our study did significantly extend somewhat caudally in patients with thoracic vertebral compression fractures (approximately one-third of one vertebral body width, on average). Some investigations have reported a positive correlation between the CMT level and increasing age. Lirk and colleagues 10 and Soleiman and colleagues 11 postulated that older people in general have a lower CMT position than younger people. But, Soleiman and colleagues hypothesized that age might not be the key variable per se, but rather that the deterioration of the spine that accompanies increasing age might be the determining patient factor. The spinal column is surrounded and supported by many structures, and decreasing spine length may be attributed to pathological changes of the intervertebral discs, perispinous ligaments, or vertebral bodies themselves. Indeed, spinal deterioration due to these changes might be a substantial factor which results in a lower CMT than what is normal. The variable effect of lumbar vs thoracic vertebral compression fractures which we found would seem to make some anatomical sense, since compression at the thoracic level, where the spinal cord is present, should cause a downward movement of the CMT, whereas this effect should be less apparent with lumbar compression below the level of the CMT. As for disorders of the lumbar vertebral discs, tearing of annular fibres and loss of hydration of the nucleus pulposus may result in disc space narrowing, rendering the length of the spine slightly shorter. However, we found no downward movement of the CMT in this population (gender excluded), which also makes some anatomical sense for the reasons mentioned above. Of note, most cases of spondylolisthesis within that cohort occurred at the L4 L5 or L5 S1 levels, which by the same reasoning would also not be expected to affect the CMT position significantly. Our results indicate that gender may affect the CMTposition in various disease states, and hence could be an important factor. Analysis between males and females with thoracic vertebral compression fractures, lumbar intervertebral disc 860

5 Spinal anaesthesia in spinal disorders BJA herniations, and bulging discs displayed a significant difference, with females having a lower CMT position in each case. Females with thoracic vertebral compression fractures, having a CMT one-third of a vertebral body plus the entire width of an intervertebral disc lower than their male counterparts (corresponding to the U1/3 of the L2 vertebral body, on average), could clearly be at risk for spinal cord injury if the L2 L3 interspace is chosen for spinal anaesthesia. Demiryürek and colleagues 12 studied 639 patients and found a difference between males and females, stating that the spinal cord tends to extend further caudally in females. Others, however, observed no difference in spinal cord length based on gender. These observations were made in the general population, and in the absence of an examination and comparison of complex patient factors, such as disease states. At this point in time, the effect of gender alone on spinal cord length is still unclear. On the basis of our data, however, it seems that the CMT position in females is more prone to the effects of spinal disease than in males. It is well established that vertebral body pathology is often the result of changes in bone mineral density, and that females seem more vulnerable in this regard. Felson and colleagues 19 evaluated the association of weight and bone mass in elderly male and female patients, and found that differences in weight displayed much less of an effect in males than in females on bone mineral density. It should be noted that the effect of spinal column disease on the CMT position may be different with lumbar, thoracic, or cervical disorders, and the absence of any high thoracic or cervical imaging in our study is a limitation of our analysis. Secondly, despite a normal age distribution in each disease cohort, our sample population was overall elderly and consisted of outpatients with pain complaints but generally normal spinal anatomy. Thus, our findings may reflect this patient population instead of the general population. Thirdly, the number of degenerative scoliosis cases which we were able to capture was too small to analyse the differences between males and females. Expansion of this and any other disease cohort would improve the overall analysis. Other patient factors, such as height and weight, were summarily excluded from this analysis. In conclusion, our data suggest that it is important for the anaesthesiologist to give ample consideration to the site of spinal needle insertion when spinal column disease is present. This is especially true for female patients with thoracic vertebral compression fractures, who may be at an increased risk for spinal cord injury if spinal anaesthesia is attempted at the L2 L3 interspace level. Interestingly, all published reports of the spinal cord damage with spinal anaesthesia to date involve female patients. Clearly, however, other complex factors may play a role, such as needle insertion level, equipment used, and other patient factors. 8 Acknowledgements We are most grateful to the Imaging Center of Neuroscience of Beijjng Tiantan Hospital for providing MRI images for this study and to Dr Antoun Koht and Dr Ming Lu for reviewing the manuscript and providing valuable guidance. Conflict of interest None declared. Funding This work was supported by the Department of Anaesthesiology of Capital Medical University in Beijing, China, and by the Department of Anaesthesiology of Northwestern University in Chicago, USA. References 1 Absalom AR, Martinelli G, Scott NB. 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