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MR determination of neonatal spinal canal depth
European Journal of Radiology 81 (2012) e813–e816
Contents lists available at SciVerse ScienceDirect
European Journal of Radiology journal homepage: www.elsevier.com/locate/ejrad
MR determination of neonatal spinal canal depth Owen Arthurs a,1 , Sudhin Thayyil b,∗ , Angie Wade c,1 , W.K. ‘Kling’ Chong d , Neil J. Sebire e,1 , Andrew M. Taylor f,1 a
Centre for Cardiovascular MR, Great Ormond Street Hospital for Children, London WC1N 3JH, UK Academic Neonatology, Institute for Women’s Health, London WC1E 6AU, UK Centre for Paediatric Epidemiology and Biostatistics, UCL Institute of Child Health, London, UK d Paediatric Neuroradiology, Great Ormond Street Hospital for Children, London, UK e Histopathology, Great Ormond Street Hospital for Children, London WC1E 6AU, UK f Centre for Cardiovascular MR, Cardiorespiratory Unit, Great Ormond Street Hospital for Children & UCL Institute of Cardiovascular Science, London WC1E 6AU, UK b c
a r t i c l e
i n f o
Article history: Received 26 November 2011 Received in revised form 10 February 2012 Accepted 12 February 2012 Keywords: Spinal cord Canal depth Lumbar puncture Foetus Post mortem
a b s t r a c t Objectives: Lumbar punctures (LPs) are frequently performed in neonates and often result in traumatic haemorrhagic taps. Knowledge of the distance from the skin to the middle of the spinal canal (mid-spinal canal depth – MSCD) may reduce the incidence of traumatic taps, but there is little data in extremely premature or low birth weight neonates. Here, we determined the spinal canal depth at post-mortem in perinatal deaths using magnetic resonance imaging (MRI). Patients and methods: Spinal canal depth was measured in 78 post-mortem foetuses and perinatal cases (mean gestation 26 weeks; mean weight 1.04 kg) at the L3/L4 inter-vertebral space at post-mortem MRI. Both anterior (ASCD) and posterior (PSCD) spinal canal depth were measured; MSCD was calculated and modelled against weight and gestational age. Results: ASCD and PSCD (mm) correlated significantly with weight and gestational age (all r > 0.8). A simple linear model MSCD (mm) = 3 × Weight (kg) + 5 was the best fit, identifying an SCD value within the correct range for 87.2% (68/78) (95% CI (78.0, 92.9%)) cases. Gestational age did not add significantly to the predictive value of the model. Conclusion: There is a significant correlation between MSCD and body weight at post-mortem MRI in foetuses and perinatal deaths. If this association holds in preterm neonates, use of the formula MSCD (mm) = 3 × Weight (kg) + 5 could result in fewer traumatic LPs in this population. © 2012 Elsevier Ireland Ltd. All rights reserved.
1. Introduction Lumbar punctures (LPs) are one of the most commonly undertaken procedures in a neonatal intensive care unit. Dry and traumatic taps have serious consequences, often necessitating prolonged empirical courses of parenteral antibiotics [1]. Prior knowledge of the distance from skin to the middle of the spinal canal (mid-spinal canal depth – MSCD) may be useful in assessing the required depth of insertion of an LP needle in older children,
Abbreviations: LP, lumbar puncture; ASCD, anterior spinal canal depth; MSCD, mid-spinal canal depth; PSCD, posterior spinal canal depth; SCD, spinal canal depth; USS, ultrasound; MRI, magnetic resonance imaging; W, weight. ∗ Corresponding author at: Institute for Women’s Health, University College London, Medical School Building, 74 Huntley Street, London WC1E 6AU, UK. Tel.: +44 0 203 108 2009; fax: +44 0 203 108 2009. E-mail addresses: [email protected] (O. Arthurs), [email protected] (S. Thayyil), [email protected] (A. Wade), [email protected] (W.K.‘. Chong), [email protected] (N.J. Sebire), [email protected] (A.M. Taylor). 1 Tel.: +44 0 20 7405 9200; fax: +44 0 20 7405 9200. 0720-048X/$ – see front matter © 2012 Elsevier Ireland Ltd. All rights reserved. doi:10.1016/j.ejrad.2012.02.003
thus potentially reducing the chances of dry or haemorrhagic CSF samples. Many studies have attempted to predict MSCD, but have either found poor correlations with body weight or gestation [2], used very complicated formula [3], or used height as a predictor, which is not routinely used as a clinical measure in neonatology [4]. One study derived a simple formula based on body weight (MSCD (mm) = 2 × weight (kg) + 7 mm) to accurately predict the MSCD as measured by postnatal ultrasound in neonates [5]. However, using this formula in clinical practice did not improve overall LP performance, although the results suggested an improvement in LP success in a small number of premature infants and in procedures performed by more junior doctors [6]. Accurate data is difficult to obtain from live, stable, premature infants using ultrasound, and we considered whether a different approach might yield more reliable data. We used post mortem MRI to evaluate SCD across a range of gestations in perinatal deaths to derive a simple formula based on weight to predict MSCD, for comparison with other ultrasound-derived formulas [5].
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O. Arthurs et al. / European Journal of Radiology 81 (2012) e813–e816
Fig. 1. Axial and coronal T2 -weighted images of foetal spine. Two measurements were taken from an axial image: (1) the skin to the anterior spinal canal distance (ASCD), and (2) the skin to posterior spinal canal distance (PSCD). From this, the mid-spinal canal depth (MSCD) was calculated. The sagittal image is for illustration purposes only.
2. Patients and methods
10 15 20
a
5
spinal canal range
This work was undertaken as a nested study in a large ongoing prospective study – Magnetic resonance imaging autopsy study (MaRIAS) comparing post-mortem MRI with conventional autopsy in foetuses, newborns, infants and children [7]. All patients were examined on a 1.5 T whole-body MR system (Magnetom Avanto; Siemens Healthcare Ltd., Enlargen, Germany) using a multichannel phase array head coil. Whole body 3D T1 -weighted (Fast Low Angle SHot Magnetic Resonance Imaging (FLASH), 3D Constructive Interference Steady State (CISS) (0.6 mm3 voxels), and axial and sagittal T2 -weighted Dual Echo Short TI Inversion Recovery (DESTIR) sequences were acquired of the brain, followed by 3D T1 -weighted Volumetric Interpolated Breath hold Examination (VIBE) and 2D Turbo Spin Echo (TSE) sequences of the spinal cord. We included all foetuses (born dead <24 weeks gestation), stillbirths (born dead >24 weeks gestation) and neonatal deaths (deaths within first 7 days of life) who had a normal antemortem neuro-imaging and normal neuropathological examination. We excluded cases who had an intracranial or spinal abnormality (e.g. tethered cord or spinal dysraphism) identified either on MRI or on subsequent autopsy, or cases where a genetic, chromosomal abnormality or skeletal abnormality was present.
1
2
3
b
2.1. Statistical analysis MSCD was modelled as a function of weight and gestational age. Polynomial, log and power curves were compared. The aim was to find a simple model, which gave estimates consistently within the ASCD to PSCD range. At smaller weights, the range was narrower and hence a model weighted inversely according to the range was also investigated. Model estimates are presented with 95% confidence intervals. Repeat measurements were summarised as median (range) and presented as plots. We used PASW Statistics 18.0 for Windows (SPSS Inc., Chicago, IL) for statistical analysis.
4
3. Results
weight (kg)
10 15 20
3.1. Demographics
5
spinal canal range
0
All MR images were reported by a specialist paediatric neuroradiologist. Another radiologist with 4 years paediatric radiology experience reviewed the MR datasets. The vertebral bodies were identified on sagittal reformatted MR images of the spine. Vertebral levels were defined either by counting down from C2 where visible on the same sagittal image, or T12 where 12 normal ribs were identified on coronal imaging. Where there was diagnostic difficulty, a neuro-radiologist adjudicated. An axial plane was identified at the level of the L3/4 disc space, and then two measurements were taken: the skin to posterior spinal canal distance – the posterior spinal canal depth (PSCD), and the skin to anterior spinal canal distance – the anterior spinal canal depth (ASCD). From these, the skin to mid-spinal canal depth (MSCD) can be calculated by taking the average of PSCD and ASCD. The spinal canal diameter is the difference between PSCD and ASCD (Fig. 1). In order to measure intra-observer variability, 11 randomly selected images were assessed twice by the same observer, at the beginning and end of the study. The study was approved by the local research ethics committee. MRI was performed as a part of the post-mortem investigations.
15
20
25
30
35
40
gestation (weeks) Fig. 2. Spinal depth (mm) correlates with (a) body weight (kg) and (b) gestational age in weeks.
128 datasets were examined, with gestation range from 12 to 42 weeks. 15 cases were excluded as the cord was either tethered or there was an associated spinal cord abnormality, including one sacrococcygeal teratoma. We were unable to ascertain canal depth in 35 cases, predominantly at early gestations due to the resolution of the MR or poor condition of the foetus. Data from 78 cases were used in the subsequent analysis, with mean gestation 26.5 weeks (range 14–41 weeks) at the time of death and mean weight 1039 g (range 52–4294 g). Sixty-eight cases were at or below 37 weeks
O. Arthurs et al. / European Journal of Radiology 81 (2012) e813–e816
10 0
1
2
3
4
Weight in kg
Spinal canal range
20
b
5
A linear model fit, with weight as a predictor, accounted for 79% of the variation in MSCD. Quadratic, power and log models did not give better fits. Gestational age only accounted for a further 2.6% of the variation. Hence the best fitting simple model was: MSCD (mm) = 2.9 W (kg) + 4.97, i.e. predicted MSCD increasing on average 2.9 mm (95% confidence interval (2.6, 3.2), p < 0.0005) per kilogram increase in body weight (Fig. 3a). Using the previously published MSCD based formula of MSCD (mm) = 2 W (kg) + 7 [5], the estimated depth was outside the spinal canal depth range as determined by MRI for 29/78 (37.2%) of measured cases in this study. Depth would be over-estimated in 28 of these (27 of which weighed less than 1 kg) and under-estimated in 1 (39 weeks, 2.95 kg). However, using our linear model of MSCD (mm) = 2.9 W (kg) + 4.97, the predicted depth was outside the measured spinal canal diameter range in 10 (12.8%) of cases. Errors were still more frequently seen in low weight cases, but are now more evenly spread in direction (7 above, 3 below; Fig. 3b). The slightly more user-friendly model MSCD (mm) = 3 W (kg) + 5 gave the same results. Using this relationship, estimated MSCD to predict depth of lumbar puncture insertion used would be 8 mm at 1 kg, 11 mm at 2 kg, and 14 mm at 3 kg.
5
3.3. Predicting spinal canal depth in early gestation
15
There was a clear pattern of ASCD, PSCD and MSCD (mm) all increasing with increasing weight and gestational age as expected (Fig. 2a and b). All correlation coefficients were >0.8 or highly significant (p < 0.0005). SCD correlated well with weight and gestational age (r = 0.96, p < 0.0005). The difference between ASCD and PSCD, i.e. spinal canal diameter, varied from 2 to 11.6 mm (median 4.7) and was also highly correlated to weight and gestation (r = 0.88, p < 0.0005 for both).
Spinal canal range
3.2. Relationship of spinal depth to weight and gestational age
15
20
a
10
gestation, including 32 measurements below 22 weeks; 65 cases were below 2.5 kg.
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3.4. Intra-observer variability Median (range) differences in ASCD and spinal depth (PSCDASCD) between the first and repeat measurements were 0.3 (−1.3, 2.0) and 0 (−1.4, 1.0) mm respectively. 4. Discussion In this cross-sectional study using post-mortem MRI, we demonstrate a good correlation between MSCD and body weight in foetuses and perinatal deaths, deriving the simple formula MSCD (mm) = 3 × W (kg) + 5. Using this relationship, MSCD would be 8 mm at 1 kg, 11 mm at 2 kg, and 14 mm at 3 kg. If this association holds in preterm neonates then using this formula has the potential to reduce traumatic LPs in this population. As the majority of our measurements were from premature and low birth weight infants, our MRI-based formula of MSCD = 3 W + 5 may be more robust in this cohort than the currently used formula. Cerebrospinal fluid (CSF) samples obtained by LP are used to diagnose neonatal meningitis in neonates, although the incidence is low [8]. Traumatic LPs occur at a rate of around 40% [9], and occur more frequently with prematurity, leading to prolonged antibiotic courses [1]. Our study demonstrates a 2–12 mm diameter of the spinal canal in which to place a spinal needle, which highlights the relatively small margin of error in performing clinical LPs in this cohort, consistent with previous measurements [5]. The exact mechanism of traumatic LP samples is unknown, but the direction
−3
−2
−1
0
1
Log (Weight in kg) Fig. 3. (a) Spinal depth ranges with previous USS based formula, MSCD (mm) = 2W + 7 (dotted line) and proposed new MRI-based formula of MSCD (mm) = 3W + 5 (solid line). (b) The same data plotted on a logarithmic scale demonstrates that most errors occur at lower body weights, but are more evenly spread using the MRI-based formula (solid line).
of measurements error is likely to be important. Whilst an underestimation of SCD leads to a dry tap, meaning that the LP needle can be advanced further, an over-insertion of the LP needle can lead to CSF contamination with bone marrow as well as blood [10]. This is likely to be from damage to the extensive venous plexus (Batson’s plexus) on the posterior aspect of vertebral bodies, which is present at very early gestations [11,12]. Although our study gave a very similar relationship to that found using USS neonates of MSCD = 2 W + 7 mm [5], they are not similar enough to be used interchangeably. The USS based formula would predominantly over-estimate MSCD (in 37% of our study population), which could lead to traumatic LPs. Alternatively, the MRI derived formula could be considered to be a guide for maximum distance of spinal needle insertion. However, we have not accounted for the post-mortem changes associated with loss of
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O. Arthurs et al. / European Journal of Radiology 81 (2012) e813–e816
cutaneous fluid, which may result in our MRI measurements being an underestimate of SCD. Of course, it is difficult to develop a “one size fits all” approach to neonatal spinal canal depth, as the graphs demonstrate that there is still a lack of overlap for neonates of similar weight. However the strengths of this MRI study include a larger premature sample size, and good reproducibility in MRI measurements. One of the study limitations is that post-mortem studies, whether by autopsy or MRI, may not be truly representative or normative data. Many cases will have been obtained by termination of pregnancy and may have no apparent abnormality, but some will have undergone spontaneous intrauterine death. We tried to avoid this by taking only those foetuses in whom no cause for death and no abnormality was identified at post-mortem, and excluded all those with identifiable CNS or other malformation. MRI is also traditionally performed with the post-mortem specimen lying in a supine position, whereas lumbar puncture is typically performed in the left lateral position. There may be an element of non-physiological gravity and/or decomposition on the spinal canal tissues in post-mortem specimens, and the variation of soft tissue relationships with postural change with flexion and extension is unknown. Previous measurements of SCD have been made in different positions: using CT data from patients lying supine [13], or ultrasound measurements of patients lying prone [3], but neither of these studies investigated neonates below 35 weeks gestation. These effects, as well as loss of overall tone, subtle postural changes (flexion, extension, etc.) are likely to be minimal given that our results are very similar to previous studies using different techniques. 5. Conclusion In summary, we have demonstrated that there is a clear relationship between body weight and spinal canal depth in foetuses and perinatal deaths using post mortem MRI. If this association holds in preterm neonates, use of the simple formula MSCD (mm) = 3 × weight (kg) + 5 mm, has the potential to improve the success rates of neonatal LPs, but needs to be evaluated in routine neonatal practice.
Acknowledgements We thank Professor Lyn Chitty (Fetal medicine, University College Hospital, London) for reading the manuscript and for kind suggestions. This is an independent report commissioned and funded by the Policy Research Programme in the Department of Health. The views expressed are not necessarily those of the Department. ST is funded by a clinician scientist award from the National Institute for Health Research (NIHR). AMT is funded by an NIHR Senior Research Fellowship. The funding bodies had no role in the interpretation of the data or preparation of the manuscript. References [1] Baziomo JM, Krim G, Kremp O, et al. Retrospective analysis of 1331 samples of cerebrospinal fluid in newborn infants with suspected infection. Arch Pediatr 1995;2:833–9. [2] Hasan MA, Howard RF, Lloyd-Thomas AR. Depth of epidural space in children. Anaesthesia 1994;49:1085–7. [3] Shenkman Z, Rathaus V, Jedikin R, et al. The distance from the skin to the subarachnoid space can be predicted in premature and former-premature infants. Can J Anaesth 2004;51:160–2. [4] Craig F, Stroobant J, Winrow A, et al. Depth of insertion of a lumbar puncture needle. Arch Dis Child 1997;77:450. [5] Arthurs OJ, Murray M, Zubier M, et al. Ultrasonographic determination of neonatal spinal canal depth. Arch Dis Child Fetal Neonatal Ed 2008;93:F451–4. [6] Murray MJ, Arthurs OJ, Hills MH, et al. A randomized study to validate a midspinal canal depth nomogram in neonates. Am J Perinatol 2009;26:733–8. [7] Thayyil S, Robertson NJ, Chitty LS, et al. Post-mortem magnetic resonance imaging in the fetus, infant, and child: a comparative study with conventional autopsy (UKCRN: 6794): Lancet Protocol 08PRT/5409. http://www.thelancet.com/protocol-reviews/08PRT-5409. [8] Greenberg RG, Smith PB, Cotten CM, et al. Traumatic lumbar punctures in neonates: test performance of the cerebrospinal fluid white blood cell count. Pediatr Infect Dis J 2008;27:1047–51. [9] Schreiner RL, Kleiman MB. Incidence and effect of traumatic lumbar puncture in the neonate. Dev Med Child Neurol 1979;21:483–7. [10] Kruskall MS, Carter SR, Ritz LP. Contamination of cerebrospinal fluid by vertebral bone-marrow cells during lumbar puncture. N Engl J Med 1983;308:697–700. [11] Batson OV. The function of the vertebral veins and their role in the spread of metastasis. Ann Surg 1940;112:138–49. [12] Groen RJ, Grobbelaar M, Muller CJ, et al. Morphology of the human internal vertebral venous plexus: a cadaver study after latex injection in the 21–25week fetus. Clin Anat 2005;18:397–403. [13] Abe KK, Yamamoto LG, Itoman EM, et al. Lumbar puncture needle length determination. Am J Emerg Med 2005;23:742–6.
Contents lists available at SciVerse ScienceDirect
European Journal of Radiology journal homepage: www.elsevier.com/locate/ejrad
MR determination of neonatal spinal canal depth Owen Arthurs a,1 , Sudhin Thayyil b,∗ , Angie Wade c,1 , W.K. ‘Kling’ Chong d , Neil J. Sebire e,1 , Andrew M. Taylor f,1 a
Centre for Cardiovascular MR, Great Ormond Street Hospital for Children, London WC1N 3JH, UK Academic Neonatology, Institute for Women’s Health, London WC1E 6AU, UK Centre for Paediatric Epidemiology and Biostatistics, UCL Institute of Child Health, London, UK d Paediatric Neuroradiology, Great Ormond Street Hospital for Children, London, UK e Histopathology, Great Ormond Street Hospital for Children, London WC1E 6AU, UK f Centre for Cardiovascular MR, Cardiorespiratory Unit, Great Ormond Street Hospital for Children & UCL Institute of Cardiovascular Science, London WC1E 6AU, UK b c
a r t i c l e
i n f o
Article history: Received 26 November 2011 Received in revised form 10 February 2012 Accepted 12 February 2012 Keywords: Spinal cord Canal depth Lumbar puncture Foetus Post mortem
a b s t r a c t Objectives: Lumbar punctures (LPs) are frequently performed in neonates and often result in traumatic haemorrhagic taps. Knowledge of the distance from the skin to the middle of the spinal canal (mid-spinal canal depth – MSCD) may reduce the incidence of traumatic taps, but there is little data in extremely premature or low birth weight neonates. Here, we determined the spinal canal depth at post-mortem in perinatal deaths using magnetic resonance imaging (MRI). Patients and methods: Spinal canal depth was measured in 78 post-mortem foetuses and perinatal cases (mean gestation 26 weeks; mean weight 1.04 kg) at the L3/L4 inter-vertebral space at post-mortem MRI. Both anterior (ASCD) and posterior (PSCD) spinal canal depth were measured; MSCD was calculated and modelled against weight and gestational age. Results: ASCD and PSCD (mm) correlated significantly with weight and gestational age (all r > 0.8). A simple linear model MSCD (mm) = 3 × Weight (kg) + 5 was the best fit, identifying an SCD value within the correct range for 87.2% (68/78) (95% CI (78.0, 92.9%)) cases. Gestational age did not add significantly to the predictive value of the model. Conclusion: There is a significant correlation between MSCD and body weight at post-mortem MRI in foetuses and perinatal deaths. If this association holds in preterm neonates, use of the formula MSCD (mm) = 3 × Weight (kg) + 5 could result in fewer traumatic LPs in this population. © 2012 Elsevier Ireland Ltd. All rights reserved.
1. Introduction Lumbar punctures (LPs) are one of the most commonly undertaken procedures in a neonatal intensive care unit. Dry and traumatic taps have serious consequences, often necessitating prolonged empirical courses of parenteral antibiotics [1]. Prior knowledge of the distance from skin to the middle of the spinal canal (mid-spinal canal depth – MSCD) may be useful in assessing the required depth of insertion of an LP needle in older children,
Abbreviations: LP, lumbar puncture; ASCD, anterior spinal canal depth; MSCD, mid-spinal canal depth; PSCD, posterior spinal canal depth; SCD, spinal canal depth; USS, ultrasound; MRI, magnetic resonance imaging; W, weight. ∗ Corresponding author at: Institute for Women’s Health, University College London, Medical School Building, 74 Huntley Street, London WC1E 6AU, UK. Tel.: +44 0 203 108 2009; fax: +44 0 203 108 2009. E-mail addresses: [email protected] (O. Arthurs), [email protected] (S. Thayyil), [email protected] (A. Wade), [email protected] (W.K.‘. Chong), [email protected] (N.J. Sebire), [email protected] (A.M. Taylor). 1 Tel.: +44 0 20 7405 9200; fax: +44 0 20 7405 9200. 0720-048X/$ – see front matter © 2012 Elsevier Ireland Ltd. All rights reserved. doi:10.1016/j.ejrad.2012.02.003
thus potentially reducing the chances of dry or haemorrhagic CSF samples. Many studies have attempted to predict MSCD, but have either found poor correlations with body weight or gestation [2], used very complicated formula [3], or used height as a predictor, which is not routinely used as a clinical measure in neonatology [4]. One study derived a simple formula based on body weight (MSCD (mm) = 2 × weight (kg) + 7 mm) to accurately predict the MSCD as measured by postnatal ultrasound in neonates [5]. However, using this formula in clinical practice did not improve overall LP performance, although the results suggested an improvement in LP success in a small number of premature infants and in procedures performed by more junior doctors [6]. Accurate data is difficult to obtain from live, stable, premature infants using ultrasound, and we considered whether a different approach might yield more reliable data. We used post mortem MRI to evaluate SCD across a range of gestations in perinatal deaths to derive a simple formula based on weight to predict MSCD, for comparison with other ultrasound-derived formulas [5].
e814
O. Arthurs et al. / European Journal of Radiology 81 (2012) e813–e816
Fig. 1. Axial and coronal T2 -weighted images of foetal spine. Two measurements were taken from an axial image: (1) the skin to the anterior spinal canal distance (ASCD), and (2) the skin to posterior spinal canal distance (PSCD). From this, the mid-spinal canal depth (MSCD) was calculated. The sagittal image is for illustration purposes only.
2. Patients and methods
10 15 20
a
5
spinal canal range
This work was undertaken as a nested study in a large ongoing prospective study – Magnetic resonance imaging autopsy study (MaRIAS) comparing post-mortem MRI with conventional autopsy in foetuses, newborns, infants and children [7]. All patients were examined on a 1.5 T whole-body MR system (Magnetom Avanto; Siemens Healthcare Ltd., Enlargen, Germany) using a multichannel phase array head coil. Whole body 3D T1 -weighted (Fast Low Angle SHot Magnetic Resonance Imaging (FLASH), 3D Constructive Interference Steady State (CISS) (0.6 mm3 voxels), and axial and sagittal T2 -weighted Dual Echo Short TI Inversion Recovery (DESTIR) sequences were acquired of the brain, followed by 3D T1 -weighted Volumetric Interpolated Breath hold Examination (VIBE) and 2D Turbo Spin Echo (TSE) sequences of the spinal cord. We included all foetuses (born dead <24 weeks gestation), stillbirths (born dead >24 weeks gestation) and neonatal deaths (deaths within first 7 days of life) who had a normal antemortem neuro-imaging and normal neuropathological examination. We excluded cases who had an intracranial or spinal abnormality (e.g. tethered cord or spinal dysraphism) identified either on MRI or on subsequent autopsy, or cases where a genetic, chromosomal abnormality or skeletal abnormality was present.
1
2
3
b
2.1. Statistical analysis MSCD was modelled as a function of weight and gestational age. Polynomial, log and power curves were compared. The aim was to find a simple model, which gave estimates consistently within the ASCD to PSCD range. At smaller weights, the range was narrower and hence a model weighted inversely according to the range was also investigated. Model estimates are presented with 95% confidence intervals. Repeat measurements were summarised as median (range) and presented as plots. We used PASW Statistics 18.0 for Windows (SPSS Inc., Chicago, IL) for statistical analysis.
4
3. Results
weight (kg)
10 15 20
3.1. Demographics
5
spinal canal range
0
All MR images were reported by a specialist paediatric neuroradiologist. Another radiologist with 4 years paediatric radiology experience reviewed the MR datasets. The vertebral bodies were identified on sagittal reformatted MR images of the spine. Vertebral levels were defined either by counting down from C2 where visible on the same sagittal image, or T12 where 12 normal ribs were identified on coronal imaging. Where there was diagnostic difficulty, a neuro-radiologist adjudicated. An axial plane was identified at the level of the L3/4 disc space, and then two measurements were taken: the skin to posterior spinal canal distance – the posterior spinal canal depth (PSCD), and the skin to anterior spinal canal distance – the anterior spinal canal depth (ASCD). From these, the skin to mid-spinal canal depth (MSCD) can be calculated by taking the average of PSCD and ASCD. The spinal canal diameter is the difference between PSCD and ASCD (Fig. 1). In order to measure intra-observer variability, 11 randomly selected images were assessed twice by the same observer, at the beginning and end of the study. The study was approved by the local research ethics committee. MRI was performed as a part of the post-mortem investigations.
15
20
25
30
35
40
gestation (weeks) Fig. 2. Spinal depth (mm) correlates with (a) body weight (kg) and (b) gestational age in weeks.
128 datasets were examined, with gestation range from 12 to 42 weeks. 15 cases were excluded as the cord was either tethered or there was an associated spinal cord abnormality, including one sacrococcygeal teratoma. We were unable to ascertain canal depth in 35 cases, predominantly at early gestations due to the resolution of the MR or poor condition of the foetus. Data from 78 cases were used in the subsequent analysis, with mean gestation 26.5 weeks (range 14–41 weeks) at the time of death and mean weight 1039 g (range 52–4294 g). Sixty-eight cases were at or below 37 weeks
O. Arthurs et al. / European Journal of Radiology 81 (2012) e813–e816
10 0
1
2
3
4
Weight in kg
Spinal canal range
20
b
5
A linear model fit, with weight as a predictor, accounted for 79% of the variation in MSCD. Quadratic, power and log models did not give better fits. Gestational age only accounted for a further 2.6% of the variation. Hence the best fitting simple model was: MSCD (mm) = 2.9 W (kg) + 4.97, i.e. predicted MSCD increasing on average 2.9 mm (95% confidence interval (2.6, 3.2), p < 0.0005) per kilogram increase in body weight (Fig. 3a). Using the previously published MSCD based formula of MSCD (mm) = 2 W (kg) + 7 [5], the estimated depth was outside the spinal canal depth range as determined by MRI for 29/78 (37.2%) of measured cases in this study. Depth would be over-estimated in 28 of these (27 of which weighed less than 1 kg) and under-estimated in 1 (39 weeks, 2.95 kg). However, using our linear model of MSCD (mm) = 2.9 W (kg) + 4.97, the predicted depth was outside the measured spinal canal diameter range in 10 (12.8%) of cases. Errors were still more frequently seen in low weight cases, but are now more evenly spread in direction (7 above, 3 below; Fig. 3b). The slightly more user-friendly model MSCD (mm) = 3 W (kg) + 5 gave the same results. Using this relationship, estimated MSCD to predict depth of lumbar puncture insertion used would be 8 mm at 1 kg, 11 mm at 2 kg, and 14 mm at 3 kg.
5
3.3. Predicting spinal canal depth in early gestation
15
There was a clear pattern of ASCD, PSCD and MSCD (mm) all increasing with increasing weight and gestational age as expected (Fig. 2a and b). All correlation coefficients were >0.8 or highly significant (p < 0.0005). SCD correlated well with weight and gestational age (r = 0.96, p < 0.0005). The difference between ASCD and PSCD, i.e. spinal canal diameter, varied from 2 to 11.6 mm (median 4.7) and was also highly correlated to weight and gestation (r = 0.88, p < 0.0005 for both).
Spinal canal range
3.2. Relationship of spinal depth to weight and gestational age
15
20
a
10
gestation, including 32 measurements below 22 weeks; 65 cases were below 2.5 kg.
e815
3.4. Intra-observer variability Median (range) differences in ASCD and spinal depth (PSCDASCD) between the first and repeat measurements were 0.3 (−1.3, 2.0) and 0 (−1.4, 1.0) mm respectively. 4. Discussion In this cross-sectional study using post-mortem MRI, we demonstrate a good correlation between MSCD and body weight in foetuses and perinatal deaths, deriving the simple formula MSCD (mm) = 3 × W (kg) + 5. Using this relationship, MSCD would be 8 mm at 1 kg, 11 mm at 2 kg, and 14 mm at 3 kg. If this association holds in preterm neonates then using this formula has the potential to reduce traumatic LPs in this population. As the majority of our measurements were from premature and low birth weight infants, our MRI-based formula of MSCD = 3 W + 5 may be more robust in this cohort than the currently used formula. Cerebrospinal fluid (CSF) samples obtained by LP are used to diagnose neonatal meningitis in neonates, although the incidence is low [8]. Traumatic LPs occur at a rate of around 40% [9], and occur more frequently with prematurity, leading to prolonged antibiotic courses [1]. Our study demonstrates a 2–12 mm diameter of the spinal canal in which to place a spinal needle, which highlights the relatively small margin of error in performing clinical LPs in this cohort, consistent with previous measurements [5]. The exact mechanism of traumatic LP samples is unknown, but the direction
−3
−2
−1
0
1
Log (Weight in kg) Fig. 3. (a) Spinal depth ranges with previous USS based formula, MSCD (mm) = 2W + 7 (dotted line) and proposed new MRI-based formula of MSCD (mm) = 3W + 5 (solid line). (b) The same data plotted on a logarithmic scale demonstrates that most errors occur at lower body weights, but are more evenly spread using the MRI-based formula (solid line).
of measurements error is likely to be important. Whilst an underestimation of SCD leads to a dry tap, meaning that the LP needle can be advanced further, an over-insertion of the LP needle can lead to CSF contamination with bone marrow as well as blood [10]. This is likely to be from damage to the extensive venous plexus (Batson’s plexus) on the posterior aspect of vertebral bodies, which is present at very early gestations [11,12]. Although our study gave a very similar relationship to that found using USS neonates of MSCD = 2 W + 7 mm [5], they are not similar enough to be used interchangeably. The USS based formula would predominantly over-estimate MSCD (in 37% of our study population), which could lead to traumatic LPs. Alternatively, the MRI derived formula could be considered to be a guide for maximum distance of spinal needle insertion. However, we have not accounted for the post-mortem changes associated with loss of
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cutaneous fluid, which may result in our MRI measurements being an underestimate of SCD. Of course, it is difficult to develop a “one size fits all” approach to neonatal spinal canal depth, as the graphs demonstrate that there is still a lack of overlap for neonates of similar weight. However the strengths of this MRI study include a larger premature sample size, and good reproducibility in MRI measurements. One of the study limitations is that post-mortem studies, whether by autopsy or MRI, may not be truly representative or normative data. Many cases will have been obtained by termination of pregnancy and may have no apparent abnormality, but some will have undergone spontaneous intrauterine death. We tried to avoid this by taking only those foetuses in whom no cause for death and no abnormality was identified at post-mortem, and excluded all those with identifiable CNS or other malformation. MRI is also traditionally performed with the post-mortem specimen lying in a supine position, whereas lumbar puncture is typically performed in the left lateral position. There may be an element of non-physiological gravity and/or decomposition on the spinal canal tissues in post-mortem specimens, and the variation of soft tissue relationships with postural change with flexion and extension is unknown. Previous measurements of SCD have been made in different positions: using CT data from patients lying supine [13], or ultrasound measurements of patients lying prone [3], but neither of these studies investigated neonates below 35 weeks gestation. These effects, as well as loss of overall tone, subtle postural changes (flexion, extension, etc.) are likely to be minimal given that our results are very similar to previous studies using different techniques. 5. Conclusion In summary, we have demonstrated that there is a clear relationship between body weight and spinal canal depth in foetuses and perinatal deaths using post mortem MRI. If this association holds in preterm neonates, use of the simple formula MSCD (mm) = 3 × weight (kg) + 5 mm, has the potential to improve the success rates of neonatal LPs, but needs to be evaluated in routine neonatal practice.
Acknowledgements We thank Professor Lyn Chitty (Fetal medicine, University College Hospital, London) for reading the manuscript and for kind suggestions. This is an independent report commissioned and funded by the Policy Research Programme in the Department of Health. The views expressed are not necessarily those of the Department. ST is funded by a clinician scientist award from the National Institute for Health Research (NIHR). AMT is funded by an NIHR Senior Research Fellowship. The funding bodies had no role in the interpretation of the data or preparation of the manuscript. References [1] Baziomo JM, Krim G, Kremp O, et al. Retrospective analysis of 1331 samples of cerebrospinal fluid in newborn infants with suspected infection. Arch Pediatr 1995;2:833–9. [2] Hasan MA, Howard RF, Lloyd-Thomas AR. Depth of epidural space in children. Anaesthesia 1994;49:1085–7. [3] Shenkman Z, Rathaus V, Jedikin R, et al. The distance from the skin to the subarachnoid space can be predicted in premature and former-premature infants. Can J Anaesth 2004;51:160–2. [4] Craig F, Stroobant J, Winrow A, et al. Depth of insertion of a lumbar puncture needle. Arch Dis Child 1997;77:450. [5] Arthurs OJ, Murray M, Zubier M, et al. Ultrasonographic determination of neonatal spinal canal depth. Arch Dis Child Fetal Neonatal Ed 2008;93:F451–4. [6] Murray MJ, Arthurs OJ, Hills MH, et al. A randomized study to validate a midspinal canal depth nomogram in neonates. Am J Perinatol 2009;26:733–8. [7] Thayyil S, Robertson NJ, Chitty LS, et al. Post-mortem magnetic resonance imaging in the fetus, infant, and child: a comparative study with conventional autopsy (UKCRN: 6794): Lancet Protocol 08PRT/5409. http://www.thelancet.com/protocol-reviews/08PRT-5409. [8] Greenberg RG, Smith PB, Cotten CM, et al. Traumatic lumbar punctures in neonates: test performance of the cerebrospinal fluid white blood cell count. Pediatr Infect Dis J 2008;27:1047–51. [9] Schreiner RL, Kleiman MB. Incidence and effect of traumatic lumbar puncture in the neonate. Dev Med Child Neurol 1979;21:483–7. [10] Kruskall MS, Carter SR, Ritz LP. Contamination of cerebrospinal fluid by vertebral bone-marrow cells during lumbar puncture. N Engl J Med 1983;308:697–700. [11] Batson OV. The function of the vertebral veins and their role in the spread of metastasis. Ann Surg 1940;112:138–49. [12] Groen RJ, Grobbelaar M, Muller CJ, et al. Morphology of the human internal vertebral venous plexus: a cadaver study after latex injection in the 21–25week fetus. Clin Anat 2005;18:397–403. [13] Abe KK, Yamamoto LG, Itoman EM, et al. Lumbar puncture needle length determination. Am J Emerg Med 2005;23:742–6.