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Anatomy of the epidural space
REGIONAL ANAESTHESIA
Anatomy of the epidural space
Learning objectives After reading this article you should be able to describe the: C borders of the epidural space C contents of the epidural space C shape and distribution of the posterior epidural space
Jon L Westbrook
Abstract Despite the importance of anatomical knowledge in the safe and effective conduct of thoraco-lumbar epidural anaesthesia there are widespread misunderstandings about the detailed anatomy of this complex region. Current textbooks and teaching continue to propagate misleading information. Much of the difficulty in studying the lumbar epidural region is that the epidural space is a ‘potential’ space occupying only a small volume at rest but capable of expanding to accommodate large volumes of injected solution. This results in considerable distortion during some anatomical studies. However new imaging and dissection techniques have given us a much clearer insight into the anatomy and the relationships of the dura, epidural fat, blood vessels and spinal canal. This article seeks to provide an up-to-date account of current knowledge to give the reader a clearer idea of the true anatomy and how it might influence their epidural practice.
This distorts the relative volumes of these components of the spinal canal. There may also be tissue autolysis depending on the time between death and the study. An investigation that requires the injection of solution into the epidural space (e.g. latex or contrast solution) will also distort the anatomy by expansion of the epidural space and compression of the dural sac. Early epiduroscopic studies were performed on cadavers5 but fine flexible scopes now allow epiduroscopy in live subjects6 but this still requires injectate to expand the space. Advances in MRI techniques and other sophisticated methods such as cryomicrotome sectioning of freshly frozen cadavers have given as much clearer understanding of the relevant anatomy and challenged accepted ideas.
Basic anatomy of the epidural space Keywords Epidural/extradural anatomy
The posterior longitudinal ligament (PLL) borders the epidural space anteriorly as it overlies the vertebral bodies and intervertebral discs. Laterally the medial aspect of the pedicles and the intervertebral foramina demarcate the space. The exact point within the foramina that the epidural space ends is not well defined. Posteriorly the borders are alternately the anterior aspect of the laminae and the ligamentum flava. The canal is oval in the cervical region becoming almost triangular in the lumbar region. The epidural space surrounds the dural sac containing the spinal cord and CSF. The spinal cord typically terminates at lumbar level of 1 or 2 whereupon the nerve roots of the cauda equina continue through the epidural space to their respective foramina. The epidural space ends superiorly where the dura blends with the periosteum at the foramen magnum preventing intracranial spread of epidural solutions. Inferiorly it ends at the sacrococcygeal membrane. The dural sac ends at the approximate level of S1, 2. The ligamentum flavum comprises fibres running in the long axis of the spine between each lamina. They are inserted at the upper border and posterior aspect of the lamina below and the lower half and posterior aspect of the lamina above. At each level there are two separate ligaments (left and right), which usually meet in the mid-line. However the ligament is occasionally deficient in the mid-line when it blends with the anterior fibres of the interspinous ligament. In one study7 the ligamentum flavum was deficient in 11% of subjects in the midlumbar region. Small veins may traverse the ligamentum in the mid-line. Zarzur’s8 detailed examination of the ligamentum flavum in cadavers found that it extends laterally out to the intervertebral foramina where it merges with the capsule of the articular processes. Thickness ranged from 3 to 5 mm although clinical studies often report greater distances probably because of the oblique angle that many needles traverse the ligament.
Royal College of Anaesthetists CPD Matrix: 2G02 3A09
Introduction The performance of any invasive medical procedure is improved by a clear understanding of the relevant anatomy. However the anatomy of the epidural space is not well described in standard medical texts nor is it clearly understood by many anaesthetists. The reason for this is that the epidural region has proved to be very difficult to study and many of the early papers produced misleading information. New study techniques have now provided much clearer detail about the anatomy, which may have wider implications for clinical practice. The reason the epidural space has proved so difficult to study is that it is a potential space, a term first coined by Bromage1 to explain the small resting volume of the epidural space that at the same time is able to accommodate large volumes of an injected solution. The epidural space is contained entirely within the rigid spinal canal and so any injected solution either causes displacement of the dural sac or passes out through the intervertebral foramina. Early studies included dissection of cadavers sometimes following injection of liquid resin into the epidural space.2e4 Cadaver studies inevitably produce artefact as the CSF pressure is lost after death and the venous pressure increases.
Jon L Westbrook MB BS MRCP FRCA FFICM is a Consultant Anaesthetist and Honorary Senior Clinical Lecturer at Oxford University Hospitals, UK. Conflicts of interest: none declared.
ANAESTHESIA AND INTENSIVE CARE MEDICINE 13:11
551
Ó 2012 Published by Elsevier Ltd.
REGIONAL ANAESTHESIA
The angle subtended by the two ligaments in the mid-line is usually 90 or less. The anterior internal vertebral venous plexus drains the spinal cord, meninges and vertebrae and lies in the antero-lateral aspect of the spinal canal between two thin layers of the PLL. These veins connect to the azygos and intercostal systems and so may become engorged during inferior vena cava compression caused by a gravid uterus or the prone position for spinal surgery. Arteries enter the spinal canal through the intervertebral foramina as branches of the segmental vessels. They divide to follow the nerve roots and supply the cord along with the larger anterior spinal artery. Epidural fat differs significantly from the majority of body fat. It has little metabolic function but may provide some physical protection to the contents of the spinal canal and exiting nerve roots. There is no correlation between total body fat and the volume of fat in the posterior epidural space.9,10 Hogan11 has shown with his detailed studies that the fat is very liquid with only a few septae. This means that it is easily displaced by solutions injected outside the fat pad. Epidural catheters rarely enter this fat pad12 but usually lie outside its capsule. The fat pads are tethered loosely to the walls of the spinal canal and so injected solution such as contrast or local anaesthetic can compress the fat pad and this gives the impression of membranes or septae within the epidural space when imaged by epiduroscopy or CT epidurograms. Both Hogan13 and Plaisant14 have studied the anterior epidural space and have shown it to be a complex and segmented compartment. There is a medial and lateral anterior venous plexus, which is partially incorporated into layers of the PLL. There are fibrous structures connecting the dura mater to the PLL producing an anterior medial compartment, which is obliterated at the level of the intervertebral disc. This complex arrangement of fibrous bands and membranes may prevent the easy passage of solution across the mid-line in the anterior epidural space.
The more recent introduction of real-time ultrasound imaging of the lumbar region during epidural needle insertion can provide an accurate measure of the depth of the epidural space, which may aid introduction of the Tuohy needle.19
Segmented epidural space The most important and yet least understood aspect of epidural anatomy is the segmented distribution of the epidural space. Studies using CT epidurography,20 and MRI21,22 and cryomicrotome dissection techniques13 have clearly demonstrated that the posterior epidural space is divided into segments at the levels of the ligamentum flavum (Figure 1) and is obliterated between these levels as the dura lies adjacent to the anterior aspect of the lamina (Figure 2). However at these levels there is easy passage of both epidural catheters and any injected solution. There are small pockets of epidural fat within each intervertebral foramen. This is well illustrated by the three-dimensional (3D) diagram in Hogan’s publication23 (Figure 3). Unfortunately most standard textbooks represent the posterior epidural space as a smooth, continuous compartment extending from the sacrum up to the foramen magnum anterior to the ligamentum flavum. Close examination of the cranial end of any individual posterior segment of epidural space will show that a Tuohy needle passing through the ligamentum at just below the lamina above could deliver the epidural catheter against the bony barrier of the lamina. This could cause difficulty in passing the
Depth of the epidural space There have been several studies to measure the distance between the skin and the epidural space to establish some correlation between the depth and surrogate markers such as body weight or body surface area. This is potentially useful as the operator and trainer are blind to the depth of the space, which makes it a difficult technique to teach. Many factors also influence the measured distance such as vertebral level, direction of approach to the space, pregnancy and the degree of spinal flexion. Harrison15 showed that in the pregnant woman the median distance was 4.7 cm with a range of 3.0e7.4 cm with the distance greatest at L3,4. Sutton16 in a retrospective note review was able to show that the shallower the epidural space the greater the risk of accidental dural punctures whilst Narang has shown that the deeper the space the greater the chance of an uneven block.17 An attempt at a multivariate model was able to predict the depth to within an accuracy of only 8 mm.18 The study did demonstrate interracial variations with oriental subjects having shallower spaces than others but otherwise concluded that there is unlikely to be a formula sufficiently accurate for clinical use.
ANAESTHESIA AND INTENSIVE CARE MEDICINE 13:11
Figure 1 Sagittal mid-line T1 MRI of lumbar spine demonstrating the segmented posterior epidural space. Epidural fat appears as white, CSF dark. (a) Section through posterior segment of epidural space; (b) dura lying immediately anterior to the epidural space containing CSF. The dura lies adjacent to the bone at the level of the lamina with the epidural space obliterated; (c) ligamentum flavum lying immediately posterior to the epidural space.
552
Ó 2012 Published by Elsevier Ltd.
REGIONAL ANAESTHESIA
catheter as this technique delivers the catheter into the caudal end of the epidural segment where it will not impact on the lamina above. Equipped with an accurate understanding of epidural anatomy the trainee or experienced ‘epiduralist’ can refine their technique to optimize the successful and accurate placement of epidural catheters or spinal cord stimulators. A
REFERENCES 1 Bromage. Anatomy. In: Epidural analgesia. Philadelphia: WB Saunders, 1978. pp. 8e20. 2 Parkin IG, Harrison GR. The topographical anatomy of the lumbar epidural space. J Anat 1985 Aug; 141: 211e7. 3 Harrison G, Parkin I, Shah J. Resin injection studies of the lumbar extradural space. Br J Anaesth 1985 Mar 1; 57: 333e6. 4 Husemeyer R, White D. Topography of the lumbar epidural space. A study in cadavers using injected polyester resin. Anaesthesia 1980; 35: 7e11. 5 Blomberg R. Technical advantages of the paramedian approach for lumbar epidural puncture and catheter introduction. A study using epiduroscopy in autopsy subjects. Anaesthesia 1988 Oct 1; 43: 837e43. 6 Igarashi T, Hirabayashi Y, Shimizu R, Saitoh K, Fukuda H, Suzuki H. The fiberscopic findings of the epidural space in pregnant women. Anesthesiol 2000 Jun 1; 92: 1631e6. 7 Lirk P, Moriggl B, Colvin J, et al. The incidence of lumbar ligamentum flavum midline gaps. Anesth Analg 2004 Apr 1; 98: 1178e80. table of contents. 8 Zarzur E. Anatomic studies of the human ligamentum flavum. Anesth Analg 1984 May 1; 63: 499e502. 9 Reina M, Pulido P, Castedo J, Villanueva M, Lopez A, Sola R. Characteristics and distribution of normal human epidural fat. Rev Esp Anestesiol Reanim 2006 Jun 1; 53: 363e72. 10 Alicioglu B, Sarac A, Tokuc B. Does abdominal obesity cause increase in the amount of epidural fat? Eur Spine J 2008 Oct 1; 17: 1324e8. 11 Hogan QA. New look at lumbar epidural anatomy. Anesthesiol 1991 Sep 1; 75: A716. 12 Hogan Q. Epidural catheter tip position and distribution of injectate evaluated by computed tomography. Anesthesiol 1999 Mar 31; 90: 964e70. 13 Hogan QH. Epidural anatomy examined by cryomicrotome section. Influence of age, vertebral level, and disease. Reg Anesth 1996 Aug; 21: 395e406. 14 Plaisant O, Cosnard G, Gillot C, Schill H, Lassau J. MRI of the epidural space after gelatin/gadolinium venous injection. Surg Radiol Anat 1994; 16: 71e5. 15 Harrison G, Clowes N. The depth of the lumbar epidural space from the skin. Anaesthesia 1985 Jul 1; 40: 685e7. 16 Sutton DN, Linter SP. Depth of extradural space and dural puncture. Anaesthesia 1991 Feb; 46: 97e8. 17 Narang V, Linter S. Failure of extradural blockade in obstetrics. A new hypothesis. Br J Anaesth 1988 Mar 1; 60: 402e4. 18 Segal S, Beach M, Eappen S. A multivariate model to predict the distance from the skin to the epidural space in an obstetric population. Reg Anesth 1996 Sep 1; 21: 451e5. 19 Perlas A. Evidence for the use of ultrasound in neuraxial blocks. Reg Anesth Pain Med 2010 Feb; 35(2 suppl): S43e6.
Figure 2 Axial T2 MRI of lumbar spine at the level of a lamina. CSF appears white. (a) Vertebral body; (b) dural sac containing CSF with nerve roots lying posteriorly; (c) dura lying against the anterior aspect of the lamina with no epidural space at this level.
catheter, requiring twisting and manipulation of the catheter or even complete obstruction to catheter insertion necessitating reinsertion of the Tuohy needle. Studies have shown that in comparison to the paramedian approach catheters inserted by the mid-line route are less likely to remain in the mid-line.5 Not infrequently the catheter lies in the lateral or anterior epidural space12 or passes out through the intervertebral foramina into the paravertebral space potentially increasing the risk of paraesthesia, venous puncture or unilateral blocks. This contrasts with the unimpeded insertion of a paramedian
Figure 3 Three-dimensional diagram demonstrating segmented nature of the epidural space. (a) Posterior segment of epidural space; (b) epidural fat in intervertebral foramina; (c) dural sac in spinal canal. (From Hogan23).
ANAESTHESIA AND INTENSIVE CARE MEDICINE 13:11
553
Ó 2012 Published by Elsevier Ltd.
REGIONAL ANAESTHESIA
20 Savolaine E, Greenblatt S, Conover S, Lipton S. Computed tomography application to lumbar epidurography. J Comput Tomogr 1987 Apr 1; 11: 193e9. 21 Westbrook JL, Renowden SA, Carrie LE. Study of the anatomy of the extradural region using magnetic resonance imaging. Br J Anaesth 1993 Oct; 71: 495e8.
ANAESTHESIA AND INTENSIVE CARE MEDICINE 13:11
22 Capogna G, Celleno D, Simonetti C, Lupoi D. Anatomy of the lumbar epidural region using magnetic resonance imaging: a study of dimensions and a comparison of two postures. Int J Obstet Anesth 1997 Apr; 6: 97e100. 23 Hogan QH. Lumbar epidural anatomy. A new look by cryomicrotome section. Anesthesiol 1991 Nov; 75: 767e75.
554
Ó 2012 Published by Elsevier Ltd.
Anatomy of the epidural space
Learning objectives After reading this article you should be able to describe the: C borders of the epidural space C contents of the epidural space C shape and distribution of the posterior epidural space
Jon L Westbrook
Abstract Despite the importance of anatomical knowledge in the safe and effective conduct of thoraco-lumbar epidural anaesthesia there are widespread misunderstandings about the detailed anatomy of this complex region. Current textbooks and teaching continue to propagate misleading information. Much of the difficulty in studying the lumbar epidural region is that the epidural space is a ‘potential’ space occupying only a small volume at rest but capable of expanding to accommodate large volumes of injected solution. This results in considerable distortion during some anatomical studies. However new imaging and dissection techniques have given us a much clearer insight into the anatomy and the relationships of the dura, epidural fat, blood vessels and spinal canal. This article seeks to provide an up-to-date account of current knowledge to give the reader a clearer idea of the true anatomy and how it might influence their epidural practice.
This distorts the relative volumes of these components of the spinal canal. There may also be tissue autolysis depending on the time between death and the study. An investigation that requires the injection of solution into the epidural space (e.g. latex or contrast solution) will also distort the anatomy by expansion of the epidural space and compression of the dural sac. Early epiduroscopic studies were performed on cadavers5 but fine flexible scopes now allow epiduroscopy in live subjects6 but this still requires injectate to expand the space. Advances in MRI techniques and other sophisticated methods such as cryomicrotome sectioning of freshly frozen cadavers have given as much clearer understanding of the relevant anatomy and challenged accepted ideas.
Basic anatomy of the epidural space Keywords Epidural/extradural anatomy
The posterior longitudinal ligament (PLL) borders the epidural space anteriorly as it overlies the vertebral bodies and intervertebral discs. Laterally the medial aspect of the pedicles and the intervertebral foramina demarcate the space. The exact point within the foramina that the epidural space ends is not well defined. Posteriorly the borders are alternately the anterior aspect of the laminae and the ligamentum flava. The canal is oval in the cervical region becoming almost triangular in the lumbar region. The epidural space surrounds the dural sac containing the spinal cord and CSF. The spinal cord typically terminates at lumbar level of 1 or 2 whereupon the nerve roots of the cauda equina continue through the epidural space to their respective foramina. The epidural space ends superiorly where the dura blends with the periosteum at the foramen magnum preventing intracranial spread of epidural solutions. Inferiorly it ends at the sacrococcygeal membrane. The dural sac ends at the approximate level of S1, 2. The ligamentum flavum comprises fibres running in the long axis of the spine between each lamina. They are inserted at the upper border and posterior aspect of the lamina below and the lower half and posterior aspect of the lamina above. At each level there are two separate ligaments (left and right), which usually meet in the mid-line. However the ligament is occasionally deficient in the mid-line when it blends with the anterior fibres of the interspinous ligament. In one study7 the ligamentum flavum was deficient in 11% of subjects in the midlumbar region. Small veins may traverse the ligamentum in the mid-line. Zarzur’s8 detailed examination of the ligamentum flavum in cadavers found that it extends laterally out to the intervertebral foramina where it merges with the capsule of the articular processes. Thickness ranged from 3 to 5 mm although clinical studies often report greater distances probably because of the oblique angle that many needles traverse the ligament.
Royal College of Anaesthetists CPD Matrix: 2G02 3A09
Introduction The performance of any invasive medical procedure is improved by a clear understanding of the relevant anatomy. However the anatomy of the epidural space is not well described in standard medical texts nor is it clearly understood by many anaesthetists. The reason for this is that the epidural region has proved to be very difficult to study and many of the early papers produced misleading information. New study techniques have now provided much clearer detail about the anatomy, which may have wider implications for clinical practice. The reason the epidural space has proved so difficult to study is that it is a potential space, a term first coined by Bromage1 to explain the small resting volume of the epidural space that at the same time is able to accommodate large volumes of an injected solution. The epidural space is contained entirely within the rigid spinal canal and so any injected solution either causes displacement of the dural sac or passes out through the intervertebral foramina. Early studies included dissection of cadavers sometimes following injection of liquid resin into the epidural space.2e4 Cadaver studies inevitably produce artefact as the CSF pressure is lost after death and the venous pressure increases.
Jon L Westbrook MB BS MRCP FRCA FFICM is a Consultant Anaesthetist and Honorary Senior Clinical Lecturer at Oxford University Hospitals, UK. Conflicts of interest: none declared.
ANAESTHESIA AND INTENSIVE CARE MEDICINE 13:11
551
Ó 2012 Published by Elsevier Ltd.
REGIONAL ANAESTHESIA
The angle subtended by the two ligaments in the mid-line is usually 90 or less. The anterior internal vertebral venous plexus drains the spinal cord, meninges and vertebrae and lies in the antero-lateral aspect of the spinal canal between two thin layers of the PLL. These veins connect to the azygos and intercostal systems and so may become engorged during inferior vena cava compression caused by a gravid uterus or the prone position for spinal surgery. Arteries enter the spinal canal through the intervertebral foramina as branches of the segmental vessels. They divide to follow the nerve roots and supply the cord along with the larger anterior spinal artery. Epidural fat differs significantly from the majority of body fat. It has little metabolic function but may provide some physical protection to the contents of the spinal canal and exiting nerve roots. There is no correlation between total body fat and the volume of fat in the posterior epidural space.9,10 Hogan11 has shown with his detailed studies that the fat is very liquid with only a few septae. This means that it is easily displaced by solutions injected outside the fat pad. Epidural catheters rarely enter this fat pad12 but usually lie outside its capsule. The fat pads are tethered loosely to the walls of the spinal canal and so injected solution such as contrast or local anaesthetic can compress the fat pad and this gives the impression of membranes or septae within the epidural space when imaged by epiduroscopy or CT epidurograms. Both Hogan13 and Plaisant14 have studied the anterior epidural space and have shown it to be a complex and segmented compartment. There is a medial and lateral anterior venous plexus, which is partially incorporated into layers of the PLL. There are fibrous structures connecting the dura mater to the PLL producing an anterior medial compartment, which is obliterated at the level of the intervertebral disc. This complex arrangement of fibrous bands and membranes may prevent the easy passage of solution across the mid-line in the anterior epidural space.
The more recent introduction of real-time ultrasound imaging of the lumbar region during epidural needle insertion can provide an accurate measure of the depth of the epidural space, which may aid introduction of the Tuohy needle.19
Segmented epidural space The most important and yet least understood aspect of epidural anatomy is the segmented distribution of the epidural space. Studies using CT epidurography,20 and MRI21,22 and cryomicrotome dissection techniques13 have clearly demonstrated that the posterior epidural space is divided into segments at the levels of the ligamentum flavum (Figure 1) and is obliterated between these levels as the dura lies adjacent to the anterior aspect of the lamina (Figure 2). However at these levels there is easy passage of both epidural catheters and any injected solution. There are small pockets of epidural fat within each intervertebral foramen. This is well illustrated by the three-dimensional (3D) diagram in Hogan’s publication23 (Figure 3). Unfortunately most standard textbooks represent the posterior epidural space as a smooth, continuous compartment extending from the sacrum up to the foramen magnum anterior to the ligamentum flavum. Close examination of the cranial end of any individual posterior segment of epidural space will show that a Tuohy needle passing through the ligamentum at just below the lamina above could deliver the epidural catheter against the bony barrier of the lamina. This could cause difficulty in passing the
Depth of the epidural space There have been several studies to measure the distance between the skin and the epidural space to establish some correlation between the depth and surrogate markers such as body weight or body surface area. This is potentially useful as the operator and trainer are blind to the depth of the space, which makes it a difficult technique to teach. Many factors also influence the measured distance such as vertebral level, direction of approach to the space, pregnancy and the degree of spinal flexion. Harrison15 showed that in the pregnant woman the median distance was 4.7 cm with a range of 3.0e7.4 cm with the distance greatest at L3,4. Sutton16 in a retrospective note review was able to show that the shallower the epidural space the greater the risk of accidental dural punctures whilst Narang has shown that the deeper the space the greater the chance of an uneven block.17 An attempt at a multivariate model was able to predict the depth to within an accuracy of only 8 mm.18 The study did demonstrate interracial variations with oriental subjects having shallower spaces than others but otherwise concluded that there is unlikely to be a formula sufficiently accurate for clinical use.
ANAESTHESIA AND INTENSIVE CARE MEDICINE 13:11
Figure 1 Sagittal mid-line T1 MRI of lumbar spine demonstrating the segmented posterior epidural space. Epidural fat appears as white, CSF dark. (a) Section through posterior segment of epidural space; (b) dura lying immediately anterior to the epidural space containing CSF. The dura lies adjacent to the bone at the level of the lamina with the epidural space obliterated; (c) ligamentum flavum lying immediately posterior to the epidural space.
552
Ó 2012 Published by Elsevier Ltd.
REGIONAL ANAESTHESIA
catheter as this technique delivers the catheter into the caudal end of the epidural segment where it will not impact on the lamina above. Equipped with an accurate understanding of epidural anatomy the trainee or experienced ‘epiduralist’ can refine their technique to optimize the successful and accurate placement of epidural catheters or spinal cord stimulators. A
REFERENCES 1 Bromage. Anatomy. In: Epidural analgesia. Philadelphia: WB Saunders, 1978. pp. 8e20. 2 Parkin IG, Harrison GR. The topographical anatomy of the lumbar epidural space. J Anat 1985 Aug; 141: 211e7. 3 Harrison G, Parkin I, Shah J. Resin injection studies of the lumbar extradural space. Br J Anaesth 1985 Mar 1; 57: 333e6. 4 Husemeyer R, White D. Topography of the lumbar epidural space. A study in cadavers using injected polyester resin. Anaesthesia 1980; 35: 7e11. 5 Blomberg R. Technical advantages of the paramedian approach for lumbar epidural puncture and catheter introduction. A study using epiduroscopy in autopsy subjects. Anaesthesia 1988 Oct 1; 43: 837e43. 6 Igarashi T, Hirabayashi Y, Shimizu R, Saitoh K, Fukuda H, Suzuki H. The fiberscopic findings of the epidural space in pregnant women. Anesthesiol 2000 Jun 1; 92: 1631e6. 7 Lirk P, Moriggl B, Colvin J, et al. The incidence of lumbar ligamentum flavum midline gaps. Anesth Analg 2004 Apr 1; 98: 1178e80. table of contents. 8 Zarzur E. Anatomic studies of the human ligamentum flavum. Anesth Analg 1984 May 1; 63: 499e502. 9 Reina M, Pulido P, Castedo J, Villanueva M, Lopez A, Sola R. Characteristics and distribution of normal human epidural fat. Rev Esp Anestesiol Reanim 2006 Jun 1; 53: 363e72. 10 Alicioglu B, Sarac A, Tokuc B. Does abdominal obesity cause increase in the amount of epidural fat? Eur Spine J 2008 Oct 1; 17: 1324e8. 11 Hogan QA. New look at lumbar epidural anatomy. Anesthesiol 1991 Sep 1; 75: A716. 12 Hogan Q. Epidural catheter tip position and distribution of injectate evaluated by computed tomography. Anesthesiol 1999 Mar 31; 90: 964e70. 13 Hogan QH. Epidural anatomy examined by cryomicrotome section. Influence of age, vertebral level, and disease. Reg Anesth 1996 Aug; 21: 395e406. 14 Plaisant O, Cosnard G, Gillot C, Schill H, Lassau J. MRI of the epidural space after gelatin/gadolinium venous injection. Surg Radiol Anat 1994; 16: 71e5. 15 Harrison G, Clowes N. The depth of the lumbar epidural space from the skin. Anaesthesia 1985 Jul 1; 40: 685e7. 16 Sutton DN, Linter SP. Depth of extradural space and dural puncture. Anaesthesia 1991 Feb; 46: 97e8. 17 Narang V, Linter S. Failure of extradural blockade in obstetrics. A new hypothesis. Br J Anaesth 1988 Mar 1; 60: 402e4. 18 Segal S, Beach M, Eappen S. A multivariate model to predict the distance from the skin to the epidural space in an obstetric population. Reg Anesth 1996 Sep 1; 21: 451e5. 19 Perlas A. Evidence for the use of ultrasound in neuraxial blocks. Reg Anesth Pain Med 2010 Feb; 35(2 suppl): S43e6.
Figure 2 Axial T2 MRI of lumbar spine at the level of a lamina. CSF appears white. (a) Vertebral body; (b) dural sac containing CSF with nerve roots lying posteriorly; (c) dura lying against the anterior aspect of the lamina with no epidural space at this level.
catheter, requiring twisting and manipulation of the catheter or even complete obstruction to catheter insertion necessitating reinsertion of the Tuohy needle. Studies have shown that in comparison to the paramedian approach catheters inserted by the mid-line route are less likely to remain in the mid-line.5 Not infrequently the catheter lies in the lateral or anterior epidural space12 or passes out through the intervertebral foramina into the paravertebral space potentially increasing the risk of paraesthesia, venous puncture or unilateral blocks. This contrasts with the unimpeded insertion of a paramedian
Figure 3 Three-dimensional diagram demonstrating segmented nature of the epidural space. (a) Posterior segment of epidural space; (b) epidural fat in intervertebral foramina; (c) dural sac in spinal canal. (From Hogan23).
ANAESTHESIA AND INTENSIVE CARE MEDICINE 13:11
553
Ó 2012 Published by Elsevier Ltd.
REGIONAL ANAESTHESIA
20 Savolaine E, Greenblatt S, Conover S, Lipton S. Computed tomography application to lumbar epidurography. J Comput Tomogr 1987 Apr 1; 11: 193e9. 21 Westbrook JL, Renowden SA, Carrie LE. Study of the anatomy of the extradural region using magnetic resonance imaging. Br J Anaesth 1993 Oct; 71: 495e8.
ANAESTHESIA AND INTENSIVE CARE MEDICINE 13:11
22 Capogna G, Celleno D, Simonetti C, Lupoi D. Anatomy of the lumbar epidural region using magnetic resonance imaging: a study of dimensions and a comparison of two postures. Int J Obstet Anesth 1997 Apr; 6: 97e100. 23 Hogan QH. Lumbar epidural anatomy. A new look by cryomicrotome section. Anesthesiol 1991 Nov; 75: 767e75.
554
Ó 2012 Published by Elsevier Ltd.