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Applied epidural anatomy Jonathan Richardson MD MRCP FRCA FIPP Gerbrand J Groen MD PhD Anatomy of the epidural space Key points Clinicians performing epidural procedures should have a good knowledge of the relevant anatomy. Radiological screening is helpful for difficult epidural entry. Maximal neck flexion can almost double the depth of the cervical epidural space. Spinal endoscopy adds another dimension to epidural catheterization. At present, its main application is in relation to the management and diagnosis of chronic radicular pain. Jonathan Richardson MD MRCP FRCA FIPP Department of Anaesthetics Bradford Royal Infirmary Bradford BD9 6RJ Tel: 01274 364066 Fax: 01274 366548 E-mail: [email protected] (for correspondence) Gerbrand J Groen MD PhD Division of Perioperative and Emergency Medicine Department of Anaesthesiology and Pain Treatment University Medical Centre Utrecht The Netherlands 98 Vertebral column There are 24 individual vertebrae: seven cervical, 12 thoracic and five lumbar. The five (fused) sacral vertebrae and the coccyx (made up of 3–5 rudimentary vertebrae) are not always classed as being a part of the vertebral column. Vertebral anatomy varies according to each level. The atlas and the axis are highly atypical and the first ‘recognizably normal’ vertebra is C3. Atlas and axis anatomy are relevant to anaesthetists in that the odontoid process (dens) of the axis should be closely applied (2–3 mm) to the anterior arch of the atlas in flexion and extension (the gap is filled with cartilage). Rheumatoid arthritis and trauma may affect this relationship and the ability to recognize the relevant anatomy on lateral radiographs is important. From C3 downwards, the vertebrae (although they vary) have a recognizable anterior body, posterolateral pedicles (Latin for ‘little feet’), transverse processes and posterior laminae (‘thin layers’), which fuse to form the spinous processes. The spinal canal enclosed within these structures is also known as the epidural space, apart from the central portion occupied by the dural sac and its contents. The dura mater contains the arachnoid mater. Between the arachnoid and the pia mater, which is applied to the spinal cord, is cerebrospinal fluid. As the vertebral column grows, it leaves behind the spinal cord so that by adulthood the cord ends at the lower border of L1 (although this can vary by one vertebra). The dural sac generally ends at the lower border of S2 below which it continues as the filum terminale, a structure clearly and frequently seen with spinal endoscopy. The dural sac contains the anterior and posterior spinal nerve roots, collectively know as the cauda equina. Spinal nerves Spinal nerves exit at each level and are numbered according to the thoracic, lumbar or Table 1 Boundaries of the epidural space Superiorly Inferiorly Anteriorly Laterally Posteriorly Fusion of the spinal and periosteal layers of dura mater at the foramen magnum1 Sacrococcygeal membrane Posterior longitudinal ligament, vertebral bodies and discs Pedicles and intervertebral foraminae Ligamentum flavum, capsule of facet joints and laminae Table 2 Definition of the cervical, thoracic, lumbar and sacral epidural spaces Cervical Thoracic Lumbar Sacral Fusion of the spinal and periosteal layers of dura mater at the foramen magnum1 to lower margin of C7 Lower margin of C7 to upper margin of L1 Upper margin of L1 to upper margin of S1 Upper margin of S1 to sacrococcygeal membrane sacral vertebra above. As there are eight cervical spinal nerves and seven vertebrae, the nerves in this region only are numbered according to the vertebra below. The only exception is spinal nerve C8 that leaves between vertebra C7 and T1. Epidural space The boundaries of the epidural space are summarized in Table 1 and the definitions of the cervical, thoracic, lumbar and sacral regions are defined in Table 2. The epidural space contains fat, the dural sac, spinal nerves, blood vessels and connective tissue (Table 3). Blood supply Inside the spinal canal there is an anterior and posterior arterial (and venous) arcade formed from the spinal arteries entering at each level through the interventricular foramina. These arise from the vertebral arteries superiorly and then thoracic and the lumbar aorta. They anastomose with the anterior spinal artery, running on the surface of the spinal cord arising initially from the vertebral arteries at the circle of Willis. The nerve root (with the Continuing Education in Anaesthesia, Critical Care & Pain | Volume 5 Number 3 2005 ª The Board of Management and Trustees of the British Journal of Anaesthesia [2005]. All rights reserved. For Permissions, please email: [email protected] doi 10.1093/bjaceaccp/mki026 Applied epidural anatomy Table 3 Contents of the epidural space Table 4 Epidural space location according to vertebral level Structure Comment Level Notes Fat Varies in direct proportion to the rest of the body.1 Not uniform in distribution; exists in bands at level of intervertebral foramina Ends at approximately S2. Contains the spinal cord (to the lower border of L1) and cauda equina In pairs. Dorsal root ganglia in lateral recesses See text Variable dorsomedian folds, median fold. After leakage of nucleus pulposus, surgery or previous epidural catheterization may be heavy scar tissue Cervical Loss of resistance poorly appreciated as the ligamentum flavum is thin and soft.1 ‘Hanging drop’ method in the sitting position often employed. Depth of space only 1.5–2 mm at C7.1 Increases to 3–4 mm with neck flexion.3 Should be performed by the experienced practitioner only Kyphotic apex at T6. Slight right scoliosis common and normal. Avoid midline approach between T5–T8 Enter if possible below L2 to avoid the cord Dural sac Spinal nerves Vessels Connective tissue exception of the dorsal root ganglion) has a poor blood supply compared with the spinal cord. There is a watershed area half way along its length where branches from the conus medullaris meet the supply from the thoracic and lumbar arteries. Despite anastomoses throughout the vertebral canal, arterial trauma can compromise the blood supply of the cord itself. Venous drainage is via the valveless vertebral venous plexus of Batson, which, fortunately for the epiduralist, is predominantly an anterior spinal canal structure. The posterior venous plexus is variable in size at the lumbar level but generally increases in the thoracic and cervical areas.2 Drainage is into the intracranial venous sinuses and, at a local level, into the thoracic and lumbar veins through the intervertebral foramina. The veins in the lumbar area drain into the ascending lumbar veins lying on the anterior surface of the transverse processes. These empty into either the iliac veins inferiorly or the hemiazygous or azygous veins on the left and right, respectively. As the whole system is valveless, increased intrathoracic or intra-abdominal pressure (e.g. ascites, pregnancy) can lead to major congestion and vessel enlargement within the spinal canal. Nerve supply The spinal canal and its contents have their own innervation. The anterior dura is heavily innervated;2 fortunately for spinal anaesthesia, the posterior dura is sparsely supplied. The nerve supply of the spinal canal is via direct branches from the sympathetic chain and via the sinu-vertebral nerves that originate from the rami communicantes. The periosteum is pain sensitive but the ligamentum flavum is not. Thorax Lumbar Distance from ligamentum flavum to dura. Table 5 Overview of methods of epidural entry Interlaminar (transflaval) Transforaminal Transsacral Direct vision Paravertebral The ‘usual’ method. Loss of resistance methods. Can use ‘hanging drop’ or other techniques in the cervical region Directs solution to the anterior epidural space. Radiological guidance mandatory. Specialist use only Simple entry, aspiration and injection. Up to 40% in the adult incorrectly placed without radiological guidance4 Spinal endoscopy Frequent epidural blockade5 Locating the epidural space The standard methods of detection of entry into the epidural space will not be discussed in detail here; they are covered in many other texts. Some helpful suggestions are offered according to the anatomical level of entry (Table 4). The various methods are summarized in Table 5. Pathology affecting epidural entry Anatomical abnormalities affecting epidural catheterization are either congenital or acquired. Congenital abnormalities that cause difficulties include achondroplasia, congenital adolescent scoliosis and spina bifida. The use of epidurals in achondroplasia and congenital adolescent scoliosis is controversial. As spina bifida is frequently associated with a meningocoele, which may be close to the surface, and with the failure of fusion of the laminae with attendant ligamentum flavum abnormalities, epidural location should not be attempted. Acquired difficulties include ligamentum flavum hypertrophy, often contributing to spinal stenosis, foraminal stenosis and disc prolapse. The latter two are not contraindications, but could make satisfactory entry or catheterization difficult. X-ray guidance may be of help. The effect of previous epidural catheterization, spinal level and respiration on epidural space structures (as determined by epiduroscopy) is summarized in Table 6. Lymphatics Lymphatics are present around the region of the nerve root and function to remove foreign material. They are absent in the nerve root itself. Epiduroscopy Although receiving considerable recent attention, spinal endoscopy or epiduroscopy is not a new technique. It has been carried Continuing Education in Anaesthesia, Critical Care & Pain | Volume 5 Number 3 2005 99 Applied epidural anatomy Table 6 The effect of previous epidural catheterization, spinal level and respiration on epidural space structures (from the work of Igarashi and colleagues6–8) Structure No previous epidural catheterization Previous epidural entry Increasing age Cephalad Inspiration Fat Segmental Less Less Less Connective tissue Fragile median and dorso-median folds Needle damage in 23% Good Adhesions and granulation tissue ND Poor No effect Less Drawn out through intervertebral foraminae Nil Nil More ND More Less More Vascularity Patency ND ¼ not determined. out since the 1930s using rigid instruments and transflaval approaches. Nowadays, most epiduroscopy is carried out using flexible instruments introduced through the sacrococcygeal route providing a direct path to more cranial structures. Spinal endoscopy is different from other imaging techniques in that it has a major interactive element with the patient, allowing examination of appropriate areas, which may be causing pain. Examination of the contents of the dural sac is easy as they are suspended in clear cerebro-spinal fluid. With recent advances in instrumentation, especially involving fully steerable, flexible instruments, along with a saline delivery system, a multilevel detailed examination of the epidural space can be satisfactorily achieved. High quality views aid the examiner in exactly identifying the nerve roots that may be implicated in pain generation.9 10 Conditions that have so far been diagnosed using this technique include: cysts and tumours, fibrosis, ischaemia and tethering of nerve roots, arachnoiditis, tuberculosis meningitis and acute and traumatic events associated with epidural catheterization. References 1. Bromage PR. Anatomy. In: Bromage PR, ed. Epidural Analgesia. Philadelphia: WB Saunders, 1978; 8–20 100 2. Groen GJ, Baljet B, Drukker J. The innervation of the spinal dura mater. Anatomy and clinical implications. Acta Neurochir (Wien) 1988; 92: 39–46 3. Reynolds AF, Roberts PA, Pollay M, et al. Quantitative anatomy of the thoracolumbar epidural space. Neurosurgery 1985; 17: 905 4. Renfrew DL, Moore TE, Kathol MH, el-Koury GY, Lemke JH, Walker CW. Correct placement of epidural steroid injections: fluoroscopic guidance and contrast administration. Am J Neuroradiol 1991; 12: 1003–7 5. Richardson J, Lönnqvist PA. Thoracic paravertebral blockade. A review. Br J Anaesth 1998; 81: 230–8 6. Igarashi T, Hirabayashi Y, Shimizu R, Saitoh K, Fukuda H, Mitsuhata H. The lumbar extradural structure changes with increasing age. Br J Anaesth 1997; 78: 149–52 7. Igarashi T, Hirabayashi Y, Shimizu R, Saitoh K, Fukuda H. Thoracic and lumbar extradural structure examined by extraduroscope. Br J Anaesth 1998; 81: 121–5 8. Igarashi T, Hirabayashi Y, Shimizu R, et al. Inflammatory changes after extradural anaesthesia may affect the spread of local anaesthetic within the extradural space. Br J Anaesth 1996; 77: 347–51 9. Richardson J, McGurgan P, Cheema S, Prashad R. Gupta S. Spinal endoscopy in chronic low-back pain with radiculopathy. A prospective case series. Anaesthesia 2001: 56; 447–84 10. Geurts JW, Kallewaard JW, Richardson J, Groen GJ. Targeted methylprednisolone/hyaluronidase/clonidine injection after diagnostic epiduroscopy for chronic sciatica: a prospective, 1-year follow-up study. Reg Anesth Pain Med 2002; 27: 343–52 See multiple choice questions 78–80. Continuing Education in Anaesthesia, Critical Care & Pain | Volume 5 Number 3 2005