Download Applied epidural anatomy

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
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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