2. Extradural Space Flashcards
Basic Anatomy
The epidural space posteriorly is an actual space whose posterior
dimensions can be seen easily on any coronal CT or MR scan of the vertebral
column.
This actual space is the area surrounding the dural sheath as it lies within the
vertebral canal.
It extends from the foramen magnum superiorly (where the dura is fused to the
skull) to the sacral hiatus inferiorly
. So, in contrast to a ‘total
spinal’, a patient with a high cervical epidural block may stop breathing but will not
be unconscious.
Space boundaries
The space is traversed by the dural sheath, whose thickness in the lumbar region is
about 0.3–0.5 mm, and which comprises the membranes of the dura and arachnoid
maters, the subarachnoid space containing CSF, the spinal nerves of the cauda equina
and the filum terminale.
Anteriorly, the epidural space is bounded by the bodies of the vertebrae and by the
intervertebral discs, over which lies the posterior longitudinal ligament. The anterior
dura mater and the posterior longitudinal ligament are closely apposed, and so
anteriorly the epidural space is ‘potential’.
Laterally, the epidural space is bounded by the pedicles and the intervertebral
foramina
Posteriorly, it is bounded by the laminae of the neural arches.
Ligamenta flava
Ligamenta flava: these are not continuous
Each ligament extends from the lower part of the anterior surface
of the lamina above to the posterior surface of, and upper margin of, the lamina
below. Their fibres run in a perpendicular direction, but when viewed in the sagittal
plane the ligaments are triangular in shape, with the apex of the triangle formed at the
upper lamina.
Dural space at a vertebrae
At the level of a typical lumbar vertebra, for example L3, the space contains the spinal
nerves, each of which is invested with a cuff of dura, with loosely packed fat, areolar
connective tissue, lymphatics and blood vessels. These vessels include the rich valveless
vertebral venous plexus of Batson. (The lack of valves means that they will engorge as
intra-abdominal pressure increases, for example during a contraction in labour.)
Depth space
The depth of the posterior epidural space (between the ligamenta flava and the dura)
varies with the vertebral level. In the mid-cervical region it is only 1.0–1.5 mm wide,
and at T6 it is deeper, at around 2.5–3.0 mm. The greatest depth is at the L2
interspace in men, in whom this is 5.0–6.0 mm.
Complications Associated with the Procedure
These include inadvertent dural puncture and
subsequent post-dural puncture headache (PDPH) (incidence of 0.5%);
failure (1%);
unilateral or patchy block (5–10%);
inadvertent subdural block (0.1%);
intravascular injection;
retention of a fragment of needle or catheter; epidural haematoma. The risk of permanent neurological sequelae is very small.
The incidence is quoted at 1 in 10–15,000 epidurals but many of these
complications are relatively minor, comprising, for example, little more than a patch
of residual numbness, and even this figure is likely to be too high because childbirth
itself may cause permanent neurological deficit (1 in 2,000).
In the NAP 3 report on
complications of central neuraxial blockade in the UK (2009), the incidence of
permanent harm, considering the ‘pessimistic’ interpretation, was 1 in 161,550.
There is finally no evidence, despite much debate fueled by the lay press, that routine
epidurals lead to chronic back pain.
Complications Associated with Drugs That Are Injected
These include hypotension owing to sympathetic block; high spinal block; evidence
of systemic toxicity of local anaesthetic; urinary retention; pruritus, nausea and
vomiting (usually associated with extradural opiate); respiratory depression. A total
spinal may follow inadvertent intrathecal injection, depending on the epidural dose.
There are many case reports of accidental injection of the wrong solution. Numerous
substances have been administered in this way, including various antibiotics;
solutions of total parenteral nutrition (which apparently provided good quality
analgesia); chlorhexidine, with catastrophic sequelae; and thiopental.
do not have any influence on caesarean section rates but do increase the
likelihood of instrumental delivery by up to 14%.
PDPH Features
Diagnosis: the incidence of inadvertent dural puncture should not exceed 0.5%, and
the incidence is usually quoted at between 0.5% and 1.0%. The incidence of PDPH is
highest in obstetric patients, more than 80% of whom will develop symptoms. These
are probably caused by traction on intracranial pain-sensitive structures such as the
tentorium and blood vessel, and by adenosine-mediated reflex intracerebral vasodilatation.
The low-pressure headache results from the failure of the choroid plexus to
produce sufficient CSF to compensate for the loss through the breach in the dura.
The onset is variable, with the headache commonly starting after about 12–24 hours.
It can occur earlier or later. The headache may be frontal or occipital rather than
global, but typically it is postural and relieved by recumbency or abdominal pressure.
It may also be associated with photophobia, visual disturbance, neck and shoulder
stiffness, and tinnitus. If the patient also complains of anorexia, nausea and vomiting,
this is an indication that there is significant sagging of intracranial contents, with
pressure on the brain stem at the foramen magnum
includes migraine, pre-eclampsia (which can present
post-delivery), intracranial haemorrhage (associated with severe hypertension), meningitis
and cortical vein thrombosis
Management of severe PDPH:
Management of severe PDPH: assuming the failure of initial conservative treatment,
advising recumbence when headache supervenes and simple analgesia, management
may move on to other treatment
Cerebral vasoconstrictors such as caffeine and
sumatriptan may improve symptoms, but they will not address the cause
corticosteroids, and accordingly there is some
evidence to support the administration of ACTH analogues such as tetracosactrin
(Synacthen), which stimulate glucocorticoid release
small. The only technique that is likely to
provide immediate relief is an extradural blood patch (EBP). This will abolish symptoms
in almost all patients, but in at least 30% of mothers the procedure will need to be
repeated. EBP has been associated with the development of chronic low back pain, and
this risk must be weighed against those of persistent long-term headache or of neurological
disaster (such as subdural haemorrhage), of which there are numerous reports in
the literature
Inadvertent Subdural Block
catheter or needle may deposit solution in the subdural space between the dura
and arachnoid mater. Radiologists maintain that during myelography there is a 1% incidence of subdural injection. It is much less commonly diagnosed in clinical
anaesthesia. Some authorities cite an incidence of 1 in 1,000.
Subdural block is often patchy; it may be extensive and unilateral, may extend very
high (the subdural space extends into the cranium) and it often spares the sacral
roots. The dura and arachnoid are more densely adherent to each other anteriorly,
and so there may be a relative sparing of motor fibres. Sympathetic block may be
minimal, and analgesia may be delayed. Horner’s syndrome may be apparent.
The use of a multi-holed catheter may further confuse the picture, because it is
theoretically possible for the catheter to lie partly within the epidural and partly
within the subdural space. Slow injection will favour emergence of the solution from
the proximal epidural holes; more vigorous injection will favour dispersal through
the distal subdural hole
High Block or Total Spinal
A high block or developing total spinal is characterized by the development of
paraesthesia and weakness of the upper limbs,
respiratory embarrassment owing to intercostal paralysis,
a weak voice and cough,
and sensory loss over the skin of the
neck and eventually the jaw.
If the block is a total spinal then apnoea and unconsciousness
will supervene. Pupils dilate. It is usually asserted that a high sympathetic
block will lead to hypotension and bradycardia because of local anaesthetic effects on
the cardiac accelerator fibres (T1–T4).
In practice, the cardiovascular changes are by
no means always so predictable. High blocks regress quickly, whereas it might be
some hours before a total spinal has worn off to the point at which comfortable
respiration will be possible. Until this happens, anaesthesia must be maintained to
prevent awareness.
Thoracic Epidural Analgesia
angled downwards and overlap such that the process of one thoracic
vertebra is opposite the transverse process of the one below. This means that if a
midline approach to the epidural space is used, the advancing needle will have to
be at an angle of at least 45 to the spinal column, depending on which
interspace is being used. For this reason, many anaesthetists prefer the paramedian
approach
thoracic epidural can provide effective analgesia following thoracic
and major abdominal surgery, can attenuate the stress response to surgery and
decreases adverse peri-operative cardiac events.
Evidence for thoracic epi
The conclusions of a systematic
review and meta-analysis of 125 trials comparing epidural with systemic opioid
analgesia and published in 2014 were unequivocal
decreased the risks of death, of atrial fibrillation and other supraventricular
tachycardias, of deep venous thrombosis, respiratory depression, atelectasis and
pneumonia, and post-operative nausea and vomiting. It also decreased ileus and
accelerated the recovery of bowel function
Predictably the technique increased the risk of arterial hypotension, pruritus, urinary retention and motor block, but these complications, some of which are readily manageable, would not seem to outweigh
the substantial advantage
Key points
A sound knowledge of epidural anatomy is
essential for clinicians to perform safe and
effective regional anaesthesia.
The anatomy of individual vertebrae varies according to their level.
The ligamentum flavum fuses in the midline.
Areas of absent fusion may account for failure to
recognise loss of resistance.
Surface landmarks are useful but not reliable for
identifying spinal levels accurately.
Ultrasound may be used to help identify access to
the epidural space and for orientation in cases of
altered anatomy.
