Page 353 - Clinical Anatomy

ECA6 7/18/06 6:54 PM Page 338

338 The central nervous system

The extradural (or epidural) space is the compartment between the dural
sheath and the spinal canal. It extends downwards from the foramen
magnum (above which the dura becomes two-layered) to the sacral hiatus.
It is filled with semiliquid fat and contains lymphatics (although there are
no lymphatics within the nervous system deep to the dura), together with
arteries and large, thin-walled veins. These can be considered equivalent
to the cerebral venous sinuses which lie between the two layers of
cerebral dura.
Whereas the arteries of this space are relatively insignificant, the
extradural veins form a plexus which communicate freely and also receive
the basivertebral veins, which emerge from each vertebral body on its poste-
rior aspect. In addition, the veins link up with both the pelvic veins below
and the cerebral veins above—a pathway for the spread of both bacteria and
tumour cells. This accounts, for example, for the ready spread of prostatic
cancer to the sacrum and vertebrae (Batson’s ‘valveless vertebral venous
plexus’).

Clinical features

Lumbar puncture to withdraw C.S.F. from the spinal subarachnoid space
must be performed well clear of the termination of the cord. A line joining
the iliac crests passes through the 4th lumbar vertebra (see Fig. 42) and
therefore the intervertebral spaces immediately above or below this land-
mark can be used with safety. The spine must be fully flexed (with the
patient either on his side or seated) so that the vertebral interspinous spaces
are opened to their maximum extent (Fig. 239). The needle is passed
inwards and somewhat cranially exactly in the midline and at right angles
to the spine; the supraspinous and interspinous ligaments are traversed
and then the dura is penetrated, the latter with a distinct ‘give’. Occasion-
ally root pain is experienced if a root of the cauda equina is impinged upon,
but usually these float clear of the needle.
At spinal puncture C.S.F. can be obtained for examination; antibiotics,
radio-opaque contrast medium or anaesthetics may be injected into the
subarachnoid space, and the C.S.F. pressure can be estimated (normal,
when lying on the side, 80–180mm C.S.F.). Ablock in the spinal canal above
the point of puncture, produced, for example, by a spinal tumour, can be
revealed by Queckenstedt’s test as follows:
Pressure is applied to the neck in order to compress the internal jugular
veins; this reduces venous outflow from the cranium and raises the
intracranial pressure. Consequently, C.S.F. is displaced into the spinal sac
and the C.S.F. pressure, as determined by lumbar puncture and manome-
try, rises briskly by at least 40mm. This rise in pressure is not seen if a spinal
block is present.
Extradural block.◊The extradural space can be entered by a needle passed
either between the spinal laminae or via the sacral hiatus (caudal or sacral
anaesthesia, see page 132).