Blood supply of the spinal cord

The dura mater is a loose sheath around the spinal cord representing the inner layer of the cerebral dura. The outer periosteal layer is represented by the periosteum of the vertebral bodies w i th the extradural - or epidural - space between these layers. The extradural (epidural) space contains loose areolar tissue, fat and a plexus of veins. It extends laterally for a short distance beyond the intervertebral foramina along the spinal nerves.

The dural sac or thecal sac extends inferiorly as far as the S2 vertebral level. Below this the arachnoid and dura blend w i th the pia on the filum terminale.

THE BLOOD SUPPLY OF THE SPINAL CORD

(Figs 3. 17 and 3. 18) The spinal cord is supplied by two posterolateral spinal arteries that supply the posterior white columns and part of the grey matter of the dorsal horns, and a single midline anterior spinal artery that supplies the remainder (i. e. twothirds of the cross-sectional area) of the spinal cord. At the lower end of the cord (i. e. the conus) the anterior artery divides in two and anastomoses w i th each posterior spinal artery. A small ' t w i g' continues on the filum terminale.

The posterolateral spinal arteries arise by one or two branches on each side from the vertebral artery or, more commonly, its posterior inferior cerebellar branch. The anterior spinal artery also arises from the vertebral artery by union, in the midline, of a branch from each side. These arteries receive further supply at intervals along the length of the cord from branches of the deep cervical branch of the subclavian artery, the cervical part of the vertebral artery, and by branches from the intercostal and first lumbar arteries.

These segmental branches pass into the intervertebral foramen and along the nerve roots towards the spinal cord.

Branches along the anterior nerve root may arise with or separately from those to the posterior nerve root. The majority of these are radicular arteries (i. e. supply mainly the nerve root), but some are radiculomedullary arteries and give a significant supply to the cord. The level of origin and number of radiculomedullary arteries is variable, but there are usually four to nine and they arise mainly in the lower cervical, lower thoracic and upper lumbar regions. One of these is often larger than the others and is called the arteria radicularis magna or the artery of Adamkiewicz. This arises at the lower thoracic level between T8 and T12 (on the left side in two-thirds of cases) and supplies the cord both above and below this level. Radiculomedullary arteries reinforce the supply of both the anterior and posterior spinal arteries, and their supply to the cord is critical to its function.

Venous drainage of the spinal cord

The spinal cord drains to a plexus of veins anterior and posterior to the cord, which in turn drains along the nerve roots to segmental veins. The plexus communicates w i t h: • The veins of the medulla at the foramen magnum; • The vertebral veins in the neck; • The azygous veins in the thorax; and • The lumbar veins in the lumbar region.

Radiological features of the spinal cord, its meninges and blood supply

Myelographic technique Lumbar puncture for myelography is performed at a level below the termination of the cord, that is, below L1/2 and above the likely level of abnormality - often L5/S1. The highest point of the iliac crest is at L4, and this is used as a landmark to identify the interspaces above and below it. The anatomy of both cisternal and cervical puncture has already been described.

Nerve roots leave the cord anterolaterally; they are therefore foreshortened in AP views and are best seen in oblique views.

Inadvertent injection of contrast other than into the CSF in the subarachnoid space causes characteristic patterns owing to the anatomy of the spinal meninges. Extradural injection between the dura and the vertebral bone causes collection of contrast around the dura and extension of contrast along the nerve roots to beyond the intervertebral foramina. Subdural injection between the dura and the arachnoid gives an angular collection of slow-moving contrast on AP views, which extends along the nerve roots only as far as the intervertebral foramen. In both subdural and extradural injection contrast is dense because it is not mixed w i th CSF. Injection into an epidural vein is recognized by the rapid disappearance of contrast w i th blood flow. Myelographic appearance Cervical On the AP view the cervical expansion, maximal at C6, can be seen. The cord occupies 50-75% of the interpedicular distance. Nerve roots and their sheaths can be seen leaving the cord almost horizontally. The odontoid peg and spinal vessels, vertebral arteries or posterior inferior cerebellar arteries (PICA) may cause normal filling defects. On lateral views, the transverse ligament of the atlas may cause a filling defect posterior to the odontoid peg.

Thoracic On the AP view a thin line of contrast on each side outlines the spinal cord, which fills most of the interpedicular distance. The path of the nerve roots is, as a result, short but somewhat more vertical than in the cervical region. On lateral views the cord is seen to be more anterior in position than in the cervical region.

Lumbar (Fig. 3. 19) The spinal cord tapers suddenly to end at L1/L2 level as the conus medullaris. The filum terminale can be seen as a long, thin, central filling defect extending from the conus to the end of the thecal sac. It is surrounded by the nerve roots - the cauda equina - which lie laterally until they reach their exit points. The S1 nerve root can be identified passing to the first sacral foramen. The lumbar roots pass around beneath the pedicle of the correspondingly numbered vertebra. Roots are compressed by a protrusion of the disc above the exit level (e. g. L5 root is compressed by a disc at L4/5 level).

The level at which the thecal sac ends is variable and may lie at L5/S1 or extend to S2. The thecal sac fills 30-80% of the interpedicular distance and is also variable on the lateral view, usually lying close to the vertebral bodies and discs, but occasionally a wide epidural space separates the thecal sac from the L5/S1 disc space and may hide a disc protrusion here.

In lumbar myelography in the young subject the cord is seen to extend to a lower level than in the adult. The normal conus may have a bulbous appearance in the child.

Computed tomography (see Figs 3. 10 and 3. 11) CT of the spinal cord can be performed w i th or without intrathecal contrast. In non-contrast views low-density epidural fat outlines the thecal sac. The CSF is of water density and the higher-density cord is seen within it. The cord is elliptical in cross-section, w i th its wide axis lying transversely in the cervical region and in the centre of the subarachnoid space. In the thoracic region the cord is circular in cross-section and lies more anteriorly. No cord is visible in cuts taken below the level of L2.

Without intrathecal contrast, nerve roots are difficult to see within the thecal sac; however, at the intervertebral foramen they can be seen, as here they are outlined by fat. For the same reason, the dorsal root ganglion may also be seen here. With contrast, the sheath of the nerve roots f i ll and the individual roots of the cauda equina may also be seen.