SPECIFIC LABORATORY TESTS

While the diagnostic sieve usually involves the above modalities to narrow down the differential diagnosis, there is a place for specific blood tests, which either confirm or refute specific diagnostic possibilities. An example of this would be dementia, where it would be appropriate to perform a blood screen to look for treatable causes of dementia, such as vitamin B12 deficiency and thyroid function. Depending on the clinical presentation, there may be a need to look for specific diseases. For example, if abnormal movements accompanied dementia, this would raise the possibility of Huntington’s disease, and therefore genetic screening for this disorder would be necessary. In similar vein, if there were a family history of dementia, migraine, and stroke-like episodes, and if characteristic white matter lesions were seen on MRI, then it would be appropriate to test for the notch 3 mutation on chromosome 19, which would lead to a diagnosis of cerebral autosomal dominant arteriopathy with subcortical infarcts and leucoencephalopathy (CADASIL).

41

INVESTIGATING THE SPINAL CORD

NORMAL ANATOMY

The vertebral column comprises 7 cervical, 12 thoracic, 5 lumbar, 5 sacral, and 4 coccygeal vertebrae. The sacral bones fuse to form the sacrum and the coccygeal bones the coccyx (42).

With the exception of both the atlas (C1) and the axis (C2), all vertebrae share common anatomical features. Each is made up of a body, pedicles, and pairs of facets. The superior facet articulates with the inferior facet by means of the pars interarticularis (pars: part; inter: between; articularis: articulating surface). Finally, each vertebra possesses two laminae and a spinous process. The spinal cord, conus, and cauda equina lie within the vertebral canal and emerging nerve roots pass within the neural foramina. The C1 nerve root

passes superiorly above the atlas, the C5 root passes above the body of C5 into the C4–5 neural foramen, the C6 root passes above C6 into the C5–6 foramen, and so on. The lowest cervical nerve root, C8, passes below the body of C7 and outwards through the neural foramen of C7–T1. The presence of the C8 root means that the T1 thoracic nerve root passes below the body of T1 through the foramen of T1–2 and all subsequent nerve roots in this manner. The last (T12) nerve root similarly passes below its vertebral body and then through the foramen of T12–L1. The first lumbar root likewise passes below L1 into the L1–2 neural foramen and the L5 root passes below the body of L5 into the L5–S1 neural foramen.

In adults, the tip of the conus lies at or above the L2–L3 intervertebral disc space. The filum terminale, a thin glial-ependymal cord, is a direct continuation of the conus terminating in the posterior aspect of the coccyx. It measures <2 mm in diameter and, if larger, is regarded as a thickened filum often associated with developmental neurological deficits (spinal dysraphism). Not infrequently, the filum contains a small amount of fat tissue, considered a normal anatomical variant of no clinical significance, though large masses, spinal lipomas, can occur.

Understanding the relationships between the vertebral bodies and emerging nerves is indispensable for the correct correlation between the clinical examination and radiological findings.

Knowledge of spinal cord blood supply is helpful in the diagnosis of ‘spinal stroke’ but also in the interpretation of spinal angiography, a highly specialized infrequently performed investigation (43, 44). Two paired posterior spinal arteries supply the spinal cord, arising from the posterior inferior cerebellar artery and merging to form a plexus of vessels on the posterior surface of the cord. This rich blood supply ensures protection against ischaemia and explains the relative sparing of the posterior spinal cord (dorsal columns) in spinal strokes. The arterial blood supply to the anterior two-thirds of the spinal cord is much more vulnerable, being through a single vessel. The anterior spinal artery arises by fusion of a branch from each vertebral artery. Seven to 10 unpaired radicular arteries leave it as it descends the cord. The largest, variably arising between T9–T12 levels, is called the artery of Adamkiewicz. This less efficient circulation renders the spinal cord most vulnerable to ischaemia at the ‘watershed level’ T8.

RADIOLOGY

The role of plain radiology in the diagnosis of spinal cord disease has become less valuable, particularly with the increasing accessibility of CT and MRI. Degenerative changes can be anticipated with increasing age and the radiological presence of cervical or lumbar spondylosis does not, in isolation, result in a clinical diagnosis. A complete radiological cervical spine assessment consists of lateral, anteroposterior (AP), open-mouth, and right and left oblique views. The thoracic spine series consists of an AP view and a lateral view, though the upper part of the thoracic spine is very difficult to visualize on the lateral view because of the overlying shoulders. A complete lumbosacral spine series consists of AP, lateral, coned-down lateral, and right and left oblique views (oblique views are useful in evaluating the pars interarticularis for spondylolysis). Guidelines on the use of plain films must be followed, thus limiting unnecessary radiation exposure.

CONTRAST MYELOGRAPHY

Contrast myelography in combination with CT remains a useful investigation if MRI is contraindicated due to a pacemaker or other reason (45). Indeed, some neurosurgeons still rely on this investigation to visualize small disc fragments in the lateral spinal recesses. Nonionic water-soluble contrast agents are used. A history of allergy or prior contrast reaction should be enquired into (premedication with antihistamines and steroids may be necessary). Contrast is injected at the L3/4 level slowly over 1–2 minutes. Alternatively, a lateral C1–2 puncture can be performed with care to prevent intracranial flow (acute neurotoxicity). Postmyelography, patients are maintained at a 30–45° head up position to further prevent this.

SPINAL CT

CT is still preferred in evaluating acute spinal trauma. Fractures are identified as lucencies without sclerotic margins, and bone impingement on the spinal canal is clearly visualized (46). Images are generally obtained in the axial plane with angulation of the scanning gantry where necessary. The spine is best viewed when the gantry is perpendicular to the area of interest. In the lumbar region, this necessitates considerable angulation to evaluate the lumbar discs. Images may be obtained using 1–10 mm-thick slices and sagittal reconstruction can be made; however, spinal MRI has all but ruled out the need for these. CT can provide helpful information on vertebral

Cervical arteries arise from vertebral and subclavian vessels, form plexuses and supply the cervical and upper thoracic cord

T1 Intercostal artery branches supply

the midthoracic cord

Anterior spinal artery is at its

narrowest at T8. This level of the 7 spinal cord is liable to damage

during hypertension – watershed area

10

Artery of Adamkiewicz, the largest radicular artery, supplies the low thoracic and lumbar cord. It usually arises around T9–T12 levels and is on the left side in 70% of the population

L5

Sacral arteries arise from the hypogastric artery and supply the sacral cord and cauda equina

Anterior radicular branches joining anterior spinal artery

43 Sagittal diagram of the blood supply to the spinal cord.

Virtually no anastomotic communication

Anterior spinal artery territory

Penetrating branches – anterior and part of posterior grey matter

Circumferential branches – anterior white matter.

– Anterior two-thirds of spinal cord

44 Axial figure of the spinal cord, illustrating anterior and posterior spinal artery territories.

body disease (tumours and infections) and may be complementary to MRI, but the latter is generally the spinal imaging modality of choice.

SPINAL MRI

MRI has revolutionized the investigation of disease of the spinal cord, and recent advances have shortened data acquisition times and reduced artefacts. Its noninvasive ability to image the spinal cord and surrounding structures (CSF, dura, and ligaments) makes it indispensable in the investigative workup. Classically there are three locations for spinal neurological disease, extradural, intradural/ extramedullary, and intramedullary (47). MRI has the capability to evaluate all three locations.

A good working knowledge of basic MR imaging principles and spine anatomy is essential for understanding the imaging observations seen in various spine lesions.

For example, because of their anterior oblique orientation, the neural foramina in the cervical spine are exceedingly difficult to see on a sagittal image and are best seen on oblique sagittal or axial views. Conversely in the lumbosacral region, the neural foramina are laterally orientated and therefore are properly visualized on a sagittal T1-weighted image (48). Anatomic changes inevitably occur to the neural foramen with ageing. Posterior disk bulge, lateral

herniated disk, together with vertebral body osteophytes can encroach on the neural foramen and its contents. Although MRI is poor in showing dense normal cortical bone, it shows infiltrative/infective disease well. Contrast-medium enhanced imaging can help differentiate tumours, inflammation, and active demyelinating diseases.

A common mistake is failure to order views that display the likely site(s) of the lesion. In most cases, careful history and examination will decide which of cervical, thoracic, or lumbosacral levels are to be imaged. However, this is not always possible and ‘full spinal imaging’ should then be requested. Some disease processes can be multilevel (multiple sclerosis [MS]) and, on occasion, the common (lumbar disc disease) and the rare (thoracic meningioma) may coexist. In any patient with a spinal cord syndrome of unknown cause, the clinician must consider the possibility of a lesion at the level of the foramen magnum, a level that is neither spinal nor cranial and consequently is often overlooked. Imaging here should pay particular attention to the possibility of malformations (Chiari malformations) and the location of the odontoid process (rheumatoid arthritis). A benign tumour of the foramen magnum such as meningioma can be confused with MS or degenerative cord diseases and, prior to correct diagnosis, progressive disability passes beyond the point of reversibility.

SPINAL ANGIOGRAPHY

While in the head CT and MR angiography have increasingly replaced digital subtraction angiography (DSA), this is not the case with the spine. Resolution and the complexity of the spinal circulation mean that conventional angiography still remains the gold standard. However, the indications for spinal angiography are few (spinal vascular lesions such as dural fistula). This infrequently performed investigation requires considerable technical skill, and on occasion, selective cannulation of several radicular vessels may be required to demonstrate a dural fistula. CT angiography may show large dural fistulas (49).

NEUROPHYSIOLOGY

Neurophysiological investigations such as EMG can be helpful in localizing the level of a lesion if this is not apparent on imaging. It is a valuable adjunct to imaging where there is uncertainty about the true significance of a compressed nerve root (cervical or lumbar radiculopathy).

Nerve conduction studies are of value in differentiating peripheral nerve from nerve root (radicular) lesions. Radicular lesions cause a segmental distribution of muscle involvement, i.e. affect muscles supplied by the same root, whereas peripheral nerve lesions may affect muscles supplied by that nerve, which may be innervated by several nerve roots. Reduction in nerve conduction velocity may indicate peripheral neuropathy. Fasciculation and fibrillations can be detected in muscles that