Infective lesions

Infections of the spine are uncommon, but can usefully be classified as either vertebral osteomyelitis or intraspinal infection. Vertebral osteomyelitis is the more common variety of infection and can lead to intraspinal infection. "Pure" intraspinal infection implies no associated infection of the vertebral column and includes extradural, subdural, or intramedullary abscess in descending order of frequency.36,37 Intraspinal infections occur at a frequency of approximately one per million per year in United Kingdom neurosurgical units and are predominantly caused by pyogenic organisms, usually Staphylococcus sp., whereas in Asia or Africa Mycobacterium tuberculosis is the common infecting organism.38 An abscess may be found anywhere within the spinal epidural space but the most common location is posterior where the epidural space is most prominent and thoracic and lumbar levels are most commonly affected (Figure 10.5). The spinal epidural space does not communicate with the

Figure 10.5 MRI (1-5T Siemens Magnetom, T1 weighted scan with gadolinium enhancement). Spinal epidural abscess in the lumbar spine presenting as a complete cauda equina syndrome

intracranial extradural space and has no true intracranial analogue because at the foramen magnum the outer, endosteal layer of the intracranial dura adheres to bone and becomes continuous with the pericranium. The spinal dura is a single layer continuous with the inner, meningeal layer of the intracranial dura. Cervical epidural abscess is uncommon and is usually associated with vertebral osteomyelitis. The abscess may extend over few or many vertebral levels, and it is well recognised that non-contiguous abscesses may occur.39,40 A spinal epidural abscess is a neurosurgical emergency and is one of the few instances where the history and clinical examination may provide the pathological diagnosis. The patient may or may not present with systemic infective illness, but as the abscess enlarges and compression of the spinal cord occurs then myelopathic symptoms gradually develop, usually over the course of a small number of days.41-43 The outstanding clinical feature is spinal pain and the marked local tenderness of the spine that may be elicited by lightly tapping the spinous processes with a tendon hammer. Several studies have reported the frequent misdiagnosis of this condition in its early stages. A complaint of spinal pain, particularly in younger patients, and of thoracic pain in all patients should be regarded seriously and not be attributed to minor "mechanical dysfunction". The presence of neurological symptoms may occur before neurological signs and it is at this stage that the chance of surgical cure without residual disability is highest. The patient presenting with paraplegia should be regarded as a failure of diagnosis, as symptoms are frequently present for several days before this end stage.

The most valuable investigation is high grade MRI.44 This confirms the diagnosis of epidural abscess, indicates its upper and lower limits, demonstrates whether or not there is an associated primary osteomyelitis, and in most cases differentiates between epidural abscess and subdural abscess. Myelography with CT is the next best diagnostic investigation, although the distinction between abscess, haematoma or even in some cases metastatic tumour is not always possible.

Investigations should be carried out without delay and managed by immediate surgical decompression. As abscesses are very commonly located in the posterior extradural space, albeit extending laterally on either side, a decompressive laminectomy is the operation of choice. Attention should be paid to the upper and lower limits of the abscess, although the laminectomy may not need to be taken to these extremes should the pus be liquid. The laminectomy may need to be taken to the limits if the compressive material is semisolid, infected granulation tissue.

The poor results of surgery in severely affected or paraplegic patients underline the urgency of timely referral. The mortality rate for this condition has diminished over the years with improving diagnosis, but nevertheless remains substantial for a spinal condition and is reported at between 17% and 36%.38,45

Vertebral osteomyelitis is a relatively uncommon condition, but familiar to spine surgeons in the United Kingdom. The commonest organisms involved are staphylococci, streptococci, Escherichia coli, and occasionally an unusual organism such as Salmonella or Brucella sp. For the most part the spinal cord is not affected and the problem is one of structural integrity in

Figure 10.6 MRI (1-5T Siemens Magnetom, T2 weighted scan). Vertebral osteomyelitis secondary to tuberculosis and associated with anterior cord compression at C3 and C4

the vertebral column.46 Both pyogenic and tuberculous osteomyelitis may lead to the formation of an extradural abscess lying anterior to the spinal cord, which may cause acute spinal cord compression in fashion not dissimilar to a "pure" epidural abscess. The gradual and progressive nature of bone infection produces a more protracted clinical history, including spinal pain, aches, malaise, discomfort, and systemic signs of infection culminating in neurological deterioration (Figure 10.6).

Plain radiographs of the spine may give the diagnosis by revealing bone erosion and loss of height in contiguous vertebrae with destruction of the intervertebral disc. The diagnosis of coexisting extradural abscess is made by MRI or by CT myelography done as a matter of some urgency. It requires only a few millilitres of localised liquefied pus in the anterior cervical spinal canal to cause tetraparesis. Recovery is entirely possible with expeditious diagnosis and decompression. Scanning also reveals the extent of vertebral and paravertebral infection and can usually distinguish infection from metastatic disease.47

For pyogenic osteomyelitis, which predominates in the United Kingdom, direct ventral spinal canal decompression is often required. In the cervical spine this involves an anterior cervical decompression procedure carried out through an intervertebral disc space or by a vertebrectomy. Division of the posterior longitudinal ligament usually results in the egress of a small volume of liquefied pus that is sometimes less than impressive considering the degree of paralysis that it has produced. In these acute situations the surgeon is best advised not to incorporate spinal reconstruction, but rather to return the patient to a period of external spinal stabilisation, usually by cervical traction, accompanied by appropriate intravenous antibiotic therapy. Once the infection has been treated in such a way for a period of several days, more definitive spinal stabilisation can be established. Once the pyogenic infection is under control, bone grafting almost always results in a solid fusion. In selected cases it is accepted practice to use internal metal fixation while continuing antibiotic treatment. As in osteomyelitis at any location, a prolonged course of antibiotics is required. The erythrocite sedimentation rate (ESR) and C-reactive protein (CRP) are useful parameters to follow during the course of treatment.

In the thoracic and lumbar spine surgical decompression involves more difficult access, either by costotransversectomy or posterior thoracotomy in the thoracic region or the extraperitoneal route in the lumbar region. In tuberculous osteomyelitis a limited posterolateral decompression by costotransversectomy to release indolent purulent material is a satisfactory means of decompression, with reconstruction and stabilising surgery reserved for later if necessary.48

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