Degenerative pathology

Degenerative change within the spine causes acute spinal cord compression rarely, usually having a more prolonged course of progressive neurological symptoms.49 Protrusion of an intervertebral disc into the spinal canal, whether in the presence of existing osteophytic compression or not, can produce a rapidly developing myelopathy. Acute disc protrusions most commonly occur in the lumbar spine with 90% being located either at the L4-L5 or L5-S1 level. Central compression of the thecal sac at this level causes an acute cauda equina syndrome, which is a spinal emergency. Isolated or "pure" cervical intervertebral disc protrusion is much less common than compression due to osteophyte formation, although it is common to find a combination of both. "Pure" cervical disc prolapse can occur at any age, and in some cases a pre-existing mild myelopathy rapidly becomes much worse because of protrusion of cervical disc a few millimetres further into the spinal canal. Acute myelopathy may present in older patients who have degenerative changes superimposed on a developmentally narrow cervical spinal canal (< 10 mm anteroposterior diameter). In these patients the usual mechanism is violent extension of the neck.50 "Pure" thoracic disc protrusion is uncommon and these patients present to neurosurgical units at a frequency of one per million population per year.51 It is common to find that the so-called thoracic "disc" is in fact a combination of osteophyte and calcified disc material, rather than degenerate nucleus pulposus that has prolapsed into the spinal canal.

Acute disc protrusion at any level commonly presents with spinal pain, usually accompanied by nerve root pain. The level of root pain provides a good indication of which intervertebral disc is the culprit. When the disc protrudes sufficiently into the spinal canal to cause spinal (or cauda equina) compression, neurological symptoms and signs accompany the pain. The sensory level gives a good indication of the level of thoracic disc prolapse and the distribution of pain, numbness, or "dropped" reflexes in the upper limbs provides a good clinical indicator of the level of cervical disc disease. When the patient presents with loss of manual dexterity the compression is often at the level of the C3-C4 intervertebral disc. In the lumbar spine, compression of the cauda equina produces loss of sensation across the sacral dermotomes, either unilaterally or bilaterally, associated with nerve root pain in both legs and loss of bladder sensation and sphincter function. The clinical diagnosis of an acute disc

Figure 10.7 MRI (1-5T Siemens Magnetom, T1 weighted sagittal image). Acute L5-S1 disc herniation presenting as an acute cauda equina syndrome.

The image shows severe compression of the thecal sac by soft disc

Figure 10.7 MRI (1-5T Siemens Magnetom, T1 weighted sagittal image). Acute L5-S1 disc herniation presenting as an acute cauda equina syndrome.

The image shows severe compression of the thecal sac by soft disc prolapse in the cervical and lumbar spine is relatively straightforward, but misdiagnosis often occurs in the thoracic region. A background history of thoracic spine pain should raise suspicion, but several studies have confirmed that, probably because of the rarity of thoracic disc disease, other pathological diagnoses are often given prior consideration.

The diagnosis of disc prolapse is confirmed radiologically by MRI of the appropriate spinal region (Figures 10.7, 10.8). CT myelography may also be used though with modern CT scanners contrast is usually not required to confirm lumbar disc prolapse.52

Acute myelopathy or cauda equina syndrome secondary to disc prolapse requires urgent surgical decompression. In the

Figure 10.8 MRI (1-5T Philips T2 weighted scan). Anterior spinal cord compression by a degenerative disc at C3/4 presenting as a rapidly progressive myelopathy

lumbar spine this may be carried out through a posterior approach as for elective microdiscectomy because the thecal sac will tolerate some retraction to reach the disc space. Occasionally the access has to be increased by either a hemilaminectomy or, on rare occasions, a full laminectomy, especially in the presence of coexisting lumbar canal stenosis. The risk of a postsurgical cauda equina syndrome is about one in 500 lumbar disc operations.53

In the cervical spine, the disc must be approached anteriorly as the spinal cord cannot be retracted at this level. The standard approach to the anterior subaxial spine is the plane between the carotid sheath laterally and the pharynx/larynx medially. When the compression is due to disc prolapse alone, a simple disc excision may be sufficient. This is carried out using an operating microscope with excision of the disc material and the cartilaginous endplates down to the level of the posterior longitudinal ligament and laterally to the medial part of the uncovertebral joints. It is necessary to open the posterior longitudinal ligament to directly visualise and explore the extradural space for disc material that can find its way through the longitudinally orientated fibres of this ligament. Fusion may be carried out either by a variety of techniques using iliac crest graft or specially designed implants but is not always necessary. The use of a bone fusion does not confer additional neurological recovery, although it may be required (or preferred) if spinal stability is significantly degraded.

For those patients who develop an acute myelopathy due to a hyperextension injury superimposed on a narrow spinal canal, there is little, if any, convincing evidence that surgical decompression improves neurological recovery. About 60% of these patients improve neurologically in the natural course of the condition. A few patients who have a narrow cervical spinal canal develop an acute non-traumatic myelopathy. Laminectomy or multiple level anterior decompression and fusion are the surgical alternatives, although the presumed ischaemic pathogenesis does not incline to the favourable outcome associated with slowly developing spondylotic myelopathy.49

For a thoracic disc protrusion the surgical access must be either lateral or anterior or a combination of both. The recommended approaches to the thoracic spinal canal give access to the disc prolapse without requiring retraction of the spinal cord and allow the disc material to be removed in a direction away from the spinal dura. They are pediculectomy, costotransversectomy, or transthoracic partial vertebrectomy.51,54,55 A laminectomy is only indicated for thoracic disc prolapse if the disc material is entirely free and is located lateral to the spinal cord. This does happen, but rarely. A laminectomy approach to an anteriorly placed thoracic disc prolapse invites major neurological deterioration.

For patients who present with an acute disc prolapse causing cauda equina syndrome, prognosis for recovery is primarily based on the severity of the preoperative neurological deficit rather than the duration of neurological symptoms, although it is difficult to separate these two components entirely.56 The

Figure 10.9 Axial CT scan. Bone windows through a metastatic breast tumour in the thoracic spine. Destruction of the posterior elements and pedicles is demonstrated

same is likely to apply to prognosis for recovery from myelopathy secondary to cervical or thoracic discs. Good recovery can only be expected for expeditiously diagnosed and optimally managed patients.

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