Chronic disk degeneration is the most common cause of cervical spondylotic myelopathy in patients over age 55 years.3,4 Patients with a congenitally narrow vertebral canal (less than 13 mm anteroposterior [AP] diameter compared with the normal diameter of 17 to 18 mm) may be predisposed to cervical spondylotic myelopathy; the two conditions often occur together.7,8 The degree of compression is often proportional to the severity of the clinical presentation, yet remains unpredictable.8
A high clinical suspicion should guide further investigation. Patients with mild symptoms may present with complaints of increasing gait or balance disturbance, or difficulty negotiating fine-motor tasks such as buttoning clothing, writing, or handling a telephone or cutlery.2,7,8 Although most patients present with a pure myelopathy, as many as 41% may present with a concomitant radiculopathy, causing pain, paresthesias, or weakness in a dermatomal pattern, possibly with muscle wasting and hyporeflexia.10 Signs of pure myelopathy include upper limb hyperreflexia (depending on whether the level of compression is at or above the corresponding upper limb nerve root), atrophy of the intrinsic muscles of the hand, difficulty with rapid grip and release exercises of the hands, and a Hoffman sign (thumb and finger flexion when the nail bed of the ipsilateral long finger is flicked). Other common signs of significant myelopathy include gait disturbance with lower extremity spasticity and a scissor gait. Lower limb reflexes are generally exaggerated with an upgoing toe to plantar or lateral malleolus stimula-tion, known as Babinski and Chaddock signs, respectively. Ankle clonus also may be present.3,4,7 Patients with advanced and severe myelopathy may have bowel or, more commonly, bladder urgency and incontinence. Sensory signs and symptoms often are a less-prominent feature of myelopathic disease. When present in the upper extremities, features include signs of lateral spinothalamic tract involvement with impaired pain and/or temperature discrimination. In the lower extremities, difficulty with tandem gait, a positive Romberg sign, and/or impairment of vibratory or joint position sense indicate involvement of the dorsal column.1,7 Funicular, or central, burning, lancinating pain also may be present (Table 1).
SCORING OF SEVERITY
The Nurick, Japanese Orthopaedic Association and modified Japanese Orthopaedic Association scores, Cooper myelopathy scale, Prolo score, and European myelopathy score are commonly used scoring systems to objectively quantify disease severity. These are valuable tools in the initial assessment and subsequent evolution of a patient's condition and may be used in concert with a detailed neurologic examination.10-12
Physician assistants (PAs) must remain attuned to other disease processes with presentations similar to cervical spondylotic myelopathy. The differential diagnosis includes peripheral neuropathy, brachial or lumbar plexopathy, multiple sclerosis, and motor neuron diseases such as amyotrophic lateral sclerosis. Trauma, Lyme disease, neurosyphilis, vascular insufficiency, subacute combined degeneration of the cord secondary to pernicious anemia, and HIV-associated myelopathy also may present with myelopathic features. These conditions can be distinguished from cervical spondylotic myelopathy via history and examination, nerve conduction tests, electromyography, and/or hematobiochemical studies.1,3,6,7
Plain radiographs of the cervical spine often are the first diagnostic test acquired. Radiographs are useful for assessing degree of spondylosis, spondylolisthesis, loss of lordosis, spontaneous fusion, osteophyte formation, and ossification of the ligamentum flavum or posterior longitudinal ligament.1,4,7 Radiographs obtained in flexion and extension are important to evaluate for cervical spine instability that can cause dynamic compression of the spinal cord or nerve roots. Stenosis also can be noted with plain radiographs by measuring the vertebral canal AP diameter; a measurement of less than 10 mm is considered absolute stenosis.7 Relative stenosis is defined as less than 13 mm diameter; normal sagittal diameter is 17 to 18 mm.7 The Torg-Pavlov ratio often is used to evaluate stenosis by measuring ratio of the AP diameter of the vertebral canal at the mid vertebral body level to the AP dimension of the vertebral body at the same level. A Torq-Pavlov a ratio of less than 0.82 indicates stenosis.13
Oblique radiographs allow direct visualization of the neural foramen to diagnose bony foraminal stenosis. The gold standard for diagnostic testing in cervical spondylotic myelopathy is MRI because of its ability to direct-ly demonstrate the degree and location of spinal cord compression. In reviewing the relationship between bony structures, intervertebral disks, and neural tissue, the level or levels of compression can be confirmed. The MRI also allows visualization of ligamentous hypertrophy and edema in the spinal cord.4 In patients with a contraindication to MRI or metallic imaging artifact, CT with myelography may be used. This test is more invasive than MRI and involves the injection of contrast dye into the subarachnoid space via the lumbar or cervical spine.
The natural history of cervical spondylotic myelopathy remains incompletely understood. This makes it difficult to predict the effectiveness of conservative treatment. Conservative therapy options consist of a rigid cervical collar, physiotherapy, pharmacotherapy, behavior modification, a “wait and watch” strategy of clinical and radiographic surveillance, or any combination of these options.1,4,6,7 Conservative therapy generally is reserved for patients who are mildly affected, stable, and without significant neurologic signs. This therapy also is reserved for neurologically stable patients with serious comorbid conditions that would place them at increased surgical risk.1,7
The decision to proceed with surgery requires an open and comprehensive discussion with the patient, explaining that the objective of surgery is to prevent disease progression and to stabilize the cervical spine; returning the patient to prepathology baseline is sometimes an unattainable outcome. This is especially true in patients with nonprogressive disease. A prime indication for surgical intervention or spinal cord decompression is progressive, symptomatic myelopathy—that is, worsening neurologic deficit with radiologic confirmation of cervical spondylotic myelopathy and exclusion of other possible contributing pathologies.1,4,6,7,9 An upgoing plantar response is another important sign of the need for surgery.
Decompressive surgery for treatment of cervical spondylotic myelopathy can be performed either via anterior or posterior approaches and may require spinal fusion. In general, the accepted trend is that patients with ventral compression, kyphosis, and/or pathology at one to three levels be treated anteriorly.1,7,9,10,14 Anterior decompression necessitates removal of the intervertebral disk. Historically, fusion was not always performed and patients would do well until they developed disk space collapse with subsequent kyphosis and foraminal stenosis, leading to recurrent symptoms. Contemporary anterior decompression includes anterior fusion to prevent late failures. The disk or vertebral body is replaced with an intervertebral cadaveric bone graft or iliac crest autograft. A rigid cervical collar can be considered on one-level anterior diskectomies, but internal fixation via anterior plating eliminates the need for an orthosis, and most patients choose to have anterior plating (Figures 4 and 5).
For multilevel anterior fusions, instrumentation advances have led to better fusion rates. More complex pathology consisting of irreducible kyphosis with or without significant osteophytic involvement, particularly of the posterior longitudinal ligament, may require a corpectomy. Corpectomy involves removal of a vertebral body, to be subsequently replaced with a graft or cage to maintain vertebral height and maintain lordosis. This is followed by anterior plating.1,4,7,9,14 Several choices for vertebral body replacement are used and include iliac crest autograft, fibular allograft, and polyetheretherketone (PEEK) or titanium cages.
Patients with multilevel disease, significant ligamentum flavum hypertrophy, or a congenitally narrow vertebral canal are more likely candidates for a posterior approach.1,7,9,10 Posterior decompression may be completed by laminectomy (often with fusion) or laminoplasty. A laminectomy involves removing the posterior elements including bilateral laminae, spinous processes, and ligamentum flavum in order to maximize the vertebral canal diameter. This often is accompanied by placement of lateral mass screws and connecting rods to accomplish a fusion for stability. Fusion is especially important to consider in patients with kyphosis, loss of lordosis, or if instability has been demonstrated on dynamic radiographs preoperatively (Figure 3). In patients with well-preserved lordosis, a laminoplasty may be an effective choice. A laminoplasty similarly expands the vertebral canal diameter, but rather than removing vertebral elements, the laminae are hinged open, displaced laterally or posterolaterally, and held open either by incorporating a graft or fixing the lamina to the adjacent structures with sutures or plates. This maintains both mobility and a level of protection to the thecal sac posteriorly.1,4,7,10,14
Laminectomy or laminoplasty may be performed as standalone procedures in patients with maintained cervical lordosis and no evidence of instability. Although some experts have suggested that laminoplasty offers an advantage in preservation of lordosis compared with laminectomy, meta-analysis data suggest that patients undergoing either of these procedures are at risk of developing kyphotic deformity postoperatively. This risk can be eliminated by pairing laminectomy or laminoplasty with a fusion via lateral mass screws and rods.14,15
In some cases, combining an anterior and posterior approach is indicated (Figure 5). These more difficult cases include patients with compressive pathology both anteriorly and posteriorly; those who need a multilevel corpectomy; those with significant kyphotic deformity, compromised bone integrity due to osteoporosis, renal disease, diabetes, or a history of tobacco smoking; and those with other factors resulting in diffuse vertebral involvement contributing to cervical spondylotic myelopathy.14,16
Significant risks associated with either anterior or posterior approaches must be taken into consideration, including permanent neurologic deficit, osteomyelitis or diskitis, cerebrospinal fluid leak, meningitis, pseudoarthrosis, gait disturbance, bowel or bladder dysfunction, injury to the carotid and/or vertebral arteries, injury to the trachea and/or esophagus, recurrent laryngeal nerve palsy, dislodgement of bone grafts, dislodgement of plates and screws, adjacent segment disease, need for future surgery, or failure of the operative procedure to improve symptoms.1,7,14,16 Higher rates of complications are associated with perioperative blood loss, increased intraoperative time, combined anterior and posterior approach, and increased patient age.1,14,17
Several studies concluded that, over time, there was no significant difference in outcome in patients who were treated conservatively versus those treated surgically.1,3,6,18,19 However, other studies have found that the patients with cervical spondylotic myelopathy who undergo surgical decompression of the cervical spine, anteriorly, posteriorly, or circumferentially, show significant improvement, even in patients over age 75 years.14,20-22 The conflicting outcomes may be related to the highly variable constellation of signs and symptoms, degree of disease progression at the time of patient presentation, and unpredictable clinical course, all of which present challenges for researchers. Current practice appears to be trending toward earlier surgical intervention. Most recently, a prospective multicenter study revealed a 75% positive response rate in patients treated surgically, defined as a more than 2-point increase in modified Japanese Orthopaedic Association score, and a 49% success rate, defined as an increase to 17 or 18 on the scoring system at 12 months postoperatively.14,20,23 Improvement was demonstrated in all outcome measures including an average 2.88-point improvement in modified Japanese Orthopaedic Association score and 1.59-point improvement in Nurick grade. Improvement in modified Japanese Orthopaedic Association scores was significantly higher in patients with severe disease versus mild or moderate disease; no significant difference in degree of improvement in other outcome measures was found between patient groups.20
Positive predictors of postoperative outcome include younger age, shorter duration of symptomatic pathology, increased compression ratio (defined as a greater than 40% increase in postoperative AP diameter of the spinal cord), and resolution of abnormal spinal cord signal change on postoperative MRI of the cervical spine.1,4,6,14
The general consensus is that patients with clinically progressive myelopathic signs and symptoms in addition to radiographic evidence of cervical spondylotic myelopathy are candidates for decompressive surgery. In the instances of patients continuing to deteriorate neurologically postoperatively, despite MRI evidence of adequate and complete decompression, further investigations may be required to rule out additional causes.
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Keywords:Copyright © 2015 American Academy of Physician Assistants
cervical spondylotic myelopathy; spine; central vertebral canal; stenosis; decompressive surgery; laminectomy