Management of Cervical Spondylotic Myelopathy

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Chapter 156 Management of Cervical Spondylotic Myelopathy

Cervical spondylotic myelopathy (CSM) is caused by a reduction of the sagittal diameter of the cervical spinal canal as a result of congenital and degenerative changes in the cervical spine.13 It is the most common type of spinal cord dysfunction in patients over the age of 55 years and it is the most common cause of acquired spastic paraparesis (quadriparesis) in adults.4,5 Risk factors for spondylosis include cigarette smoking, frequent lifting, and diving.6

Degenerative changes of cervical spondylosis occur in the five articulations that comprise the cervical motion segment: the intervertebral disc, the two facet joints, and the two false uncovertebral joints (of Luschka). The normal aging process of the spine results initially in disc desiccation and results in loss of disc height. This change is thought to bring the uncovertebral joints into contact, thereby disrupting the normal biomechanics of the facet joints. Osteophyte formation, ligamentum flavum hypertrophy, and facet and uncovertebral joint eburnation may then occur as a reaction to the abnormal biomechanics.6,7 These degenerative changes most commonly occur at C5–C6 and C6–C7.8 White and Panjabi classify these changes as “static” factors involved in the pathogenesis of cervical spondylotic myelopathy.9 Also included in this category are congenital spinal canal stenosis (less than 13 mm anteroposterior diameter) and disc herniation. White and Panjabi also describe “dynamic” factors, which are abnormal forces placed on the spinal column and cord during flexion and extension under normal physiologic loads. For example, repetitive traumatic compression of the spinal cord against an osteophyte during normal flexion and extension of the cervical spine is defined as a dynamic factor.4,10,11

In addition to the direct mechanical compression of neural elements, CSM may also be the result of spinal cord ischemia. Ischemia can result from compression of large arterial feeders to the spinal cord such as the anterior spinal artery, reduced flow through the penetrating arteries of the spinal cord or the pial plexuses, or impairment in venous outflow resulting in venous hypertension.4,1214

The natural history of CSM appears to be one of progressive disability.15,16 It should be noted that there are some limitations in determining this natural history; studies are hampered by bias, heterogeneous patient populations, lack of long-term follow-up, and the use of qualitative rather than quantitative outcome measures.17 In fact, Clarke and Robinson asserted that once the disorder was recognized, neurologic function did not return to normal.3 In their series, 75% had episodic progression; 20% had slow, steady progression; and 5% had a rapid onset of symptoms followed by a prolonged period of stable disease. Nurick observed that patients with mild disability tended to have a better prognosis while patients over the age of 60 years tended to progress.18 Moreover, patients who have had symptoms longer than 2 years show no improvement despite treatment.4,19

Symptom duration may have a negative impact on natural history. Sadasivan et al. reported on 22 patients with symptomatic CSM for an average 6.3 years.20 All patients were reported to be Nurick II myelopathy at diagnosis. All progressed over the study period, one to grade III, 17 to grade IV, and 4 to grade V. Gait was involved in 100%, weakness in 45%, and determity in 72% of patients.

Pathologic Anatomy and Cervical Spondylotic Myelopathy

The patient with cervical spondylosis presents to the physician with complaints of neck pain related to degenerative changes, neurologic symptoms from spinal cord and nerve root compression, or a combination of complaints. CSM is the clinical result of spinal cord compression, and there are many factors with the potential to contribute to decreased space available for the spinal cord within the spinal canal.

There are static, degenerative factors, including intervertebral disc bulging, dorsal vertebral body osteophytes, ossification of the posterior longitudinal ligament, uncovertebral and facet joint hypertrophy, ligamentum flavum, and facet joint capsule laxity and infolding. Collapse of the intervertebral disc space has the potential to lead to a degenerative cascade. As the disc-space height decreases, the uncovertebral joints approximate and are loaded abnormally, increasing the likelihood of degenerative spurring. Disc space collapse also causes a rostral–caudal translation, leading to similar degenerative spurring in the facet joints and laxity and buckling of the facet capsules and ligamentum flavum. All of these contribute to static narrowing of the spinal canal (Fig. 156-1).

Dynamic factors include abnormal translation or angulation during flexion and extension of the cervical spine. Flexion can increase spinal cord compression in the presence of disc protrusions or posterior vertebral body osteophytes as the cord is draped over these structures. Extension/hyperextension may narrow the spinal canal and increase compression on the cord as well by infolding or buckling of the ligamentum flavum and facet joint capsules. If there is existing loss of disc space height, this dorsal soft-tissue buckling is usually exaggerated. The morphology of the spinal cord has been shown to change with flexion and extension. Flexion tends to stretch the spinal cord, which can be magnified in the presence of disc protrusions and vertebral body osteophytes. Cervical spine extension causes the spinal cord to shorten and thicken, making it more susceptible to compression from a buckling ligamentum flavum and other injuries as well.

In addition to these static and dynamic causes for spinal cord compression, congenital narrowing can be associated with myelopathy. Congenital cervical spine stenosis is said to exist if the ventrodorsal diameter of the spinal canal is less than 13 mm. The normal adult canal diameter in the subaxial cervical spine is approximately 17 to 18 mm. Congenital stenosis, with superimposed degenerative and/or dynamic compressive pathology, increases the likelihood of developing myelopathy. Recognizing the presence of congenital stenosis in the face of significant degenerative changes is crucial, as it may affect surgical decision making.

Patient Presentation and Physical Examination

The patient with CSM can present with any number of subjective complaints and objective findings related to spinal cord compression. Subjective findings can range from subtle paresthesias of the hands to significant problems with upper extremity dexterity and gait/balance difficulties. Often, the patient will attribute early symptoms such as numbness in the fingers or mild balance disturbance to expected aging changes. The patient may also complain of axial neck pain due to the degenerative changes of spondylosis.

Cervical spondylosis can be divided into three groups based on clinical presentation: (1) myelopathy, (2) radiculopathy, and (3) myeloradiculopathy. Vascular or ischemic pathology is also a known cause of cervical myelopathy; however, this chapter will discuss myelopathy and myeloradiculopathy secondary to spondylosis. Myelopathy in CSM is related to spinal cord compression manifested by vague paresthesias and upper motor neuron signs. The patient with myeloradiculopathy presents with a combination of findings due to myelopathy and radiculopathy (nerve root compression and related lower motor neuron findings).

During the assessment of the patient with suspected CSM, a directed review of systems (ROS) is an important portion of the evaluation, since patients may dismiss or not think to volunteer mild symptoms. Questions are generally directed toward the upper extremity, lower extremity, and bladder function. This is consistent with the myelopathy grading system described by Benzel (modification of the myelopathy scale devised by the Japanese Orthopaedic Association).21 Upper extremity dysfunction can be ascertained by assessing for handwriting deterioration, increasing frequency of dropping objects, difficulty with fine motor skills such as fastening clasps or buttons, and the feeling of numbness and tingling in the hands. The term “myelopathy hand” had been used to describe nondermatomal paresthesias in the hands, a sense of clumsiness in the hands, and interosseous wasting.22,23 Myelopathy hand can be confused with peripheral nerve compression syndromes such as carpal tunnel, cubital tunnel, and thoracic outlet syndrome. Questioning regarding lower extremity function is directed toward balance and gait dysfunction, which is related in part to spasticity. Paresthesias and weakness in the lower extremities may also be a prominent complaint. Finally, inquiring about bladder function is important. Bladder dysfunction is not as common a finding as extremity problems but may be present in the form of incontinence or retention.24

The physical examination of the patient with suspected CSM includes a generalized neurologic examination, which can then focus more specifically based on the history. A thorough examination can differentiate CSM from those problems stemming rostral and caudal to the cervical spine and from motor neuron disease and peripheral neuropathy. The examination begins with observation of the patient’s posture, gait, and general appearance and fluidity of movements. Posture assessment may demonstrate sagittal and/or coronal imbalances. Gait examination may show a wide-based, spastic gait, and subtle changes may manifest only when the patient is asked to increase the pace of walking. Cranial nerve evaluation is included. Extremity function includes a motor and sensory examination from C4 to T1 and L2 to S1 bilaterally. Note that when assessing thoracic sensory levels, patients with cervical myelopathy can have variable sensory levels localizing to the thoracic spine.25 Deep tendon reflexes (DTRs) are typically hyperactive. This is not always the case because coexisting peripheral neuropathy such as diabetes can alter DTRs. Coexisting lumbar spinal stenosis can present with hyperactive upper extremity reflexes but normal or hypoactive lower extremity reflexes. Additional reflexes include Babinski’s reflex and assessment for clonus in the lower extremities, and Hoffman’s reflex, the inverted radial reflex, and finger escape sign in the upper extremities. Other reflexes that may help localize the site of compression are the pectoralis reflex, scapulohumeral reflex, and jaw jerk. Finally, cervical spine range of motion may be restricted. Associated findings may include a positive Spurling’s sign and Lhermitte’s sign.

Radiologic Evaluation

Once an appropriate history and thorough physical examination are completed, a radiographic assessment is needed to complete the evaluation. Imaging studies by themselves do not diagnose myelopathy, because the levels of spondylosis and cord compression do not always correlate clinically with patient complaints and physical findings. Imaging studies used in the workup of CSM include plain radiographs, magnetic resonance imaging (MRI), and computed tomography (CT) scan with or without myelography.

Plain radiographs are still an important part for the evaluation of the patient with CSM. The views obtained include anteroposterior (AP), lateral, flexion/extension, and obliques. AP radiographs allow for the evaluation of coronal alignment (scoliosis) and for the presence of anomalies such as cervical ribs or large C7 transverse processes (sometimes associated with fibrous bands). The lateral view provides the most information. It demonstrates the sagittal balance or the amount of lordosis/kyphosis. The lateral view also demonstrates the amount of disc space narrowing, vertebral body osteophytes, the AP diameter of the spinal canal, and sometimes the ossification of the posterior longitudinal ligament. The Pavlov ratio, defined as the ratio of the sagittal diameter of the spinal canal to the sagittal diameter of the vertebral body, is determined using lateral radiographs.26 A ratio of less than 0.8 has been associated with an increased risk for developing myelopathy. Lateral flexion/extension views evaluate for translation and angulation abnormalities, which can provide information regarding dynamic cord compression. Flexion/extension views, combined with a static lateral view, demonstrate the ability of the patient to achieve lordosis, either globally or focally, which can be important for deciding between surgical approaches. Lastly, oblique views provide information about foraminal narrowing, particularly due to uncovertebral joint spurring.

MRI is invaluable for evaluation of soft-tissue compressive structures as well as assessing the spinal cord itself. The intervertebral disc and ligamentous and capsular structures are all well visualized in the axial and sagittal planes. Perhaps the greatest advantage of MRI in CSM is its ability to directly visualize the spinal cord. Parenchymal changes such as myelomalacia signal changes and syrinx formation are readily identified (Fig. 156-2). MRI may also have a prognostic role. High-signal-intensity abnormalities on T2-weighted images and low-signal-intensity changes on T1-weighted MRIs are not uncommon findings in patients with CSM. A recent retrospective review correlating MRI findings to operative outcomes suggested that low-signal-intensity changes on the T1-weighted images preoperatively indicated a poor prognosis.27 A recent guideline article focusing on predictors of outcome found consensus that the presence of low signal of preoperative T1-weighted images, multisegment and focal high signal on T2-weighted images and the presence of cord atrophy are indicators or poor outcome.28,29 The addition of gadolinium is useful in cases of suspected infection or tumor and in patients with prior spine surgery.

Myelography combined with CT can serve several important roles. The myelogram can be evaluated with the cervical spine in flexed and extended postures, providing details regarding dynamic compression. The CT provides the most detailed information about bony pathology such as osteophytes and ossified posterior longitudinal ligament (OPLL), and in the case of significant OPLL, can influence the decision on surgical approach. It is better at differentiating bony from soft tissue than MRI, and it has been suggested that CT myelography and MRI should be considered complementary, not interchangeable.30

Somatosensory and Motor-Evoked Potential Monitoring

Somatosensory (SSEP) and motor-evoked potential recording (MEP and EMG have been used as tools of prognostic indication for CSM. Lyu et al. performed preoperative MEP and SEP in 39 patients with CSM who were to undergo surgical decompression.31,32 The authors judged improvement postoperatively by using pre- and post-operative JOA scores and 6-month neurologic recovery rates. The mean recovery rate was 51%, and this did not correlate with sex, arm or leg MEP or tibial SEP recordings. Neurological recovery rate, however, significantly correlated with normal SEP results. Morshita et al.32 performed pre- and post-operative median nerve SEPs in 14 patients with CSM with underwent cervical decompression. The authors demonstrated a statistically significant correlation between 1-week postoperative improvement in median nerve SEPs and 12-week postoperative JOA score for recovery rate. On the other hand, a lack of improvement in the median nerve SSEP in the early decompression period was associated with a poor neurological outcome. This data and others was judged to provide class II evidence supporting the use of median nerve SSEPs in predicting outcome following treatment of CSM.33

The use of EMG has not been shown to be convincingly predictive of recovery in CSM.28 It should be noted that EMG evidence of radiculopathy has been shown to be predictive of developing myelopathy in patients with cervical stenosis that is asymptomatic.17

Nonsurgical Treatment

Controversy exists regarding the natural history of patients with spondylotic myelopathy. This stems from the lack of randomized prospective studies on the issue and the reflection that the majority of studies that exist in the literature contain a heterogeneous population of patients. The natural history of cervical myelopathy is a slow, stepwise progressive deterioration. Lees and Turner reported episodic, stepwise deterioration in 75% of their patients.16 Patients with mild symptoms had the best prognosis with regard to lack of progression.

Conservative treatment consists of measures aimed at decreasing symptoms and increasing function. Options include cervical orthoses, physical therapy, medication, and epidural steroids. Medications include anti-inflammatories, analgesics (opioids), muscle relaxants, and steroids. Some or all of these conservative measures may lead to some improvement in 30% to 50% of patients, depending on the grading criteria utilized.34,35 This is especially true for those with only mild symptoms, that is, mild hand and arm symptoms but without impairment of daily tasks.

Careful conservative management may be considered for those with mild myelopathy, but caution should be taken with those with moderate to severe symptoms. No difference at 2 years was observed between those who received surgical decompression and those who did not in patients with only mild myelopathy.36 In contrast, those with severe myelopathy rarely improve with conservative measures. Fourteen of fifteen patients with severe disability remained so in one series.16

Ultimately, most patients with cervical spondylotic myelopathy undergo surgery. Those with moderate or severe symptoms should undergo early surgical decompression. Many surgical studies have demonstrated that shortening symptom duration prior to decompression results in improved outcome.37 Patients with only mild symptoms may be closely followed and decompression performed when there is deterioration or lack of improvement.

Operative Management

Ventral Approach

The ventral approach to the cervical spine to address spondylosis has been successfully used for nearly 50 years.3840 This approach provides direct access for spinal cord and nerve root decompression secondary to herniated intervertebral discs, dorsal vertebral body osteophytes, ossified posterior longitudinal ligament, and uncovertebral joint hypertrophy. Another important benefit of the ventral approach is the ability to correct kyphotic deformity through discectomy or corpectomy.41 Although some advocate ventral discectomy without arthrodesis in certain clinical situations, decompression in the setting of CSM is accompanied by a stabilization procedure with the goal of arthrodesis (artificial disc placement would be an exception). Ventral decompression options include discectomy (either single or multiple levels), corpectomy (either single or multiple levels), or a combination of discectomy and corpectomy. Arthrodesis can be accomplished by using autograft or allograft bone, or a combination. A variety of metallic and carbon fiber cages have been used, typically packed with autograft or allograft bone, and all of these options can be performed with or without a ventral cervical plate (Fig. 156-3).

Discectomy versus Corpectomy

There is little debate that when spinal compressive pathology is isolated to the level of the disc space, ventral discectomy and fusion is the treatment of choice. This includes disc herniations, spondylotic, vertebral body spurring, and OPLL. If spinal cord compression is present at multiple levels but still localized to the disc space, multilevel discectomies with bone graft at each disc space is appropriate. In the case of a degenerated, narrowed disc space with posterior osteophytes, consideration should be given to decompressing through the narrowed disc space. These large osteophytes can be associated with OPLL, extending beyond the disc space, either rostrally, caudally, or both. Attempting to reach behind the vertebral body through a collapsed disc space may risk neurologic injury. Distraction of the disc space to improve exposure prior to osteophyte removal may compromise the neural structures as well. In this case, corpectomy may provide safer and more complete decompression. When addressing compression due to OPLL, preoperative imaging must define the anatomy of the ligament. The ossification may occur in segments or could be a continuous bony bar, which may necessitate multiple corpectomies or consideration of a posterior decompressive procedure.

Another issue when considering discectomy versus corpectomy is the fusion rate. Certainly, the literature demonstrates successful fusion rates for single-level ventral discectomy as high as 96%.42 However, as the number of discectomies increases, so does the nonunion rate.43,44 A corpectomy procedure has the advantage of having fewer sites to fuse (one rostral and one caudal), compared with a multilevel discectomy procedure (rostral, caudal at all intervening sites). In one retrospective review, the corpectomy group had a significantly higher fusion rate when compared with a multilevel discectomy group. However, there were more graft-related complications in the corpectomy group, the clinical outcomes were not significantly different, and these procedures were done without instrumentation.45 Multiple-strut instrumented fusions have a theoretical mechanical and fusion potential advantage, particularly when utilizing intermediate points of fixation.41

Autograft versus Allograft

The use of autograft bone for ventral cervical spine arthrodesis has long been the standard with which to compare other techniques. Structural autograft is typically in the form of tricortical iliac crest or fibula. However, the morbidity associated with these harvest procedures has lead to the use of structural allograft alternatives. Iliac crest bone graft harvest has been associated with a high incidence of local pain, lasting up to a year in one third of patients. Other potential pitfalls include injury to the lateral femoral cutaneous nerve, fracture of the ilium, hematoma formation, and infection. Donor site complications have been reported to be as high as 20%.46

The literature provides mixed results when comparing autograft and allograft for ventral discectomy and fusion procedures. Some investigators report equivalent rates of fusion between autograft and allograft recipients while others demonstrate clearly superior results when using autograft.4750 A common finding, whether using autograft or allograft, is that increasing the number of discectomies per patient also increases the pseudoarthrosis rate.

The comparison of autograft and allograft for ventral cervical corpectomy is less well studied compared with single-level discectomy. Corpectomy autograft options are tricortical iliac crest and fibula. The predominant corpectomy allograft is the fibular strut graft, although various allograft options exist. Presently, the literature supports the use of both auto-graft and allograft for cervical corpectomy.5153

Instrumentation versus No Instrumentation

The addition of ventral plating to the cervical spine stemmed from the interest to provide immediate stability, increase arthrodesis rates, and prevent graft dislodgement. Biomechanical testing has demonstrated the ability of ventral cervical plating to enhance construct stability. This increased stability has translated into improved fusion rates clinically, in single as well as multilevel ventral discectomy procedures.5456 However, the use of ventral plates for cervical corpectomies has not been as promising, especially for multilevel corpectomies. Graft displacement is reported to be the most common complication following cervical corpectomy (Fig. 156-4). One study failed to show any benefit by the application of a plate to patients with a single-level corpectomy, while others demonstrated satisfactory outcomes with plating in single- and multilevel corpectomies.53,57

The specifics of each plate may play a role in outcomes. For example, a static plate (one that does not allow settling) will offload the graft and will be a load-bearing device instead of a load-sharing device. Fusion at the rostral and caudal graft-host interface relies on axial loading. If the implant bears the entire load, this will tend to decrease graft loading and increase implant failure. Newer plate systems allow for some settling, sharing of the load between the implant and graft, and more normal loading characteristics during flexion and extension (Fig. 156-5). Published trials using these implants in corpectomy cases are not yet available. One fact that seems to be agreed upon is if decompression requires three or more contiguous corpectomies, supplemental dorsal stabilization should be seriously considered to help limit ventral graft-related complications.

Cervical Arthroplasty

Cervical arthroplasty has been applied to cervical degenerative disease. This has included myelopathy. There, however, is a concern of continued motion may result in microtrauma to the spinal cord and negatively affect outcome. Buchowski et al. reviewed the IDE trials of two commercially available artificial discs and their use for the treatment of myelopathy.58 The authors reported on 199 patients in which 106 underwent arthroplasty and 93 underwent arthrodesis. All demonstrated myelopathy. Valid outcome measures were utilized. The authors found that both groups demonstrated improvement following surgery. Improvement was similar between groups, with no worsening of myelopathy in the arthroplasty group. The study presents class II data that arthroplasty appears equivalent to arthrodesis for the treatment of single-level compression at the disc space. Other pathologies such as retrovertebral compression or OPLL were not assessed and no conclusions can be drawn.

Evidence

Fehlings and Arvin have provided an excellent editorial relative to the anterior approaches to the spine for CSM.28 Evidence has shown that there is no superiority of anterior cervical discectomy (ACD) and addition of fusion (ACDF) or instrumentation, or corpectomy with plating in the treatment of CSM.59 It appears that ACD and ACDF are equally effective in terms of long term outcome; however, ACDF results in more rapid reduction in neck pain and radiculopathy, and reduces the risk of kyphosis. For multi-level ventral decompression, the addition of fusion to ACD appears superior to ACD alone. The addition of instrumentation reduces the risk on nonunion and improves arm pain, but does not appear to improve long-term outcome versus ACD alone.

Ryken et al. have reviewed the evidence regarding fusion substrate and anterior cervical approaches.60 The use of autologous iliac crest bone, allograft, PEEK, and titanium appear equivalent with regard to fusion. BMP-2 is an option worth discussion for anterior surgery. There have been concerns regarding safety of this osteobiologic in anterior cervical fusions. This has been largely related to swelling after surgery.61 This complication has been reported to be much lower with the usage of lower doses.62

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