Cervical Spondylotic Myelopathy

Cervical spondylotic myelopathy is the most common spinal cord disorder in persons over 55 years of age today. Although the disease has probably been present since humankind came into existence, it has been recognized as a distinct nosologic entity only in the last six decades. Even to this date, a few cases are misdiagnosed as multiple sclerosis or amyotrophic lateral sclerosis. The stooped, short -stepped, shuffling gait that typifies the so-called senile gait may indeed represent an end-stage spondylotic myelopathy. Better life expectancy due to improvements in health care and nutrition throughout the world had led to a greater proportion of elderly individuals; thus we may expect to see a higher incidence of cervical spondylotic myelopathy in the decades to come.

The pathogenesis of cervical spondylotic myelopathy is incompletely understood, yet the influence of certain factors is well established. An important predisposing factor is the constitutive size of the spinal canal. Of patients presenting with myelopathy, 72 percent had a congenitally narrow spinal canal, as indicated by the ratio of the sagittal diameter of the canal to the anterior-posterior diameter of the vertebral body. Other mechanical factors that cause compression of the cord may be grouped under static or dynamic elements. Large posteriorly projecting osteophytes or sharp spurs from an ossified posterior longitudinal ligament are examples of static elements. Infolding of thickened, inelastic ligamenta flava during extension, and antero­ or retroluxations of vertebral bodies with a consequent "pincer effect" with compression of the cord against the posterior elements of the spinal column, constitute the dynamic elements. Vascular factors have been implicated in the pathogenesis of myelopathy, but the ischemic etiology seems to come into play only in the end stage of the disease. This is fortunate. because if appropriate surgical therapy is undertaken and if the causative mechanical factors are corrected before irreversible myelomalacia from vascular ischemia sets in, then the neurological deficit should be reversible.

Surgical Treatment

There is a universal perception among neurosurgeons that the currently available surgical methods yield suboptimal results in the treatment of cervical spondylotic myelopathy in contrast to cervical radiculopathy. Cervical laminectomy, the oldest and perhaps the most frequently used of the procedures, seems to offer the least favorable results. Gratifying results have been reported by enthusiastic proponents of the procedure, but the composite data obtained from pooling most major reports over the past five decades have emphasized the "arrest of progression" of the disease, rather than actual improvement with this procedure. Various additional technical manoeuvres that were intended to enhance the favourable results, such as sectioning of dentate ligaments, durotomy with duroplasty, and foraminotomy with curettage of osteophytes, have not drawn enthusiastic followers. Critical analysis of the factors that explain the failure of improvement after laminectomy point to the following: (1) The anterior elements that cause the compression of the spinal cord are not dealt with directly at all. Although the dural sac is expected to migrate posteriorly and may well be demonstrated to have done so by postoperative myelography, experimental studies show that ventral compression is insufficiently relieved by posterior decompression. (2) The chronic spinal instability evidenced by multilevel subluxation continues to be present and in fact may worsen after laminectomy. and thus continues to cause dynamic mechanical compression of the spinal cord. (3) Although laminectomy offers immediate decompression, and symptoms ameliorate transiently in the postoperative period, in the long run a dense unyielding epidural fibrous membrane forms at the site of laminectomy: thus. bony compression is replaced by fibrous compression.

Anterior discectomy, osteophytectomy and interbody fusion, by either the Cloward or Smith-Robinson technique, have yielded results that are somewhat superior to laminectomy, but only if the disease process is confined to one or two spaces. Neurosurgeons are generally reluctant to deal with more than two levels. presumably because of (1) concerns of causing a kyphotic or swan neck deformity; (2) the increased morbidity from a prolonged operation; and (3) reluctance to reach the high cervical levels because of the unfamiliarity of the anatomy. Even if all the involved levels are fused, the results continue to be less than ideal because the decompression is confined to the intervertebral spaces and does not extend to the midbodies of the vertebrae. Thus, if the patients have constitutive spinal stenosis, as some 72 percent of them do, then the spinal canal is not adequately decompressed. Additionally, a discectomy and fusion procedure does not deal with the thickened, calcified. or ossified posterior longitudinal ligament situated directly behind the vertebral bodies. which may continue to cause spinal cord compression.

A canal-expansive laminoplasty is certainly an appealing procedure that seems to disrupt the normal anatomy very little. Numerous ingenious variations in the technique have been described. Although primarily used in the management of ossification of the posterior longitudinal ligament, it has been used to treat spondylotic myelopathy as well. But the operative results from laminoplasty for spondylotic myelopathy are similar to those of simple decompressive laminectomy. although certain postoperative imaging studies show dramatic improvement after laminoplasty. The same factors that unfavourably influence the outcome after laminectomy apply to laminoplasty as well-namely, failure to remove the disc and osteophytes and failure to correct the spinal hypermobility, subluxation. and instability.

One can thus conclude that there are two fundamental attributes of an ideal surgical procedure to ameliorate the symptoms of cervical spondylotic myelopathy. First. the procedure should decompress the entire longitudinal extent of the involved portion of the cervical spinal cord. Second, it should offer stabilization of the spine. Partial median vertebrectomy with fibular grafting seems to satisfy these two major requirements.