CERVICAL SPINAL STENOSIS AND MYELOPATHY

This is also known as Cervical Spondylotic Myelopathy and refers to the narrowing of the spinal canal of the vertebral column in the neck from osteoarthritis and the damage to the spinal cord that may result from this.

INTRODUCTION

Marked degenerative changes occur in the cervical spine of the majority of people over 50 years old. Only a very small minority of these will develop the neurological symptoms of myelopathy. On the other hand, it is also true that in the same age group the most common cause of cervical myelopathy is this same cervical spinal stenosis.

FUNCTIONAL ANATOMY OF THE CERVICAL SPINAL CANAL

The cervical spinal column has a number of functions. One of these is to protect the spinal cord on its way from the head to the chest.

In the neck, the spinal column is represented by a strong flexible pillar which consists of a row of sturdy vertebral bodies. The protective function for the cord is performed by a bony canal which is made up of a row of arches of bone, the vertebral arches, which extend backwards from each of the row of vertebral bodies. This row of arches forms a strong flexible tube called the spinal canal. The protective wall of this canal consists of a number of components. In front there is the posterior surface of the row of vertebral bodies and their intervertebral discs. On both sides there are the round pedicles and the paravertebral joints of each vertebra. At the back are the flat laminae separated by the bases of the spinous processes and between them a fibrous sheet of ligament, the tallow ligament (fig.1).

This anatomical construction creates a chain of strong, articulated bony and fibrous rings which protects the spinal cord, at the same time carrying the head and allowing it to be moved in all directions. (fig.2) There are apertures between these rings which allow the cervical nerves to pass out of the canal on their way to the arms. The muscles of the neck that are responsible for balance, posture and movement of the neck are attached to outside projections of the vertebrae.

CERVICAL STENOSIS

Common causes for intrusion into the canal with resultant potential pressure on the cervical spinal cord include also trauma, tumor, abscess and hematomas. These will not be dealt with here, as it is the purpose of this article to only discuss the degenerative processes which cause spinal stenosis which may lead to spinal cord compression. These processes are usually caused by the normal ageing of the joints and ligaments of the spinal column.

The essential feature of spinal stenosis is that it ultimately may cause compression of the spinal cord. This occurs as soon as the size of the space available for the cord in the spinal canal becomes too small for the thickness of the cord and pressure develops. This pressure causes the cord to blanch for loss of blood flow in the compressed area and this loss of blood flow leads to a lack of oxygen available to the spinal nerve structures (ischemia). This ischemia leads to tissue damage in the cord and constitutes the main mechanism of the eventual loss of function of the cord.

Since it is a mismatch in cross sectional space in the cervical canal that is the main underlying mechanism for myelopathy, it is important to know that, where as the size of the spinal cord varies only to some extent from person to person, the size of the spinal canal can varies quite significantly. It stands to reason then that a person born with a small canal will be significantly more prone to the effects of stenotic canal narrowing than the person who is protected by a very wide canal.

There are a number of disease processes that can narrow the canal. One of these processes results from the common osteoarthritic degeneration of the intervertebral disc and paravertebral joints which occurs mainly with ageing.

Usually, osteoarthitis in the spine starts with the gradual collapse of the perishing discs. This in itself is not a problem, the boney surfaces of the vertebral body on either side of the discs as well as the worn discs themselves, spread circumferentially in an attempt to increase the load-bearing surface of the intervertebral joint. These spreading osteophytes develop in all directions, also backwards and thus towards the spinal cord. This part of the spread contributes significantly to the narrowing of the spinal canal. (fig.3)

The paravertebral joints on either side of the canal also enlarge by the formation of osteophytes and by the thickening of the joint capsule. This constitutes additional intrusion into the spinal canal decreasing its size even further.

Finally, as a result of degeneration and the loss of disc height, the ligaments which interconnect the laminae, thicken and fold into the canal, taking further space away.

It has been found that once the spinal canal has narrowed by 30%, symptoms of cervical myelopathy usually appear. Of all the intrusions the front-to-back reduction in the diameter of the spinal canal is usually the most damaging.

These processes continue until the spinal cord needs to deform in order to accommodate to the new shape of the canal. Once this causes deprivation of blood supply (ischemia), myelopathy develops.

In some cases the degenerative changes of the spinal column are further complicated by the development of instability. This instability develops because the osteoarthritis in the perishing joints causes an inability to hold the vertebrae in alignment during movement and at rest. A forwards-and-backwards slippage takes place. This can change the space available to the cord quite rapidly and significantly during movements of the neck (fig.4).

Another way in which a large anterior osteophytic ridge can do damage is by bowstringing the spinal cord during flexion of the neck.

A loss of alignment of a large segment of the cervical spinal column from osteoarthritis may lead to a fixed flexion deformity which will have a similar detrimental effect on the cord. (Fig 5)

Occasionally, a relatively sudden onset of cervical myelopathy is observed in cases of large, often spontaneous, intervertebral disc herniation onto the cord.

An unrelated entity, in which a ligament anterior in the spinal canal becomes growing bone (OPLL), occurs only rarely here, but relatively frequently in countries like Japan.

THE EFFECTS OF STENOSIS

Stenosis causes pressure to be exerted on the tissues of the spinal cord. Initially the spinal cord takes on the shape of the remaining space in the canal and deforms without loss of function. When the pressure becomes severe enough to compress the small blood vessels of the cord, no oxygen reaches the nervous tissue and the spinal cord becomes ischemic. This impairs nerve function. If the pressure is relieved quickly enough, no damage follows. Initially the compression is intermittent and the cord continues to recover between cycles of compression. Later on, as the pressure becomes more intense and longer lasting, the insult caused by the repeated episodes of ischemia results in permanent tissue damage. This damage may not recover well or, at times, not at all, even after surgical relief of the compression.

This lack of oxygen (ischemia) is the root cause of the spinal cord damage which is referred to as myelopathy. Once fixed damage has occurred, it can often be clearly seen on magnetic resonance scans (MRI).

THE CLINICAL PICTURE OF MYELOPATHY

Although severe degenerative changes occur in the cervical spine of the majority of people over 50, only a small minority develop neurological symptoms.

Myelopathy leads to impairment of function of the cervical spinal cord. The symptoms and signs of myelopathy are quite distinct from the neck pain of cervical osteoarthritis which is movement dependant and can radiate to the head and the shoulders. Such neck pain is usually not from cervical myelopathy. Cervical myelopathy is often associated with relatively little or no pain.

Direct pressure on the spinal cord may create a sensation of pins-and-needles (paraesthesia) in the whole body. Initially this happens on flexion of the neck only and later becomes more continuous as the condition worsens.

The symptoms in the arms experienced by the patient as a result of the myelopathy constitute a combination of cord failure at the affected level and of compression and irritation of the cervical nerve roots which leave the neck at this site. Numbness, weakness, paraesthesia and particularly clumsiness of the hands may be present. The objective clinical findings on examination will help to distinguish between what originates from the cord and what from the nerve roots. The definitive diagnosis is based on the characteristics and the distribution of the sensory and motor findings at examination.

The findings in the body below the neck depend on the configuration and the severity of the ischemic lesion in the substance of the spinal cord. Usually there are varying measures of tightness of gait and numbness of the legs. These variations are an expression of the exact distribution of the myelopathy in the spinal cord of the individual patient.

A transverse lesion of the spinal cord will affect all the modalities of cord transmission with spasticity of gait and increased tendon reflexes, with loss of appreciation of temperature, pain and joint sense and often with impairment of bladder and bowel control. Some, more localized lesions may affect predominantly only one side of the cord .This will impair the motor function and joint sense on the side of the lesion and the appreciation of temperature and pain on the side opposite to the lesion. Alternatively, a central lesion of the cord may present with predominantly hand symptoms.

These findings will help with the evaluation of the imaging studies and other auxiliary tests that may be performed. This will then play a significant role in determining the type and extent of the treatment decided upon.

INVESTIGATIONS

Plain X-rays are important because they give a good overview of the anatomy of the vertebral column of the neck and may give an early warning of other major pathology. They also show the overall degree of osteoarthritis. At specific levels one looks for malalignment of the spine (particularly in flexion), subluxation (spondylolysthesis), significant osteophytes and an indication of the size of the canal (< than 9 mm likely to have myelopathy and > 13mm unlikely to have myelopathy).

The MRI (magnetic resonance imaging) is the most informative examination for myelopathy. It also excludes other causes for the symptoms similar to those of stenosis, like tumor, motor neuron disease or multiple sclerosis. The MRI is the best way to show the degree, extent and site of cord compression as well as the shape of the cord, the extent of scarring or edema even though the “T2 recovery” images of the MRI are inclined to exaggerate the extent of any compression significantly. The presence of increased signal in the cord (edema or scarring) does not necessarily correlate accurately with a less favorable outcome.

At myelogram an iodine-containing contrast material is injected into the spinal canal with a spinal needle for radiological visualization of the fluid spaces of the cervical spinal canal. This also demonstrates bony details better. It allows x-rays to be taken to show the spinal cord and canal during movement of the neck. Its disadvantage is that it does not give details of the structure of the spinal cord.

The evaluation of Somato-sensory Evoked Potentials (SSEP) is an electro-physiological method of measuring the function of the spinal cord. This is done by way of evaluating the characteristics of the electric changes in the brain waves following the application of specific impulses at the feet or the arms. It is used to assist with the diagnosis of spinal cord malfunction when interpreting an MRI scan in a case of cervical spinal stenosis

TREATMENT

The accepted aim of any treatment is to improve the neurological impairment due to the myelopathy, but often the best result of treatment is merely to halt the progress of the condition and to prevent further cord damage. There are times when it is possible to foresee that the onset of myelopathy is inevitable and it would then be appropriate to try and prevent it altogether at that early stage. It is usually not necessary to treat cervical spinal stenosis on its own in the absence of clinical myelopathy because the treatment is not primarily intended to correct any bony abnormalities or malalignment.

However, this correction is exactly what will happen once surgery is indicated because the condition is essence caused by these abnormalities.

NON - SURGICAL TREATMENT

This consists of treatment with medication and of non-surgical treatment of a physical nature.

The choice of the medicines that are effective is limited. Anti-inflammatory and analgesic medication is more of a help for the pain from the concomitant osteoarthritis or the nerve roots than for the myelopathy. This form of treatment is also more beneficial in acute than in chronic cases. In practice it is often used even if the benefit may be limited. Systemic cortisone, on the other hand, has been shown not to be of any help.

Physical methods like local heat may help for neck discomfort, though it does not alter the course of the illness significantly.

Rigid neck bracing is useful, also in the long term, but needs to be conscientiously applied by the patient.

Prolonged neck traction by neck halter or by skull traction has been shown to be of benefit.

Physiotherapy in the acute phase and biokinetic exercise treatment in the chronic phase are ost useful measures to allow mal-alignment, neck mobility, muscular strength and spinal balance to be corrected in the short and long term.

Epidural injections of cortisone or other substances have not been proven to be of lasting benefit and are therefore still controversial. Severe complications have been described.

A careful “wait and watch” approach is also a form of treatment in certain cases.

Non–surgical treatment is applied mindful of the following clinical concepts.

The course of the illness without treatment is not well known.

There is evidence that some patients improve spontaneously, even after a period of deterioration. Others just stop getting worse at a given time and stabilize. This occurs presumably because, in these cases, the stenosis does not progress further due to one of the natural outcomes of osteoarthritis. Osteoarthritis tends to lead to an increasing stiffness which in the end results in a spontaneous cessation of movement at the affected levels in the neck. The percentage figure given for this cohort of patients who improve or stabilize without surgery varies between 30% and 60%. These patients usually have a mild degree of myelopathy, a long history of the condition or a slow progress of the impairment.

Another reason for considering non-surgical treatment is that many of the patients with cervical spondylotic myelopathy are elderly. Elderly patients may suffer from significant co–morbidity (other, unrelated illnesses).This would increase the risk of surgery to an extent that a more conservative approach would have to be considered until such time that the risks of not operating exceed the risks of operating.

SURGICAL TREATMENT

Surgery is the most reliable way of removing the compression on the spinal cord. This compression is in its essence physical and can therefore be dealt with best by physical i.e. surgical means. Factors which would mitigate towards an operation would be:

  • severe disability,
  • a relentlessly progressive course (40%– 75%)
  • a rapid deterioration of the myelopathy,
  • a significant instability between two vertebrae on plain flexion – extension X-ray films: a shift of more than 3 mm, particularly if this can be shown to have increased with time would suggest surgical instability
  • a more marked narrowing of the canal on imaging (> 30% narrowing),
  • a “banana” shaped deformity of the cord on MRI,
  • a marked increased signal in the cord on MRI, markedly abnormal somato-sensory evoked potentials
  • the absence of contra-indication, like significant co-morbidity and substantial remaining life-expectancy.

However, many of the factors mitigating towards surgery also portend a less than satisfactory outcome namely severe pre-operative deficit, abnormal signal in the cord, spinal cord atrophy and severe radiological cord compression.

In the absence of properly conducted, prospective randomized trials, the issue of what constitutes the most appropriate management strategy in any specific individual is still not settled. This is because the present literature provides no certainty in the prediction of a particular patient’s expected natural clinical course. It is therefore important that the surgical option be considered carefully in each case and be correlated with the patient’s evaluation of the situation. The patient needs to be properly appraised of all these facts.

THE SURGICAL OPTIONS

At surgery, the spinal cord is routinely approached from either the front or the back of the neck. In only very exceptional cases both approaches are employed at the same operation. It is important to be aware of the fact that it is very dangerous to the cervical spinal cord to be retracted at all during surgery for fear of producing cord damage. This means that the operation should be planned in such a way that the major compressive agent has to be dealt with from the direction of the impingement onto the cord.

THE ANTERIOR APPROACH

The approach from the front of the neck (anterior) is usually favored because the major compression is most often caused by osteophytes, with or without additional disc protrusion, pushing from the front onto the cord. This is also the best approach for the simultaneous correction of the cervical spinal alignment (lordosis). A further advantage of the anterior approach is that it is less traumatic and less likely to be followed by chronic neck pain than the posterior operation. The procedure itself is also less painful.

In this operation a horizontal incision is made to the front of the neck and the anterior surface of the cervical spine is exposed in a way that has been used for the past 50 years because it is so well designed and atraumatic. The remnants of the discs and the associated osteophytes are removed at all the involved levels. After restoration of the natural curve of the spine, the discs are replaced with closely fitting natural bone blocks or artificial implants. A titanium plate is attached with small screws to the bone, as a permanent internal support for the fusion. This is the normal procedure followed if the stenosis involves a limited number of levels (discs), if there is instability at a limited number of levels or if a large disc protrusion has not settled on conservative treatment.

THE POSTERIOR APPROACH

The posterior approach, on the other hand, is through an incision at the back of the neck. It is used in the more difficult cases where the anterior approach would not deal adequately with the compression. This would be if the stenosis is present in a person with a congenitally narrow spinal canal or is due to the ossification of the posterior longitudinal ligament (OPLL). It is also often used if the disease involves more than 3 levels because under which circumstances the anterior approach may reach its limit of technical appropriateness. The posterior approach is less appropriate if there is loss of the normal spinal alignment and the spine is kinked forward (kyphotic) from the osteoarthritis. However, the use of screws and rods, like those used of the lumbar spine, is now also possible in the neck and this has improved the chances that a fusion with a realignment of the cervical curvature from behind will be successful.

A laminectomy is the most common operation from behind and consists of the removal of the laminae at the affected levels. One level above and one level below the affected levels are often included in the operation. The complete removal of the laminae can, unfortunately, lead to the development or aggravation of deformities (swan neck deformity). Therefore other ways of increasing the size of the spinal canal to decompress the spinal cord were devised. For instance it may be useful in some cases to split the laminae longitudinally and bent individually outwards, to then be fixed in this position (laminoplasty). This would also help to maintain the muscle attachments to the vertebra posteriorly and in this way reduce post-operative pain and deformity. The laminectomy remains the most effective procedure to be performed from behind but it may have to be secured by a posterior fusion with internal screw-and-rod fixation. This would, however, negate one of the other advantages of the posterior approach which is that the movement of the cervical spine is maintained at the operated levels.

COMPLICATIONS

All operations entail risks and complications.

Any operation under general anesthesia carries a risk and this is particularly so in the elderly.

The operations for spinal stenosis in particular may be complicated by a post-operative wound hematoma, infection or a cerebro-spinal fluid leakage. There may be damage to the spinal cord or the nerve roots. There may be prolonged pain in the neck wound or in the wound over the iliac crest from where the transplanted bone was harvested. The anterior operation may further cause swallowing difficulties and hoarseness which, though mostly temporary, may be permanent. On the whole, however, these complications are infrequent and the operations are regarded as relatively safe.

OUTCOME

The operation is primarily intended to halt the progress of the neurological deterioration. In spite of this, good improvement is quite often found (48%) if the operation takes place within one year from onset of symptoms and only 16% if performed later. Generally, the outcome is less good than expected by the patient. Late deterioration may set in from ongoing disease at an adjacent level or even for no established reason.

Surgery has, on the whole, a worthwhile outcome if performed on the right candidate for the right reasons. It then also has an acceptable risk profile.

CONCLUSION

Cervical spondylotic myelopathy is an important disease of the aged. It develops insidiously and is often either not noted for some time or ascribed to other, more harmless conditions. The patients are usually over 50 years of age and note progressive loss of function and pin-and-needles in the hands and stiffness and insecure gait in the legs. Significant impairment of function may result. Early recognition is important. Conservative treatment may be effective and may be all that is needed. An operation may in this way, be avoided. If the neurological deficit gets bad then surgery needs to be considered. At that stage often only an operation will expected to stop the progress of the disease. Surgery usually helps even if, in some instances, it only prevents the condition from getting worse. On the whole, the operations are frequently performed and regarded as relatively safe and effective. They need not to be unreasonably feared

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