Spinal stenosis
Definition
Narrowing or stricture of the spinal canal causing neurological symptoms or dysfunction.
Anatomy
The cross sectional shape of the spinal canal has been classified into three shapes:
1) Round
2)Ovoid
3)Trefoil.
The mean AP diameter of the spinal canal is 12mm.
Stenosis exists when there is less than 10mm diameter, or a cross sectional area of less than 100mm2.
A minimum cross sectional area of 77 +/- 13mm2 is required to accomodate average sized neural elements.
Men have narrower spinal canals at the L3-L5 levels hence a higher rate of stenosis.
Arrangements of nerve roots within the thecal sac
The most posterior roots are always the fifth sacral roots.
The other roots are added progressively anteriorly.
At the L5-S1 disc level the most anterior nerve root is the first sacral nerve root.
The motor fibres are antero-medial within the roots and the sensory fibres postero-lateral
The dorsal root ganglion lies in the neuroforamen, with a small motor component anteriorly and a larger sensory component posteriorly.
Blood supply of the cauda equina
The blood supply of the cauda equina and spinal nerve roots is from cephalad to caudad
As the nerve root traverses the neuroforamen the blood supply is from caudad to cephalad.
Aetiology
Long term consequence of disc degeneration and OA
There is hypertrophy of the posterior disc margin and the facet joints
Pathoanatomy of disc degeneration
The annulus fibrosis is composed of around 50% type I collagen and 50% type II collagen.
The nucleus pulposus is comprised exclusively of type-II fibres.
With age the water content of the intervertebral discs decreases.
The proteoglycan content of the entire disc decreases as well.
Dehydration of the nucleus pulposus decreases its ability to distribute stress and results in fissures and tears within the annulus.
The discs therefore lose height. Annular bulging, disc herniation and early osteophyte formation occurs.
These changes increase stresses on the facet joints, which causes arthritic changes and on occasion instability to develop.
Pathophysiology
Compression alone does not cause pain, although it will result in paraesthesias, sensory deficits, motor loss and reflex abnormalities.
Inflammation may be caused by stretching of the nerve; in a normal person the S1 nerve root may move as much as 5mm within the neuroforamen upon straight leg raising. Degeneration may prevent movement of the nerve and cause an increase in internal tension and disruptions of neural architecture.
Nerve compression affects the afferent fibres first, while recovery is quickest in the efferent (motor) fibres.
The cause of the pain may be venous congestion (Arnoldi) or ischaemia.
Classification of Causes of Spinal Stenosis (Arnoldi 1976)
Congenital or developmental
Idiopathic
Achondroplastic
Osteopetrosis
Acquired
Degenerative
Central
Lateral recess and foraminal
Degenerative spondylolisthesis
Spondylolytic
Iatrogenic
Postlaminectomy
Post arthrodesis
Post discectomy
Traumatic
Miscellaneous
Acromegaly
Paget disease
Fluorosis
Ankylosing spondylitis
Combined
The commonest cause in the elderly is a degenerative L4/5 spondylolisthesis. This is much more common in females.
Incidence
F>M
Usually over 50; mean age for women is 73, slightly less for men
Occurs earlier in achondroplastics (around 40)
1.5-6%
History
The early symptoms are often insidious in onset.
Vague complaints of low back pain and stiffness are often the initial symptoms
Aching, heaviness or numbness of the buttocks, thighs and legs, particularly the posterior aspect and posterior lateral aspect of the legs.
Typically starts in the back and radiates down but may go the other way.
The symptoms are often unilateral which suggest an asymmetrical stenosis (root canal stenosis).
Symptoms worse with extension position: which narrows the spinal canal further- hence standing, walking down hills are worse: sitting, walking up hills are better.
This symptom complex of weakness, pain and paraesthesia worse with standing / walking but better with sitting / crouching is referred to as neurogenic claudication.
Radicular pain is found in around 20%, and is seen in patients with severe foraminal and lateral recess stenosis.
Radicular pain corresponds to the L5 nerve root in almost 90% of cases and the S1 root is involved in around 60% of cases.
Thus, around half of patients have double root involvement, one third have single root involvement and one sixth have triple root involvement.
Symptoms may be asymmetrical and inconsistent, and may vary from side to side and from day to day.
Classically the pain is worse with spine extension and better with flexion; the pain may come on after standing upright or walking for 5-10 minutes and be relieved by walking uphill or pushing a trolley and worsened by walking downhill. Pain brought on by walking is termed neurogenic claudication.
Men are often unable to stand up straight to shave.
May be previous history of disc prolapse or chronic backache.
Sudden worsening suggests disc prolapse or other acute decrease in area of the spinal canal.
Urinary dysfunction is uncommon.
Examination
May reveal neurological deficit but frequently normal.
Loss of lumbar lordosis is typical.
Most common neurological abnormality is weakness in the distribution of L5 (EHL).
Straight leg test is characteristically negative.
Can be made worse by getting the patient to walk first, until the pain is felt. This is called a stress neurological test.
Note: loss of knee and ankle jerks can be a normal part of aging, but the loss is normally symmetrical. Asymmetrical loss of reflexes may be significant.
Differential diagnosis
Vascular claudication
-Symptoms are primarily in the calf
-Relieved fairly promptly by sitting
-Skin changes and decreased pulses are found on examination
Lumbar spondylosis
Peripheral neuropathy
-Pain occurs more in a glove and stocking pattern than a radicular pattern
Diabetic amyotrophy
– characteristically affects femoral nerve
Sacroiliac joint pathology
Hip pathology
Knee pathology
Motor neuron disease
Abdominal aortic aneurysm
Renal disease
Depression
Compensation issues.
Radiology
Plain XR may show degenerative spondylolisthesis or advanced disc degeneration and OA.
Flexion and Extension views should be taken to reveal instability.
CT scan
MRI reveal the cross sectional diameter of the canal
MRI is preferred to CT scan because MRI sagittal scans can show the full extent of the canal and can reveal tumours within the canal.
If CT scan is used it should be combined with a myelogram. Riew performed a study where he concluded that a post myelogram CT provided more useful information than an MRI, but there are side effects to consider, such as headaches, meningitis and abscesses.
Severity of symptoms doesn’t necessarily correlate with the magnitude of compression seen on imaging studies. Patients can have radiologically significant stenosis without symptoms
Neurophysiology only useful to delineate peripheral neuropathy.
Treatment
Natural history
Few studies.
These show that severe debilitating neurological deterioration in patients who have been managed non-operatively is rare.
Complete resolution of symptoms is also rare.
Thus the decision to operate should be made on the basis of a decrease in the patient’s quality of life and an increase in symptoms rather than subtle neurological findings.
Atlas et al published a longitudinal cohort study in Spine 2000 following up surgically and nonsurgically treated patients over 4 years.
The surgically treated cohort had worse clinical and radiological features initially, but had better outcomes than the nonsurgically treated group. This was most apparent at one year, and still apparent although less strong at 4 years.
The outcomes for the nonsurgically treated patients improved modestly and remained stable over 4 years.
Nonoperative treatment
1. Education
2. Reassurance
3. Analgesics – NSAIDS, consider tricyclic antidepressants.
4. Exercise – particularly on a stationary bicycle
A program of aerobic conditioning can improve overall muscle tone and truncal balance, and reduce weight, which is important in the treatment of obese patients with spinal stenosis.
Epidural steroid injection – may gain short-term symptomatic improvement although there is no level 1 evidence that this is of long-term benefit. ESI may be particularly effective in those patients with predominant lower limb symptoms.
Use of a brace long term can lead to truncal deconditioning and is not encouraged, although it may be used in the short term in patients with instability.
Surgical treatment
It must be emphasized to the patient that the operation is designed to relieve pain in the legs, not the back.
Decompressive laminectomy with nerve root decompression is the treatment of choice.
The decompression should include all the levels that were involved on the MRI scan, not just the levels where there is neurological dysfunction.
Arthrodesis may be required. If there is no instability then arthrodesis is not required (not shown to improve outcome).
Surgery more indicated if:
Cauda Equina syndrome
Progressive neurological deformity
Unable to walk because of severity of weakness / pain
The decompression:
Usually a posterior midline approach.
To assess nerve root decoompression: pass an angled dural elevator out the foramen around the nerve.
If not planning to fuse, need to:
Exclude segmental instability in the AP and ML planes on dynamic xrays
Defined as >3mm of motion between vertebrae on dynamic views.
Not create instability with decompression:
Facets: maintain 50%
Pars: maintain at least 5mm
Indications for arthrodesis
If there is a spondylolisthesis
In selected cases of pre-existing scoliosis or kyphosis
If there has been documented curve progression
If there is correctability of the curve on side bending radiographs
If there is a scoliosis with radiculopathy on the concave side of the curve
If there is lateral spondylolisthesis
If there is loss of lumbar lordosis such that the patient is not in sagittal balance
Indications for addition of instrumentation
1. Correction or stabilization of scoliosis or kyphosis
2. Arthrodesis of two or more motion segments
3. Recurrent spinal stenosis with iatrogenic spondylolisthesis
4. Spinal instability (translation of 4mm or 10 degrees of angular motion)
