What is spinal stenosis?
Spinal stenosis means narrowing of the spinal canal. This in turn puts pressure on the spinal nerves and can cause symptoms in the legs. The most common cause of spinal stenosis is degenerative arthritis. This is another term for lumbar spondylosis or normal age-related changes. As the facet joints become arthritic they tend to enlarge as well, just like the knuckle joints in the hand. This tends to narrow the spinal canal from side to side. The picture below shows one normal facet joint and an enlarged, irregular, arthritic joint (arrow).
Bulging of the disc narrows the canal from front to back and when these occur together, as they usually do, the narrowing can be so severe that there is very little room left for the nerves.
What is a disc bulge?
Ageneralised disc bulge is a different situation and is due to loss of height of the disc due to loss of water in the nucleus. As the nucleus shrinks and the disc becomes narrower, the fibres of the annulus remain the same length and consequently bulge out to the side, just like a shirt sleeve that is pushed up the arm. In this picture, the disc can be seen bulging all the way around the margin of the bone, rather than in one focal area as is seen with a prolapse. A disc bulge like this does not usually cause sciatica but it can contribute to spinal stenosis.
The diagram below shows the normal spinal canal with the nerves in red (main group of nerves centrally and one nerve on each side exiting the spinal canal to join other nerves and form the sciatic nerve).
The next diagram shows the effects of enlargement of the facet joints and bulging of the disc, causing generalised narrowing of the spinal canal.
Looking from the side, the hole through which the nerve exits the spine (foramen) can also be significantly compromised in spinal stenosis. This is called foraminal stenosis and typically produces pain radiating down the leg (sciatica). The picture below shows the normal state with the nerve (red) exiting through a hole (foramen) that is more than adequate in size.
With degenerative change, the disc narrows and the foramen becomes much smaller, squashing the nerve.
Bulging of the disc can further aggravate this.
What symptoms does it produce?
Spinal stenosis tends to produce a fairly typical pattern of symptoms. There will very often be a degree of low back pain due to the arthritic change in the spine but more importantly the narrowing of the spinal canal produces symptoms in the legs which are usually worse with walking. People will often say that they can walk a particular distance without problems but then develop cramps in their legs or numbness or pins-and-needles or even weakness. The weakness is often described as a feeling like the legs have gone to jelly. These symptoms usually resolve quite quickly with sitting. This is unlike most other spinal problems where the symptoms tend to be worse with sitting. After a period of rest the patient with spinal stenosis can usually walk again, only to find the symptoms recurring in much the same way as before. They will therefore very often break up their walks with regular spells to sit down. Leaning forward slightly will also temporarily increase the size of the spinal canal and make it easier for someone with spinal stenosis to continue walking. Consequently they will very often find it easier to walk while pushing a shopping trolley. These symptoms can often be mimicked by vascular problems causing poor blood supply to the legs and it is therefore important to make sure that the foot pulses are normal before embarking on spinal surgery. Sometimes this may require an assessment by a vascular surgeon or an ultrasound to look at the blood supply to the legs.
This is usually by CT scan which will show the narrowing of the spinal canal and determine just which levels are affected. Sometimes, but not always, an MRI scan will also be done. The CT below shows gross expansion of the facet joints and consequent narrowing of the central canal outlined in red dots (compare with the CT of the disc bulge where the central canal is still relatively preserved).
How is spinal stenosis treated?
Anti-inflammatory medications may help the back pain, as can physiotherapy, in particular an exercise program to reduce weight and improve the strength of the back and abdominal muscles which help to support the spine. The most commonly utilised program at present is the Pilates method. Your physiotherapist can instruct you on this or you can do the exercises at home with the help of a book or video. The stenosis will not go away and although the leg symptoms may sometimes be helped temporarily by things such as epidural steroids, only surgery will restore the canal dimensions.
Facet joint injections
When the facet joints are very degenerate, some relief of back pain may be obtained from injection of steroid into the joint space. This is done by a radiologist under CT control, usually with some sedation. If the injections produce relief for a reasonable period, they may be repeated.
Injection of steroid into the epidural space (the space around the nerves in the spinal canal) can sometimes be very beneficial for sciatica but is of no benefit for back pain. In spinal stenosis, the results are not usually as good as for acute sciatica due to disc prolapse.
Surgery is aimed at relieving the pressure on the nerves and this is usually very successful at relieving leg symptoms. As the cause of stenosis is usually degenerative change, there will still be back pain due to this and surgery cannot be expected to fix this.