What is it?
The word ‘stenosis’ means narrowing. Spinal stenosis means narrowing of the spinal canal causing compression or squashing of the spinal cord and nerves. When it occurs in the lumbar spine or low back region, the nerves to the legs are affected.
Why does it happen?
The most common cause is degeneration, which is part of the ageing process. As our backs age, the discs dry out and lose height, causing them to bulge like a flat tyre. Some of the stenosis is caused by the discs bulging into the spinal canal. As the discs bulge, they become a little unstable or wobbly, and lead to increased movement between the vertebrae. The body responds by trying to restabilise or re-stiffen the back. The small joints in the back called the facet joints enlarge and grow spurs, and the ligament between the vertebrae called the ligamentum flavum thickens. These changes lead to further narrowing of the spinal canal. Sometimes the instability becomes worse and the vertebrae can slip forwards (degenerative spondylolisthesis), or sideways (degenerative scoliosis), causing kinking of the spinal canal. All these changes cause the spinal canal to become narrower and the nerves to become more compressed.
What does it cause?
The most common symptom is an ache or heaviness in the legs together with numbness, pins and needles or a feeling of ants crawling on the legs. This happens after walking or standing for a few minutes. It goes away with sitting down or leaning forward. This is because the nerve compression is worst when the spine is extended backward and is improved when it is flexed forward. This symptom is called neurogenic claudication.
Sometimes one nerve is more severely compressed, causing sciatica, or severe constant pain down the leg.
What happens to the stenosis?
With time, the stenosis gradually worsens. Usually the symptoms gradually deteriorate as well. However, the symptoms can sometimes improve temporarily and then suddenly worsen. This is because the pain is caused by inflammation of the nerves, which can change quickly. Sometimes the stenosis worsens suddenly because of a disc prolapse.
Can it be serious?
Most often, long term pressure on the nerve leads to chronic pain and gradual mild weakness and numbness. Very occasionally, severe stenosis can worsen quickly and it can damage the nerves, causing permanent leg weakness and a foot drop, meaning that the foot flops when walking. Complete sudden paralysis is very rare.
How is it diagnosed?
The diagnosis is suspected if there is leg pain and numbness or pins and needles when walking. X-rays and scans are used to confirm the diagnosis. An MRI scan is more accurate but sometimes a CT scan is ordered.
How is it treated?
Most cases of spinal stenosis cause mild symptoms initially. They can be controlled with tablets such as pain killers and anti-inflammatories. A physiotherapist can offer advice about activities and positions to avoid, and design an exercise programme to improve back posture and strength. A cortisone injection into the spine called an epidural steroid injection is often effective in the early stages, and can give up to six months of relief.
If leg pain is severe and walking distance is limited, surgery in the form of a laminectomy is considered.
What is it?
A laminectomy is an operation to remove some bone and ligament in order to make more room for the nerves. The bone at the back of the spine which forms part of the spinal canal is removed. The idea is similar to making a trench to unblock a pipe. The bone that is removed is not replaced, but the spine is protected by a thick layer of muscle and fascia. If the spine is stable, then removing this small amount of bone does not cause any problems. If there is instability seen on the x-rays and scans, an additional procedure called a fusion is often performed, to prevent the vertebrae moving or slipping after surgery.
The aim of the laminectomy surgery is to relieve leg pain. Some back pain usually continues, because the underlying degeneration or wear and tear that caused the stenosis cannot be reversed with the laminectomy operation. Most people have at least 75% improvement in their pain, and can get back to walking long distances normally.
How is it done?
You are admitted to hospital on the day of the procedure. You are given a general anaesthetic, a catheter is inserted in your bladder and you are placed on the operating table face down. The skin over your low back is cleaned. A needle is placed in your back and an x-ray is taken to make sure the correct levels are operated on. A 10 to 15 cm incision is made in the midline of your low back and the muscles are separated from the bone and held apart with a retractor. A bone drill or oscillating saw is used to cut the lamina which is the arch of bone at the back of the spine. (The number of lamina removed depends on how many levels of the spine are affected by the stenosis.) This exposes the dura (the spinal sac containing the nerves and the spinal fluid). The spurs are carefully removed along the central spinal tunnel and the small individual nerve tunnels to make sure that the nerves are no longer compressed. It is usually not necessary to remove any of the disc bulges. A drain is placed in the wound to collect any bleeding and a small tube is placed under the skin which will allow local anaesthetic to be slowly put in the wound for pain relief. The muscles and skin are closed with dissolving sutures.
What happens afterwards?
Your physiotherapist will help you get up and walk soon after the operation. The next day, the drain in your wound, your pain tube, your IV drip and your catheter will be removed. Your physiotherapist will give you advice about activity, and start an exercise programme. When your pain is controlled and you are mobile, you will be discharged, usually two or three days after surgery. After three weeks, you can drive and start going out normally. After six weeks you can get back to most of your activities gradually. You will see the physiotherapist at three weeks for more exercises, and again at six weeks when you come back to see the doctor.
What could go wrong?
You may have increased back pain which usually improves with time and exercise but may persist. You may have some residual leg pain or weakness if the nerves were damaged by the stenosis. Ongoing degeneration in nearby parts of the spine can cause new symptoms of back or leg pain. Instability between the vertebrae may develop, causing them to slip out of line, and sometimes a fusion needs to be performed at some time in the future. Unfortunately, nothing can be done at the time of surgery to prevent more degeneration in the future.
During surgery, there can be damage to the dura, causing a spinal fluid leak. This is repaired and usually causes no problems. Very occasionally a serious complication can occur during surgery or soon after, such as nerve damage, bleeding causing nerve compression, or infection. Before deciding whether to have an operation, you need to discuss these risks with your surgeon.