Low back pain: Causes, symptoms, and diagnosis

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Low back pain: Causes, symptoms, and diagnosis

Oh my aching back! If you've never uttered that lament, you are one of the lucky few who have been spared. Some 70%–80% of men (and nearly as many women) have endured a bout of moderate to severe low back pain — and lots of us have suffered through many such episodes. All in all, back pain is the fifth most common problem that sends Americans to their physicians. And the problem goes well beyond pain and inconvenience; the annual cost of medical care and lost productivity is more than $50 billion.

It would be encouraging to report that this investment of time and money results in recovery. But in most cases, back pain will resolve as quickly (or as slowly) without medical attention as it will with a doctor's care. In most cases, in fact, you can take care of your back yourself. The trick is to know what to do and to recognize symptoms that really do call for prompt medical tests and treatments.

The normal back

Like every part of the human body, the back is complex. Your "backbone," of course, is not a bone at all but a column of 24 individual bones, the vertebral bodies, or vertebrae. The lowest five vertebrae are in the lumbosacral spine, which is just above the sacrum. The sacrum, in turn, is attached to two iliac bones at the sacroiliac joints, thus forming the pelvis (see figure 1). Most back pain occurs in the vicinity of these structures.

Figure 1: Your back The back is a complex structure that depends on bones, joints, muscles, and ligaments for stability and strength. Pain can originate from any of these structures.

The vertebral bodies are separated by intervertebral discs, which act as shock absorbers. Each disc has a gelatinous core surrounded by a capsule of fibrous tissue. Nerve roots run out from the spinal cord, passing between the vertebral bodies. These spinal nerves transmit the commands responsible for muscle movement, and they carry back the signals of sensation.

A group of sturdy ligaments holds the vertebral bodies together, and strong muscles run along the sides of the spinal column to provide additional support and to power the back through its bending, twisting, and straightening motions. Not to be forgotten are the abdominal, pelvic, and hip muscles, which help maintain the normal curvature of the back and support a share of the body's weight.

Stressful situations

The back has a hard job to do. It must be strong and stable enough to permit us to stand on our own two feet, yet flexible enough to allow us to bend and twist. Much of the stress falls on the low back. More than that, our back must withstand additional force when we lift heavy objects. And when we hold an object away from the body, the arms act as a lever, dramatically increasing the pressure on the back. A man who lifts a 30-pound weight at arm's length, for example, puts 450 pounds of stress on his low back. It's a lot to ask, and as people age, their discs lose elasticity, their muscles lose strength, and their ligaments become stiffer. Considering all this, it's a wonder that low back pain isn't even more common.

Triggering pain

Back pain can start gradually, with a nagging ache that builds slowly, or it can hit abruptly with quite a bang. Some men can point to a single event that sets it off, such as lifting a heavy object or making a sudden movement. More often, though, the triggering event is much less dramatic; a simple sneeze, bending to tie a shoe or pick up a pencil, and standing up from the toilet are among the many mundane events that can set off a bout of back pain. But in many cases, the pain begins without rhyme or reason.

What causes back pain?

There are almost as many theories as there are backs. In the vast majority of cases, low back pain has a mechanical cause that cannot be pinpointed precisely. Needing some explanation, doctors attribute garden-variety back pain to muscle inflammation and spasms, strains or sprains, pinched nerves, sciatica, bulging discs, poor posture, and abnormal alignment of the spine. It's probable that ordinary acute low back pain is caused by a combination of these factors — by a cycle of pain, muscle spasm, inflammation, and nerve irritation.

Although 70% of low back pain is maddeningly nonspecific, discrete problems can be identified in some patients. Degenerated intervertebral discs and arthritis of the joints between the vertebrae account for almost 10% of cases. Most common in older people, these problems typically cause persistent, nagging, mild to moderate pain. In about 4%, a herniated ("slipped") disc causes relatively severe pain in the back or pain that radiates into the buttocks or down a leg ("sciatica"). If the bony spurs of arthritis narrow the spinal canal enough to put pressure on the nerves, spinal stenosis can cause back, buttock, and leg pain that often flares with walking (pseudoclaudication) but not with biking or other activities that flex the spine. Spinal stenosis is a disease of the elderly that accounts for perhaps 3% of all low back pain. Slightly more common, at 4%, are compression fractures of the vertebrae themselves. This pain is usually abrupt and severe; because osteoporosis is the culprit, compression fractures are more common in women than men.

Figure 2: Intervertebral discs Your discs act as shock absorbers between your vertebral bodies. But a bulging disc can press on nerve roots, causing back pain, leg pain, or both.

When to worry

A variety of other causes are responsible for occasional cases of low back pain. The possibilities include infections, vascular disease, tumors, and disorders of the abdomen, rectum, pelvis, or prostate that refer pain to the back. You don't have to know them all, much less try to diagnose yourself. Nine times out of ten, low back pain is caused by mechanical problems that don't need special tests or sophisticated treatment. But you should know the "red flags" that suggest serious problems needing prompt medical attention (see table).

It's a long list of warning signs, and while every back patient should know about them, few people will experience any of them. And the things that worry patients most are not on the list of things that worry doctors. The severity of pain is not related to the seriousness of the problem; patients with terrible pain are no more likely to have complications than those with mild to moderate pain. Similarly, the inability to straighten up and stand or walk normally is not a cause for alarm, though it certainly causes major distress.

X-rays, scans, and lab tests

It's a natural instinct. Most people in the throes of back pain expect their doctors to schedule x-rays "to see what's wrong" — and in our high-tech society, many assume an MRI is warranted. But in the case of acute back pain, imaging studies are not useful. Careful research shows that x-rays don't improve the outcome of patients with ordinary back pain, and a 2003 trial showed that early MRIs are no better.

Trekking in for x-rays when you're in pain is likely to do more harm than good. But if your symptoms are unusual or the pain hangs on for four to six weeks, your doctor may include a series of x-rays along with a few simple lab tests, such as a complete blood count, erythrocyte sedimentation rate, and urinalysis. More elaborate tests are rarely helpful for ordinary acute low back pain, but thorough testing is important if warning signs are present (see table).

Computed tomography (CT scanning) is a powerful imaging technique that has revolutionized many aspects of medical care. For back pain, unfortunately, it's been less valuable; in general, CTs should be used only when detailed imaging is necessary but MRIs cannot be done. If CTs are used, they are best performed with a myelogram, which involves a lumbar puncture ("spinal tap") and a dye injection.

Magnetic resonance imaging (MRI) provides a superb look at the back, including the vertebral bones and joints, the discs, and the nerve roots. It's a noninvasive technique that doesn't require a spinal tap, but it's not suitable for people with pacemakers or certain metallic implants. MRIs are expensive and can trigger claustrophobia. They may also be very hard on men who cannot lie still because of pain.

MRIs have another drawback: They can be too sensitive. In three studies, volunteers who were entirely free of back pain received MRIs of the spine. Even in those folks, 46%–93% had degenerative discs, 24%–79% had bulging discs, and 22%–40% had herniated discs. If these people had complained of back pain, doctors would be tempted to blame it on disc disease, perhaps starting them down the path to invasive therapy. The same could happen to you. If you have back pain and your MRI shows arthritis or disc disease, there is no assurance that the anatomic abnormality is actually responsible for the pain.

MRIs are rarely indicated for uncomplicated low back pain. But they can be critically important for patients with warning signs or with pain that is prolonged or unusually severe. In most cases, MRIs should be performed only if surgery is a serious consideration.

What to expect

Expect to recover. About 90% of people with ordinary mechanical low back pain get over it without special therapy. But it takes time to recover; about 30% of patients are substantially improved in just a week, but another 60% take up to eight weeks to get better. And about 40% will have another bout of pain within six months of the first. It doesn't mean they have "bad backs" that will plague them for life, but it does mean they need good, commonsense back care.

The outlook for patients with herniated discs is nearly as good. Recovery tends to be a bit slower, but by the end of six weeks only 10% of patients have enough pain to warrant consideration of disc surgery. And MRI studies show that two-thirds of these damaged discs actually heal within six months. In contrast, the pain of arthritis tends to linger, and spinal stenosis is likely to cause persistent discomfort or to gradually worsen.

If you have low back pain, it's comforting to know that your outlook is good. But it may seem like small comfort while you are enduring days or weeks of pain. Time is on your side, but you can take measures to reduce your discomfort while you are waiting to recover. In addition, if you learn to take care of your back, you may be able to reduce your risk of recurrent episodes of pain.

Last updated:	September 05, 2008