The diagnostic challenge

So-called 'mechanical' low back or leg pain accounts for 97% of all back pain consultations. The most frequent (70%) are called sprains or strains, like those that often occur in other joints such as the ankle or shoulder. They probably result from a mechanical injury to the muscles and/or ligaments of the lumbar spine, but direct evidence of this is missing in most of these cases. Age-related degeneration of cartilage (i.e. osteoarthritis) can affect the disks between the vertebrae as well as the small facets where the vertebrae move against each other; this accounts for approximately 10% of consultations.

Three more severe conditions - a herniated disk, spinal stenosis and compression fracture due to osteoporosis - represent approximately 4% of low back pain cases seen in general practice, but compression fracture and spinal stenosis are more common in the elderly.

A 'herniated disk' means part of the disk has slipped out of place between two vertebrae, and this can press on the nerves on one side. Spinal stenosis is due to thickening of connective tissues around the spinal cord, and can also press on the cord. Both these conditions are often accompanied by pain in one leg. Low back pain may be a symptom of diseases that do not originate in the spine. For instance, diseases of the pelvic organs (prostate, ovaries, uterus) or abdominal organs (kidney, aorta, pancreas, gallbladder, stomach, duodenum) account for 2% of low back pain cases in general practice; however, its important for your physician to enquire carefully about such possible causes.

Only three types of serious diseases involve the musculo-skeletal lumbar spine, and they are rare (1%): cancerous growth (multiple myeloma, metastatic carcinoma, spinal cord tumors), abnormal metabolism of bone (osteopenia, Paget's disease of bone) and inflammatory conditions (osteomyelitis, abscess formation). One should also remember that shingles may produce pain similar to that seen with other causes of low back (and leg) pain.

The time pain occurs, or what provokes it, are not very helpful in making the correct diagnosis. Of course, pain (or numbness) going down the back of the thigh into the foot, and pain worsened by coughing, sneezing or straining all strongly suggests nerve compression.

Testing the muscle strength in the big toe and ankle, along with the knee-jerk and ankle-jerk reflexes, can tell the doctor if there is nerve involvement. In such cases, if symptoms persist more than 2 to 3 weeks it becomes important to identify the actual cause with certainty.

Worsening of cramp-like pain or numbness in lower legs during walking, which is relieved by spinal flexion (curling up) or aggravated by extension (stretching out flat) suggests there may be some spinal stenosis, especially after the age of 60.

Natural course of low back pain

Fortunately, the great majority of cases presenting to the general practitioner have a benign cause, evolution and outcome. Up to 70% of them are diagnosed as a strain or sprain without serious cause, and complete recovery occurs within 2 weeks in 90% of those after an acute first attack.

In some patients, low back pain tends to become chronic or recurrent, along with a depressed mood or symptoms of depression. A good recovery is also the rule in these cases, especially when adequate therapy and support are provided. Because of the overall good prospects in low back pain, patients should be fully informed of the cause at the first acute episode, and reminded of this if and when there is a recurrence.

A herniated disk with leg pain improves more slowly than a sprain, but only about 10% require surgery if marked pain persists for 6 weeks. CT and MRI exams have shown that, in 2/3 of cases, herniated disks improve anatomically without any surgical intervention.

Spinal stenosis remains unchanged in 70% of cases, progresses in 15%, and improves on its own in 15%.

What next: standard X-ray, CT (Computerized Tomography) or MRI (Magnetic Resonance Imaging) ?

CT and MRI are more sensitive than a standard X-ray exam in detecting spinal infections, cancer, a herniated disk and spinal stenosis. These exams should be done in those patients with a suspicion of cancer, infection or persistent neurological signs and symptoms beyond 3 to 6 weeks, or for those elderly at risk of osteoporosis or with symptoms suggesting spinal stenosis. A history of fever, loss of weight, injury, alcohol or injection-drug abuse in someone over 50 with low back pain mans that a CT or an MRI should be probably be done.

However, strict adherence to these guidelines can lead to overuse of expensive examination techniques. Moreover, almost 40% of adults over 60 without low back pain have one of the following abnormalities on MRI: a herniated, torn, bulging or degenerative disk, or spinal stenosis. Therefore, CT or MRI detection of spinal lesions requires skilful interpretation, as the presence of such injuries may not necessarily be the cause of pain, and therefore treating them (e.g. surgery) will be unlikely to yield a beneficial result. Sound medical judgment is clearly needed.

There are more sophisticated examinations that can be used in diagnosis: electromyography or somato-sensory evoked potentials (electrical stimulation of nerves and muscles) can help define how much damage has been done to individual nerves and also to distinguish nerve compression from other nervous diseases. In most cases, however, CT and/or MRI are fully adequate diagnostic tests. Nowadays, myelography (injection of an X-ray opaque dye into the spinal cord space) is much less used than before, but still performed just before surgery in selected cases.

Recommendations

Only a very few cases of low back pain are serious. Whatever the cause, it is important for your doctor to establish the correct diagnosis so that the most suitable treatment is given. A correct diagnosis requires skill, experience and sometimes consultation or collaboration with other physicians and the use of imaging or other specialized investigations.

Low back pain is rarely disabling. A return to work or other physical activity after an acute episode is influenced by socio-economical as well as clinical factors, and will be the subject of a future article in this series.