According to the Clinical Practice Guideline for Understanding Acute Low Back Problems, published in 1994 by the Department of Health and Human Services Agency for Health Care Policy and Research, the precise cause of back pain is seldom determined, despite the advent of sophisticated diagnostic techniques. Although x rays and other imaging tests typically fail to disclose the reason for back pain, they may be important in ruling out serious conditions demanding specific treatment.
As with most other neurologic conditions, the cornerstone of diagnosis is the history, or analysis, of the patient's complaints, and the physical and neurologic examination. Additional diagnostic testing is needed in only about 1% of individuals with acute back pain. If symptoms do not improve in four to six weeks, further testing may be indicated.
The history focuses on a description of the pain and other symptoms, the circumstances in which the pain first occurred, and conditions that tend to make it better or worse, as well as any injuries and a general medical history. The physical examination should begin with a general medical examination and should include finding areas of back tenderness, testing spinal range of motion and flexibility, and measuring strength, sensation, and reflexes in the legs.
Specialized maneuvers include the straight leg-raising test. While the patient is lying flat on the back, pain in the low back or leg caused by raising a straight leg off the examining table suggests sciatica.
If there is suspicion of a serious cause for back pain, imaging or other tests may be done right away. Reasons for immediate testing include sudden back pain after a fall, suggesting fracture; back pain at night, suggesting a tumor, fever, or other signs of back infection; or loss of bowel or bladder control or progressive leg weakness, suggesting compression of the spinal cord or nerve roots. Cancer patients who develop back pain should have testing to determine if cancer has spread to the spine, which can lead to spinal cord compression and permanent paralysis if not treated promptly. Children with back pain unrelated to backpacks or sports injuries should also be tested sooner rather than later.
X rays are typically performed first as they are readily available and do a good job of visualizing bony structures, fractures, and deformities. However, they do not usually detect injuries of the muscles or other soft tissues. If x rays are negative and the doctor suspects a tumor, infection, or fracture not easily seen on x ray, bone scans may be helpful. In this test, injecting a low-dose radioactive medication into a vein allows the doctor to study bone structure and function using a special scanning camera.
Because magnetic resonance imaging (MRI) provides sharp, clear images of bones, discs, nerves, and soft tissues, it is the best test to show disc herniation and nerve compression. This test uses magnetic signals in water rather than x rays, and therefore poses no risk to the patient other than that associated with a contrast dye, which is not needed in most cases. Although the MRI may show disc bulging, this does not necessarily mean that the disc bulge is causing the back pain or that it needs to be treated. In about half of people without back pain, the MRI shows disc bulges. On the other hand, a bulging disc directly compressing a spinal nerve is more significant and may be causing pain and associated symptoms.
Computed tomography (CT) scan of the spine uses a computer to reconstruct cross-sectional x-ray images. A CT scan is good at visualizing bone problems like spinal stenosis, but it is not as sensitive as the MRI in diagnosing soft tissue injuries, and it has the added disadvantage of considerable x-ray exposure.
Because they are painful and carry a small risk of injury to the patient, certain tests are only done in patients who are about to have surgery so that the surgeon can plan the operation better. In myelography, dye is injected into the spinal canal and the patient is then tilted in different directions on a special table, allowing dye to outline the spinal cord and nerve roots and to show areas of compression. In discography, dye is injected into a disc space thought to be causing the pain, allowing the surgeon to confirm that an operation on that disc will likely relieve pain.
If there is evidence of nerve root compression on CT, MRI, history, or physical examination, electromyography (EMG), nerve conduction velocity (NCV), and evoked potential (EP) studies help determine the motor and sensory function of the involved nerve(s). These tests are also useful in diagnosing myopathy or neuropathy. During the EMG, fine needles inserted into the muscle determine how rapidly and forcefully the muscle contracts when stimulated. By applying a series of weak electrical shocks over areas supplied by a particular nerve, the NCV helps determine sensory function. Both tests are helpful in pinpointing specific patterns of nerve involvement.
In special cases, thermography and ultrasound imaging may provide additional information. Thermography uses infrared sensing devices to measure differences in temperature in body regions thought to be the source of pain. Ultrasound uses high-frequency sound waves to show tears in ligaments, muscles, tendons, and other soft tissues.
Laurie Barclay, The Gale Group Inc., Gale, Detroit,