Routine blood tests help determine the cause of back problems in a few situations. Results may reflect the presence of inflammation and/or an infection. For example:
• Blood levels of alkaline phosphatase (an enzyme released by bone-forming cells called osteoblasts) are often extremely high in people with active Paget’s disease (which can cause vertebral compression fractures and spinal stenosis). • Blood calcium levels are elevated in people with hyperparathyroidism (which, along with osteoporosis, can lead to vertebral compression fractures). • Prostate-specific antigen (PSA), which is elevated in men with prostate cancer, can be measured by a blood test to determine whether back pain is due to the spread of prostate cancer. • Abnormal proteins in blood and urine are often present in people with multiple myeloma (cancer of the plasma cells, a type of white blood cell).
Imaging Tests of the Back
Diagnostic imaging studies of the back provide a view of the bones and the soft tissues, which comprise muscles, ligaments, cartilage, tendons, and blood vessels. These studies are usually done when surgery might be necessary, when the doctor suspects a serious condition may be responsible for the back pain, or when you are experiencing severe pain.
Experts advise against routine imaging studies when no particular cause of the pain is identified through the physical examination, patient history, and laboratory tests. When imaging is ordered, the study chosen depends, in large part, on what is suspected to be causing the pain.
For chronic pain and for new-onset back pain lasting longer than four to six weeks, x-rays should be the first imaging study. They are especially useful for detecting fractures and invasion of bone by multiple myeloma. Although x-rays tend to be overused, they are useful for people over age 50, because this age group has a greater risk of malignancy and vertebral fractures. Modern x-rays are low in radiation but should be used only when necessary. Conventional x-rays are available almost anywhere and are relatively low in cost.
Computed Tomography (CT) Scans of the Back
During a CT scan, also called a computerized axial tomography (CAT) scan, a thin x-ray beam is rotated around a specific area of the body to create a cross-sectional picture. CT scans are 10 to 20 times more sensitive than x-rays; they provide better soft-tissue detail and good detail of the vertebrae. Although CT myelograms (see below) or magnetic resonance imaging (MRI) scans are even more useful for examining the soft tissues (for example, herniated disks), CT scans are still best for studying bone problems—for example, bone destruction due to infection. CT scans are considerably more expensive and deliver more radiation than conventional x-rays.
CT Myelogram for the Back
This procedure combines a CT scan and a myelogram (also known as myelography), a diagnostic tool in which contrast dye is injected into the fluid of the spinal canal. The contrast dye illuminates the spinal canal, cord, and nerve roots during imaging. A CT scan is taken after the dye is injected; the images produced clearly show both the bony structures of the spine and the nerve structures.
The injection carries the risk of infection and side effects such as nausea, headaches, and pain or discomfort at the site of injection. People are required to sit or lie with their head elevated for six to eight hours afterward. These scans offer the best detail of bone and soft tissue. CT myelograms are usually performed only prior to surgery or after failed surgery.
MRI of the Back
During an MRI, the body is surrounded by a powerful magnet. The magnet causes atomic nuclei in the body to vibrate and give out characteristic signals, which are then converted into two- and three- dimensional images. No x-rays are involved. This test provides the clearest images of soft tissues and is completely safe. Despite its precision, studies have shown that MRI should be reserved for certain specific indications.
MRI may not generate a useful image of the bones in the spine. Therefore, it is not recommended for early diagnosis of back pain unless the doctor suspects a serious condition, such as cauda equina syndrome, sciatica, or spinal stenosis. Even when an MRI is warranted, some drawbacks must be considered.
First, the abnormalities revealed may not necessarily be the cause of the back pain. Second, because people must lie perfectly still for a long period (anywhere from 30 minutes to two hours) in a relatively small space, the procedure may be uncomfortable and difficult to tolerate, especially for people who are claustrophobic. (An open MRI produces less claustrophobia than a closed machine but generally does not provide high-quality images and is not recommended if you can tolerate a standard MRI scanner.) Third, MRI cannot be used in people with certain devices, such as implantable cardioverter-defibrillators, pacemakers, or aneurysm clips. If you have any of these devices or questions about the safety of an MRI scan in your case, it’s best to speak with your doctor about it.
Bone Scans of the Back
Bone scans measure the amount of radioactivity emitted from bone after an injection of a bone-seeking compound that contains technetium, a radioactive metal. The bone is scanned two to three hours after the injection, and information about the distribution of the radioactive compound in the bone is fed into a computer, which produces a one- or two-dimensional image of the bone.
An area of bone that shows an increased uptake of technetium has a higher rate of bone turnover, which can be the result of a fracture or nonunion that was not identified on an x-ray. This finding can also indicate the presence of metastatic cancer, Paget’s disease, or osteomyelitis (a bacterial infection of the spine).
The amount of radiation delivered is similar to that of a CT scan. This technique can be used to identify fractures in both the central and peripheral areas of the body.
Electrical Tests for the Back
Three basic types of electrical tests may be used in difficult cases involving leg pain that has persisted for over a month. These tests may help the doctor pinpoint the source of pain or numbness or suggest the cause of pain (for example, whether it is due to disk herniation or nerve damage from an unrelated disorder, such as diabetes).
Electromyography: In this test, electrodes are used to measure muscle abnormalities.
Nerve conduction and sensory-evoked potentials. These two tests examine nerve function by measuring how fast the nerves conduct impulses.
All three of the electrical tests are used only when the diagnosis is in doubt. But there is still some debate over their usefulness.