Diagnostic Studies

Prior to ordering imaging studies the clinician should have noted either the emergence of a red flag or physiologic evidence of tissue insult or neurologic dysfunction.15

Plain x-rays of the lumbar spine are recommended for ruling out fractures in patients with acute low back problems when any of these red flags are present: recent significant trauma (any age), recent mild trauma (patient age over 50), history of prolonged steroid use, osteoporosis, or patient age over 70. Oblique views are not necessary.14 Laboratory tests such as erythrocyte sedimentation rate (ESR), complete blood count (CBC), and urinalysis (UA) can be useful to screen for nonspecific medical diseases (especially infection and tumor) of the low back.15 Plain x-rays in combination with CBC and ESR may be useful for ruling out tumor or infection in patients with acute low back problems when any of these red flags are present: prior cancer or recent infection, fever over 100°F, IV drug abuse, prolonged steroid use, low back pain worse with rest, or unexplained weight loss. If tumor or infection is suspected, CT or MRI may be considered in the presence of red flags, even if plain films are negative. In the presence of red flags suggesting cauda equina syndrome or progressive major motor weakness, prompt use of CT, MRI, myelography, or CT-myelography is recommended. Because these conditions may require timely surgical intervention, planning and choice of study is best carried out in consultation with a surgeon. In patients with acute back problems who have had prior back surgery, MRI with contrast is the imaging test of choice because it can help distinguish disk herniation from scar tissue from previous surgery.14 A bone scan can detect physiologic reactions to suspected spinal tumor, infection, or occult fracture.15

When the neurologic examination is less clear, however, further physiologic evidence of nerve root dysfunction should be considered before ordering an imaging study. Electromyography (EMG), including H-reflex tests, may be useful to identify subtle focal neurologic dysfunction in patients with leg symptoms lasting longer than 3 to 4 weeks. Sensory evoked potentials (SEPs) may be added to the assessment if spinal stenosis or spinal cord myelopathy is suspected.15

There is a 20 to 40 percent false-positive rate with plain films, CT, MRI, and myelogram. The altered anatomic structures identified may not be responsible for the patient's pain. Most findings on these studies are consistent with normal degenerative changes and are also found in asymptomatic individuals. Any imaging abnormalities must correlate with the patient's symptoms and signs to be of value in guiding treatment.

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