Diagnostic Evaluation of Low Back Pain with Emphasis on Imaging

Low back pain is a pervasive problem that affects two thirds of adults at some time in their lives. It ranks among the top 10 reasons for visits to internists (1, 2) and is the most common and expensive reason for work disability in the United States (3). Most often, back pain is benign and self-limited. However, it is occasionally the presenting symptom of such systemic diseases as cancer or infection. Some causes of back pain, especially those with neurologic symptoms, are surgically remediable. Thus, the major diagnostic task is to distinguish the 95% of patients with simple back pain from the 5% with serious underlying diseases or neurologic impairments. Differential Diagnosis The differential diagnosis of low back pain can be divided into mechanical causes (no primary inflammatory or neoplastic cause), visceral causes (no primary involvement of the spine), and all others (4) (Table 1). A definitive diagnosis cannot be made in as many as 85% of patients because symptoms and pathologic changes are not closely associated (9). Many cases of uncomplicated low back pain are assumed to result from muscle sprains and strains, ligamentous injuries, and spinal degenerative changes. Table 1. Differential Diagnosis of Low Back Pain Disc herniation with nerve root compression or irritation is the most common cause of neurologic abnormalities. Spinal stenosis may also be associated with leg symptoms and neurologic abnormalities, often involving both sides of the body and multiple nerve roots. Spinal instability (in the absence of fractures or spondylolisthesis) remains a controversial diagnosis. It is often identified by the finding of vertebral slippage on flexion radiographs (10). The prevalence of spinal instability in asymptomatic persons is unclear, as is the degree to which this condition causes pain. The diagnosis of internal disc disruption, identified by provocative discography (injection of contrast material into the disc with simultaneous assessment of pain), is even more controversial. Discography frequently generates pain in asymptomatic adults (11), and symptoms attributed to internal disc disruption often improve spontaneously (12). The true significance and appropriate management of this condition remain unclear. When the precise anatomic sources of pain cannot be determined, early diagnostic evaluation that focuses on three basic questions is useful: 1) Is there underlying systemic disease? 2) Is there neurologic impairment that might require surgical evaluation? 3) Is social or psychological distress amplifying or prolonging the pain? (7). For most young or middle-aged adults, these questions can be answered on the basis of history and physical examination alone; diagnostic testing is infrequently required. Pretest Probability of Disorders that Cause Back Pain Systemic Diseases In primary-care settings, about 0.7% of patients with back pain have metastatic cancer. About 0.01% have spinal infections, and 4% have osteoporotic compression fractures. Only about 0.3% have ankylosing spondylitis (7). Taking a history is more useful than physical examination in screening for underlying malignancy, at least in the early stages (Table 2) (7, 10). A history of cancer mandates further evaluation. The most common sources of metastatic cancer are the breast, lung, and prostate; these areas should be examined when cancer is suspected. Table 2. Estimated Accuracy of the History in the Diagnosis of Spinal Diseases That Cause Low Back Pain Spinal infections are usually acquired hematogenously from other sites. Common underlying infections are related to injection drug use, urinary tract infection, or skin infection. Compression fractures in older adults usually result from osteoporosis. Only about 30% of such patients have identifiable trauma. The prevalence of this condition is substantially associated with race: African-American and Mexican-American women have 25% fewer compression fractures than white women (18). Ankylosing spondylitis and other inflammatory spondyloarthropathies occur rarely, and the predictive value of positive findings is low (19). Tests of sacroiliac joint tenderness are poorly reproducible and inaccurate in distinguishing ankylosing spondylitis from mechanical spinal conditions (7). Neurologic Impairment The first clinical clue to neurologic impairment usually is a history of sciatica: sharp pain radiating down the posterior or lateral aspect of the leg, often associated with numbness or paresthesia. Pain radiating below the knee as opposed to pain limited to the buttocks or thigh is more likely to represent true radiculopathy. Pain is sometimes aggravated by coughing, sneezing, or the Valsalva maneuver. The most common cause of sciatica is a herniated intervertebral disc, which occurs most often between the ages of 30 and 55 years. Imaging identifies herniated discs in many persons with low back pain (20-23); thus, only a minority of these discs are therapeutically important (14, 24). More than 95% of clinically important lumbar disc herniations occur at the two lowest discs and involve the L5 or S1 nerve roots. Thus, the most common neurologic syndromes are weakness of the ankle and great toe dorsiflexors and sensory loss along the medial foot (L5), or weakness of ankle plantar flexion, diminished ankle reflex, and sensory loss along the lateral aspect of the foot (S1) (Table 2). Combinations of findings have not been evaluated but are probably more useful than any single finding (7, 25). Spinal stenosis may be caused by bone (for example, facet hypertrophy), soft tissue (for example, bulging disc or thickened ligamentum flavum), or both. Like other degenerative conditions, it is most common in older adults. As many as 20% of asymptomatic adults age 60 years or older have imaging evidence of spinal stenosis (20), but the prevalence of symptomatic stenosis is unknown. The classic symptom of spinal stenosis is neurogenic claudication, which is leg pain that mimics arterial claudication (26). Compared with arterial claudication, neurogenic claudication is more likely to occur simply with standing. Numbness and tingling are common, and symptoms often worsen with coughing or sneezing. Perhaps the most useful finding is a history of no pain when the patient is seated with the spine flexed (Table 2) (16). Another neurologic condition is the cauda equina syndrome, which may result from a massive midline disc hernia that causes compression of the cauda equina. It is a surgical emergency that requires immediate referral. The syndrome represents only 1% to 2% of lumbar disc herniations that require surgery. Prevalence among all patients with low back pain has been estimated at 0.0004 (7). The most consistent symptom is urinary retention. Unilateral or bilateral sciatica, sensory and motor deficits, and abnormal straight-leg raising are common. Sensory deficits over the buttocks, thighs, and perineal region (saddle anesthesia) and reduced anal sphincter tone occur in about 75% of patients (7). Diagnostic Test Description To review the diagnostic imaging literature, we performed a MEDLINE search of articles published between January 1966 and September 2001. Methods used for the search strategy are available in the Appendix. We sequentially reviewed all article titles (n = 1468). We then read the abstracts of 568 articles that seemed pertinent and the full text of 150 of these 568 articles. The authors and their affiliations were masked. Disagreements on whether particular articles should be included (approximately 15% of total articles) were settled by consensus. The data for this article were collected only from articles in the MEDLINE search. In primary care settings, the most common spine imaging tests are plain radiography, computed tomography (CT), magnetic resonance imaging (MRI), and bone scanning. Other tests (myelography, discography, and positron emission tomography) are usually ordered by specialists before surgical intervention and were not reviewed. The estimated diagnostic accuracy of these imaging techniques are given in Table 3. Table 3. Estimated Accuracy of Imaging Technique for Lumbar Spine Conditions Biases were common in the studies reviewed. The most common biases were failure to apply a single reference test to all patients, test review bias (study test was reviewed with knowledge of the final diagnosis), diagnosis review bias (determination of the final diagnosis was affected by the study test), and spectrum bias (only severe cases of disease were included). Most studies had several potential biases, and estimates of sensitivity and specificity must be considered imprecise. Plain Radiography Overview Low cost and ready availability make plain radiography the most common spinal imaging test. Several investigators (27, 28) have recommended discontinuing use of routine oblique and spot lateral views because they do not provide adequate clinically relevant findings. This position was adopted in the Agency for Health Care Policy and Research (AHCPR) guidelines (29). The anteroposterior and lateral views demonstrate alignment, disc and vertebral body height, and gross assessment of bone density and architecture; however, soft tissue structures are not evaluated extensively by these views. Oblique views show the pars interarticularis in profile and are useful for diagnosing spondylolysis when clinical evidence exists. Other special views include flexion and extension views to assess instability and angled views of the sacrum to assess sacroiliac joints for ankylosing spondylitis. Lumbar radiography may be harmful because it exposes the gonads to ionizing radiation, especially with oblique views or repeated exposures. This is a particular concern for younger female patients. The radiation exposure of oblique views is double the exposure of standard views, which alone are equivalent to the female gonadal radiation of daily chest radiography for several years (30-32). Plain radiography ide