92 Low Back Pain
Key Points• Nonspecific low back pain is usually self-limited and of short duration: about 60% of cases will resolve within 1 week and 90% within 2 to 6 weeks.
• Clues pointing to inflammatory, infectious, and oncologic disorders as possible causes of low back pain may be subtle and easily missed.
• Testing should be directed toward specific diagnostic concerns rather than screening studies.
• The primary treatment option for back pain is nonsteroidal antiinflammatory drugs along with judicious opioid use.
• Skeletal muscle relaxants and steroids have not been shown to improve outcomes and have significant side effects, although some patients do report relief.
• Patients who have pain longer than 2 weeks or in whom leg weakness, bowel or bladder dysfunction, fever, or new adverse symptoms develop at any time should be reevaluated.
Epidemiology
Most adults will experience LBP at some point in their lifetime, and about one in five adults experience LBP within a single year. LBP costs billions of dollars per year. About 85% to 90% of patients with LBP in a variety of outpatient settings are considered to have nonspecific LBP.1 This may lead to physician complacence in the evaluation of a presumably benign disorder.
Conversely, LBP can be incapacitating, and the patient may perceive the problem as a harbinger of death or disability, regardless of the cause. Patient expectations of a specific diagnosis of the pain source and a complete cure are rarely met and are probably unrealistic. Psychologic, social, and economic factors play a role in the natural history of many pain-related conditions, including progression of LBP from an acute to a chronic condition. LBP often becomes a chronic condition subject to exacerbations, akin to asthma and diabetes mellitus.2 These issues combine to make LBP a source of frustration for patients and physicians alike.
Pathophysiology
LBP is often classified as specific or nonspecific, mechanical or nonmechanical, or primary or secondary or is classified on the basis of presumed etiology (structural, neoplastic, referred pain or visceral, infectious, inflammatory, or metabolic). All these classification systems recognize that in the majority of cases a specific pathoanatomic diagnosis cannot be assigned. Indeed, clear correlation between a specific anatomic abnormality and pain is rarely established in patients, and proposed mechanisms of pain in the medical literature have been fraught with controversy. For example, many asymptomatic subjects have evidence of bulging, prolapsed, or herniated intervertebral disks.3
Differential Diagnosis and Medical Decision Making
Nonmechanical causes must be distinguished from mechanical causes of LBP. The key to such distinction rests on eliminating systemic (infectious, neoplastic, metabolic, or inflammatory) and visceral causes. Figure 92.1 shows one diagnostic approach, and Box 92.1 lists the differential diagnosis for LBP.
Box 92.1
Differential Diagnosis of Low Back Pain*
Mechanical Lower Back or Leg Pain (97%)†
Nonmechanical Spine Conditions (≈1%)**
From Jarvik JG, Deyo RA. Diagnostic evaluation of low back pain with emphasis on imaging. Ann Intern Med 2002;137:586–97.
* Figures in parentheses indicate the estimated percentages of patients with these conditions among all adult patients with low back pain in primary care. Diagnoses shown in italics are often associated with neurogenic leg pain. Percentages may vary substantially according to demographic characteristics or referral patterns in a practice. For example, spinal stenosis and osteoporosis are more common in geriatric patients, spinal infection in injection drug users, and so forth.
† The term mechanical is used here to designate an anatomic or functional abnormality without underlying malignant, neoplastic, or inflammatory disease. Approximately 2% of cases of mechanical low back or leg pain are accounted for by spondylolysis, internal disk disruption, or discogenic low back pain and presumed instability.
‡ Strain and sprain are nonspecific terms with no pathoanatomic confirmation. Nonspecific low back pain or idiopathic low back pain may be a preferable term.
§ Spondylolysis is as common in asymptomatic persons as in those with low back pain; thus its role in causing low back pain remains ambiguous.
¶ Internal disk disruption is diagnosed by provocative diskography (injection of contrast material into a degenerated disk with assessment of pain at the time of injection). However, diskography often causes pain in asymptomatic adults, and the condition in many patients with positive diskogram findings improves spontaneously. Thus the clinical importance and appropriate management of this condition remain unclear. The term diskogenic lower back pain is used more or less synonymously with the term internal disk disruption.
¶ Presumed instability is loosely defined as greater than 10 degrees of angulation or 4 mm of vertebral displacement on lateral flexion and extension radiographs. However, the diagnostic criteria, natural history, and surgical indications remain controversial.
** Scheuermann disease and Paget disease of bone probably account for less than 0.01% of nonmechanical spinal conditions.
Specific causes of LBP are shown in Table 92.1, together with red flag signs or symptoms suggesting these diagnoses. When the history and physical examination suggest a nonmechanical cause of LBP, appropriate diagnostic testing or specialty consultation (or both) is necessary to confirm or rule out the suspected specific cause or causes.
Table 92.1 “Red Flags” in the History and Physical Examination of Patients with Low Back Pain
| DISORDER | HISTORY | PHYSICAL EXAMINATION |
|---|---|---|
| All | Duration of pain >1 mo Bed rest with no relief Age < 20 or >50 yr* |
|
| Cancer | Age ≥ 50 yr Previous cancer history Unexplained weight loss† |
Neurologic findings‡ Lymphadenopathy |
| Compression fracture | Age ≥ 50 years (≥60 yr more specific) Significant trauma§ History of osteoporosis Corticosteroid use Substance abuse‖ |
Fever (>100° F [38° C]) Tenderness of spinous processes |
