This case presents a common problem, low back pain (LBP), which is typically benign and self-limited. However, it is occasionally the presenting symptom of serious systemic disease, such as cancer or infection, or a surgical emergency, such as with cauda equina compression.(1) The major challenge is to distinguish the vast majority of patients who have benign musculoskeletal pain from the small minority with a serious, specific disease process requiring timely intervention. Table 1 summarizes the major causes of LBP.
The initial task is to assess the likelihood of a serious underlying systemic disease without over-testing those with benign musculoskeletal pain. History is usually the key to early detection of serious causes of LBP.(2) Suspicion should be particularly high in patients whose pain is unrelieved in any position.(3-5)
The classic "red flags" for malignancy include age older than 50, previous history of cancer, unexplained weight loss (greater than 4.5 kg over 6 months), failure to improve after 1 month of therapy, and no relief with bedrest.(6) Malignancy accounts for less than 1% of cases seeking care for LBP.(7) The only finding specific enough to significantly increase the odds of malignancy is a previous history of cancer (Table 2 summarizes test characteristics for red flags in evaluating LBP). The other red flags, when present, only modestly raise the odds.(2) Pain worse at night or with recumbency, particularly when patients sleep in a chair to avoid pain, is very worrisome for malignancy or infection, though the precise sensitivity and specificity are unknown.(4,8) The most sensitive red flag is no relief with bedrest; when this is absent (i.e., pain is relieved with lying down), it significantly reduces the odds of malignancy. However, this finding is rather nonspecific; most patients who report a lack of relief with recumbency/rest will still have benign backache.(2)
In patients with none of the red flags, the probability of malignancy approaches zero.(2) LBP patients with one or more "red flags" have a pretest probability of serious systemic disease of between 1%–10%.(9) The physical examination is less helpful than the history for identifying malignancy. The majority of spinal malignancies are metastatic from breast, lung, or prostate, so these areas should be carefully evaluated when malignancy is suspected.(10) Focal bony tenderness in the midline is a moderately reliable finding for malignancy and should be explored as well.(11)
Intravenous drug use, urinary tract infection, indwelling urinary catheters, and skin infections raise the likelihood of infective spondylitis.(6) Fever strongly suggests infection but remains an insensitive marker.(4,10) Thus, while the presence of fever should raise strong concern for infective spondylitis, the absence of fever does not significantly lower the odds of infection.(4) This is particularly true when patients are taking acetaminophen or nonsteroidal anti-inflammatories for pain, as these can mask fever.
With an aging population and better treatment options for osteoporosis, compression fractures as a cause of LBP are becoming more important to recognize.(1) Compression fractures make up about 4% of LBP cases.(3) Being less than 50 years of age significantly lowers the odds of compression fracture, while being over 70 increases the odds of compression fracture.(2) A history of trauma is not particularly useful, and it does not markedly alter the odds of compression fracture.(2) Corticosteroid use is a fairly specific risk factor for compression fractures; compression fracture needs to be strongly considered in any patient using corticosteroids who presents with LBP.(2)
Cauda equina and spinal cord compression syndromes are the most important neurological entities in the differential diagnosis of LBP as they represent surgical emergencies. Cord compression can occur in the setting of spinal tumors or epidural abscesses or with massive midline intervertebral disc herniation (IDH). Fortunately, this entity is quite rare, accounting for an estimated 0.04% of LBP cases. Unilateral or bilateral leg pain, numbness, and/or weakness are common, each occurring in over 80% of cases.(10) Urinary retention is fairly sensitive and specific, with a high positive likelihood ratio and low negative likelihood ratio.(10)
In this case, the patient is young with few or no concerning features for malignancy, although we are not told enough about the features of his pain to know if it was mechanical or not. The report of fever is worrisome, and the lack of elevated temperature on examination should not be reassuring, particularly with his report of taking acetaminophen. The frequent lack of objective physical and imaging findings in patients with mechanical back pain complicates the evaluation. In this case, a significant red flag is the history of probable injection drug use (which might be misinterpreted as a red flag for "drug-seeking behavior" rather than as a clue to serious systemic illness).
Standard recommendations for the work-up of patients with "red flags" include a complete blood count, erythrocyte sedimentation rate (ESR), urinalysis, and plain radiography of the spine.(6,9) Plain radiographs of the spine have high specificity for malignancy but are relatively insensitive.(2) Infective spondylitis can be difficult to diagnose with plain radiography (2), particularly early on in the course of disease. Bone scanning has good sensitivity for infection but modest specificity.(2) Magnetic resonance imaging (MRI) has excellent sensitivity and specificity and is the test of choice in patients with a high clinical suspicion for infective spondylitis.
A detailed cost effectiveness analysis of different diagnostic strategies recommended advanced imaging (MRI or bone scan followed by MRI if the bone scan is abnormal) in patients with one or more red flags if they have either a worrisome radiograph (lytic or blastic lesion seen) or an ESR greater than 50. This strategy found 88% of all the findable cases at a cost of $40 per patient, $9525 per case found, and with only 1.6 false positives per 1000 patients.(12) The notable exception was patients with a personal history of cancer: because their pretest probability is relatively high at 10%–15%, and cord compression is a major concern, moving directly to MRI is probably warranted, even without a worrisome radiograph or an elevated ESR.
The incidence of infective endocarditis among intravenous drug users is estimated at 1%–5% per year with Staphylococcus aureus being the most common organism.(13) Clinical data described as helpful in identifying infective endocarditis include fever, anorexia, weight loss, and back pain.(14) Back pain is present (but may not be the chief complaint or a prominent symptom) in up to 43% of cases of endocarditis—in one case series of Staphylococcus aureus endocarditis, back pain was the chief presenting complaint in almost 10%.(15,16)
The pathogenesis of back pain with infective endocarditis is often not known but can include septic embolization, renal or splenic infarction, myalgias/arthralgias related to the inflammatory response, or infective spondylitis with or without epidural abscess.(15,17) While frank infective spondylitis has been reported to be rare in endocarditis (17), in one recent series it was present in 15% of cases.(15) In this case, the patient rapidly progressed to severe systemic infection, which may or may not have started as infective spondylitis.
The major risk factor contributing to this patient's serious systemic illness may have contributed to the missed diagnosis—his injection drug use. Care of patients with substance abuse is challenging. These patients may be regarded as "sociopaths, a burden to society, manipulative, and not intelligent enough to make a choice or decision."(18) As a result, health care teams may assign little priority to the evaluation and treatment of pain in these patients.(18) Effective principles for engaging drug users in health care relationships include a respectful approach to substance users, understanding the medical and behavioral sequelae of addiction, use of multidisciplinary teams, and refraining from moralistic judgments.(19) More careful attention to the patient's history of fever and injection drug use at the first two visits might well have led to a more timely diagnosis.
Maintaining proper vigilance for potentially "dangerous" causes of back pain without performing unnecessary diagnostic work-ups in the large numbers of patients seeking health care for simple back pain is a difficult task. The Clinical Practice Guideline published by the then Agency for Health Care Policy and Research on "Acute Low Back Problems in Adults" (6) continues to be a helpful and important resource with simple algorithms that remain highly relevant aids for physicians faced with decisions of diagnostic triage in the acute setting.
| Regional Mechanical Low Back Pain (greater than or equal to 90%) |
| Nonspecific mechanical low back pain (sprain, strain, lumbago, etc.) |
| Degenerative changes in discs and/or facet joints |
| Osteoporotic compression fractures |
| Traumatic fractures |
| Deformity (severe scoliosis, kyphosis, spondylolisthesis) |
| Mechanical Low Back Pain with Neurogenic Leg Pain (7%–10%) |
| Intervertebral disc herniation |
| Spinal stenosis |
| Spinal stenosis associated with degenerative spondylolisthesis |
| Non-Mechanical Spine Disorders (less than or equal to 1%) |
| Neoplasia (metastases, lymphoid tumors, spinal cord tumors, etc.) |
| Infection (infective spondylitis, epidural abscess, endocarditis, herpes zoster, lyme) |
| Seronegative spondyloarthritides (ankylosing spondylitis, psoriatic arthritis, reactive arthritis, Reiter's syndrome, inflammatory bowel disease) |
| Other: usually present with other accompanying symptoms |
| Pelvic (prostatitis, endometriosis, pelvic inflammatory disease) |
| Renal (nephrolithiasis, pyelonephritis, renal papillary necrosis) |
| Aortic aneurysm |
| Gastrointestinal (pancreatitis, cholecystitis, peptic ulcer disease) |
| Paget's disease |
| Parathyroid disease |
| Hemoglobinopathies |
