Back Pain: What Could Be Causing It?

The Scope of Back Pain as a Clinical Problem

Back pain is not a diagnosis. It is a symptom with a differential list that spans orthopedic, neurological, rheumatologic, oncologic, vascular, and visceral categories. The 2021 Global Burden of Disease study ranked low back pain as the leading cause of years lived with disability across 204 countries, affecting an estimated 619 million people in 2020.

In the United States, back pain accounts for more than 2.6 million emergency department visits per year according to CDC National Hospital Ambulatory Medical Care Survey data. Direct annual healthcare costs exceed $12 billion, and indirect costs from lost productivity push total economic impact well above $100 billion [1]. Despite its frequency, back pain remains poorly managed in many settings. A 2020 analysis in The Lancet noted that low-value care (early imaging, opioid prescribing, unnecessary surgery) persists across high-income countries [2].

The first clinical question is not "what treatment should I try?" The first question is "what structure or process is generating this pain?" That distinction shapes everything from imaging decisions to prognosis.

Mechanical and Nonspecific Back Pain: The 85% Category

The overwhelming majority of back pain is mechanical. Roughly 85% of patients who present to primary care receive a diagnosis of "nonspecific low back pain," meaning imaging and lab work reveal no identifiable structural pathology [3]. That label is frustrating but clinically meaningful: it tells the clinician that no dangerous cause is present.

Common mechanical contributors include lumbar muscle strain, ligamentous sprain, facet joint irritation, and early degenerative disc changes. These overlap significantly on physical exam, which is why the American College of Physicians (ACP) 2017 guideline discourages routine imaging for acute nonspecific low back pain. Dr. Amir Qaseem, then vice president of clinical policy at the ACP, stated: "Imaging does not improve outcomes for patients without red-flag features and frequently identifies incidental findings that lead to unnecessary procedures" [4].

Mechanical pain typically worsens with specific movements (flexion, extension, rotation) and improves with rest or position change. Pain duration matters. Acute episodes (<4 weeks) carry a favorable natural history. Subacute pain (4 to 12 weeks) warrants closer monitoring. Chronic pain (>12 weeks) shifts the management approach toward multidisciplinary rehabilitation.

Risk factors for progression from acute to chronic mechanical back pain include obesity (BMI ≥30), smoking, sedentary occupation, psychosocial distress, and catastrophizing thought patterns. A 2019 systematic review in The BMJ identified early return to activity and structured exercise as the strongest protective factors against chronicity [5].

Disc Herniation and Radiculopathy

When back pain radiates below the knee in a dermatomal pattern, disc herniation with nerve root compression enters the differential. The L4-L5 and L5-S1 levels account for more than 90% of lumbar herniations [6]. Sciatica (pain along the sciatic nerve distribution) is the hallmark symptom, though numbness, tingling, and focal weakness can also appear.

The straight-leg raise test remains a useful bedside screen. A positive result (reproduction of radicular pain at 30 to 70 degrees of passive leg elevation) has a sensitivity of approximately 91% for L5 or S1 root involvement according to a Cochrane review [7]. Specificity is lower (26%), so a positive test prompts further evaluation rather than confirming a surgical diagnosis.

MRI is the imaging modality of choice when radiculopathy is suspected and symptoms persist beyond six weeks or when progressive motor deficit is present. A key nuance: disc herniations are common on MRI in asymptomatic individuals. A landmark study by Jensen et al. (1994) found disc bulges in 52% and protrusions in 27% of adults with no back pain at all [8]. Correlation between imaging findings and clinical symptoms is mandatory before attributing a patient's pain to a visible disc abnormality.

Most radiculopathy resolves without surgery. The Spine Patient Outcomes Research Trial (SPORT) showed that surgical discectomy provided faster pain relief than nonoperative care, but outcomes converged by four years [9]. Surgery is typically reserved for cauda equina syndrome, progressive motor weakness, or pain refractory to 6 to 12 weeks of conservative management.

Spinal Stenosis

Spinal stenosis is the progressive narrowing of the central canal or neural foramina, most often from a combination of disc degeneration, facet hypertrophy, and ligamentum flavum thickening. It is the most common indication for spine surgery in adults over 65 [10].

The classic presentation is neurogenic claudication: bilateral leg heaviness, pain, or paresthesias that worsen with standing or walking and improve with sitting or forward flexion. This "shopping cart sign" (patients lean on a cart for relief) distinguishes neurogenic from vascular claudication, which is tied to lower-extremity arterial insufficiency.

Severity grading on MRI uses the Schizas classification or cross-sectional area of the dural sac. Moderate-to-severe stenosis corresponds to a dural sac area below 100 mm² [11]. The SPORT trial for stenosis demonstrated that decompressive laminectomy provided greater symptom improvement than nonoperative treatment at two years (SF-36 bodily pain improvement of 18.5 points vs. 11.1 points) [12].

Conservative options include physical therapy emphasizing flexion-based exercises, epidural steroid injections, and activity modification. Gabapentinoids (gabapentin, pregabalin) are sometimes used off-label for associated neuropathic pain, though evidence for their efficacy in stenosis-specific claudication is mixed.

Inflammatory and Autoimmune Causes

Not all chronic back pain is mechanical. Inflammatory back pain (IBP) suggests an underlying spondyloarthritis, most commonly ankylosing spondylitis or non-radiographic axial spondyloarthropathy. IBP affects roughly 5 to 6% of chronic back pain patients [13].

Four features distinguish IBP from mechanical pain: onset before age 40, insidious onset (not acute injury), improvement with exercise, and no improvement with rest. Morning stiffness lasting more than 30 minutes is a fifth feature recognized by the Assessment of SpondyloArthritis International Society (ASAS) criteria [14]. Meeting four of five criteria yields a sensitivity of 77% and specificity of 91.7% for IBP.

HLA-B27 testing is a useful adjunct. Roughly 90% of patients with ankylosing spondylitis are HLA-B27 positive, though the allele is present in 6 to 8% of the general white population [15]. MRI of the sacroiliac joints can detect bone marrow edema and early inflammatory changes before radiographic sacroiliitis becomes visible on plain films.

Diagnostic delay for ankylosing spondylitis averages 7 to 10 years from symptom onset. Dr. Atul Deodhar, professor of rheumatology at Oregon Health & Science University, has noted: "The average patient with axial spondyloarthritis sees four to five physicians and waits nearly a decade before receiving the correct diagnosis" [16]. This gap matters because early treatment with TNF-alpha inhibitors or IL-17 inhibitors can slow radiographic progression and preserve spinal mobility.

Visceral and Referred Causes

The lumbar spine shares segmental innervation with several abdominal and retroperitoneal organs, which means back pain can originate far from the spine itself.

Renal pathology is among the most common visceral mimics. Kidney stones produce acute, colicky flank pain that radiates to the groin. Pyelonephritis causes constant flank pain with fever and costovertebral angle tenderness. A 2018 analysis in the Annals of Emergency Medicine found that 9% of patients presenting to the ED with isolated back pain had a urological diagnosis [17].

Abdominal aortic aneurysm (AAA) is a life-threatening cause of back pain in patients over 60, particularly male smokers. The pain is typically deep, constant, pulsatile, and not affected by spinal movement. The U.S. Preventive Services Task Force recommends one-time abdominal ultrasound screening for AAA in men aged 65 to 75 who have ever smoked [18]. Ruptured AAA carries a mortality rate above 80% if not surgically repaired within hours.

Other visceral sources include pancreatitis (epigastric pain radiating to the back), endometriosis (cyclical low back pain in premenopausal women), and retroperitoneal lymphadenopathy from lymphoma or testicular cancer. The clinical clue for visceral origin is pain that does not change with spinal movement and does not reproduce on palpation of paraspinal structures.

Red-Flag Symptoms That Require Urgent Evaluation

Most back pain is safe to observe for four to six weeks with conservative measures. A small subset of presentations carries serious risk. The mnemonic "TUNA FISH" is used in some residency training programs to capture the major red flags: Trauma, Unexplained weight loss, Neurological deficit (progressive), Age over 50 with new onset, Fever, IV drug use, Steroid use (chronic), and History of cancer.

Cauda equina syndrome (CES) is the most time-sensitive spinal emergency. It results from massive central disc herniation or other compressive lesion affecting the cauda equina nerve roots below the conus medullaris (typically L1-L2). Symptoms include bilateral leg pain or weakness, saddle anesthesia (numbness of the perineum, buttocks, and inner thighs), and bladder or bowel dysfunction (urinary retention is the most specific finding). CES requires MRI and surgical decompression within 24 to 48 hours to prevent permanent neurological injury [19].

Spinal infection (vertebral osteomyelitis, epidural abscess) should be suspected in patients with back pain plus fever, recent bacteremia, injection drug use, or immunosuppression. Blood cultures, inflammatory markers (ESR, CRP), and contrast-enhanced MRI are the diagnostic triad. Staphylococcus aureus causes roughly 50 to 60% of vertebral osteomyelitis cases [20].

Metastatic spinal disease affects 5 to 10% of cancer patients. Back pain that is worse at night, unresponsive to position change, and accompanied by unintentional weight loss of more than 4.5 kg warrants cancer screening. Breast, lung, prostate, kidney, and thyroid cancers account for the majority of spinal metastases [21].

How Back Pain Is Diagnosed

The diagnostic evaluation of back pain follows a tiered approach: history and physical examination first, imaging second (and only when indicated), and laboratory testing when inflammatory, infectious, or malignant causes are suspected.

History should capture pain onset (acute vs. insidious), duration, location, radiation pattern, aggravating and relieving factors, associated neurological symptoms, constitutional symptoms (fever, weight loss, night sweats), trauma history, and prior cancer history. The 2007 joint guideline from the ACP and the American Pain Society stratified patients into three categories: nonspecific low back pain, back pain with radiculopathy, and back pain with suspected serious underlying condition [22].

Physical examination includes inspection of posture and gait, palpation of spinous processes and paraspinal musculature, range-of-motion testing, straight-leg raise, crossed straight-leg raise, and focused neurological assessment (motor strength, sensation, deep tendon reflexes at L4, L5, and S1). Provocative tests for sacroiliac joint pain (FABER, compression, distraction) add specificity when three or more are positive [23].

Imaging guidelines are conservative. The American College of Radiology Appropriateness Criteria rates plain radiographs as "usually not appropriate" for acute low back pain without red flags [24]. MRI is indicated when red flags are present, when symptoms fail to improve after six weeks of conservative care, or when surgical planning is needed. CT is an alternative when MRI is contraindicated (pacemaker, severe claustrophobia).

Lab work is situation-dependent. ESR and CRP are sensitive (though not specific) for infection and malignancy. HLA-B27 and sacroiliac MRI are appropriate when inflammatory spondyloarthritis is suspected. Complete blood count, basic metabolic panel, and urinalysis screen for systemic disease and renal pathology.

Treatment for Back Pain: First-Line and Beyond

Treatment selection depends entirely on the identified or suspected cause. For nonspecific mechanical low back pain, the ACP 2017 guideline recommends nonpharmacologic therapy first: superficial heat, massage, acupuncture, or spinal manipulation for acute pain, and structured exercise, cognitive behavioral therapy, or multidisciplinary rehabilitation for chronic pain [4].

When medication is needed, NSAIDs (ibuprofen 400 to 600 mg every 6 to 8 hours, naproxen 250 to 500 mg twice daily) are first-line pharmacotherapy. A 2017 Cochrane review found NSAIDs provided modest but significant pain reduction compared to placebo (mean difference of 7.3 points on a 100-point scale) [25]. Skeletal muscle relaxants (cyclobenzaprine, tizanidine) may be added short-term for acute spasm.

Opioids have a limited role. The ACP guideline lists tramadol and duloxetine as second-line options and advises against opioid therapy unless all other approaches have failed. A 2018 JAMA trial (the SPACE trial) randomized 240 patients with chronic back or hip pain to opioid vs. nonopioid therapy for 12 months and found no significant difference in pain-related function (mean difference 0.1 on a 0-10 scale) while opioid-treated patients reported more side effects [26].

For radiculopathy, epidural steroid injections may provide short-term relief (2 to 6 weeks), though long-term benefit is not established [27]. Physical therapy focusing on nerve-glide exercises and core stabilization is the backbone of nonsurgical management.

Surgical options vary by diagnosis. Microdiscectomy for symptomatic disc herniation, decompressive laminectomy for stenosis, and spinal fusion for confirmed instability each have specific indications. The trend in spine surgery favors minimally invasive approaches that reduce blood loss, hospitalization, and recovery time. Total surgical rates remain below 5% of all back pain presentations.

The Role of Hormonal and Metabolic Factors

Hormonal status influences spinal health in ways that are underappreciated in routine back pain evaluations. Estrogen deficiency accelerates disc degeneration and reduces bone mineral density, increasing fracture risk. Postmenopausal women have a two to fourfold higher rate of vertebral compression fractures compared to age-matched premenopausal women [28].

Testosterone deficiency in men is associated with reduced paraspinal muscle mass, lower bone density, and increased fat infiltration of the lumbar multifidus, a stabilizing muscle whose atrophy correlates with chronic low back pain recurrence. A 2020 cross-sectional analysis found that men with total testosterone below 300 ng/dL had significantly higher odds of reporting chronic low back pain (OR 1.43, 95% CI 1.12 to 1.82) after adjusting for age, BMI, and activity level [29].

Vitamin D deficiency (serum 25-hydroxyvitamin D <20 ng/mL) is another metabolic contributor. A meta-analysis in Pain Physician (2017) pooling 29 observational studies found a significant association between vitamin D deficiency and chronic low back pain [30]. Whether supplementation improves pain outcomes remains under investigation, but screening for 25-hydroxyvitamin D in patients with chronic back pain and known risk factors (limited sun exposure, darker skin, obesity) is reasonable.

GLP-1 receptor agonists, now widely prescribed for obesity, may indirectly benefit mechanical back pain by reducing body weight and axial loading on the lumbar spine. In the STEP-1 trial (N=1,961), semaglutide 2.4 mg produced 14.9% mean body weight loss at 68 weeks vs. 2.4% with placebo [31]. While no randomized trial has directly measured back pain as a primary endpoint with GLP-1 therapy, the biomechanical logic is straightforward: every 1 kg of weight loss removes approximately 4 kg of compressive force from the lumbar spine during walking.