What could be the cause of unilateral low back pain and stiffness in the absence of an injury?

Unilateral Low Back Pain Without Injury: Differential Diagnosis

In the absence of trauma, unilateral low back pain with stiffness is most commonly nonspecific mechanical low back pain arising from musculoskeletal structures (facet joints, intervertebral discs, muscles, or fascia), though you must systematically exclude serious pathology through red flag assessment before settling on this diagnosis. 1, 2

Initial Diagnostic Categorization

The American College of Physicians recommends classifying patients into three categories: 1, 3

• Nonspecific low back pain (most common) - pain primarily in the back with no signs of serious underlying condition, spinal stenosis, radiculopathy, or another specific spinal cause 2
• Back pain with radiculopathy or spinal stenosis - characterized by leg symptoms
• Back pain with another specific spinal cause - identified through red flags

Critical Red Flag Assessment

Before attributing unilateral pain to mechanical causes, you must actively screen for serious pathology: 1

Malignancy Indicators

• History of cancer (increases posttest probability from 0.7% to 9%) 1
• Unexplained weight loss 1
• Age older than 50 years 1
• Failure to improve after 1 month 1

Infection Indicators

• Fever 1
• Recent infection 1
• Intravenous drug use 1
• Immunocompromised status 1

Cauda Equina Syndrome (Requires Urgent Imaging)

• Urinary retention (90% sensitivity) 1, 3
• Fecal incontinence 1
• Saddle anesthesia 1
• Motor deficits at multiple levels 1

Vertebral Compression Fracture

• Older age 1
• History of osteoporosis 1
• Steroid use 1

Specific Etiologies for Unilateral Presentation

Facet Joint Pain

Mechanical pain from facet joints commonly presents unilaterally and is considered part of nonspecific low back pain. 4 Degenerative changes on imaging correlate poorly with symptoms and are usually nonspecific. 2

Sacroiliac Joint Dysfunction

While not extensively detailed in the guidelines, sacroiliac joint pain is a recognized source of unilateral low back pain. 5

Myofascial Pain

Recent evidence demonstrates that quadratus lumborum muscle stiffness correlates with pain intensity and central sensitization in chronic low back pain. 6 Greater muscle softness (paradoxically) is linked to higher pain intensity and poorer physical quality of life. 6

Discogenic Pain

Nonspecific low back pain can arise from intervertebral discs, though MRI-documented disc spaces that appear normal should not be considered as a pain source. 4 Degenerative changes are common and often asymptomatic. 2

Radiculopathy (If Leg Symptoms Present)

If the patient has leg pain in a typical lumbar nerve root distribution, consider herniated disc causing radiculopathy. 3 Assess for:

• L4 nerve root: knee strength and reflexes 3
• L5 nerve root: great toe and foot dorsiflexion strength 3
• S1 nerve root: foot plantarflexion and ankle reflexes 3

Spinal Stenosis (If Pseudoclaudication Present)

Leg pain and weakness with walking or standing, relieved by sitting or spinal flexion, suggests neurogenic claudication from spinal stenosis. 2

Psychosocial Factors (Critical for Prognosis)

Psychosocial factors are stronger predictors of low back pain outcomes than physical examination findings or pain severity. 1, 3 Screen for: 1

• Depression
• Passive coping strategies
• Job dissatisfaction
• Higher disability levels
• Disputed compensation claims
• Somatization

Imaging Recommendations

When NOT to Image

• Avoid routine imaging for nonspecific low back pain without red flags - it does not improve outcomes and exposes patients to unnecessary radiation. 1
• Imaging should be delayed for at least 4-6 weeks in nonspecific cases. 7, 8

When to Image Immediately

• Red flags present (do not delay the usual 4-6 weeks) 1
• MRI is preferred over plain radiography for soft tissue evaluation and nerve root visualization 3
• Plain radiography is appropriate for suspected fracture and bony abnormality 7

Common Pitfalls to Avoid

• Do not over-interpret imaging findings: Degenerative changes are poorly correlated with symptoms and are often present in asymptomatic individuals. 2, 9
• Do not use discography as a stand-alone test: It requires both concordant pain response and morphological abnormalities before directing treatment. 4
• Do not ignore psychosocial factors: They predict outcomes more strongly than physical findings. 1
• Do not assume unilateral presentation excludes serious pathology: Systematically assess red flags regardless of pain laterality. 1