Differential Diagnoses for Radicular Back Pain
When evaluating radicular back pain, you must systematically categorize patients into three primary groups: (1) radiculopathy from herniated disc or spinal stenosis, (2) serious underlying conditions requiring urgent workup (infection, cancer, cauda equina syndrome), and (3) non-spinal mimics of radicular symptoms.
Primary Diagnostic Categories
1. Mechanical Radiculopathy (Most Common)
The most frequent causes of true radicular pain include 1:
- Herniated disc with nerve root compression - Look for sciatica (pain radiating below the knee in sciatic distribution), positive straight-leg raise test (30-70 degrees), dermatomal sensory changes, and specific motor weakness
- Spinal stenosis - Presents with neurogenic claudication (leg pain/weakness with walking or standing, relieved by sitting or spinal flexion)
- Radiculopathy - Characterized by dysfunction of a nerve root with pain, sensory impairment, weakness, or diminished deep tendon reflexes in a nerve root distribution
2. Serious Underlying Conditions (Red Flags)
These require prompt imaging with MRI or CT 1:
Immediate evaluation needed for:
- Cauda equina syndrome - Urinary retention (90% sensitivity), fecal incontinence, bilateral lower extremity weakness, saddle anesthesia
- Vertebral infection/osteomyelitis - Fever, IV drug use, recent infection, immunosuppression
- Cancer with spinal cord compression - History of cancer (increases probability from 0.7% to 9%), unexplained weight loss, age >50 years, failure to improve after 1 month, ESR ≥20 mm/h 1
- Severe or progressive neurologic deficits - Motor deficits at multiple levels, rapidly worsening symptoms
Additional serious causes:
- Spinal epidural abscess or hematoma (especially with anticoagulation) 2
- Vertebral compression fracture - Consider in patients with osteoporosis or steroid use 1
- Ankylosing spondylitis - Prevalence 0.3-5% in primary care 1
3. Non-Spinal Mimics of Radicular Pain
Pelvifemoral and orthopedic conditions that can present identically to lumbar radiculopathy 3:
- Hip pathology (labral tears, osteoarthritis, avascular necrosis)
- Piriformis syndrome
- Greater trochanteric pain syndrome
- Sacroiliac joint dysfunction
- Meralgia paresthetica
Systemic/referred causes 4:
- Vascular: Aortic aneurysm, peripheral vascular disease
- Visceral: Pancreatitis, nephrolithiasis, pelvic pathology
- Metabolic: Diabetic radiculopathy
Less common spinal causes 5:
- Synovial cysts of facet joints
- Sacral insufficiency fractures (often mistaken for metastases)
- Fatigue fractures of pars interarticularis
- Modic type 1 erosive osteochondrosis with bone marrow edema
Clinical Approach Algorithm
Step 1: Rule out emergencies
- Check for cauda equina symptoms (urinary retention is key)
- Assess for severe/progressive neurologic deficits
- Screen for infection risk factors (fever, immunosuppression)
- Evaluate cancer risk (prior history, age >50, weight loss)
Step 2: Characterize the pain pattern
- True radicular pain: Sharp, shooting pain following dermatomal distribution below the knee, with positive straight-leg raise
- Pseudoclaudication: Pain with walking/standing, relieved by sitting/flexion (suggests stenosis)
- Atypical patterns: Groin pain, lateral hip pain, or pain not following dermatomes (consider non-spinal mimics)
Step 3: Timing of imaging 6, 1
- Immediate MRI/CT: Red flags present, severe/progressive deficits
- MRI at 3 months: Persistent radicular symptoms despite conservative management in surgical candidates
- Earlier referral (within 2 weeks): Severe disabling radicular pain or neurological deficit 6
- No routine imaging: Nonspecific back pain without radicular features
Critical Pitfalls to Avoid
Do not assume degenerative changes on imaging explain symptoms - MRI findings like disc bulges are extremely common in asymptomatic individuals and correlate poorly with symptoms 1. Clinical correlation is essential.
Do not miss non-spinal causes - When symptoms are atypical (groin pain, pain not following dermatomes, lack of response to typical treatments), systematically evaluate for hip pathology, sacroiliac dysfunction, and referred visceral pain 3.
Do not delay workup in high-risk patients - For patients with history of cancer, direct MRI may be more appropriate than stepwise evaluation 1. Delayed diagnosis of infection, cancer, or cauda equina worsens outcomes 1.
Recognize that most radiculopathy improves - Natural history shows improvement within 4 weeks with conservative management in most cases 1. This informs the timing of advanced interventions.