What are the differential diagnoses for an adult patient with back pain radiating to the right outer thigh?

Differential Diagnoses for Back Pain Radiating to Right Outer Thigh

The most likely diagnosis is L2-L3 lumbar radiculopathy from disc herniation or foraminal stenosis, as the lateral thigh distribution corresponds to the L2-L3 nerve root dermatome, though meralgia paresthetica (lateral femoral cutaneous nerve entrapment) and referred pain from lumbar facet joints or sacroiliac joint pathology must also be considered. 1, 2

Primary Neurologic Differentials

L2-L3 Radiculopathy

• Most common structural cause for radiation to the lateral/outer thigh, typically from disc herniation, foraminal stenosis, or degenerative changes at the L2-L3 level 2
• Pain follows dermatomal distribution along the anterolateral thigh 2
• May be accompanied by weakness in hip flexion or knee extension if severe 2
• Responds to conservative management in most cases within 4-6 weeks 3, 1

Meralgia Paresthetica

• Entrapment of the lateral femoral cutaneous nerve, causing burning pain or paresthesias in the lateral thigh distribution 2
• Purely sensory symptoms without motor involvement 2
• Often related to tight clothing, obesity, or prolonged standing 2
• Does not originate from spinal pathology despite similar radiation pattern 2

Musculoskeletal Differentials

Lumbar Facet Joint Pain

• Facet arthropathy can produce referred pain to the lateral thigh without true radiculopathy 1, 2
• Important caveat: morphologic imaging changes of facet osteoarthritis do not correlate with pain symptoms 1
• Pain typically worse with extension and rotation movements 2
• Responds to facet joint injections if this is the primary pain generator 3

Sacroiliac Joint Dysfunction

• Can refer pain to the buttock and lateral/posterior thigh 2
• Typically provoked by single-leg stance or stair climbing 2
• Positive provocative maneuvers on physical examination (FABER test, Gaenslen's test) 2

Myofascial Pain Syndrome

• Most likely in otherwise healthy individuals after overexertion, presenting with trigger points in paraspinous or gluteal muscles 1
• Referred pain patterns can mimic radiculopathy but without dermatomal distribution 2
• Self-limited and responsive to conservative management within 4 weeks 1

Serious Pathology Requiring Urgent Evaluation (Red Flags)

Spinal Malignancy

• Consider if: history of cancer, unexplained weight loss, age >50 years, constant unrelenting pain 1, 4
• Posttest probability increases from 0.7% to 9% with history of cancer 4
• Requires urgent MRI without and with IV contrast 4

Spinal Infection/Epidural Abscess

• Red flags include: fever, recent infection, IV drug use, immunosuppression 1, 4
• Accounts for only 0.01% of back pain cases but has high morbidity if missed 4
• Requires urgent MRI with contrast and laboratory studies (CBC, ESR, CRP) 4

Compression Fracture

• High risk in patients >65 years or on chronic corticosteroids, even with minimal trauma 1
• Midline tenderness on examination is a key finding 1
• MRI can identify fractures even when radiographs are negative and determine acuity via marrow edema 1

Vascular Emergency

Aortic Dissection

• Must be ruled out first in sudden-onset thoracic back pain, particularly with hypertension or after sudden exertion 1
• Presents with sudden severe tearing pain, may radiate between shoulder blades 1
• Requires immediate cardiovascular imaging if suspected 1

Referred Pain from Non-Spinal Sources

Intra-abdominal or Retroperitoneal Pathology

• Kidney stones, pyelonephritis, or retroperitoneal hemorrhage can refer pain to the flank and lateral thigh 5
• Pain typically constant and not positional 5

Hip Pathology

• Hip osteoarthritis or trochanteric bursitis can refer pain to the lateral thigh 2
• Pain worse with weight-bearing and hip range of motion 2

Diagnostic Algorithm

Initial Assessment (No Red Flags)

• No imaging indicated for acute pain (<4 weeks) without red flags 1, 6
• Begin conservative management with NSAIDs, activity modification, and physical therapy 3, 6
• Remaining active is more effective than bed rest 3

Subacute/Chronic Pain (4-12 weeks) with Failed Conservative Management

• MRI lumbar spine without IV contrast is the imaging study of choice 1
• Evaluates for disc herniation, foraminal stenosis, and other structural causes 1

Presence of Red Flags at Any Time

• Stop conservative management and obtain urgent evaluation 3, 4
• MRI without and with contrast for suspected infection or malignancy 4
• Plain radiographs for suspected fracture, though MRI may still be needed if negative 1

Critical Pitfalls to Avoid

• Do not assume radiating leg pain always indicates nerve root compression—referred pain from discs, facets, or SI joints can produce similar patterns without true radiculopathy 7, 2
• Do not delay imaging if myelopathy signs emerge (spasticity, hyperreflexia, gait disturbance) at any point during treatment 3
• Do not assume negative radiographs rule out fracture in elderly patients or those on chronic steroids—MRI may be necessary 1
• Do not order imaging during the initial 4-week period unless red flags develop—most cases resolve with conservative management alone 1, 6
• Thoracic disc abnormalities are common in asymptomatic patients, so imaging findings do not always correlate with symptoms 1