What are the potential causes and management options for a patient with acute, intermittent left lateral thigh pain without back pain, after a blood clot has been ruled out, and who has a history of dark urine?

Differential Diagnosis for Acute Lateral Thigh Pain with History of Dark Urine

The combination of acute lateral thigh pain with recent dark urine that has resolved strongly suggests rhabdomyolysis with possible meralgia paresthetica, and you should immediately check creatine kinase (CK), serum creatinine, and urine myoglobin to assess for muscle breakdown and renal injury.

Critical Initial Laboratory Workup

• Check CK levels immediately to evaluate for rhabdomyolysis, as dark urine that clears suggests myoglobinuria from muscle breakdown, which can cause acute kidney injury and requires urgent intervention 1
• Obtain serum creatinine and BUN to assess renal function, as myoglobinuria can lead to acute tubular necrosis 1
• Perform urinalysis with microscopy to differentiate between hemoglobinuria, myoglobinuria, and hematuria—the dipstick will be positive for blood but microscopy will show no red blood cells if myoglobinuria is present 1
• Measure serum potassium, calcium, and phosphate as rhabdomyolysis commonly causes electrolyte disturbances that can be life-threatening 1

Primary Diagnostic Considerations

Rhabdomyolysis (Most Likely Given Dark Urine)

• Dark urine that has cleared is pathognomonic for myoglobinuria from muscle breakdown, even without reported strenuous activity 1
• Lateral thigh pain may represent the affected muscle group, with rhabdomyolysis causing localized muscle pain and tenderness 1
• Common causes include medications (statins, fibrates), infections, electrolyte abnormalities, or occult trauma the patient doesn't recall 1
• If CK is elevated >1000 U/L with positive urine myoglobin, initiate aggressive IV hydration immediately to prevent acute kidney injury 1

Meralgia Paresthetica (Lateral Femoral Cutaneous Nerve Entrapment)

• Presents as pain, burning, or dysesthesia specifically in the anterolateral thigh without back pain, caused by nerve entrapment under the inguinal ligament 2, 3
• The intermittent nature of pain fits this diagnosis, as symptoms worsen with hip extension or tight clothing 2, 3
• Physical examination should reveal decreased sensation to light touch over the lateral thigh in the distribution of the lateral femoral cutaneous nerve (L2-L3) 2, 3
• Risk factors include tight belts, weight gain, pregnancy, or limb length discrepancy 2, 3
• Nerve conduction studies can confirm the diagnosis but are not necessary if clinical presentation is classic 2

Occult Musculoskeletal Pathology

• If initial labs are normal, obtain plain radiographs of the hip and femur as the first imaging study to exclude stress fracture, bone lesion, or tumor 4
• Radiographs have 87% sensitivity for tibial fractures and can identify most significant bone pathologies 4
• If radiographs are negative but pain persists beyond 2-3 weeks, proceed with MRI without contrast to evaluate for occult fractures, soft tissue pathology, or nerve compression 4, 5

Secondary Considerations (Less Likely but Important)

Thrombotic Disorders

• While DVT was ruled out in the ER, consider renal vein thrombosis if proteinuria is present on urinalysis, as this can cause flank/thigh pain and dark urine from hemoglobinuria 6
• Check urinalysis for proteinuria—if present with dark urine history, obtain renal ultrasound with Doppler to evaluate renal veins 6
• Membranous nephropathy can present with hypercoagulability and thromboembolism 6

Peripheral Arterial Disease (PAD)

• Perform bilateral pulse examination and ankle-brachial index (ABI) testing if exertional component to pain or vascular risk factors present 5
• An ABI <0.90 confirms PAD diagnosis 5
• However, acute intermittent pain without claudication pattern makes this less likely 5

Algorithmic Approach

1. Immediate labs: CK, creatinine, urinalysis with microscopy, electrolytes 1
2. If CK elevated: Diagnose rhabdomyolysis, start aggressive IV hydration, monitor renal function 1
3. If labs normal: Examine for lateral thigh sensory changes suggesting meralgia paresthetica 2, 3
4. If meralgia paresthetica suspected: Trial conservative management (remove tight clothing/belts, NSAIDs, avoid hip hyperextension) for 2-4 weeks 3
5. If no improvement or atypical features: Obtain plain radiographs 4
6. If radiographs negative and pain persists >2-3 weeks: Proceed to MRI without contrast 4

Critical Pitfalls to Avoid

• Do not dismiss dark urine as insignificant—this is a red flag for myoglobinuria requiring urgent evaluation 1
• Do not assume all lateral thigh pain is musculoskeletal—meralgia paresthetica is frequently misdiagnosed as low-back pain or radiculopathy 3
• Do not delay advanced imaging beyond 2-3 weeks if symptoms persist with negative initial workup, as up to 41% of stress fractures are not visible on initial radiographs 4
• Do not overlook proteinuria on urinalysis—this can unmask thrombophilia and hypercoagulable states causing thrombotic complications 6