What is the differential diagnosis and management for back pain radiating into the left arm and left leg?

Differential Diagnosis for Back Pain Radiating to Left Arm and Left Leg

This unusual presentation of back pain radiating simultaneously to both the left arm and left leg suggests either two separate pathologies affecting both the cervical and lumbosacral spine, or a systemic process such as malignancy with multilevel spinal involvement.

Key Differential Diagnoses

Most Likely: Dual-Level Pathology or Systemic Disease

• Metastatic disease with multilevel spinal involvement is the primary concern when pain radiates to both upper and lower extremities, particularly in patients over age 50, with history of cancer, unexplained weight loss, or night pain 1, 2, 3.

• Cervical radiculopathy (C5-T1 distribution) explains the left arm radiation, typically from disc herniation or foraminal stenosis causing dermatomal pain distribution 2.

• Lumbosacral radiculopathy or plexopathy (L1-S4 distribution) explains the left leg radiation, with pain following nerve root or plexus distribution 1.

• Cervical myelopathy with concurrent lumbar pathology should be considered if there are bilateral symptoms, gait disturbance, bowel/bladder dysfunction, or hyperreflexia indicating spinal cord compression 2.

Less Common but Critical Diagnoses

• Spinal infection (epidural abscess, discitis, osteomyelitis) can cause multilevel symptoms with fever, elevated inflammatory markers, and progressive neurologic deficits 1, 3.

• Brachial and lumbosacral plexopathy from infiltrative tumor, radiation injury, or inflammatory conditions causes pain with neuropathic character in multiple peripheral nerve distributions 1.

• Thoracic spine pathology with referred pain can occasionally cause both upper and lower extremity symptoms, though this is uncommon 4.

Immediate Red Flags Requiring Urgent Evaluation

Perform immediate MRI if any of the following are present:

• Progressive motor deficits in either upper or lower extremities 1, 2, 3
• Suspected myelopathy (gait disturbance, hyperreflexia, bowel/bladder dysfunction) 2
• History of cancer with new-onset back pain 1, 3
• Age >50 with unexplained weight loss or night pain 1, 2
• Fever or elevated inflammatory markers suggesting infection 1
• Severe pain unresponsive to initial conservative measures 2

Diagnostic Approach

Initial Imaging Strategy

• MRI of both cervical and lumbar spine is the preferred initial test when multilevel symptoms are present with red flags, as it provides superior visualization of soft tissue, nerve roots, spinal cord, and vertebral marrow without ionizing radiation 2, 3.

• Erythrocyte sedimentation rate (ESR) has 78% sensitivity and 67% specificity for cancer and should be obtained if malignancy is suspected 1, 3.

• Plain radiography is NOT recommended as initial imaging in the absence of red flags, as it does not improve outcomes and poorly correlates with symptoms 1, 2.

Clinical Assessment Details

• Cervical examination: Assess for dermatomal sensory loss (C5-T1), motor weakness in specific myotomes, reflex changes (biceps, triceps, brachioradialis), and Spurling's test for radiculopathy 2.

• Lumbar examination: Assess for dermatomal sensory loss (L1-S4), motor weakness (hip flexion, knee extension, ankle dorsiflexion/plantarflexion), reflex changes (patellar, Achilles), and straight leg raise test 1, 4.

• Myelopathy signs: Test for Hoffman's sign, clonus, Babinski sign, gait abnormalities, and bowel/bladder dysfunction 2.

• Systemic symptoms: Document weight loss, fever, night sweats, history of malignancy, immunosuppression, or intravenous drug use 1, 3.

Management Algorithm

If Red Flags Present:

1. Obtain urgent MRI of cervical and lumbar spine within 24-48 hours 2, 3
2. Check ESR and complete blood count if infection or malignancy suspected 1, 3
3. Immediate surgical referral for progressive motor deficits, suspected myelopathy, or cauda equina syndrome 2
4. Oncology referral if metastatic disease identified 3

If No Red Flags Present:

• Maintain activity rather than bed rest, as activity is more effective for radicular pain 2
• First-line pain control: NSAIDs or acetaminophen based on pain severity and functional deficits 2
• Physical therapy and exercise therapy for persistent symptoms beyond the acute phase 2
• Reassess at 4-6 weeks: If symptoms persist despite conservative therapy, obtain MRI of both cervical and lumbar spine and consider specialist referral 1, 2, 5

Critical Pitfalls to Avoid

• Do not assume this is simple mechanical back pain when radiation occurs to both arm and leg simultaneously—this pattern is atypical and warrants thorough investigation 1, 4.

• Do not delay imaging in patients with cancer history, as delayed diagnosis of spinal cord compression leads to poorer outcomes 1, 3.

• Do not obtain routine imaging without red flags, as this identifies many abnormalities that correlate poorly with symptoms and may lead to unnecessary interventions 1, 2.

• Do not miss cervical myelopathy, which requires urgent surgical evaluation to prevent permanent neurologic injury 2.