How do you test for focal weakness in back injuries?

Testing for Focal Weakness in Back Injuries

Perform a systematic motor examination testing specific muscle groups corresponding to nerve root levels, focusing on hip extensors, knee flexors/extensors, ankle dorsiflexors/plantarflexors, and toe extensors, with grading using the British Medical Council (0-5) scale. 1

Essential Clinical Assessment Components

Immediate Neurological Examination

• Test motor strength in all major lower extremity muscle groups bilaterally, documenting any asymmetry or weakness below the level of suspected injury 1

• Assess for specific patterns of weakness that localize to particular nerve root levels:

  • L2-L3: Hip flexion
  • L3-L4: Knee extension (quadriceps)
  • L5: Ankle dorsiflexion and great toe extension
  • S1: Ankle plantarflexion and knee flexion (hamstrings) 1

• Evaluate sacral sensation and function including perianal sensation, rectal tone, and voluntary anal sphincter contraction, as these predict neurological recovery and bladder function 1

Critical Red Flag Assessments

• Test for saddle anesthesia (perianal or perineal numbness) which indicates cauda equina syndrome requiring urgent imaging 1
• Assess for progressive neurologic deficits through serial examinations, as worsening weakness mandates immediate advanced imaging 1
• Document ankle spasticity in patients with thoracolumbar fractures, as this highly predicts neurogenic bladder dysfunction 1

Specialized Testing Techniques

Provocative Testing for Lumbar Stenosis

• Perform pre- and post-walking strength testing by examining lower extremity strength at rest, then having the patient walk 400 feet, and retesting strength within 60 seconds of completing the walk 2
• Retest after 2 minutes of supine rest to document recovery of strength, as transient paraparesis that resolves with rest confirms symptomatic lumbar spinal stenosis 2
• This provocative walking test detects weakness that patients may not subjectively perceive, particularly affecting hip extensors and knee flexors bilaterally 2

Standardized Grading Systems

• Use the ASIA Impairment Scale (AIS) for thoracolumbar injuries with neurological deficits, as entry AIS grade predicts functional outcomes 1
• Grade muscle strength using the 0-5 British Medical Council scale for objective documentation and serial comparison 1, 2
• Test abductor hallucis (AbH) motor function specifically, as this predicts neurological recovery in thoracolumbar fractures 1

Key Clinical Pitfalls to Avoid

Do not rely solely on patient-reported symptoms of weakness, as many patients with significant neurological compromise are unaware of objective motor deficits until formal testing reveals them 2. The provocative walking test demonstrates this phenomenon clearly in lumbar stenosis patients.

Do not skip sacral examination in any patient with back injury and suspected neurological involvement, as sacral sparing versus involvement fundamentally changes prognosis and management urgency 1.

Do not perform flexion-extension radiographs acutely in patients with neck pain and muscle spasm, as these are unlikely to yield additional diagnostic information and pose injury risk 1.

Documentation Requirements

• Record specific muscle groups tested, strength grades bilaterally, and any asymmetry to enable comparison on serial examinations 1
• Document presence or absence of reflexes at multiple levels (patellar, Achilles) as hyperreflexia suggests upper motor neuron involvement 1
• Note any dissociation between imaging findings and clinical deficits, as neurological examination localizes injury more accurately than imaging alone for predicting recovery 1