What is the initial diagnostic step for a patient presenting with bilateral leg weakness, should an Electromyogram (EMG) be ordered?

Diagnostic Approach for Bilateral Leg Weakness: EMG Role

For a patient presenting with bilateral leg weakness, an EMG should not be the initial diagnostic test; instead, a thorough clinical assessment followed by MRI of the spine and/or brain should be performed first to rule out structural causes before proceeding to electrodiagnostic studies. 1

Initial Diagnostic Approach

1. Clinical Assessment:

   • Evaluate pattern of weakness (proximal vs. distal, symmetric vs. asymmetric)
   • Check for sensory symptoms (paraesthesias, numbness)
   • Assess deep tendon reflexes (decreased/absent reflexes suggest peripheral causes)
   • Look for cranial nerve involvement
   • Evaluate for autonomic symptoms (blood pressure fluctuations, bladder/bowel dysfunction)
   • Determine onset and progression (acute, subacute, or chronic)

2. First-line Imaging:

   • MRI of the spine (without and with contrast) to rule out:
     • Spinal cord compression
     • Transverse myelitis
     • Demyelinating lesions
     • Nerve root impingement
   • MRI of the brain if upper motor neuron signs are present or if suspecting conditions like multiple sclerosis

3. Laboratory Studies:

   • Complete blood count
   • Comprehensive metabolic panel
   • Inflammatory markers (ESR, CRP)
   • Creatine kinase (for myopathies)
   • Consider CSF analysis if suspecting Guillain-Barré syndrome (GBS)

When to Order EMG

EMG/nerve conduction studies should be ordered after initial imaging and laboratory studies, typically when:

1. Clinical features suggest a peripheral nervous system disorder
2. Imaging studies do not reveal a structural cause
3. The pattern of weakness remains unexplained
4. Differentiating between myopathy, neuropathy, and neuronopathy is necessary

EMG Utility in Specific Conditions

Guillain-Barré Syndrome

• EMG/NCS can support diagnosis but are not required for diagnosis 1
• Electrodiagnostic studies may be normal when performed early (within 1 week of symptom onset)
• Consider repeat studies 2-3 weeks later if initial studies are normal but clinical suspicion remains high

Critical Illness Neuromyopathy

• EMG/NCS can help differentiate between critical illness polyneuropathy and myopathy 1
• Positive predictive value of EMG for final diagnosis of weakness is only 50%, with negative predictive value of 89% 1

Motor Neuron Disease

• EMG/NCS are key tests in diagnosing ALS but should follow imaging to exclude other conditions 1
• Look for evidence of both upper and lower motor neuron involvement

Pitfalls to Avoid

1. Premature EMG testing: EMG performed too early in GBS may yield false-negative results 1
2. Relying solely on EMG: EMG should be an extension of thorough clinical assessment, not a replacement 2
3. Failure to correlate with imaging: EMG findings should be interpreted alongside MRI results 2
4. Single EMG measurement: Consider serial examinations for conditions with evolving patterns 2
5. Ignoring technical factors: Electrode placement, muscle temperature, and cross-talk signals can affect results 2

EMG Interpretation Considerations

• EMG should be interpreted in conjunction with clinical findings 2, 3
• Sensitivity of abnormal EMG for abnormal muscle biopsy is 87%, specificity is 65% 3
• Certain conditions (mitochondrial myopathies, central core myopathies, glycogen storage disorders) may present with normal EMG despite abnormal muscle biopsy 3

In summary, while EMG is a valuable diagnostic tool for evaluating bilateral leg weakness, it should be ordered strategically after initial clinical assessment, imaging studies, and laboratory tests have been performed to maximize its diagnostic utility and avoid unnecessary testing.