Electrodiagnosis, Neuroimaging, and Differential Workup: Navigating ALS Mimics

Because no single definitive biofluid biomarker exists for sporadic ALS, electrodiagnostic testing, neuroimaging, and comprehensive blood panels are essential to rule out treatable "ALS mimic syndromes".

Electrodiagnostic Evaluation (EMG / NCS)

Electromyography (EMG) is the single most critical diagnostic tool in suspected ALS:

• Nerve Conduction Studies (NCS): Typically normal for sensory studies. Used primarily to exclude conduction block, entrapment, or demyelinating polyneuropathies.

• Needle EMG Features:

  1. Active Denervation: Spontaneous fibrillations, positive sharp waves, and complex fasciculation potentials.

  2. Chronic Reinnervation: Neurogenic voluntary Motor Unit Action Potentials (MUAPs) showing increased amplitude, prolonged duration, polyphasia, and reduced recruitment.

                 ESSENTIAL LABORATORY & IMAGING WORKUP
                                   │
 ┌─────────────────────────────────┼─────────────────────────────────┐
 ▼                                 ▼                                 ▼
Blood Panels                       CSF Analysis                      Neuroimaging (MRI)
• ESR, CRP, CBC, LFTs, CK    • Cell count, Protein, Glucose    • Brain/Spinal MRI to rule
• Electrolytes, Ca2+, PO4    • Oligoclonal bands        out structural lesions
• TSH, Free T3/T4, Vit B12/Folate                           • Corticospinal hyperintensity
• SPEP / Immunofixation                                       on T2/FLAIR (non-specific)
• Autoantibodies (GM-1, HIV, Lyme)

Major ALS Mimic Syndromes

A clinical history lacking disease progression or demonstrating prominent atypical features should prompt evaluation for mimic syndromes:

Category	Primary ALS Mimic Conditions	Distinguishing Features / Diagnostic Differential
Structural / Mechanical	Cervical Spondylotic Myelopathy, Syringomyelia, Radiation Plexopathy, Brainstem Tumor	Sensory deficits, focal dermatomal pain, clear MRI structural compression.
Immune / Inflammatory	Multifocal Motor Neuropathy with Conduction Block (MMN), CIDP, Myasthenia Gravis, Polymyositis	Conduction block on NCS, elevated anti-GM1 antibodies, fluctuating fatigability, response to IVIG.
Hereditary / Metabolic	Spinobulbar Muscular Atrophy (Kennedy Disease), Hereditary Spastic Paraparesis, Acid Maltase Deficiency	X-linked inheritance, gynecomastia, prominent sensory neuronopathy, specific enzyme/gene defects.
Toxic / Infectious	Lathyrism, Organophosphate toxicity, Heavy Metal poisoning, Lyme disease, HIV-associated MND	Exposure history, positive serologies, occupational toxic hazards.

Key Takeaways

• EMG is the premier tool to confirm widespread LMN denervation and chronic reinnervation.

• Sensory NCS are typically normal in ALS.

• Multifocal Motor Neuropathy (MMN) and Cervical Spondylotic Myelopathy are the two most common ALS mimics.