What is the diagnostic approach for an adult patient with acute onset of muscle weakness and areflexia, minimal sensory involvement, and recent history of respiratory or gastrointestinal infection, suspected of having the Acute Motor Axonal Neuropathy (AMAN) variant of Guillain-Barré Syndrome (GBS)?

Diagnostic Algorithm for AMAN Variant of Guillain-Barré Syndrome

Initial Clinical Recognition

Begin by identifying the pure motor phenotype: bilateral ascending weakness without sensory symptoms or signs, which is the hallmark of AMAN. 1, 2

Step 1: Confirm Core Clinical Features

• Bilateral symmetrical motor weakness starting in legs and ascending to arms, with no sensory involvement (no paresthesias, no sensory loss on examination) 1, 3
• Areflexia or hyporeflexia in affected limbs—though critically, AMAN can present with normal or even exaggerated reflexes in a minority of cases, which is atypical for other GBS subtypes 1
• Acute or subacute onset with progression to maximum disability within 2 weeks 2, 4
• Recent infection history within 6 weeks (particularly Campylobacter jejuni, which is strongly associated with AMAN) 1, 4

Step 2: Exclude Alternative Diagnoses Through Targeted Examination

• Assess for sensory level on trunk: absence confirms peripheral nerve process rather than transverse myelitis 5
• Check for early bladder/bowel dysfunction: if present at onset, strongly reconsider diagnosis toward transverse myelitis 5
• Evaluate for marked persistent asymmetry: if present, alternative diagnoses should be considered 2
• Cranial nerve examination: facial weakness may occur but is less common in pure AMAN; ophthalmoplegia suggests Miller Fisher syndrome instead 1, 2

Confirmatory Testing

Step 3: Cerebrospinal Fluid Analysis

• Perform lumbar puncture to identify albumino-cytological dissociation (elevated protein with normal cell count <5 cells/μL) 2, 6
• Timing matters: CSF protein may be normal in the first week, so do not dismiss GBS based on normal early CSF protein 2
• Protein elevation typically appears at end of first week and may persist until third week 6
• Marked CSF pleocytosis (>50 cells/μL) should prompt reconsideration of diagnosis toward infectious or inflammatory myelitis 2

Step 4: Electrodiagnostic Studies (Definitive for AMAN)

Nerve conduction studies are essential to distinguish AMAN from demyelinating subtypes and confirm the diagnosis. 2, 7

AMAN-Specific Electrodiagnostic Criteria:

• Reduced motor amplitudes (compound muscle action potentials) indicating axonal loss 6, 3
• Normal or near-normal conduction velocities (no demyelination) 6, 3
• Absence of conduction blocks, temporal dispersion, or prolonged distal latencies (these indicate demyelination, not AMAN) 2, 7
• Normal sensory nerve action potentials throughout (pure motor involvement) 3
• No evidence of demyelination on neurophysiologic testing is the key distinguishing feature from AIDP 3

Step 5: Supportive Laboratory Testing

• Complete blood count, glucose, electrolytes, kidney and liver function to exclude metabolic causes of weakness 2
• Serum creatine kinase: may be elevated but is nonspecific 2
• Anti-ganglioside antibody testing (particularly anti-GM1, anti-GD1a) has limited clinical utility for typical AMAN and should not delay treatment, but may support diagnosis if positive 7
• Test for hepatitis E virus, Campylobacter jejuni, and other GBS-associated pathogens if clinically indicated 4, 3

Critical Assessment for Complications

Step 6: Immediate Respiratory and Autonomic Monitoring

Assess respiratory function immediately as AMAN can progress to respiratory failure requiring mechanical ventilation. 2

• Measure vital capacity, negative inspiratory force, and maximum inspiratory/expiratory pressures at presentation and serially 2
• Apply the 20/30/40 rule: patient at risk if vital capacity <20 mL/kg, maximum inspiratory pressure <30 cmH₂O, or maximum expiratory pressure <40 cmH₂O 2
• Single breath count ≤19 predicts need for mechanical ventilation 2
• Continuous cardiac monitoring for arrhythmias and blood pressure instability from dysautonomia 2

Diagnostic Pitfalls to Avoid

• Do not wait for CSF protein elevation in the first week—it may be normal early 2
• Do not exclude AMAN based on preserved reflexes—a minority of pure motor/AMAN cases maintain normal or brisk reflexes throughout 1
• Do not confuse with chronic inflammatory demyelinating polyradiculoneuropathy (CIDP)—if progression continues beyond 8 weeks, reconsider diagnosis toward A-CIDP 7
• Do not delay treatment waiting for antibody results—treatment should begin based on clinical suspicion 2
• Recognize that AMAN has better prognosis than demyelinating forms with potential for spontaneous recovery, though severe cases still require immunotherapy 3