What differentiates Acute Motor Axonal Neuropathy (AMAN) from Acute Inflammatory Demyelinating Polyneuropathy (AIDP) in a pediatric patient?

Differentiating Features Between AMAN and AIDP

The primary distinction between AMAN and AIDP lies in electrodiagnostic findings: AMAN shows reduced compound muscle action potential (CMAP) amplitudes with preserved conduction velocities and no demyelinating features, while AIDP demonstrates demyelinating characteristics including prolonged distal latencies, slowed conduction velocities, temporal dispersion, and conduction blocks. 1

Electrodiagnostic Differentiation

AMAN (Acute Motor Axonal Neuropathy)

• Decreased or absent CMAP amplitudes without demyelinating features 2
• Normal or near-normal conduction velocities despite reduced amplitudes 3
• Absence of temporal dispersion and prolonged distal latencies 1
• Motor conduction blocks may appear transiently in early stages but disappear in follow-up studies, which can initially mimic AIDP 2
• Markedly increased refractoriness (threshold increases during relative refractory period), indicating reduced safety margin for impulse transmission 3

AIDP (Acute Inflammatory Demyelinating Polyneuropathy)

• Prolonged distal motor latencies across multiple nerves 1, 4
• Slowed conduction velocities with relatively uniform involvement 4
• Abnormal temporal dispersion reflecting non-uniform slowing across nerve fibers 4
• Partial motor conduction blocks that persist on repeat testing 1
• Normal refractoriness despite marked prolongation of distal latencies 3
• "Sural sparing" pattern (normal sural sensory response with abnormal median/ulnar responses) is highly characteristic 4

Clinical Progression Patterns

AMAN Characteristics

• Rapid progression with early nadir (mean 11.5 days from onset) 5
• Shorter interval between symptom onset and first examination (mean 4.2 days) 5
• 88% of patients with mild initial disability reach nadir by first examination 5
• More frequent lower limb predominance of weakness 6
• Predominantly distal immune attack targeting axolemma and nodes of Ranvier 3

AIDP Characteristics

• Slower, prolonged progression with later nadir (mean 18.0 days from onset) 5
• Longer interval to first examination (mean 5.3 days) 5
• Only 65% reach nadir at first examination; remaining 35% progress over 1-2 weeks and lose ability to walk 5
• Ascending pattern with more gradual proximal spread 1
• Immune attack targets myelin with macrophage-mediated demyelination 7

Geographic and Epidemiologic Patterns

• AIDP predominates in Europe and North America (90-95% of GBS cases) 1
• AMAN is more common in Asia, Latin America, and the Caribbean (30-65% of cases) 1
• AMAN shows stronger association with Campylobacter jejuni gastroenteritis, particularly in endemic regions 1
• In pediatric populations, AIDP represents 70% and AMAN 16% of cases in Western studies 8

Prognostic Differences

AMAN Prognosis

• Associated with worse outcomes and more frequent sequelae 8, 6
• Lower probability of complete recovery at 3 months (30% vs 56% for AIDP) 6
• Approximately 80% present with severe GBS (disability score >2) in regions where AMAN predominates 1
• Sequelae strongly correlated with axonal subtype 8

AIDP Prognosis

• Better recovery profile with higher rates of complete recovery 6
• 40-60% present with severe GBS in regions where AIDP predominates 1
• More favorable long-term outcomes in pediatric populations 8

Critical Diagnostic Pitfalls

• Repeat electrodiagnostic studies are essential because motor conduction blocks in early AMAN can mimic AIDP, but these blocks disappear on follow-up while CMAP amplitudes remain reduced 2
• Studies performed within the first week may be normal in 30-34% of patients despite active disease, necessitating repeat testing 2-3 weeks later if clinical suspicion remains high 4
• Short interval between symptom onset and hospitalization correlates with worse outcomes, particularly in axonal forms 8
• AIDP patients require careful monitoring as 35% continue progressing after initial examination and may lose ambulation over 1-2 weeks 5