Can the Acute Motor Axonal Neuropathy (AMAN) form of Guillain-Barré Syndrome (GBS) present with involvement of only the proximal lower limbs without initial involvement of the upper limbs?

Can AMAN Present with Isolated Proximal Lower Limb Involvement?

Yes, AMAN can present with weakness limited to the lower limbs initially, including proximal distribution, though this represents an atypical variant pattern that may later progress to involve upper limbs or remain as a paraparetic variant.

Clinical Presentation Patterns in AMAN

The pure motor variant of GBS, which includes AMAN, can manifest with weakness limited to specific body regions without following the classic ascending pattern 1. Recognized GBS variants include:

• Paraparetic variant: Weakness specifically limited to the lower limbs, which is an established clinical variant that does not necessarily progress to involve upper extremities 1
• Pure motor variant (5-70% of cases): Motor weakness without sensory signs, which can have variable distribution patterns 2

Distribution Characteristics in Lower Limb Weakness

Research demonstrates that lower extremity weakness in GBS is characteristically proximal in 68% of patients, contrasting with upper extremity weakness which tends to be distal 3. This proximal lower limb predominance is:

• A recognized pattern in GBS that differs from typical polyneuropathies 3
• Helpful in differential diagnosis, especially in early disease stages 3

Temporal Evolution and Diagnostic Considerations

In 80% of GBS patients, muscle weakness starts in the lower limbs, though 4-limb involvement becomes most frequent at presentation (96%) 3. However, this does not mean all cases must progress to upper limb involvement:

• The progressive phase typically lasts days to 4 weeks, usually less than 2 weeks 1
• AMAN specifically can present with normal or even exaggerated reflexes throughout the disease course, distinguishing it from classic GBS 1
• Young children (<6 years) may present with atypical features including refusal to bear weight and poorly localized pain before obvious weakness patterns emerge 1

Critical Diagnostic Pitfalls

Do not dismiss AMAN based on atypical distribution alone. Key considerations include:

• Electrophysiological measurements might be normal when performed early (within 1 week) or in patients with initially proximal weakness 1
• Repeat electrodiagnostic studies 2-3 weeks later can be helpful if clinical suspicion remains high 1
• Marked persistent asymmetry of weakness should prompt reconsideration of the diagnosis toward alternative conditions 1, 2

Pathophysiological Context

AMAN is strongly associated with Campylobacter jejuni infection and targets gangliosides (GM1, GM1b, GD1a, GalNAc-GD1a) on motor axolemma 4, 5. The axonal damage pattern can produce:

• Variable weakness distribution depending on which motor axons are preferentially affected 5
• Better prognosis than demyelinating forms with potential for spontaneous recovery, though severe cases still require immunotherapy 4

Clinical Management Implications

Even with isolated lower limb involvement:

• All grades of GBS warrant complete workup given potential for rapid progression to respiratory compromise 2
• Immediate respiratory function assessment is mandatory, as approximately 20% develop respiratory failure 2, 6
• Neurology consultation should be obtained for all suspected cases 2