What are the differential diagnoses for a 68-year-old woman with subacute rapidly progressive pure motor weakness that began in the hands, spread proximally and to the legs, causing inability to walk, with minimal ankle reflexes, no sensory loss, and preceding intermittent dysphagia?

Differential Diagnoses

This presentation is most consistent with Guillain-Barré syndrome (GBS), specifically the pure motor variant, though myasthenia gravis and acute intermittent porphyria must also be strongly considered given the atypical features.

Primary Consideration: Guillain-Barré Syndrome (GBS)

The clinical picture strongly suggests GBS with atypical features 1. While the hand-first onset is unusual, GBS can present atypically with weakness starting in the arms rather than the classic ascending pattern from legs 1. Key supporting features include:

• Rapidly progressive bilateral motor weakness over days to weeks (subacute timeframe fits GBS) 1
• Absence of sensory deficits consistent with pure motor variant of GBS 1
• Hyporeflexia (1+ ankle reflexes) - though not completely absent, reflexes are decreased as expected in GBS 1
• Preceding dysphagia - cranial nerve dysfunction can precede limb weakness in atypical GBS presentations 1
• Proximal and distal involvement - GBS can present with asymmetrical or mixed proximal/distal patterns 1

Critical caveat: The pure motor variant of GBS (AMAN subtype) can present with normal or even exaggerated reflexes throughout the disease course 1, so the presence of 1+ reflexes does not exclude this diagnosis.

Alternative Diagnoses to Exclude

Myasthenia Gravis (MG)

Must be considered given 2:

• Preceding intermittent dysphagia (bulbar involvement)
• Progressive limb weakness
• Can present with atypical distal weakness rather than classic proximal pattern 2
• However, the lack of fatigability and absence of ocular symptoms make this less likely
• Distinguishing feature: MG typically shows fluctuating weakness that worsens with activity, not the relentless progression seen here

Acute Intermittent Porphyria (AIP)

Critical to exclude given 3:

• Subacute motor-predominant neuropathy
• Can present with proximal weakness and sensory deficits
• Key distinguishing features to assess:
  • Abdominal pain (chronic or acute)
  • Autonomic dysfunction (tachycardia, hypertension, presyncope)
  • Neuropsychiatric symptoms
  • Weight loss
• If any of these systemic features are present, urgently check urinary porphobilinogen and aminolevulinic acid 3

Spinal Cord Pathology

Must exclude given 4:

• Anterior spinal artery infarction can initially mimic peripheral neuropathy
• Red flags that would suggest central process:
  • Hyperreflexia or spasticity (not present here)
  • Sensory level
  • Bowel/bladder dysfunction beyond what GBS causes
  • Babinski sign
• The hyporeflexia argues strongly against spinal cord pathology 4

Other Considerations

• Chronic inflammatory demyelinating polyneuropathy (CIDP): Less likely given the rapid progression (CIDP evolves over >8 weeks)
• Botulism: Would expect descending paralysis and autonomic features
• Dyskalemic myopathy: Check electrolytes urgently 5
• Diabetic neuropathy: Too rapid for typical diabetic neuropathy

Immediate Diagnostic Approach

Urgent workup required 1:

1. Lumbar puncture: Look for cytoalbuminologic dissociation (elevated protein with normal cell count) - classic for GBS but can be normal early in disease 1

2. Electrophysiology (EMG/NCS):

   • Distinguishes GBS subtypes (AIDP vs AMAN vs AMSAN) 1
   • Can confirm neuromuscular junction disorder if MG suspected 2
   • May be normal early in GBS course 1

3. Respiratory function monitoring: 20% of GBS patients develop respiratory failure requiring mechanical ventilation 1 - monitor forced vital capacity (FVC) serially

4. Serum antibodies:

   • Acetylcholine receptor antibodies if MG suspected 2
   • Anti-ganglioside antibodies (GQ1b for Miller Fisher variant) 1

5. Basic labs:

   • Electrolytes (potassium, calcium, magnesium) 6
   • Creatine kinase 6, 7
   • TSH 6
   • Urinary porphobilinogen if any systemic symptoms 3

6. MRI spine: Only if upper motor neuron signs develop or to exclude structural lesions 4

Treatment Implications

If GBS is confirmed, treatment should be initiated promptly as most patients reach maximum disability within 2 weeks 1:

• Intravenous immunoglobulin (IVIG): 0.4 g/kg daily for 5 days 1
• Plasma exchange: 200-250 ml/kg for 5 sessions 1
• Both are equally effective 1

Critical monitoring: Admit to ICU-level care given risk of respiratory failure and autonomic instability (cardiac arrhythmias, blood pressure instability) 1.