What are the causes and treatment options for pure motor neuropathy?

Causes of Pure Motor Neuropathy

Pure motor neuropathy is most commonly caused by multifocal motor neuropathy (MMN), motor-predominant Guillain-Barré syndrome (GBS), pure motor chronic inflammatory demyelinating polyneuropathy (CIDP), and motor neuron diseases, with MMN being the most treatable form requiring early IVIG therapy.

Primary Causes of Pure Motor Neuropathy

Immune-Mediated Causes

1. Multifocal Motor Neuropathy (MMN)

   • Characterized by asymmetric distal weakness, typically affecting arms first 1
   • Associated with anti-GM1 IgM antibodies in 80-90% of cases 2
   • Electrophysiological findings show multifocal partial motor conduction blocks 1
   • Responds well to high-dose intravenous immunoglobulin (IVIG) therapy 1, 2

2. Pure Motor Guillain-Barré Syndrome (GBS)

   • Acute onset with rapidly progressive weakness
   • Can present as acute motor axonal neuropathy (AMAN) subtype 3
   • May show normal or exaggerated reflexes throughout disease course 3
   • Typically follows infection (especially Campylobacter jejuni) 3

3. Pure Motor Chronic Inflammatory Demyelinating Polyneuropathy (CIDP)

   • Relapsing-remitting course with selective motor fiber involvement 4
   • Often steroid-unresponsive but improves with immunoglobulins 4
   • Electrophysiological testing shows demyelinating neuropathy without sensory involvement 4

Neurodegenerative Causes

1. Motor Neuron Diseases
   • Amyotrophic Lateral Sclerosis (ALS)
   • Kennedy's Disease (X-linked spinobulbar muscular atrophy)
   • Other lower motor neuron diseases (inherited or acquired) 5

Toxic/Metabolic Causes

1. Chemotherapy-Induced Motor Neuropathy

   • Vincristine can cause severe motor involvement 3
   • Motor impairment with bortezomib occurs in approximately 10% of patients 3

2. Diabetic Motor Neuropathy

   • Though rare as a pure motor form, diabetes can cause motor-predominant neuropathy 3, 6

3. Other Toxic Causes

   • Heavy metal poisoning
   • Alcohol abuse 3, 6

Diagnostic Approach

Clinical Assessment

• Evaluate pattern of weakness (distal vs. proximal, symmetric vs. asymmetric)
• Assess deep tendon reflexes (typically reduced or absent)
• Confirm absence of sensory symptoms and normal sensory examination
• Look for cranial nerve involvement (especially in GBS variants) 3

Electrophysiological Testing

• Nerve conduction studies to identify:
  • Conduction blocks (hallmark of MMN)
  • Demyelinating vs. axonal patterns
  • Normal sensory potentials (confirming pure motor involvement) 7
• Electromyography to assess for denervation patterns

Laboratory Testing

• Anti-GM1 IgM antibodies (positive in 80-90% of MMN cases) 2
• Cerebrospinal fluid analysis (protein elevation in CIDP and GBS)
• Screen for underlying causes:
  • Vitamin B12 levels
  • Thyroid function
  • Renal function
  • Serum protein electrophoresis 3, 6

Treatment Approaches

For MMN

• First-line: IVIG at 2g/kg initially, with maintenance therapy in responders 2
• Second-line: Cyclophosphamide (for severe cases unresponsive to IVIG) 2

For Pure Motor GBS

• IVIG (0.4g/kg/day for 5 days) or plasma exchange
• Supportive care and monitoring for respiratory compromise 3

For Pure Motor CIDP

• IVIG (responds better than to steroids)
• Consider interferon alpha as an alternative therapy 4

For Chemotherapy-Induced Motor Neuropathy

• Dose reduction or discontinuation of offending agent
• Consider steroids for mild symptoms 3
• IVIG for AIDP-type picture 3

Prognosis and Monitoring

• MMN: Typically chronic but treatable with good response to IVIG in up to 80% of patients 2
• Pure motor GBS: Usually monophasic with good recovery if treated early
• Pure motor CIDP: Relapsing-remitting course requiring long-term immunomodulatory therapy 4
• Motor neuron diseases: Generally progressive with poorer prognosis

Key Pitfalls to Avoid

1. Misdiagnosis as ALS: Pure motor presentations, especially MMN, can be mistaken for ALS, leading to missed treatment opportunities. Always perform thorough electrophysiological testing to look for conduction blocks.

2. Overlooking treatable causes: Screen for vitamin deficiencies, thyroid dysfunction, and other metabolic causes even when presentation seems typical for an immune-mediated neuropathy.

3. Delayed treatment: Early initiation of IVIG in MMN and GBS is associated with better outcomes. Don't wait for complete diagnostic workup if clinical suspicion is high.

4. Inadequate follow-up: Pure motor CIDP and MMN often require long-term therapy with periodic reassessment of treatment response.