Multifocal Motor Neuropathy with Conduction block

Related Subjects: |Neurological History taking |Motor Neuron Disease (MND-ALS) |Miller-Fisher syndrome |Guillain Barre Syndrome |Multifocal Motor Neuropathy with Conduction block |Multiple Sclerosis (MS) Demyelination |Inclusion Body Myositis |Cervical spondylosis |Anterior Spinal Cord syndrome |Central Spinal Cord syndrome |Brown-Sequard Spinal Cord syndrome |Spinal Cord Compression |Spinal Cord Haematoma |Spinal Cord Infarction

⚡ Multifocal Motor Neuropathy (MMN) is crucial to recognize because it can mimic Motor Neurone Disease (MND). ✅ Unlike MND, MMN is treatable, making early diagnosis vital.

ℹ️ About

• 🧠 MMN with Conduction Block: An acquired, immune-mediated demyelinating neuropathy.
• ♻️ Slowly progressive weakness that may resemble MND but is potentially reversible with treatment.
• 📉 Much rarer than MND, typically affects younger men (mean age ~41).

🧬 Aetiology

• 🔬 Immune-mediated demyelination of motor nerves in the peripheral nervous system.
• 🧪 Often associated with anti-GM1 antibodies, which damage motor nerves.

Clinical Features 🚨

• 💪 Limb weakness, often asymmetric and more in the arms than legs.
• ⚡ Fasciculations (muscle twitching) may be seen.
• 👨 Higher prevalence in men (≈3:1 ratio).
• 🔎 Reflexes: Reduced or absent, but sensation is preserved (key clue vs other neuropathies).
• 🗣️ Cranial nerves and bulbar muscles are usually spared → helps differentiate from MND.

Investigations 🧪

• 🧬 Anti-GM1 antibodies: Elevated in ~80% of cases → important diagnostic marker.
• ⚡ Nerve conduction studies (NCS): Show motor conduction block (esp. ulnar & median nerves). Sensory studies = normal.
• 💧 CSF analysis: Usually normal (helps exclude CIDP).

Differential Diagnosis 🔍

• ❌ Motor Neuron Disease (MND): Progressive, untreatable, often with bulbar involvement.
• 🔄 CIDP: Affects both sensory + motor nerves; CSF protein usually ↑.
• ☠️ Lead poisoning: Heavy metal neuropathy can mimic MMN.
• 🧬 Hexosaminidase A deficiency: Rare inherited metabolic neuropathy with similar features.

MMN 🟦 vs MND 🟥 – At a Glance

Management 🩺

• 💉 IV Immunoglobulin (IVIG): First-line; 0.4 g/kg/day → often dramatic improvement in strength & function.
• 🧪 Cyclophosphamide: Second-line (oral or IV) if IVIG insufficient.
• 🚫 Steroids: Can worsen symptoms → should be avoided.
• 📈 Outcome: Many patients improve significantly with treatment → highlighting the importance of recognition.

Cases — Multifocal Motor Neuropathy with Conduction Block (MMN)

• Case 1 — Asymmetric Distal Weakness ✋: A 45-year-old man develops progressive weakness in his right hand over 12 months, struggling with grip and finger extension. No sensory loss. Exam: asymmetric distal weakness, fasciculations absent, reflexes preserved. Nerve conduction study: conduction block in the right ulnar nerve. Diagnosis: MMN. Management: IV immunoglobulin (IVIG) — usually responsive; physiotherapy for hand function.
• Case 2 — Misdiagnosed as MND ❌: A 52-year-old woman is referred with “possible motor neurone disease” due to progressive left foot drop and calf wasting. However, sensation is intact, and EMG shows conduction block (not widespread denervation). Anti-GM1 antibodies positive. Diagnosis: MMN initially mislabelled as ALS. Management: IVIG (good clinical response); immunosuppressants (cyclophosphamide, rituximab) if refractory.
• Case 3 — Upper Limb Predominant Weakness 💪: A 38-year-old man presents with slowly progressive weakness in both arms, particularly wrist extensors. No bulbar or respiratory involvement. Sensory testing normal. Nerve conduction: multifocal conduction block in motor nerves. Diagnosis: MMN with upper limb–predominant involvement. Management: Regular IVIG infusions; supportive physio; avoid steroids (can worsen MMN).

Teaching Commentary 🧠

MMN is a chronic immune-mediated neuropathy causing asymmetric distal limb weakness without sensory loss. Key features: - Weakness > wasting, asymmetric, often upper limb first. - No sensory involvement (distinguishes from CIDP). - Conduction block on nerve conduction studies is diagnostic. - Anti-GM1 antibodies positive in ~50%. ⚡ Important differential: MND/ALS — but MMN is treatable! Treatment: IVIG (first-line), sometimes rituximab/cyclophosphamide. Steroids and plasma exchange are ineffective or may worsen. Prognosis: good functional outcomes if diagnosed early.