Makindo Medical Notes

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 important to recognize because it can mimic Motor Neurone Disease (MND), but unlike MND, it is treatable.

About

• Multifocal Motor Neuropathy with Conduction Block (MMN): An acquired, immune-mediated demyelinating neuropathy.
• Characterized by slowly progressive weakness that can resemble MND, but with important differences such as treatability.
• It is much rarer than MND, typically seen in younger individuals, particularly men.

Aetiology

• MMN is caused by demyelination of motor nerves, primarily affecting the peripheral nervous system.
• It is believed to be immune-mediated, with antibodies (anti-GM1) contributing to the damage of motor nerves.

Clinical Features

• Presents with limb weakness, often accompanied by fasciculations (muscle twitching).
• Affects a younger population with a mean age of around 41 years.
• MMN typically involves the arms more than the legs.
• There is a male predominance, with men being affected three times more frequently than women.
• Reflexes are reduced or absent, but sensory function remains normal.
• Cranial nerves and bulbar muscles are usually spared, which helps differentiate it from MND.

Investigations

• Anti-GM1 antibodies: Blood tests reveal elevated anti-GM1 antibody titres in around 80% of patients, which is a key diagnostic marker.
• Nerve conduction studies (NCS): Show motor conduction block, particularly in the ulnar and median nerves. Sensory conduction studies are typically normal, which distinguishes MMN from other neuropathies.
• CSF analysis: Typically normal, helping rule out other inflammatory neuropathies.

Differential Diagnosis

• Motor Neuron Disease (MND): Unlike MMN, MND does not respond to treatment and often involves bulbar muscles and sensory loss is uncommon.
• Chronic Inflammatory Demyelinating Polyneuropathy (CIDP): CIDP typically affects both sensory and motor nerves and may have an elevated protein level in CSF.
• Lead poisoning: Heavy metal exposure can cause peripheral neuropathy that may mimic MMN.
• Hexosaminidase A deficiency: A rare inherited disorder that can cause neurological symptoms similar to MMN.

Management

• Intravenous immunoglobulin (IVIG): First-line treatment, typically given at a dose of 0.4 g/kg/day, often results in substantial improvement in muscle strength and function.
• Cyclophosphamide: Used as a second-line treatment either orally or intravenously in patients who do not respond adequately to IVIG.
• Steroids: Paradoxically, corticosteroids can worsen symptoms in MMN and should be avoided.
• Outcome: Treatment can result in significant functional improvement, making early diagnosis and intervention crucial in MMN.