Neuromuscular weakness
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|Respiratory Failure
|Non-invasive ventilation (NIV)
|Intubation and Mechanical Ventilation
|Critical illness neuromuscular weakness
|Haemodialysis
|Dobutamine
Neuromuscular Weakness refers to loss of muscle power due to disorders affecting the motor unit
(upper or lower motor neuron, neuromuscular junction, or muscle).
It may present acutely (e.g., Guillain-Barré syndrome) or gradually (e.g., muscular dystrophies),
and recognition of the anatomical level is key to diagnosis and treatment.
📖 About
- Caused by pathology in anterior horn cells, peripheral nerves, neuromuscular junction, or muscle fibres.
- Can be acute, subacute, or chronic in onset.
- Patterns of weakness (proximal vs distal, symmetric vs asymmetric) help localise the lesion.
- Respiratory and bulbar involvement are red flags for impending respiratory failure.
🧬 Physiology of the Motor Unit
- Upper Motor Neurons (UMN): Corticospinal tracts controlling voluntary movement.
- Lower Motor Neurons (LMN): Anterior horn cells and peripheral nerves transmitting signals to muscle.
- Neuromuscular Junction: Acetylcholine release, receptor activation, muscle depolarisation.
- Muscle fibres: Contract via actin-myosin interactions powered by ATP and calcium handling.
⚠️ Causes
- 🧠 Central / UMN: Stroke, multiple sclerosis, spinal cord compression, MND (mixed UMN/LMN).
- 🔌 Peripheral Nerve: Guillain-Barré, CIDP, diabetic neuropathy, toxins (lead, arsenic).
- 🔗 Neuromuscular Junction: Myasthenia gravis, Lambert-Eaton syndrome, botulism, drugs (aminoglycosides, NMBAs).
- 💪 Muscle Disorders: Muscular dystrophies, polymyositis/dermatomyositis, metabolic myopathies, thyroid disease, steroid-induced myopathy.
- ⚡ Metabolic/Electrolytes: Hypokalaemia, hypermagnesaemia, hypophosphataemia, hypocalcaemia.
📋 History
- Onset: acute (GBS, myasthenia crisis) vs chronic (dystrophies, neuropathies).
- Pattern: proximal (myopathy) vs distal (neuropathy); fatigable (myasthenia).
- Symmetry: symmetric (GBS, myopathy) vs asymmetric (MND, vasculitis).
- Associated features: sensory symptoms (neuropathy), bulbar involvement (MG, MND), pain (myositis, neuropathy).
- Drug/toxin exposure (steroids, statins, alcohol, organophosphates).
🩺 Examination
- Muscle bulk: wasting (neuropathy, MND) vs pseudohypertrophy (Duchenne’s).
- Tone: decreased in LMN/myopathy, increased in UMN.
- Reflexes: absent in neuropathy/myopathy, brisk in UMN.
- Distribution: proximal (myopathy), distal (neuropathy), patchy (vasculitis).
- Cranial nerves: ptosis/diplopia (MG), facial weakness (CIDP, MND).
- Respiratory assessment: vital capacity, paradoxical breathing.
🔎 Investigations
- Bloods: CK (↑ in myopathy, rhabdomyolysis), electrolytes, TFTs, autoantibodies (anti-AChR, ANA, ANCA).
- Nerve conduction studies: demyelinating vs axonal neuropathy.
- EMG: myopathic vs neurogenic patterns.
- Repetitive nerve stimulation: decrement in MG, increment in Lambert-Eaton.
- Muscle biopsy: dystrophy, inflammatory myopathy, mitochondrial disease.
- MRI spine/brain: exclude central lesions.
💊 Management Principles
- Stabilisation: Monitor respiratory function (FVC, NIF); ICU support if declining.
- Treat underlying cause: Immunotherapy (steroids, IVIG, plasmapheresis) in GBS, MG, myositis; replace electrolytes; stop offending drugs.
- Symptomatic: Pyridostigmine for MG; riluzole for MND; analgesia for neuropathic pain.
- Rehabilitation: Physio, OT, speech therapy (bulbar involvement), dietician input.
- Long-term care: Multidisciplinary support, assistive devices, psychological support.
📉 Prognosis
- Depends on cause: many neuropathies/myopathies are reversible with treatment (e.g., GBS, hypokalaemia).
- Progressive disorders (MND, muscular dystrophies) have poorer outcomes but supportive care prolongs survival and function.
- Early recognition of respiratory involvement is crucial for survival.
📚 References
- Aminoff MJ. Neurology and General Medicine. Elsevier.
- Kumar & Clark’s Clinical Medicine, 10th Edition.
- Preston & Shapiro. Electromyography and Neuromuscular Disorders.
