Lower Motor Neuron Weakness
February 15th, 2021
Overview
Signs of Lower Motor Neuron Weakness
- Muscle wasting
- Fasciculations
- Reduced tone
- Reduced power
- Hyporeflexia
- Downgoing plantar reflex
Interpretation
- Weakness affecting a single myotome - likely due to spinal nerve root pathology
- Weakness affecting a single peripheral nerve distribution - likely due to a focal peripheral nerve palsy
- Weakness affecting multiple peripheral nerve distributions - potentially due to a brachial plexus lesion or mononeuritis multiplex
- Generalised weakness - suggests a polyneuropathy, neuromuscular pathology or myopathy
Causes of Lower Motor Neuron Weakness
Spinal Nerve Root Pathology
- Spinal trauma
- Radiculopathy
- Tumour
Brachial Plexopathy
- Brachial plexus trauma
- Brachial neuritis
- Radiation injury
- Tumour
Focal Peripheral Nerve Pathology
- Trauma
- Nerve entrapment (e.g. carpal tunnel syndrome)
- Focal ischaemia
- Peripheral nerve tumour
- Sarcoidosis
Mononeuritis Multiplex
- Vasculitis - polyarteritis nodosa, EGPA, GPA, cryoglobulinaemia, SLE, rheumatoid arthritis, Sjogren's syndrome, scleroderma, Behcet's disease
- Diabetes
- Infection - Lyme disease, leprosy, HIV, hepatitis B / C
- Malignancy - lymphoma, small cell lung cancer
- Amyloidosis
- Sarcoidosis
Peripheral Polyneuropathy
- Endocrine / nutritional - diabetic neuropathy, hypothyroidism, B12 deficiency
- Demyelination - Guillain-Barré syndrome, Chronic inflammatory demyelinating polyneuropathy (CIDP)
- Vasculitis - rheumatoid arthritis, polyarteritis nodosa, SLE
- Infection - lyme disease, HIV neuropathy, leprosy, polio
- Paraneoplastic syndrome
- Multifocal motor neuropathy with conduction block
- Motor neurone disease
- Charcot-Marie-Tooth
- Sarcoidosis
- Drugs - ethambutol, certain chemotherapy agents, amiodarone, lithium
- Toxins - alcohol, heavy metals, organophosphates
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