Overview
Neurological causes of weakness may be delineated into upper motor neuron (brain, brainstem or spinal cord) or lower motor neuron (nerve root, plexus or peripheral nerve) causes. These two broad groups manifest with specific findings elicitable on examination.
Causes of Weakness
Upper Motor Neuron
- CNS lesions - decreased power with increased tone, clonus and reflexes.Ischaemia, SAH / ICH, tumour, trauma, encephalitis, vasculitis, MS, ADEM, PML, sarcoidosis, prions
Lower Motor Neuron
- Nerve root pathology - loss of a single myotomeTrauma, radiculopathy, neoplasm
- Focal peripheral nerve pathology - in the muscles supplied by a single nerveTrauma, entrapment, focal ischaemia, sarcoidosis, neoplasia, Bell's palsy (face)
- Axonal pathology (peripheral neuropathy)Diabetes, motor neurone disease, hypothyroidism, B12 / folate deficiency, alcoholism, connective tissue disease, amyloid / sarcoidosis, toxins
- Peripheral demyelinationGuillain-Barré syndrome, Charcot-Marie-Tooth
Other
- Neuromuscular pathologyMyasthenia gravis, amyloidosis
- Muscular pathologyPolymyositis, rhabdomyolysis
- Poor compliance with examination
The Motor Pathway
Upper Motor Nucleus
Primary motor cortex (precentral gyrus)
Primary motor cortex (precentral gyrus)
Central Pathway
Corticobulbar tract
(via cerebral peduncle)
Corticobulbar tract
(via cerebral peduncle)
Decussation
Medullary Pyramids
Medullary Pyramids
Spinal Pathway
Lateral corticospinal tract
Lateral corticospinal tract
Lower Motor Nucleus
Ventral horn at the level of entry into the spinal cord
Ventral horn at the level of entry into the spinal cord
Neuromuscular Junction
Muscle
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