Lower motor neuron lesion
|Lower motor neuron lesion|
|Classification and external resources|
Lower motor neuron in red
One major characteristic used to identify a lower motor neuron lesion is flaccid paralysis – paralysis accompanied by loss of muscle tone. This is in contrast to an upper motor neuron lesion, which often presents with spastic paralysis – paralysis accompanied by severe hypertonia.
- Muscle paresis or paralysis
- hypotonia or atonia- Tone is not velocity dependent.
- Areflexia or hyporeflexia -Along with deep reflexes even cutaneous reflexes are also decreased or absent
- Strength -weakness is limited to segmental or focal pattern, Root innervated pattern
The extensor Babinski reflex is usually absent. Muscle paresis/paralysis, hypotonia/atonia, and hyporeflexia/areflexia are usually seen immediately following an insult. Muscle wasting, fasciculations and fibrillations are typically signs of end-stage muscle denervation and are seen over a longer time period. Another feature is the segmentation of symptoms - only muscles innervated by the damaged nerves will be symptomatic.
Most common causes of lower motor neuron injuries are trauma to peripheral nerves that sever the axons and poliomyelitis - a virus that selectively attacks ventral horn cells. Disuse atrophy of the muscle occurs i.e., shrinkage of muscle fibre finally replaced by fibrous tissue (fibrous muscle) Other causes include Guillain-Barré syndrome, C. botulism, polio, and cauda equina syndrome; another common cause of Lower Motor neuron degeneration is Amyotrophic lateral sclerosis.
- Myasthenia gravis - synaptic transmission at motor end-plate is impaired
- Muscular dystrophy - contraction of muscle is impaired due to a cellular defect
- Amyotrophic lateral sclerosis - causes death of motor neurons, although exact cause is unknown it has been suggested that abnormal built-up of proteins proves toxic for the neurons.