LMN Lesion

A lower motor neuron lesion is a lesion which affects nerve fibers traveling from the anterior horn of the spinal cord to the relevant muscle(s) -- the lower motor neuron.[1]
One major characteristic used to identify a lower motor neuron lesion is flaccid paralysis - paralysis accompanied by muscle loss. This is in contrast to a upper motor neuron lesion, which often presents with spastic paralysis - paralysis accompanied by severe hypertonia.

Symptoms

Muscle paresis or paralysis
fibrillations
fasciculations
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.

Etiology

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, botulism and cauda equina syndrome.

Differential Diagnosis

Myasthenia gravis - synaptic transmission at motor end-plate is impaired
Muscular dystrophy - contraction of muscle is impaired due to a cellular defect