Lower motor neuron lesion

Lower motor neuron lesion
Lower motor neuron in red
Classification and external resources
Specialty
DiseasesDB	22143
Patient UK	Lower motor neuron lesion

A lower motor neuron lesion is a lesion which affects nerve fibers traveling from the ventral horn or anterior grey column of the spinal cord to the relevant muscle(s) – the lower motor neuron.

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.

Symptoms

• Muscle paresis or paralysis
• fibrillations
• fasciculations- caused by increased receptor concentration on muscles to compensate for lack of innervation.
• hypotonia or atonia- Tone is not velocity dependent.
• 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 - 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.

Differential Diagnosis

• Myasthenia gravis - synaptic transmission at motor end-plate is impaired
• 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.

See also

• Lower motor neuron
• Upper motor neuron
• Upper motor neuron lesion

External links

• http://library.med.utah.edu/neurologicexam/html/motor_anatomy.html#06