Educational video describing the classification of nerve injuries.
A penetrating cut at the wrist will affect the flexor carpi radialis tendon and most likely injure the median nerve. A nerve is a bundle of axons that are grouped into fascicles. Nerve provide a pathway for impulses to be transmitted along each of the axons to the limbs and organs.
Cross section of the nerve
•Perineurium: covers the fascicles
•fascicle
•endoneurium surrounds each axon.
•Myelin sheath allows the signal to move faster along the axon.
•Axon
Injury to the axon may be caused due to a single or double crush injury. Classification of the nerve injury depends on the nerve components affected, loss of functionality and the ability of the nerve to recover.
Classification of the nerve injury depends on the nerve components affected, loss of functionality and the ability of the nerve to recover. Two grading systems are used to define the extent of the nerve injury:
1-Seddon’s classification includes neurapraxia, axonotmesis, and neurotmesis in the order of increasing severity.
2-Sunderland’s: five grades of nerve injuries.
Seddon’s classification includes three types of nerve injuries:
•Type I: neurapraxia: prognosis is good with neurapraxia and it’s the mildest form of nerve injury and the nerve remains intact. Temporary loss of motor and sensory function due to blockage of nerve conduction. No conduction occurs across the area of injury. It may result from pressure ischemia. The signaling ability of the nerve is damaged, however, this injury usually recovers completely and there is no axonal injury here. It may take up to 12 weeks to recover. There is no fibrillation-fibrillation is only seen when there is an axonal injury. There is no axonal injury. There is the only block of the conduction. Conduction is intact in the distal segment and also the proximal segment but no conduction occurs across the area of injury.
•Type II: axonotmesis: it occurs in crush injuries or displaced bone fractures causing disruption of the nerve cell axon. The axon is damaged and the surrounding connective tissue remains intact. There will be partial or complete recovery of the nerve and Wallerian degeneration occurs distal to the injury site. Recovery depends on the regrowth of the axon and also the distance that is involved. Recovery occurs 1 mm per day or 1 inch per month. There will be fibrillation and positive sharp waves. With re-innervation and recovery, motor unit potentials will be seen. No conduction velocity distal to the lesion and axonal sprouting and regeneration usually occurs. Recovery is possible without surgery and it may take several months.
•Type III: neurotmesis: it occurs with knife lacerations, gun shot wound or severe ischemic injuries. The axon and surrounding connective tissues are damaged. Most serious nerve injury. No recovery, fibrillation is present and the injury usually requires surgery. Motor neuron unit potential is usually absent. If the motor unit potential is found, it means the axon has been spared.
Neurapraxia may be associated with brachial plexus injuries such as a “stinger” or “burner” injury commonly seen in football players. This injury usually improves.
Neurapraxia of the radial nerve or “Saturday night palsy” occurs from compression of the radial nerve at the spiral groove of the humerus. Radial nerve palsy associated with a Holstein-Lewis fracture could show signs of neurapraxia with wrist drop which later recovers. Observe the patient and get an EMG. However, open fractures of the humerus with radial nerve palsy are probably more prone to type III neurotmesis nerve injuries. The radial nerve should be explored. Because of injury to the radial nerve, the patient may be experiencing the condition known as “Wrist drop”.
Peripheral nerves have the ability to regenerate, the muscle fiber (the axon) is very long. It travels a long distance towards its target (muscle). A long time is needed to the peripheral nerve to recover so it can reach its target and achieve good function.
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