Types of Nerve Injury
Neuropraxia: physiologic block of nerve
Axonotmesis: anatomic disruption of axon with little disruption of connective tissue
Neurotmesis: anatomic disruption of axon and connective tissue
Nerve injury can be classified into three types:
Neuropraxia - physiologic block of nerve conduction within an axon without any anatomical interruption.
- Many infants with birth brachial plexus injury have neuropraxia and recover spontaneously because neuropraxia tends to disappear within 4-6 weeks.
Axonotmesis - anatomical interruption of the axon with no or only partial interruption of the connective tissue framework.
- This type of nerve injury requires regrowth of the axon to the target muscle, which takes a considerable amount of time. This regrowth can be inhibited by scar formation. Whether patients with axonotmesis will require surgical treatment depends on the number of disrupted axons and the extent of scar formation at the site of nerve injury.
- When an axon is disrupted, there is a 2-4 week delay before the axon starts to regenerate. Axons grow in adults at about 1 inch per month, which means many months will be required for the axon to grow down to the muscles in the arms. In infants, however, the axon may regenerate more rapidly, and the distance to be covered is much less. When a muscle loses its innervation, the nerve receptors will disappear over a period of 12 to 18 months. This affects the timing of neurosurgical intervention, because a repair done too late will not have receptors in the muscles for the growing nerves.
Neurotmesis - complete anatomical disruption of the both the axon and all of the surrounding connective tissue (rupture of the nerve).
- Birth brachial plexus injury is sometimes associated with neurotmesis. This is the most severe type of nerve injury and has no chance of spontaneous recovery. Early surgical treatment is necessary.