Axonal Injury

1. What is the difference between axonal and Wallerian degeneration?
2. What are the electrodiagnostic findings in axonal injury?
3. What is collateral sprouting and how does it affect the EMG?
4. What is axonal regrowth and how does it affect the EMG?
5. What occurs if the connective tissue is not intact to guide this regrowth?

Answers:
1. Axonal degeneration begins distally and ascends proximally. Wallerian degeneration proceeds distally from the injury, starting at 4-5 days and completing in 7 days for motor nerve and 11 for sensory.
2. NCS shows normal latency with decreased CV and decreased amplitude through the entire nerve. EMG shows abnormal MUAPs with decreased recruitment and abnormal insertional and resting activity.
3. Collateral sprouting occurs when sprouts off an intact axon, with small branches and thinner myelin, come over to innervate the injured motor unit. This results in poor firing synchronicity, resulting in EMG findings of polyphasic waveforms with increased amplitudes.
4. Axonal regrowth occurs when the axon regrows down its original path to the muscle fibers at a rate of 1mm/day (1 inch/month), creating axons with decreased diameter, thinner myelin, and shorter internodal distance. EMG findings include low amplitude, long duration, and polyphasic MUs, known as nascent potentials.
5. A neuroma can form with failure to reach the end organ.