Which electrodiagnostic finding would indicate denervation in a brachial plexus injury?

• A. Fibrillation potentials
• B. No denervation potentials
• C. Normal EMG
• D. Increased CMAP amplitudes

Answer: (C)

Denervation shows up on EMG as spontaneous activity in the muscle because the muscle fibers lose their nerve input and become electrically unstable. The hallmark finding is fibrillation potentials—small, spontaneous spikes from individual denervated muscle fibers that indicate active loss of motor innervation. In a brachial plexus injury with axonal disruption, these denervation signs appear as the injured nerves degenerate and the muscle fibers become hyperexcitable. Nerve conduction studies would typically show reduced CMAP amplitudes when there is axonal loss, reflecting fewer functioning motor units, while a normal EMG would not reveal denervation and increased CMAP amplitudes do not indicate denervation. So the presence of fibrillation potentials on EMG is the best sign of denervation in this context.