Denervation and Reinnervation in S1 Radiculopathy
Denervation refers to the loss of nerve supply to muscle fibers following axonal injury, characterized on EMG by spontaneous activity including fibrillation potentials and positive sharp waves, while reinnervation is the process where intact neighboring axons sprout to reconnect with denervated muscle fibers, producing polyphasic motor unit action potentials with increased duration and complexity.
Denervation: The Acute Injury Phase
When an axon is damaged in S1 radiculopathy, the muscle fibers lose their nerve supply and undergo denervation 1. This process follows a predictable timeline:
- Immediate phase: Initially, spontaneous discharges only occur when muscle fibers are directly stimulated by needle movement during EMG
- After ~1 week: Spontaneous activity develops with positive sharp waves and fibrillation potentials appearing on EMG 1
- EMG characteristics: These are short duration muscle fiber action potentials (typically <5 ms duration) with regular discharge patterns and peak-to-peak amplitudes around 200 microvolts 1
Key EMG Findings in Denervation
The hallmark features include 1:
- Fibrillation potentials: Spontaneously discharging individual muscle fibers
- Positive sharp waves: Spontaneous short duration potentials always in the positive (downward) direction
- Prolonged insertional activity: Activity extending beyond the normal 300 ms after needle movement
- Both patterns discharge regularly and indicate active denervation
Reinnervation: The Recovery Phase
Over weeks to months following the initial injury, intact neighboring axons may sprout to reinnervate adjacent denervated muscle fibers 1. This represents the body's attempt to restore function:
Mechanism of Reinnervation
- Axonal sprouting: Intact motor neurons send out new branches to reconnect with orphaned muscle fibers
- Slower conduction: Because these axonal sprouts are thin and poorly myelinated, their conduction velocity is slower than original axonal branches 1
- Asynchronous activation: The slower conduction results in muscle fibers within the same motor unit firing out of sync
EMG Characteristics of Reinnervation
The reinnervation process produces distinctive changes 1:
- Polyphasic motor unit action potentials (MUAPs): Complex waveforms with at least four zero crossings producing five phases
- Increased MUAP duration: Longer than normal due to asynchronous muscle fiber activation
- Increased MUAP amplitude: Generally produces long duration, large amplitude MUAPs in neuropathic conditions 1
- More complex waveforms: Multiple baseline crossings that must reappear consistently to distinguish from chance simultaneous firing 1
Clinical Context in S1 Radiculopathy
In a patient with S1 radiculopathy confirmed by EMG 2:
- Muscles examined: Gastrocnemius (medial and lateral heads), biceps femoris (both heads are exclusively S1 innervated), and other S1 myotome muscles
- Active denervation indicates ongoing nerve injury with fibrillations and positive waves
- Chronic denervation-reinnervation shows polyphasic potentials indicating the nerve has been injured but recovery processes are underway 3, 4
Important Clinical Caveat
The presence of reinnervation does not necessarily indicate clinical improvement 5. Denervation and reinnervation comprise a dynamic process that can occur simultaneously in progressive conditions. The finding of reinnervation on EMG simply confirms that the injury occurred weeks to months prior and that collateral sprouting has begun—it does not predict functional recovery or indicate that the pathologic process has resolved.
Correlation with Imaging
MRI using STIR sequences shows increased signal intensity that corresponds closely with spontaneous activity on EMG in denervated muscle, with 84% sensitivity and 100% specificity 6. However, EMG remains more sensitive for detecting active denervation.