Evoked Potentials (EPs) are the "stress tests" of the nervous system. By extracting microvolt-level signals from background noise, we can map the functional integrity of the visual, auditory, and somatosensory pathways with millimeter precision.
Signal Averaging Physics
The EP signal is tiny (0.1 - 10 µV) and buried in massive background EEG noise (50 µV). We solve this with Signal Averaging.
The Averaging Engine
[cite_start]Because the response is time-locked to the stimulus, repeating the test sums the signal constructively, while random noise cancels out to zero [cite: 729-731].
Visual Evoked Potentials (VEP)
The VEP tests the anterior visual pathway (Optic Nerve to Visual Cortex). [cite_start]It is the gold standard for detecting demyelination (Optic Neuritis) [cite: 741-742].
P100 Pattern Reversal
Oz-Fz Montage1 Hz Reversal
Pathology: Demyelination vs. Axonal Loss
Finding: Prolonged Latency (>115ms).
Signal is delayed because saltatory conduction fails. [cite_start]Hallmark of MS/Optic Neuritis [cite: 763-766].
Finding: Reduced Amplitude (<5µV).
Signal arrives on time, but is "quieter" because fibers are dead. [cite_start]Poor prognosis [cite: 768-771].
Brainstem Auditory (BAEP)
BAEPs are resistant to anesthesia and coma, making them ideal for intraoperative monitoring and brain death assessment. We analyze Waves I through V.
The Auditory Staircase
Clinical Pearl: Wave I is the peripheral input. Wave V is the robust central marker. [cite_start]If I-V interval is prolonged, suspect retrocochlear pathology (e.g., Acoustic Neuroma) [cite: 813-817].
Somatosensory (SSEP)
SSEP assesses the Dorsal Columns (Proprioception). We stimulate the Median (wrist) or Tibial (ankle) nerve and track the signal to the cortex.
| Waveform | Generator | Significance |
|---|---|---|
| N9 Erb's Point |
Brachial Plexus | Peripheral Input. [cite_start]If absent, the issue is in the arm/nerve, not the cord [cite: 826-828]. |
| N13 Cervical |
Cervical Cord | Stationary potential of the dorsal horn. [cite_start]Confirms signal reached the spine [cite: 829-831]. |
| N20 Cortical |
Parietal Cortex (S1) | Primary cortical reception. [cite_start]N13-N20 interval measures Central Conduction Time [cite: 834-836]. |
Intraoperative Monitoring (IOM)
In spine surgery, SSEP protects the cord. The "Alarm Criteria" is a 50% drop in amplitude or a 10% increase in latency. [cite_start]This warns the surgeon to reverse the maneuver before paralysis becomes permanent [cite: 852-853].
Conclusion
Evoked Potentials allow us to "trace the wires" of the human nervous system. From the P100 latency confirming optic neuritis to the preservation of Wave V in coma, EPs provide objective data where clinical exams fail. Mastering the interplay of anatomy and signal physics is the key to accurate diagnosis.