The Newbie’s Roadmap to NCS & EMG: Building Foundations in PM&R Diagnostics

For PM&R physicians embarking on the journey of learning Nerve Conduction Studies (NCS) and Electromyography (EMG), mastering the nuances of these diagnostic tools is essential for effective diagnosis and treatment of neuromuscular disorders.

Foundational Steps in NCS

NCS form the backbone of electrodiagnostic evaluations, paving the way for the needle EMG examination. By administering a brief electrical pulse over a peripheral nerve, we can measure the nerve’s response, providing vital information for the subsequent EMG analysis.

• Principal Nerves: Focus on the median, ulnar, and radial nerves in the upper extremity, and the peroneal, tibial, and sural nerves in the lower extremity.
• Study Varieties: Conduct motor, sensory, and mixed nerve studies based on the patient’s symptoms, with each type offering unique insights into nerve function.

Motor Conduction Studies: Prioritizing Efficiency

Motor studies are typically initiated first due to their simplicity compared to sensory or mixed nerve studies. This approach not only enhances the efficiency of the diagnostic process but also optimizes patient comfort by quickly identifying the nerve’s pathway and stimulation needs.

- Motor Response Metrics: Motor responses are measured in millivolts (mV), offering a clear signal less susceptible to interference compared to the microvolt (µV) range seen in sensory responses.

Key Technical Specifications

- Gain Settings: For motor conduction studies, adjust the gain to 2–5 mV per division, ensuring optimal signal resolution.
- Electrical Pulse Duration: Set the duration of the electrical pulse to 200 ms for motor studies, a standard parameter to adequately stimulate most nerves.
- Current Requirements: Normal nerves typically require a stimulation current of 20–50 mA to achieve supramaximal stimulation. Begin with a baseline of 0 mA and increase gradually, usually by 5–10 mA increments, to engage more nerve fibers and generate a comprehensive muscle response.

Decoding NCS Data

1. Amplitude: This reflects the nerve’s electrical signal strength, crucial for assessing the health of nerve-muscle interactions.

2. Latency: Represents the delay between stimulation and response, key for identifying conduction speed and potential delays.

3. Area and Duration: These metrics provide a fuller picture of the nerve’s function, with the area indicating the total muscle fiber response and duration assessing the synchrony of this response.

4. Conduction Velocity: Essential for distinguishing between demyelinating and axonal neuropathies, this is calculated by the distance over latency difference between two stimulation points.

Enhancing EMG Examination with NCS Insights

Leveraging NCS data, the needle EMG exam becomes a targeted exploration of neuromuscular integrity. This step-wise approach from NCS to EMG ensures a focused diagnostic process.

Practical Tips for Effective NCS and EMG

- Order Matters: Conduct motor conduction studies first to guide subsequent sensory studies efficiently.
- Optimal Electrode Placement: Employ the belly-tendon montage for accurate signal capture, placing the active electrode over the muscle belly and the reference electrode over the muscle’s tendon.
- Stimulation Strategy: Aim for supramaximal stimulation, gradually increasing the current (by another 20%) to ensure comprehensive nerve fiber engagement.

Interpreting Complex Signals

Understanding the Compound Muscle Action Potential (CMAP) and its characteristics — latency, amplitude, duration, area — unlocks deeper insights into nerve and muscle health, guiding clinical decisions.

Enhancing Diagnostic Precision in Sensory Conduction Studies

Sensory conduction studies stand as a cornerstone in the realm of electrodiagnostic testing for PM&R physicians, offering a focused lens on the functionality of sensory nerve fibers. These studies, by sidestepping the neuromuscular junction and muscle fibers, provide invaluable insights into the integrity of sensory pathways.

Navigating Technicalities with Finesse

The subtle nature of sensory responses, typically within the 1–50 µV range, necessitates an acute focus on technical precision to mitigate the influence of electrical noise.

• Gain Settings: Adjust the gain to 10–20 µV per division, optimizing the clarity of sensory nerve action potentials (SNAPs) for accurate analysis.
• Electrode Configuration: Place recording electrodes (G1 and G2) directly over the nerve, maintaining a strategic interelectrode distance of 2.5–4 cm. Position the active electrode (G1) closer to the stimulator to enhance signal detection.
• Ring Electrodes: Employ ring electrodes for sensory assessments in the fingers, ensuring precise measurement of SNAPs.

Stimulation Strategy

- Pulse Duration: Choose an electrical pulse duration of either 100 or 200 ms, tailored to the specific requirements of the sensory study at hand.
- Current Requirements: Sensory nerves generally necessitate a stimulation current ranging from 5–30 mA for supramaximal stimulation — significantly less than their motor counterparts, reflecting the sensory fibers’ lower stimulation threshold.

Decoding SNAPs

SNAPs, whether biphasic or triphasic, encapsulate the collective response of sensory fibers, analyzed for onset latency, peak latency, duration, and amplitude.

Core Measurements Unpacked

1. Onset Latency: The interval from stimulus to the first negative deflection for biphasic SNAPs or the initial positive peak for triphasic SNAPs, crucial for gauging the conduction time of the largest cutaneous sensory fibers.

2. Peak Latency: This measurement, reaching to the midpoint of the first negative peak, stands out for its consistency across individuals, though its utility in calculating conduction velocity is limited.

3. Amplitude: Represents the cumulative depolarization of sensory fibers. A diminished SNAP amplitude signals potential peripheral nerve pathology.

4. Duration: The SNAP duration, markedly shorter than motor action potential (MAP) duration, typically spans 1.5 ms compared to the 5–6 ms seen in MAPs. This significant difference aids in distinguishing genuine nerve potentials from those of muscular origin.

Sensory Conduction Velocity: Simplified Calculation

Sensory conduction velocity, derived from a single stimulation point, simplifies the assessment of the fastest, myelinated cutaneous sensory fibers by dividing the distance from stimulator to active electrode by the onset latency.

Advanced Diagnostic Insights

- Proximal Sensory Studies: While challenging due to the reduced amplitude of potentials, these studies expand the diagnostic reach to proximal nerve segments, despite the inherent complexities such as phase cancellation and temporal dispersion.

Refining Sensory Nerve Evaluation

For PM&R physicians, mastering sensory conduction studies is pivotal, not just for the procedural know-how but for the rich diagnostic insights they offer into sensory nerve function. This article, with a spotlight on critical metric and technical detail — especially the significant duration contrast between SNAPs and MAPs — aims to enhance your diagnostic prowess, ensuring a thorough and nuanced approach to neuromuscular patient care.

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📒 Compiled by — Sigrid Chen, Rehabilitation Medicine Resident Physician at Taichung Tzu Chi Hospital, Occupational Therapist, Personal Trainer of the American College of Sports Medicine.