R.NCS.T. Exam cheat sheet

Sensory latency is measured to the peak of the negative deflection; motor latency is measured to the onset of the compound muscle action potential (CMAP).

Conduction velocity = distance (mm) / latency difference (ms), reported in m/s; use proximal-distal latency difference, not absolute latency.

Sensory nerve action potential (SNAP) amplitudes are measured peak-to-peak; CMAP amplitudes are measured baseline-to-negative peak.

Upper limb median motor normal conduction velocity is roughly 49-65 m/s; lower limb peroneal and tibial nerves are approximately 40-55 m/s.

Prolonged distal latency with normal conduction velocity suggests a distal demyelinating lesion or focal entrapment; reduced amplitude with normal velocity suggests axonal loss.

F-wave latency reflects the fastest conducting motor fibers over the entire proximal segment; prolongation indicates proximal demyelination.

H-reflex is the electrical analog of the ankle jerk and evaluates the S1 afferent and efferent arc; it is typically absent or prolonged in S1 radiculopathy.

Temporal dispersion and conduction block (drop in CMAP amplitude of >50% proximal versus distal) indicate acquired demyelination.

Needle EMG findings of fibrillation potentials and positive sharp waves indicate active denervation, appearing 2-3 weeks after nerve injury.

Myelinated fibers conduct via saltatory conduction at nodes of Ranvier; conduction velocity increases with fiber diameter and degree of myelination.

Large myelinated A-alpha fibers (Ia, Ib) mediate proprioception and motor output; small myelinated A-delta and unmyelinated C fibers mediate pain and temperature.

The brachial plexus derives from ventral rami of C5-T1; the lumbosacral plexus derives from L4-S3.

The median nerve arises from the medial and lateral cords; the ulnar nerve arises from the medial cord; the radial nerve arises from the posterior cord.

The common peroneal nerve wraps around the fibular head, making it vulnerable to compression; the ulnar nerve passes through the cubital tunnel at the elbow.

The blood-nerve barrier is formed by perineurial tight junctions and endoneurial capillary endothelial cells.

Wallerian degeneration occurs distal to the injury site; remyelination proceeds at roughly 1-2 mm per day after nerve repair.

The neuromuscular junction uses acetylcholine; postsynaptic nicotinic receptors are degraded by acetylcholinesterase.

Surface temperature should be maintained at or above 32 degrees Celsius on the upper limb and 30 degrees Celsius on the lower limb; cooling slows conduction velocity by about 2 m/s per degree Celsius.

Filter settings for motor studies are typically 2-10 Hz low-frequency and 10 kHz high-frequency; sensory studies use 20 Hz to 2-3 kHz.

The active (G1) electrode is placed over the muscle belly or nerve; the reference (G2) electrode is placed over the tendon or a distant site.

Supramaximal stimulation is defined as a stimulus intensity 20-25% above the level that produces a maximal response.

Ground electrode is placed between the active recording electrode and the stimulating electrode to reduce stimulus artifact.

Sweep speed of 2-5 ms/division is typical for sensory studies; 5-10 ms/division for motor studies.

Averaging of 10-20 responses improves signal-to-noise ratio for small SNAPs; single supramaximal stimuli suffice for CMAPs.

Electrode impedance should be kept below 5 kilo-ohms; higher impedance increases noise and artifact.

Informed consent must be obtained before the procedure; the patient should be told about electrical stimulation, needle discomfort, and the purpose of the test.

Pacemakers and implanted defibrillators are a relative contraindication for direct stimulation over the chest; consult cardiology and avoid transthoracic current paths.

Needle EMG should be avoided in patients on therapeutic anticoagulation or with severe thrombocytopenia unless the benefit clearly outweighs bleeding risk.

Universal precautions apply: gloves for all needle procedures, sharps disposal in biohazard containers, and hand hygiene before and after contact.

Skin preparation with alcohol reduces impedance and infection risk; do not stimulate over broken or infected skin.

Report critical findings promptly; document procedure, findings, and any adverse events in the medical record.

Maintain patient privacy consistent with HIPAA; do not discuss identifiable patient information outside the clinical setting.

Electrical stimulation should be discontinued if the patient reports severe pain, syncope, or signs of skin burn.