6.7 Electrophysiologic Recordings (EEG, ECG, EMG, and NCV)

6.7 Electrophysiologic Recordings: EEG, ECG, EMG, and NCV

Overview

Electrophysiologic studies measure the electrical activity of excitable tissue — brain (EEG), heart (ECG), and peripheral nerve/muscle (NCV, EMG) — to localize and characterize dysfunction that plain imaging cannot show. For NBCE Part III, the highest-yield skill is knowing which study answers which clinical question, particularly using EMG/NCV to separate radiculopathy from peripheral neuropathy, and recognizing ECG findings that redirect a seemingly musculoskeletal chest or arm complaint toward emergency cardiac care.

Electromyography (EMG)

EMG records the electrical activity of muscle at rest and during voluntary contraction using a needle electrode inserted directly into the muscle. It is indicated when a lower motor neuron lesion — nerve root, plexus, peripheral nerve, or the muscle itself — is suspected and imaging or the clinical exam alone is inconclusive.

Key findings:

• Denervation (active, ongoing nerve damage): spontaneous fibrillation potentials and positive sharp waves at rest, seen in radiculopathy, peripheral nerve injury, or motor neuron disease
• Reinnervation (chronic/healing): large-amplitude, long-duration, polyphasic motor unit action potentials as surviving axons sprout to reinnervate muscle fibers
• Myopathic pattern: small-amplitude, short-duration, polyphasic motor unit potentials with early recruitment, seen in primary muscle disease rather than nerve pathology

Critical timing rule: denervation changes (fibrillation potentials) do not appear immediately after nerve injury — they typically take 2-3 weeks to develop as the distal axon undergoes Wallerian degeneration. An EMG performed too early after an acute radiculopathy can be falsely normal; the study is generally most useful when obtained 3-4 weeks after symptom onset if symptoms persist.

Nerve Conduction Velocity (NCV) Studies

NCV studies stimulate a peripheral nerve at one point and record the response at another, measuring conduction velocity, amplitude, and latency of motor and sensory responses. They help answer two key questions:

• Is the process axonal (reduced amplitude with relatively preserved velocity) or demyelinating (markedly slowed velocity, conduction block)?
• Is the lesion focal, such as an entrapment like carpal tunnel syndrome, or diffuse, such as a polyneuropathy?

EMG/NCV: Radiculopathy vs. Peripheral Neuropathy

This distinction is a classic Part III testing point:

The single most useful discriminator is the SNAP: a normal SNAP in the presence of clinical sensory loss in a dermatomal pattern strongly favors a root-level (preganglionic) lesion, since the sensory neuron cell body in the DRG — and therefore its distal axon — remains intact even when the root itself is compressed. An abnormal or diminished SNAP amplitude points to pathology distal to the DRG, meaning true peripheral neuropathy or plexopathy.

Example: carpal tunnel syndrome shows prolonged distal motor and sensory latencies across the wrist on median nerve conduction studies. In more severe or chronic cases, EMG of the abductor pollicis brevis shows denervation potentials, indicating axonal loss superimposed on the underlying demyelinating entrapment.

Electrocardiography (ECG)

ECG records the heart's electrical activity through the P wave (atrial depolarization), QRS complex (ventricular depolarization), and T wave (ventricular repolarization). For chiropractors, the essential skill is recognizing when a seemingly musculoskeletal chest, arm, jaw, or upper back complaint is actually cardiac in origin.

Reference values: PR interval 0.12-0.20 seconds, QRS duration under 0.12 seconds, and corrected QT typically under 0.44-0.46 seconds.

Red flags demanding immediate ECG/emergency referral rather than spinal evaluation:

• Exertional chest, jaw, or left arm pain, especially with diaphoresis, dyspnea, or nausea
• Chest or interscapular pain radiating in a nondermatomal or bilateral pattern
• New arrhythmia symptoms, such as palpitations with syncope or near-syncope
• ST-segment elevation or depression, a new left bundle branch block, or significant Q waves, suggesting ischemia or infarction
• A prolonged QT interval, which raises the risk of a dangerous arrhythmia, particularly relevant when certain medications are involved

A classic Part III trap is the patient presenting with what looks like thoracic spine or "T4 syndrome" pain radiating to the left arm. Cardiac ischemia must always be excluded before attributing this pain to a mechanical spinal source, particularly in patients with cardiac risk factors such as age, smoking, diabetes, hypertension, or a family history of heart disease.

Electroencephalography (EEG)

EEG records cortical electrical activity via scalp electrodes and is used far less often in a chiropractic referral context, but its indication must still be known: evaluating suspected seizure disorders, unexplained loss of consciousness, and differentiating a true seizure from syncope or a psychogenic event. It may also be used in the workup of significant head trauma when seizure activity or persistent altered consciousness is a concern. A chiropractic patient who describes a witnessed convulsive event, post-traumatic confusion with abnormal movements, or recurrent unexplained "blackouts" should be referred for neurologic evaluation, which may include EEG — this is not a study a chiropractor orders directly, but recognizing the indication is testable.

Summary Table