What Actually Happens When You Request a Nerve Conduction Study

This post is written for doctors and other healthcare professionals. It is not intended for patients — if you have been referred for a nerve conduction study and have questions, please speak to your GP or the neurophysiology department directly.

Somewhere in a hospital near you, a neurophysiologist is staring at a referral that says “?carpal tunnel” and nothing else. No clinical findings. No symptom duration. No mention of which hand. Just a question mark and a diagnosis that may or may not be correct.

This is fine. We are used to it. But it does mean that the test you ordered — the one designed to answer a specific clinical question — is now going to answer a slightly different, more generic question instead. The result may still be useful. It may also be a report that lands in your inbox and tells you approximately nothing actionable.

This guide is an attempt to fix that, by explaining what nerve conduction studies and EMG actually are, what they can tell you, and how to get the most out of them. It is not a criticism of anyone. Neurophysiology is genuinely confusing to people who do not spend their days in it, and the specialty has historically been bad at explaining itself. Consider this a peace offering.

First, the important thing: this is not an automated scan

Here is the single most useful thing to understand about neurophysiology referrals, and the thing that distinguishes them from almost every other investigation you can request.

An NCS/EMG is not like an ECG. There is no machine that independently processes the data and produces a result. The study is designed, modified, extended, and interpreted in real time by a clinical neurophysiologist — a specialist — who is making decisions throughout based on what they find. Which nerves to study, which muscles to sample, whether to add extra protocols, how to interpret borderline values — all of this depends on knowing what question is being asked.

There is a principle that applies to many things in medicine: there is no point doing a test unless you know what you are going to do with the result. Before picking your nose, in other words, you need a plan for what happens next. The neurophysiologist needs a hypothesis to work with — not a definitive answer, but a differential diagnosis, a clinical picture, a reason why this particular patient is in front of you.

A well-framed referral gives the neurophysiologist the information to tailor the study. An under-specified referral produces a generic study that may or may not answer your question. Critically, the study takes at least an hour — often longer for complex cases — and that time is spent thinking through a diagnostic problem with the patient in the room. It is a clinical consultation with electrodes, not a scan with a readout.

What the test actually involves

A nerve conduction study works by applying a small electrical stimulus to a nerve — via surface electrodes on the skin — and recording the response either from the nerve itself or from the muscle it supplies. The stimulus is mildly uncomfortable (patients often describe it as a sharp tap or a static shock), but brief. You do not need to warn your patients that it is agony, because it is not.

From these recordings, two key measurements come out:

The CMAP — Compound Muscle Action Potential is the summed electrical response of all the muscle fibres activated by stimulating a motor nerve. The amplitude tells you roughly how many functioning motor axons are present and whether the neuromuscular junction is intact. A reduced CMAP amplitude means axonal loss or neuromuscular junction failure. A normal amplitude but prolonged latency or slowed conduction velocity means the nerve fibres are intact but conducting poorly — which points toward demyelination.

The SNAP — Sensory Nerve Action Potential is the equivalent for pure sensory nerves. SNAPs are small signals and technically demanding to record reliably. A reduced or absent SNAP indicates either axonal loss in sensory fibres, or a lesion distal to the dorsal root ganglion (DRG). This last point matters a lot diagnostically: a normal SNAP in a patient with sensory symptoms in a dermatomal distribution suggests the lesion is proximal to the DRG — i.e. a root problem, not a nerve problem. We will return to this.

EMG is a separate but complementary test. A fine needle electrode — roughly the gauge of an acupuncture needle — is inserted into a muscle to record its electrical activity at rest and during contraction. At rest, a healthy muscle is electrically silent. A denervated muscle is not: it produces fibrillation potentials and positive sharp waves, which are the electrophysiological signature of a muscle that has lost its nerve supply and is not happy about it. During contraction, the size, shape, and recruitment of motor unit potentials tell you whether the muscle is recovering from a nerve injury (large, complex, reinnervating potentials), primarily diseased (small, fragmented myopathic potentials), or somewhere in between.

Axonal loss vs demyelination: the distinction that drives management

When the report comes back, the two most important words to look for are “axonal” and “demyelinating.” These describe fundamentally different pathological processes with different implications for prognosis and urgency.

Demyelination means the nerve fibres themselves are intact, but the myelin sheath surrounding them is damaged. Signals still get through — they just travel more slowly. On NCS this shows up as prolonged latencies and reduced conduction velocities, with relatively preserved CMAP and SNAP amplitudes. In focal entrapment neuropathies — carpal tunnel syndrome being the textbook example — localised demyelination at the compression site produces slowing across that segment alone. Demyelinating lesions are generally more reversible: once you decompress the nerve, or treat the underlying cause, the myelin can recover.

Axonal loss means the nerve fibres themselves have been lost. The signals are reduced because there are fewer conductors, not because each conductor is slower. On NCS this shows up as reduced CMAP and SNAP amplitudes with relatively preserved conduction velocities. Recovery from axonal loss is slow — axons regenerate at approximately 1mm per day, which means that for a proximal lesion, meaningful functional recovery may take months to years, and may never be complete.

This distinction is not academic. A report describing “severe axonal loss” in a motor nerve is telling you that the patient has lost a substantial number of nerve fibres and that the road to recovery is long. A report describing “mild demyelinating features at the wrist consistent with carpal tunnel syndrome” is describing a very different situation. Both are called “abnormal NCS” but they are not remotely the same finding.

When to refer — and when timing matters

For most peripheral nerve conditions, the optimal time to refer is at least two weeks after symptom onset, and ideally longer. This is because the electrophysiological changes that follow nerve injury — Wallerian degeneration of the distal axon, followed by denervation changes appearing in the muscle — take time to develop fully. A study done in the first two weeks after an acute mononeuropathy or suspected radiculopathy may be entirely normal, not because the patient is fine, but because the nerve has not yet had time to show its hand.

There are important exceptions:

Guillain-Barré syndrome is the main one. In suspected GBS, early NCS is clinically useful precisely because it can help confirm the diagnosis and — critically — differentiate the demyelinating subtype (AIDP) from the axonal subtypes (AMAN, AMSAN), which have different prognoses. Early electrodiagnostic testing, particularly H-reflex studies, can yield diagnostically useful information within the first week of weakness onset. Serial studies are often needed as the picture evolves. If you are suspicious of GBS, refer early and do not wait.

Myasthenia gravis and neuromuscular junction disorders are not subject to the same timing constraints as nerve injury studies. Repetitive nerve stimulation (RNS) and single-fibre EMG (SFEMG) — the tests used to assess the NMJ — can be done at any point in the disease course. In the inpatient setting, RNS can rapidly support a diagnosis of MG while antibody results are pending, making early neurophysiology referral actively useful in acute presentations with suspected NMJ disease.

Suspected motor neuron disease also warrants prompt referral, both because the EMG findings are not time-dependent in the same way and because the diagnostic urgency is high.

Acute nerve injury with suspected transection — early NCS can establish a baseline and help determine whether nerve continuity is preserved, which guides surgical decision-making.

For everything else — a slowly progressive neuropathy, a suspected entrapment, a chronic radiculopathy — there is no advantage to referring in the first two weeks, and you may simply generate an uninformative result and a frustrated neurophysiologist.

A worked example: suspected C7/C8 radiculopathy

The following is a fictional case for illustration.

A 48-year-old right-handed builder presents with six weeks of progressive right neck pain radiating into the medial forearm, ring finger, and little finger. He has noticed weakness in his grip and some wasting of the intrinsic hand muscles. Symptoms began after heavy lifting and have been constant and worsening since. MRI is pending.

The differential is: C8 radiculopathy vs ulnar neuropathy at the elbow vs lower trunk brachial plexopathy.

What the NCS will show — and why the SNAP is the key:

Here is where the DRG anatomy becomes diagnostically useful. In a C8 radiculopathy, the lesion is proximal to the dorsal root ganglion. The sensory neuron cell body sits in the DRG and is physically intact — it is only the central projection (into the spinal cord) that is compressed, not the peripheral axon. This means that the SNAP recorded from ulnar sensory fibres (which carry C8 sensory information) may be completely normal in amplitude, even in a patient with significant C8 sensory symptoms.

Contrast this with an ulnar neuropathy at the elbow, where the lesion is distal to the DRG. Here, the peripheral axon itself is damaged, and the SNAP will be reduced or absent. This single NCS finding — normal vs abnormal SNAP — is one of the most powerful ways to distinguish a root lesion from a peripheral nerve lesion, and it is why knowing your differential before the study matters so much.

The motor studies may show a reduced CMAP from abductor digiti minimi (ulnar, C8) if there is significant motor axonal loss at the root.

What the EMG adds:

The needle study is where radiculopathy is confirmed. Denervation changes in C8-innervated muscles — first dorsal interosseous, flexor carpi ulnaris, abductor pollicis brevis — combined with abnormality in the cervical paraspinal muscles at the appropriate level, and with sparing of muscles outside the C8 myotome, localises the lesion to the root. Finding changes in paraspinal muscles is particularly valuable because these muscles are supplied directly from the posterior primary rami before any plexus or peripheral nerve branching — if they are abnormal, the lesion must be at or proximal to the root.

Reading the conclusion:

A report saying “active C8 radiculopathy with severe axonal loss and ongoing denervation” is telling you there is significant structural damage and an active ongoing process. This warrants urgent surgical review. A report saying “chronic C8 radiculopathy with mild axonal loss, reinnervation, and no active denervation” describes a lesion that is old, stable, and recovering — a very different clinical situation that may be managed conservatively. If the report does not make this distinction clearly, it is entirely reasonable to ring the neurophysiology department and ask. That is what we are there for.

What to include in your referral

The following information directly changes what the neurophysiologist does in the room. The more of it you include, the better the study and report will be.

• The symptom distribution — which limb(s), proximal or distal, which fingers or dermatomes, unilateral or bilateral
• The time course — when did symptoms start, are they constant or intermittent, progressive or stable or improving
• Your differential diagnosis — even a provisional one: “?C8 radiculopathy vs ulnar neuropathy” is enormously more useful than “?peripheral nerve problem”
• Relevant clinical findings — weakness distribution, reflexes, UMN vs LMN signs
• Relevant history — diabetes, autoimmune conditions, family history of neuropathy, chemotherapy, alcohol use, relevant medications
• Investigations already done — MRI findings, blood results, previous neurophysiology

You do not need a definitive diagnosis before referring. You need a hypothesis — a reasonable clinical question that neurophysiology can help answer. The neurophysiologist will take it from there.

Next in this series: EEG for the referring clinician — why a normal EEG does not mean your patient does not have epilepsy.