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Your Report Is Full of Milliseconds. Here’s What They’re Actually Saying.
A nerve conduction study measures how fast and how strongly electricity travels along a nerve. It is a test of NERVE FUNCTION, not a picture. Three numbers carry the result: latency (delay), conduction velocity (speed) and amplitude (size). Slowed means the nerve is irritated. Smaller means fibers have been lost. “Mild” and “moderate” usually leave time for splinting. “Severe” or muscle changes on EMG mean waiting has a cost.
Two more things are worth knowing before you read your result.
- A normal study does not rule out a compressed nerve.
- And a severe-looking study does not automatically mean surgery.
The test informs the decision. It does not make it.
Here’s the problem. The report lands in your inbox as a page of numbers and abbreviations, the review appointment is weeks away, and Google gives you reference tables with no context. So people either panic over a word like “severe” or relax over a word like “normal”…and both can be the wrong call. The second one is the expensive mistake, cos a nerve that keeps losing fibers doesn’t wait for your next appointment.
What happens during a nerve conduction test (and does it hurt?)
Two parts, and people are usually only warned about the first. Allow about 30 to 60 minutes.
- Nerve conduction. Small electrical pulses on the skin, recording how long the signal takes to travel a known distance. Uncomfortable, like repeated static shocks, over in minutes.
- EMG. A fine needle into selected muscles, listening to their electrical activity at rest and on contraction. This is the part that tells you whether a muscle has lost its nerve supply, and it is sorer than the first part. A small bruise afterwards is common.
Cold hands slow conduction. If your hands were cold, say so. It affects the numbers.
Latency, velocity and amplitude: the three numbers that matter
- Latency. How long the signal takes to arrive, in milliseconds (ms). Prolonged latency means the insulation around the nerve (the myelin) is damaged. This is the earliest change in compression, and the most recoverable.
- Conduction velocity. Speed, in meters per second (m/s). Slowing across a specific segment localizes the compression. Slowing across the elbow rather than along the forearm points straight at cubital tunnel.
- Amplitude. The size of the response. Reduced amplitude means nerve fibers have been lost, not just slowed, and that is the number with prognostic weight. Latency recovers quickly after decompression. Amplitude recovers slowly, and sometimes not fully.
(Think of it like a garden hose. Latency and velocity are how long the water takes to reach the end. Amplitude is how much water comes out. A kinked hose is slow. A hose with holes in it delivers less. Holes are the bigger problem.)
What are normal nerve conduction values?
Every lab sets its own reference ranges, adjusted for age, height and skin temperature, and they’re printed next to your numbers. Use theirs. As a rough guide only:
- Conduction velocity in the arm nerves: normally around 50 m/s or faster.
- Median nerve motor latency at the wrist: most labs flag values above roughly 4.2 to 4.5 ms as prolonged.
- Ulnar nerve across the elbow: a velocity under about 50 m/s, or a drop of more than about 10 m/s compared with the forearm segment, suggests compression at the elbow.
- Sensory responses: “absent” or “not recordable” is a significant finding, not a technical glitch, unless the report says the hand was cold or swollen.
On the EMG side, terms like fibrillations and positive sharp waves mean a muscle is sitting there without a working nerve supply. Reinnervation changes describe a nerve that is growing back.
What do mild, moderate and severe carpal tunnel mean on a nerve test?
Grading varies between labs, but most follow a pattern like this:
- Mild: only the sensory fibers are slowed. Motor fibers are normal. The nerve is irritated, and in my experience most people in this group do well with a night splint and position changes.
- Moderate: sensory slowing plus a prolonged motor latency. Still mostly an insulation problem. The decision sits with your symptoms and function rather than the numbers.
- Severe: sensory responses absent, motor amplitude reduced, or EMG changes in the thumb muscles. Fibers are being lost. This is the group to discuss surgery with sooner rather than later.
See carpal tunnel syndrome for the full picture beyond the test.
Slowed is irritated. Smaller is damaged. Those two words carry most of the prognosis in the report.
Normal nerve conduction but still have symptoms?
This is the part that causes the most confusion. The test misses:
- Mild or intermittent compression. A meaningful minority of people with genuine carpal tunnel have a normal study, particularly early, and particularly when symptoms only happen at night. A normal result does not mean you imagined it.
- Small sensory nerve branches that are difficult to test reliably. See Wartenberg’s syndrome.
- Small fiber nerve damage, which causes burning and isn’t measured by standard studies. See burning hands.
- Dynamic compression that only happens in certain positions, such as thoracic outlet syndrome, where studies are often unremarkable.
- Timing after injury. Done too early after a nerve injury, the study can look misleadingly normal. The changes take time to appear, and the full picture on EMG usually needs about 3 to 4 weeks.
- Two problems at once. A neck problem and a wrist problem can coexist, and a clear finding at the wrist doesn’t exclude one higher up. See cervical radiculopathy.
Patients arrive quite regularly having been told their normal study means nothing is wrong. Their symptoms are still real, their examination still fits, and the treatment plan usually doesn’t change.
How the result changes the plan
- Mild changes, intermittent symptoms. Splinting, position and activity change. Retest only if things worsen.
- Moderate changes. The decision sits with your symptoms and function rather than the numbers.
- Reduced amplitude or EMG changes in the muscles. This is the group where waiting has a cost, because fibers already lost are what limit the final recovery. See therapy or surgery.
- Normal study, convincing symptoms. Treat the person. See peripheral neuropathy or carpal tunnel if both hands and feet are involved.
How long does it take a compressed nerve to recover?
This is where your report maps onto the Closing Window Method. Compressed nerves run on a window, and the numbers tell you roughly where you are in it.
- OPEN: slowed latency, normal amplitude. Insulation damage only. With the pressure off, by splinting or surgery, this usually recovers well. Night tingling often eases within weeks.
- CLOSING: amplitude starting to drop, or early EMG changes. Fibers are being lost. Act now. Recovery after decompression is still good, but slower: numbness can take 3 to 6 months to improve, sometimes up to a year.
- SHUT for full recovery: absent responses, muscle wasting at the thumb base. Surgery can still stop it getting worse and often helps pain and night waking. Full feeling and strength may not come back. The goal changes, not the effort.
If you’ve already had surgery and things are slow, see still numb after carpal tunnel surgery. A repeat study after surgery is usually only useful if you’re not improving by around 3 to 6 months, or getting worse.
What a hand therapist does with your report
The report is one input. The examination is the other, and it’s repeatable every visit.
- Sensory testing: monofilaments and two-point discrimination, so there’s a baseline to track.
- Strength: grip and pinch measured with a dynamometer, and a check of the thumb base muscles for wasting.
- Provocation tests: Tinel’s, Phalen’s and elbow flexion, matched against where the study found the problem.
- Splinting: a neutral wrist night splint for carpal tunnel, worn every night for 4 to 6 weeks before judging it, or an elbow night splint for cubital tunnel. See the carpal tunnel night splint.
- Nerve glides: 5 slow reps, 2 to 3 times a day, never pushed into tingling.
- Position and load changes at work and in bed, which often matter more than any exercise.
Practical notes for the day of the test
- No moisturizer on the day. It interferes with the electrodes.
- Warm hands. Bring gloves if you are traveling in air conditioning.
- Tell them about blood thinners and any pacemaker or implanted device.
- Ask for a copy of the numbers, not just the conclusion. A repeat study in a year is only useful if you can compare.
Get it checked today if
- Numbness and weakness are spreading in both hands and feet over days, or you’re struggling to walk. Go to the ER.
- You suddenly can’t lift the wrist, the fingers or the thumb.
- A finger went numb after a cut. A divided nerve needs a surgeon within days, not a nerve test in weeks.
- After the EMG, a needle site becomes hot, red and swollen, or a large painful swelling develops, especially if you’re on blood thinners.
Get it checked if
- Your report says severe, absent responses, reduced amplitude or denervation, and nobody has discussed a plan with you.
- You notice a hollow at the base of the thumb or between thumb and index.
- Numbness has become constant, or you’re dropping things.
- Your study was normal but your symptoms are getting worse.
The study measures the nerve on the day it was tested. Your symptoms measure the nerve on every other day, and both belong in the decision.
Written by Nigel Chua, hand occupational therapist in practice since 2005. Thousands of hands, wrists and elbows assessed and treated. My credentials are here. This article explains; it doesn’t examine. A report that says “severe” deserves a conversation about the plan this month, not at next year’s review.