Nerve Conduction Study Report: Carpal Tunnel and Neuropathy

Short answer: a nerve conduction study (NCS) checks how fast and how strongly electrical signals travel along your nerves. The report gives three kinds of numbers: latency (how long the signal takes to arrive), amplitude (how big it is when it gets there) and conduction velocity (its speed). Slow speed or long latency suggests damage to the nerve’s insulation, as in carpal tunnel; small amplitude suggests loss of nerve fibres, as in many cases of diabetic neuropathy. The neurologist puts it together with your symptoms.

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Your report arrives as a thick sheaf of tables. Each nerve gets a row: median, ulnar, peroneal, tibial, sural. Each row has columns of numbers, some flagged in bold or with an asterisk, and a conclusion at the end that reads like “Electrophysiological evidence of moderate right carpal tunnel syndrome and a length-dependent sensorimotor axonal polyneuropathy.” Long words. Here’s what they’re saying.

What does a nerve conduction study measure?

Nerves carry messages as tiny electrical pulses. During an NCS, a technician or doctor places sticky pads on the skin over a muscle or along a nerve, then gives short electrical taps at another point. The machine times how long the signal takes to travel between them and measures its size.

ELI5: think of a nerve as a bundle of electric wires, each wrapped in plastic insulation called myelin. If the insulation is damaged, the message still arrives, but slowly, like traffic on a road with potholes. If the wires themselves are cut or dying, fewer messages arrive, so the signal is weak, like a road with lanes closed. NCS can tell those two apart.

It’s often done together with EMG, where a fine needle records electrical activity inside a muscle. Your report may include both.

The columns on an NCS report, explained

As a rough guide, many labs expect median motor distal latency under about 4.2 to 4.4 ms and arm conduction velocities of about 50 m/s or more, leg velocities about 40 m/s or more. These are typical lab ranges, which vary by lab, age, height and skin temperature; use the range printed on your report.

What carpal tunnel looks like on the report

The median nerve runs through a narrow tunnel at the front of the wrist. When it’s squeezed, the insulation at that spot suffers first. So the report shows a long median distal latency and slow sensory speed across the wrist, while the ulnar nerve, which doesn’t pass through the tunnel, stays normal. Comparing the two on the same hand is one of the most useful tricks in the test.

Reports often grade it: mild (only sensory slowing), moderate (sensory and motor latency affected), severe (amplitudes low or absent, meaning fibres are being lost). The grade, plus your symptoms such as numb fingers at night, weak grip or wasting at the base of the thumb, guides what the doctor suggests next, from a wrist splint to surgery. For daily wrist care, see the carpal tunnel wrist stretches guide.

What diabetic neuropathy looks like on the report

Diabetes usually damages the longest nerves first, so the feet go before the hands. That’s why reports say “length-dependent.” The sural nerve (a sensory nerve at the ankle) often shows a small or absent signal early. Later, the peroneal and tibial motor amplitudes drop too. Velocities may be mildly slow. The word “axonal” means the wire itself is damaged, not just the insulation.

A normal NCS doesn’t always rule out neuropathy. The very thinnest nerve fibres, which carry pain and temperature and cause burning feet, don’t show on standard NCS. So you can have burning feet with a normal test. The guide to burning feet, B12 and diabetes covers that pattern, and hath paon sun hona explains other causes of numbness.

Other causes of an abnormal result

Low vitamin B12, an underactive thyroid, kidney disease, heavy alcohol use, some chemotherapy and other medicines, pressure on a nerve at the elbow (ulnar) or knee (peroneal), a slipped disc pressing on a root, and rarer inflammatory conditions such as Guillain-Barré syndrome can all show up. The pattern of which nerves are hit, on which side, points the doctor towards the cause, and blood tests usually follow.

Myth: the test uses dangerous electric shocks

The pulses are brief and small. They feel like a sharp tap or a snap from static electricity, and they can make a finger or toe twitch, which is startling. They don’t damage nerves or the heart. People believe the myth because “electric current” sounds frightening. If you have a pacemaker or defibrillator, just tell the team beforehand, as the settings may be adjusted.

When symptoms mean see a doctor today

How to prepare for a nerve conduction study

1. Bathe on the morning of the test and skip creams, oils and lotions on hands, arms, legs and feet; greasy skin stops the pads sticking.
2. Keep warm. Cold hands and feet slow nerves and can make results look falsely abnormal. In winter, wear socks and gloves on the way.
3. Wear loose clothes that roll up easily to the elbow and knee.
4. Tell the team about a pacemaker, blood thinners (relevant if EMG needles are used), and any recent swelling in the limbs.
5. Eat normally and take your usual medicines unless told otherwise. People with diabetes should keep their normal meal pattern.
6. Allow 30 to 90 minutes, depending on how many nerves are tested.

More report guides are gathered under Lab Reports Decoded.

Surprising facts about nerve signals

Did you know? The fastest human nerve signals travel at around 60 metres per second or more, roughly the speed of a car on a motorway. And tall people have longer nerves, so their foot latencies are naturally a little longer, which is why labs adjust for height.

Nerve conduction study questions people ask most