Nerve conduction study (NCS) for ALS diagnosis
Reviewed by Amyotrophic lateral sclerosis (ALS) is a nervous system disease that damages the nerve cells responsible for controlling voluntary muscles. As these cells stop sending signals to the muscles, they gradually weaken and waste away.
Because other conditions can cause similar symptoms, a care team will use a combination of clinical evaluations and tests to understand what is affecting communication between the nerve cells and muscles.
These assessments may include a nerve conduction study (NCS) to support an ALS diagnosis or rule out other possible causes.
Why is a nerve conduction study used for ALS?
An NCS for ALS, also called a nerve conduction velocity test, checks how well the nerves controlling the muscles are working.
Nerves carry electrical signals around the body, including messages that tell muscles when to contract. By measuring how quickly and effectively these signals travel, an NCS can help a care team identify whether nerve damage is present and where it may be in the body.
Can a nerve conduction study diagnose ALS?
An NCS cannot diagnose ALS on its own because the condition damages motor nerve cells in the brain and spinal cord.
Despite this damage, weaker electrical signals may still travel to the muscles at relatively normal speeds. When this happens, an NCS alone may not show clear signs of ALS.
Because of this, the care team mainly uses an NCS to rule out other possible causes of muscle weakness. Doctors review the results alongside electromyography (EMG) test results, clinical examinations, and other tests when diagnosing ALS.
What does a nerve conduction study measure?
An NCS measures how quickly and strongly electrical signals move through the nerves, evaluating how motor and sensory nerves respond to stimulation.
Signal speed
Signal speed reflects how quickly signals travel through the body’s muscles. When an NCS detects slower-than-normal signals, this can indicate nerve damage.
Signal strength
When nerves are working properly, they respond as expected to stimulation. Weaker signals suggest that some nerve fibers are damaged or no longer function efficiently.
Motor and sensory nerve responses
Motor nerves control voluntary movements such as walking and sitting, while sensory nerves carry information about touch, pain, and temperature to the brain and spinal cord.
Doctors examine responses controlled by both nerve types to understand the pattern and extent of the nerve damage and determine whether ALS or another condition may be involved.
How is a nerve conduction study different from EMG?
An EMG is another diagnostic test that examines the health and function of the muscles and nerves. While an NCS measures the speed and strength of electrical signals traveling through the nerves, an EMG records electrical activity in the muscles during movement and at rest.
An NCS is usually performed alongside an EMG to provide a clearer picture of how the nerves and muscles function.
To make an ALS diagnosis, the care team will not choose between a nerve conduction study and an EMG. Instead, they use both tests to determine whether a patient’s symptoms are related to the nerves, muscles, or their communication.
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| Nerve conduction study | Electromyography test | |
|---|---|---|
| What it tests | How well nerves carry electrical signals through the body | Electrical activity in muscles |
| How the test is performed | Electrodes are placed over the skin, and brief electrical pulses stimulate the nerve | A thin needle electrode enters the muscle while the patient relaxes and contracts the muscle |
| How it relates to ALS | ALS may impair motor nerve function measured by the test | ALS may impair muscle electrical activity measured by the test |
| ALS diagnosis | Cannot diagnose ALS alone; used in combination with other tests to make a diagnosis | Cannot diagnose ALS alone; used in combination with other tests to make a diagnosis |
How do patients prepare for an NCS?
Before the test, patients should avoid moisturizers, creams, or lotions, as they can interfere with the electrodes that record signals.
Depending on the area being examined, patients may need to remove clothing and other items, such as jewelry, hairpins, glasses, or hearing aids. If needed, the patient may be given a gown.
Preparation checklist:
- Ask care providers how to prepare for the test
- Discuss any questions or concerns about the test with care providers
- Avoid moisturizers, creams, and lotions before the test
- Remove jewelry, accessories, and medical devices in the area being tested
- Change into a gown, if necessary
What happens during the test?
Depending on the areas being tested, the patient sits or lies down. A healthcare professional attaches two electrodes to the skin over the target areas.
One electrode delivers a mild electrical pulse to the nerve, while the other records the nerve’s response and displays the results on a monitor.
Why several nerves may be tested
Doctors may examine nerves in different parts of the body because several conditions can cause similar muscle weakness. Checking multiple nerves helps show where changes occur, how widespread they are, and what condition they may point to.
What does a nerve conduction study feel like?
An NCS involves brief electrical pulses that stimulate the nerves. People often describe these as a quick buzzing or tingling. While the feeling can be a little uncomfortable, each pulse lasts only a moment.
If the sensation becomes too uncomfortable, patients should inform the healthcare provider, who can adjust the settings.
What do the results mean during an ALS evaluation?
NCS results help the care team understand how a person’s nerves function and whether observed changes reflect ALS or another condition.
Findings commonly seen with ALS
With ALS, nerve conduction results often look normal, especially for sensory nerves. However, motor responses may appear weaker because fewer nerve fibers remain active.
This can also occur due to other conditions, so the care team will have to consider the results alongside an EMG and other tests.
Findings that may suggest another condition
A patient’s healthcare team may look for another cause of symptoms if the electrical signals:
- travel much slower than expected
- become blocked
- show abnormal changes in sensory nerves
These patterns are more commonly associated with conditions that directly compress or damage peripheral nerves.
Normal or inconclusive results
Normal NCS results do not immediately rule out ALS, especially early in the condition.
If findings are unclear or inconclusive, the care team may consider EMG results or use other tests to better understand the symptoms.
What happens after the test?
After testing is complete, some people may have mild, temporary discomfort from the electrical pulses, but these sensations cause no lasting damage and resolve quickly.
The care team will review the NCS results along with other test findings to better understand what may be causing symptoms and determine the next steps.
Questions to ask before and after the study
Asking a few questions before and after the study can help a patient know what to expect and better understand the next course of action:
- How should I prepare for the test?
- How long will testing take?
- Do any current medications affect the test?
- Were any nerve responses unusual?
- Do the findings suggest ALS or another condition?
- What are the next steps in care?
ALS News Today is strictly a news and information website about the disease. It does not provide medical advice, diagnosis, or treatment. This content is not intended to be a substitute for professional medical advice, diagnosis, or treatment. Always seek the advice of your physician or other qualified health provider with any questions you may have regarding a medical condition. Never disregard professional medical advice or delay in seeking it because of something you have read on this website.
