Fasciculation

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Fasciculation (muscle twitch)

A fasciculation is a small, involuntary contraction and relaxation of a few muscle fibers. It often shows up as a brief flutter under the skin. Many people experience them at some point in life; about 70% of healthy adults do. Most fasciculations are harmless, but they can be related to nerve or muscle problems in rare cases. If no disease is found, it’s called benign fasciculation syndrome.

What causes it and how it works
- A fasciculation happens when a lower motor neuron fires on its own, causing all the fibers in one motor unit to twitch together.
- You might notice the twitch when the muscle is at rest. Moving the muscle usually stops the twitch, but it can come back after you stop.

Tics vs fasciculations
- Small eyelid or other single-fiber twitches aren’t tics. Tics involve a larger part of a muscle and can often be suppressed, while fasciculations usually can’t be.

Detection
- Electromyography (EMG) can detect fasciculations. Surface EMG is often more sensitive than needle EMG, especially in certain diseases like ALS (amyotrophic lateral sclerosis).

Who gets them and risk factors
- Fasciulations are common in healthy people and are usually not a sign of serious illness.
- Risk factors include age, stress, fatigue, and strenuous exercise.

Substances and medicines that can trigger them
- Anxiety, caffeine, alcohol, and thyroid problems can contribute.
- Some medicines are linked to fasciculations, including certain antihistamines (like diphenhydramine and dimenhydrinate), stimulants (like caffeine, pseudoephedrine, amphetamines), asthma inhalers (such as albuterol), and some ADHD medications.
- The muscle relaxant succinylcholine can cause brief fasciculations during use.

Helpful notes
- Even if caffeine or another substance seems related, it isn’t always the exact cause.
- A combination of a mild magnesium deficiency and caffeine might trigger twitches in some people.

Fasciculations in ALS (amyotrophic lateral sclerosis)
- There is no proven treatment that stops fasciculations caused by ALS.
- In ALS, how often fasciculations occur does not reliably predict how the disease will progress or how weak the muscles will become.
- Fasciculations are reported more often by men.

What to do
- Most fasciculations are harmless. If you have persistent, widespread twitches, or if they are accompanied by weakness, numbness, or weight loss, see a healthcare provider for a full evaluation.