The painless thumb weakness and the ultrasound finding in the muscles

A 47 year old patient is referred to your lab. The referral diagnosis is posterior interosseus syndrome. 

As you look at the posterior interosseus nerve on its course through the supinator muscle, you suddenly notice what you can see in the first video below. You get curious and look at the extensor muscles of the forearm a little more distally (second video).

1. What is the name of this phenomenon?
2. What is the underlying cause?
3. In which disorders can you find it and in which disorders is it a prominent   feature?
4. How does that now fit in with the referral diagnosis?

Answers will follow on this page by June 1st 

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ANSWERS:

1. These are fasciulations. 

2. Fasciculations are involuntary movements caused by activation of single motor units.

3. Fasciculations can be benign (acutally 70% of people experience them during life at least once). Or they appear in any disorder causing damage of the lower motor – from the anterior horn cell to the neuromuscular junction. As a prominent feature you will usually find it in motor neuron disorders like ALS and in a subtype of autoimmune neuropathies – the so-called multifocal motor neuropathy (MMN).

4. So it turned out that in this case we are dealing with MMN. The fasciculations guided us the way 🙂