These two videos are from a middle-aged man presenting with progressive bilateral lower limb and right upper limb weakness for few months. In the first video, there are prominent fasciculations over the entire right upper limb. Whilst in the second video, fasciculations can also be noted over both lower limbs, but more prominent on the left leg. This man was subsequently diagnosed to have motor neuron disease.

Discussion of the sign

Fasciculations are involuntary, non-rhythmic contraction of muscle groups caused by spontaneous firing of motor units. They appear as fine, rapid, flickering muscle contractions which are irregular in timing and location. In the presence of other lower motor neuron signs (e.g. weakness, hypotonia, hyporeflexia), the presence of fasciculations signify the presence of lower motor neuron dysfunction. In these circumstances, denervation and reinnervation of muscle fibres secondary to lower motor neuron disease causes spontaneous excitation of individual motor units. Common lower motor neuron lesions that may result in fasciculations include anterior horn cell disease (e.g. motor neuron disease, poliomyelitis), radiculopathy, and less commonly, in entrapment neuropathy and peripheral neuropathy.

Fasciculations can also be found in patients with cholinergic toxicity (e.g. organophosphate poisoning) and can also be a normal phenomenon (benign fasciculations) if no other neurological sign could be elicited.

A common pitfall of students is that they do not observe long enough for the fasciculations to appear. Sometimes, stimulating the muscle bulk gently may elicit the fasciculations. However, students often rush through this process and are simply “flicking” the muscle but not observing long enough for the fasciculations to appear.