Clasp-knife sign

What it means

Clasp-knife phenomenon refers to increased muscle tone while bending or stretching a limb, whereby there is a sudden relaxation (decrease in resistance) as the muscle continues to be stretched. This phenomenon has been likened to opening a clasp knife.

Rare diseases that can present with this1

The range is how often this sign appears among people diagnosed with each disease — it is not a probability of having any of them. Many of these signs also occur in common, non-rare conditions. A sign on this page is a conversation to have with a clinician, not a conclusion.

Part of the broader category