Treatment: Simply remove the puffer and place it into a tank without supersaturation.  An alternative is to drive off the excess nitrogen by agitating the water surface.

Background: This condition is analogous to the syndrome affecting human divers.  Its cause is the supersaturation of gas (normally nitrogen) in the tank.  More often than not, this supersaturation occurs when colder water (gas-rich, and usually from the tap) is heated quickly.  This, along with extreme aeration, can also be the culprit. The puffers breathe in this gas-rich water, which can cause a gas embolism.  This is the obstruction of blood vessels by gas bubbles.   Testing for nitrogen content is normally not cost-effective for the average aquarist.  A low-tech but effective test is the "finger test".  Simply put your dry finger into the volume of water you wish to test.  Hold it there for at least one (1) minute.  If many bubbles appear on your finger, you may have a supersaturation problem.