In the past classification was based upon changes in the level of blood potassium (which is normally between 3.5 and 5.1 mmol per litre) during, and particularly at the onset of, the attack.
The three types in this classification are:
Hypokalaemic periodic paralysis: In these attacks the blood potassium is low
Hyperkalaemic periodic paralysis: In these attacks the blood potassium is high
Normokalaemic periodic paralysis: In these attacks the blood potassium remains normal
In fact, it has recently been discovered that it is not the change in the blood potassium level that is the primary problem in periodic paralysis. The primary problem in all of these conditions is that the normal pores which exist in the walls of the muscle cells don’t work properly.
It does seem that changes in blood potassium levels can further hinder the function of these pores and that is why changes in blood potassium can be relevant. However, other factors separate from blood potassium can also worsen the function of the pores, so a change in blood potassium is not essential.