Sometimes called HypoKPP
for short, this is a relatively rare genetic disorder
are somewhere between the 1:200,000 and 1:500,000).
This disorder is characterized by occasional elevation in the working-rate of cellular sodium-potassium pumps (the reasons for which are not entirely clear), as well as an abnormal formation of calcium channals. This causes a sudden and significant drop in blood potassium level, and as a result the nervous system does not recieve the potassium it needs for its activities, which causes it to fail.
This failure in the nervous system causes an effect, visibly characterized as attacks that range from severe muscle cramps and myalgia in light attacks, to paralysis of the entire body, in severe ones.
In addition to the above during an attack there is a dramatic increase in CPK levels, as the body tries to compensate for muscle inactivity. If this is left untreated for long, it may cause permanent muscle damage.
Attacks usually begin in the proximal muscles (i.e. the arms and legs), and may spread to more vital parts (which, again, might cause permanent damage, or even, in rare occasions, death). They usually pass by themselves within several hours (the actual length of an attack changes from attack to attack and from person to person, my longest attack lasted about 36 hours), but as they may cause permanent damage by the time they're gone, patients are usually treated by potassium (or kalium) infusions, which shortens the length of an attack.
During an attack the patient is completely conscious and aware of his surroundings, and his ability to speak is intact. After the attack has passed the patient usually feels sleepy and tired for several days.
Despite the fact that most attacks happen in mornings, and the patient actually "wakes into them" HypoKPP is not to be confused with sleep paralysis, the two conditions have nothing to do with each other.
Although the actual causes that trigger the attacks of HypoKPP are not entirely clear, a connection was found between attacks and the consumption of foods rich in sodium and carbohydrates in the evenings and to large late suppers. Other possible triggers are: alcohol and hallucinogenics (and basically anything that can open the BBB and other cellular barriers).
In addition to HypoKPP, there are two other potassium-related periodic paralysis disorders: Hyperkalemic Periodic Paralysis (HyperKPP), and Normokalemic Periodic Paralysis (NormoKPP).
Until about 20 years ago, when the genes that cause these disorders were found, they were considered mental diseases and were treated accordingly (usually unsuccesfully). Currently there is no cure for these disorders, nor is there any treatment to prevent attacks.