Common examples of ions include sodium, calcium and potassium. It is the highly regulated passage of these ions into and out of the muscle cells that allows muscles to conduct the electrical signals required for muscle contraction.
It is because these ion channels do not open and close properly the muscle cannot conduct electricity in the normal way. Since muscle contractions depends on this electrical conduction patients experience weakness.
The factors which can trigger attacks of weakness [described below] seem to worsen the function of channels that are already not working properly, and this tips the balance resulting in an attack of weakness.
The blueprint for making each ion channel correctly is contained in our genes. There is a single gene for each of the three common channels i.e. the sodium channel, the calcium channel and the potassium channel.
Changes in the genetic blueprint [know as gene mutations] result in the production of channels that don’t work correctly and hence cause the conditions.
It is now possible to use a genetic classification of periodic paralysis that reflects the new genetic information. This is combined with the older classification based on potassium levels in the table below.
|Type of periodic paralysis||Channel affected||Gene responsible|
|Hyperkalaemic periodic paralysis||Sodium channel||Sodium channely gene|
|Normokalaemic periodic paralysis||Sodium channel||Sodium channel gene|
|Hypokalaemic periodic paralysis||Calcium channel||Calcium channel gene|
|Andersen's syndrome||Potassium channel||Potassium channel gene|
All the forms of periodic paralysis described are genetic. They are inherited in an autosomal dominant way (see figure below). In practice this means that there is a 50:50 chance that a child of an affected person will also develop the condition.