Hypokalemic periodic paralysis (HypoKPP)
HypoKPP is a rare, autosomal dominant channelopathy characterized by muscle weakness or paralysis with a matching fall in potassium levels in the blood (primarily due to defect in a voltage-gated calcium channel).
In individuals with this mutation, attacks often begin in adolescence and most commonly occur:
- on awakening or after sleep or rest
- following strenuous exercise (attacks during exercise are rare),
- after high carbohydrate meals,
- after meals with high sodium content,
- at sudden changes in temperature,
- with excitement,
- by noise,
- on flashing lights
- being induced by cold temperatures.
Weakness may be mild and limited to certain muscle groups, or more severe full body paralysis. Attacks may last for a few hours or persist for several days. Recovery is usually sudden when it occurs, due to release of potassium from swollen muscles as they recover. Some patients may fall into an abortive attack or develop chronic muscle weakness later in life.
To differentiate it from Guillian Barre syndrome, deep tendon reflexes are normal and 7th cranial nerve is spared. During an attack reflexes may be decreased or absent.
Some people only develop symptoms of periodic paralysis due to hyperthyroidism (overactive thyroid). This entity is distinguished with thyroid function tests, and the diagnosis is instead called thyrotoxic periodic paralysis.
Normokalemic periodic paralysis (NormoKPP)
Periodic paralysis in which the serum potassium level is within normal limits during attacks; there is often severe quadriplegia.
An inherited form of periodic paralysis in which the serum potassium concentration is within normal limits during attacks. Onset usually occurs between the ages of 2 and 5 years, often associated with severe quadriplegia. Exposure to cold may provoke attacks and over time, result in vacuolar myopathy; the attacks may be provoked by high-carbohydrate, high-sodium diets and during periods of excitement and may respond to oral potassium.
Hyperkalemic periodic paralysis (HYKPP, HyperKPP)
HyperKPPis a genetic disorder. It occurs in humans and perhaps other animals. It is an inherited autosomal dominant disorder that affects sodium channels in muscle cells and the ability to regulate potassium levels in the blood.
It is characterized by muscle hyperexcitability or weakness which, exacerbated by potassium, heat or cold, can lead to uncontrolled shaking followed by paralysis. Onset in humans usually occurs in early childhood, but still occurs with adults.
The mutation which causes this disorder is dominant on SCN4A with linkage to the sodium channel expressed in muscle. The mutation causes single amino acid changes in parts of the channel which are important for inactivation. In the presence of high potassium levels, including those induced by diet, sodium channels fail to inactivate properly.