Alternatively, the muscle weakness may be more generalised e.g. the muscles of the arms and legs. When the weakness is generalised the patient may be completely unable to move (paralysis). Even in a severe generalised attack it is very rare for the breathing, speaking or wallowing muscles to be involved. In the great majority of patients with periodic paralysis the heart muscle is not involved.
In an uncommon form of periodic paralysis called Andersen’s syndrome disturbances of the heart rhythm may occur. At the end of an attack in all forms of periodic paralysis the muscle strength returns. In some patients as the years go by they notice a degree of persistent muscle weakness between the attacks of paralysis. If this persistent muscle weakness develops it can slowly get worse. The duration of an attack of weakness varies from minutes to days depending upon the exact type of periodic paralysis.
Hypokalaemic periodic paralysis
Hypokalaemic periodic paralysis is caused by genetic changes [mutations] in the calcium channel gene. It is the commonest form of periodic paralysis. The attacks usually start between the ages of 10 and 20 years, but may vary in frequency from several times a week to once a year. In some families the attacks are milder and less frequent in woman than in men. The attacks become less frequent in middle age, but by then some patients have a degree of permanent muscle weakness. Typically the attacks develop during sleep and weakness is present on waking. Gentle exercise may abort or shorten an attack.
Attacks may be provoked by:
- strenuous exercise
- carbohydrate-rich food
- salty food
- alcohol
- emotional stress
Hyperkalaemic periodic paralysis
The attacks are very similar to those described above for hypokalaemic periodic paralysis, but they tend to be shorter and occur during the day rather than on first waking. They also tend to start at a younger age, sometimes in early infancy. In virtually all cases there is a family history of attacks.
Attacks may be provoked by:
- cold
- fasting
- resting after exertion
- stress
- pregnancy
- alcohol
As with the hypokalaemic form, some patients eventually develop permanent weakness.
Some patients with hyperkalaemic periodic paralysis, but not those with hypokalaemic or normokalaemic periodic paralysis, also experience myotonia. This means that the muscles are unable to relax immediately after activity. Such stiffness is more marked in the cold. Patients may complain that their hands feel stiff, and the doctor may notice that when the patient looks upwards and then down, that the eye lids are slow to descend.
There is a condition called paramyotonia congenita in which this cold-induced muscle stiffness is very pronounced; some of these patients also have periodic paralysis. It is now known that hyperkalaemic periodic paralysis and paramyotonia congenita are both due to the altered function of the sodium channel, but that the gene abnormality in each case is different.
Normokalaemic periodic paralysis
In some patients with periodic paralysis, the blood potassium level does not change during an attack. In other respects, the clinical features are very similar to those described above for the hypo- and hyper- kalaemic forms.
Attacks may be provoked by:
- physical activity
- cold
- alcohol
The frequency of attacks may be reduced by acetazolamide (see treatment of hypokalaemic periodic paralysis) and individual attacks may respond to salt tablets. There is now evidence that normokalaemic periodic paralysis is also caused by mutations in the sodium channel gene, the same gene that causes hyperkalaemic periodic paralysis.
Andersen’s syndrome
Patients with Andersen’s syndrome have attacks of periodic paralysis that may be hyper, hypo or normokalaemic. In addition, they have other clinical features which are not present in the forms of periodic paralysis described above
. The most important additional feature is that the heart can also be affected. Patients may develop changes in the rhythm of the heart which can sometimes be significant and they may need close monitoring and treatment by a heart specialist [cardiologist].
A less important additional feature is that they may have mild changes in the appearance of their fingers and face. The precipitants to attacks of paralysis are similar to those described above. Andersen’s syndrome is caused by genetic mutations in a potassium channel gene.