Hypokalaemic periodic paralysis
Diagnosis is not difficult in somebody with typical attacks and a positive family history. It is notable that one third of cases do not have any family history. If a blood sample can be taken during an attack, a low potassium level will help to confirm the diagnosis, but the change in potassium may be too slight to give a certain answer.
In the past if there was uncertainty about the diagnosis, the patient was admitted to hospital so that the doctors could try to provoke an attack, usually by giving glucose, either orally or by injection, and an injection of insulin.
However, this is rarely needed these days because of the advances in genetics. It is now possible to analyse the calcium channel gene from a simple blood test. If a change [mutation] is found in the calcium channel gene this confirms the diagnosis and no further diagnostic tests are needed.
Sometimes special electrical measurements of the muscles electromyography [EMG] are undertaken to provide further diagnostic information. Occasionally, a muscle biopsy is required. This is a minor procedure in which the skin is numbed with a local anaesthetic and a small piece of muscle removed for examination under the microscope. The appearances may be normal in periodic paralysis, but during and shortly after an attack, and in patients with permanent weakness, little fluid filledcavities (vacuoles) may be seen in the muscle fibres.
Hyperkalaemic periodic paralysis
The diagnosis is relatively easy if there is a clear family history and the blood potassium level can be shown to be increased during an attack. If there is doubt, previously doctors may have tried to precipitate an attack, in hospital, by giving a potassium solution to drink. However, now it is usually possible to achieve a precise diagnosis by genetic analysis on a simple blood test.
If genetic tests are inconclusive electrical tests (electromyography or EMG) may be helpful, particularly if they can be done during a spontaneous or induced attack. Muscle biopsy is occasionally helpful and the same comments as made for hypokalaemic periodic paralysis apply.
The diagnosis can be most easily achieved by genetic testing on a blood sample. A heart tracing [ECG] often shows typical changes in the heart rhythm such as extra beats.