Periodic paralysis: attacks of weakness linked to muscle ion channels

Skeletal muscle contracts when sodium, potassium and calcium move through channels in the fibre membrane in a tightly regulated sequence. In periodic paralysis, the channel proteins made from CACNA1S or SCN4A are altered. MedlinePlus describes the channels as leaky: ions flow slowly but continually into muscle cells, which reduces the ability of the muscle to contract.

Under certain conditions, the fibre membrane becomes depolarised and stops responding. The muscle is not damaged in the usual sense. It is electrically switched off, which is why strength can return to normal once the attack passes.

Clinicians group the main types by the blood potassium measured during a typical attack. The groups are not completely interchangeable. A small number of people with the hyperkalaemic type have normal potassium during attacks, and some forms show features of both.

The key is catching an attack. A blood potassium taken during weakness, ideally within hours of its onset, can show low, high or normal values, which guides classification. Outside attacks, electromyography with a specific long exercise test can show a characteristic fall in muscle response, and genetic testing for CACNA1S and SCN4A confirms the diagnosis in many people.

Other causes of recurrent weakness must be excluded, including thyrotoxicosis (which can cause low potassium paralysis), kidney or adrenal disorders, medicine effects and myasthenia. The UK page on the low potassium result can help explain what the blood test measures.

Management is mostly about recognising and avoiding triggers and treating attacks sensibly. People with the hypokalaemic form are often advised on diet and the timing of exercise, while those with the hyperkalaemic form are typically advised about regular meals, avoiding fasting and prolonged rest after heavy exercise, and keeping warm. Specific recommendations should come from a neuromuscular specialist, because inappropriate potassium supplementation or restriction can be harmful.

Some specialists prescribe medicines that reduce the frequency of attacks, such as carbonic anhydrase inhibitors, but these are individualised, require monitoring and are not described here as suitable for any particular person. Anaesthesia needs prior planning. Tell surgical teams about the diagnosis because fluids, glucose and potassium handling during an operation can precipitate attacks.

Weakness that affects breathing or swallowing, chest pain, palpitations, a new inability to stand that does not begin to improve within hours, or any episode in someone who has never had one before should be assessed urgently. Call NHS 111 for advice, and 999 for difficulty breathing, collapse or chest pain. Potassium disturbances can affect the heart rhythm, which is why an ECG is usually done in hospital during a severe attack.