# Periodic Paralysis

> Periodic paralysis for NEET-PG Medicine covering hypokalaemic, hyperkalaemic and thyrotoxic forms, triggers, potassium replacement and Asian thyrotoxic cases.

- Canonical URL: https://prepelephant.com/topics/neet-pg/medicine/periodic-paralysis
- Exam / course: NEET-PG · Subject: Medicine
- Publisher: PrepElephant (https://prepelephant.com) — Prepared and reviewed by the PrepElephant Academic Review Team
- First published: 2026-10-08
- Last updated: 2026-10-08
- Reference basis: Harrison's Principles of Internal Medicine; Adams and Victor's Principles of Neurology; Davidson's Principles and Practice of Medicine
- How to cite: "Periodic Paralysis", PrepElephant, https://prepelephant.com/topics/neet-pg/medicine/periodic-paralysis

## Direct answer

Periodic paralysis describes episodic flaccid weakness caused by abnormal potassium movement across muscle membranes, usually through ion channel mutations in familial forms. Hypokalaemic periodic paralysis is the commonest, with attacks triggered by high-carbohydrate meals, rest after exercise, stress and cold. Thyrotoxic periodic paralysis is an acquired form seen mainly in Asian men with hyperthyroidism, and it is treated by correcting potassium and controlling the thyrotoxicosis.

## What you must remember

- **Hypokalaemic familial form:** autosomal dominant, with CACNA1S or SCN4A mutations and low serum potassium during attacks.
- **Hyperkalaemic form:** SCN4A mutations, with attacks often triggered by fasting or potassium-rich food and potassium rising during episodes.
- **Thyrotoxic form:** Asian men, usually young adults, with hyperthyroidism and hypokalaemia; women are rarely affected.
- **Triggers:** high-carbohydrate meal, rest after strenuous exercise, emotional stress, cold exposure and alcohol.
- **Acute potassium:** oral potassium chloride for mild attacks; intravenous potassium for severe weakness, with cardiac monitoring because rebound hyperkalaemia can occur.
- **Thyrotoxic management:** propranolol reduces attacks quickly, and antithyroid treatment prevents recurrence.
- **Prevention:** acetazolamide or dichlorphenamide reduce attacks in familial hypokalaemic disease.
- **ECG:** hypokalaemia causes flattened T waves and prominent U waves, and attacks carry a risk of arrhythmia.

## A worked example in a young man with heavy legs

A 27-year-old man of Asian ancestry wakes unable to stand after a heavy carbohydrate meal and a day of strenuous exercise. Power is reduced symmetrically, worse in the proximal limbs, reflexes are reduced or absent, and he is fully conscious, with low serum potassium and U waves on the ECG.

| Form | Key features | Key clue |
|---|---|---|
| Hypokalaemic familial | Autosomal dominant, CACNA1S or SCN4A mutations, low potassium in attacks | High-carbohydrate meals and rest after exercise trigger attacks |
| Hyperkalaemic | SCN4A mutations, potassium rises during episodes | Fasting or potassium-rich food triggers attacks |
| Thyrotoxic | Asian men, usually young adults, with hyperthyroidism and hypokalaemia | Women are rarely affected |

The first step is to confirm hypokalaemia and check thyroid function, because a thyrotoxic cause changes the treatment.

Propranolol reduces the adrenergic drive that moves potassium into muscle, oral potassium is given with monitoring, and intravenous potassium is kept for severe weakness because large rapid doses can cause rebound hyperkalaemia. A high-carbohydrate meal is avoided during the attack, because insulin and glucose drive potassium further into cells. The family history is still taken, because familial forms also exist.

## Where the thyroid link is tested

Examiners expect the thyroid link in any young Asian man with flaccid weakness and low potassium.

- **Trap:** treating only the potassium and missing the hyperthyroidism. — **Correct:** propranolol is the correct early drug for thyrotoxic attacks, because it blocks the adrenergic trigger of potassium shift.
- **Trap:** assuming every attack is hypokalaemic. — **Correct:** a normal serum potassium during an attack points to hyperkalaemic or normokalaemic variants rather than hypokalaemia.
- **Trap:** missing secondary hypokalaemia. — **Correct:** diuretics, renal tubular acidosis, diarrhoea or primary hyperaldosteronism can produce the same weakness, so the cause must be looked for.
- **Trap:** giving rapid intravenous potassium without monitoring. — **Correct:** it risks rebound hyperkalaemia, so cardiac monitoring is needed.

## Frequently asked questions

### Why is thyrotoxic periodic paralysis more common in Asian men?

Thyrotoxic periodic paralysis is strongly linked to male sex and Asian ancestry, and the genetic susceptibility behind this pattern is not fully characterised. It presents as hypokalaemic flaccid weakness during attacks in people with hyperthyroidism, and women are rarely affected.

### How should hypokalaemic periodic paralysis be treated acutely?

Acute hypokalaemic periodic paralysis is treated with oral potassium chloride for mild attacks, while intravenous potassium is reserved for severe weakness and given with cardiac monitoring. Rebound hyperkalaemia and arrhythmias are the main hazards, so potassium should be given carefully.

### Which drugs reduce attacks in familial hypokalaemic periodic paralysis?

Acetazolamide and dichlorphenamide reduce the frequency of attacks in familial hypokalaemic periodic paralysis. They are used long term, with monitoring of potassium and acid-base status, because their effect on potassium handling in muscle is only partly understood.

### What triggers an attack of periodic paralysis?

Attacks of periodic paralysis are triggered by a high-carbohydrate meal, rest after strenuous exercise, emotional stress, cold exposure and alcohol. Insulin and glucose drive potassium into cells, so high-carbohydrate meals provoke hypokalaemia in susceptible people.