Treatment of NDM

All of the non-dystrophic myotonias can be treated with similar medications. The first line treatment is mexiletine. Myotonia may also improve with treatment with lamotrigine, carbamazepine, flecainide, acetazolamide and phenytoin. However there is less definitive evidence of the efficacy of these agents. We are happy to take referrals to see any patients who want to try any of these treatments for their myotonia

Lifestyle changes

Lifestyle changes can have a significant impact on patient function. We often advise patients to keep a diary for a month of their activity and eating habits to see if there are any specific triggers.

Patients with myotonia congenita often find that warming up before activities makes a significant difference to their function. Simple techniques such as getting up a stop earlier when on public transport can help with mobility.

Patients with paramyotonia congenita are usually very cold sensitive. Ensuring they keep their muscles warm, especially in winter can have a significant impact. Hand and feet warmers can make a big difference to function. Avoiding swimming in the cold sea is an important consideration. These patients often worsen with exercise so pacing themselves is important.

Mexiletine

Mexiletine is a sodium channel blocker that acts by binding to the activated sodium channel, switching it to the inactivated state. This therefore reduces the sustained depolarisation that causes myotonia and helps improve the clinical stiffness. 

  • There is good evidence from a number of randomised controlled trials that it is effective in treating myotonia in all types of non-dystrophic myotonia.
  • We will typically prescribe 400mg to 600mg per day of mexiletine in divided doses for adults. For children we prescribe 15mg/kg/day.
  • We will usually start very slowly in 100mg increments initially to ensure that patients tolerate the medication and it is safe.
  • It is generally well tolerated but patients often complain of gastric reflux symptoms and therefore need to take it with food and may need antacids or if severe a proton pump inhibitor to tolerate the medication.
  • All patients must have an ECG before starting and if they have a history of palpitations or cardiac disease will need to be reviewed by a cardiologist first. They will then need an ECG at each dose increase and after that every 2 years.
  • Generic mexiletine (50mg and 200mg tablets) is difficult to source in the UK but a licensed version Namuscla is now available. Due to its cost, it can only be prescribed at certain centres. We are currently able to source both types of mexiletine and prescribe it for patients under our care. Patients on mexiletine need to be seen in our clinic yearly for follow up.

Lamotrigine

Lamotrigine is also a sodium channel blocker. There is a randomised controlled trial that demonstrates it can also improve myotonia.

  • We will often use lamotrigine in patients who cannot take mexiletine, those who have not responded to mexiletine or those of child-bearing age.
  • Lamotrigine has to be started slowly at 25mg once a day and slowly built up in 25mg increments over time. It cannot be taken intermittently to treat myotonia. Patients typically need 150-300mg a day to get a significant benefit from lamotrigine.
  • Patients must be warned of the risk of rash with lamotrigine and told to stop if they develop a rash. 

Treatment of Periodic Paralysis

Lifestyle changes

Lifestyle changes in periodic paralysis can have a significant impact on attacks of paralysis. We recommend that patients keep and food, activity and attack diary for a month to try and determine key triggers for their attacks of weakness. This can be very informative in managing the disease.

In patients with HypoPP (and ATS), diet can have a significant impact on attacks. The most common trigger is large carbohydrate meals at night. We recommend that patients with significant attacks avoid eating carbohydrates after 6pm and try to eat protein based meals at night. Trying to restrict the amount of carbohydrates during the day can be helpful. Patients may find eating potassium-rich foods can be useful in aborting and minimising attacks. 

In patients with HyperPP, trying to minimise potassium-rich foods can help reduce attacks. Some patients also find that during an attack, having a sugary food or gently exercising the affected limb can abort or improve the attack. 

Acetazolamide

Acetazolamide is the mainstay of treatment for all types of periodic paralysis. It is a carbonic anhydrase inhibitor and whilst its exact mechanism of action is unknown it is clear it is frequently effective in reducing attacks of paralysis.

  • Patients with frequent or debilitating attacks of weakness should be trialled on acetazolamide.
  • We will usually start on 125-250mg a day and slowly increase up as needed. Most patients require approximately 500-1000mg a day to reduce attacks
  • The commonest side effect is tingling and pins and needles. This usually settles with time and sufficient rehydration.
  • There is a small risk of kidney stones with long term use and therefore all patients should have yearly kidney ultrasounds.

Dichlorphenamide

Dichlorphenamide also has randomised controlled trial evidence that it can reduce attack frequency in all types of periodic paralysis. Unfortunately it currently cannot be prescribed on the NHS in England due to the cost of the licensed form. Outside of England, clinicians may be able to apply with an individual funding request to their local health board for their patients. 

Other treatments for HypoPP

Potassium Supplements

Patients with HypoPP often improve with oral potassium tablets. These can be taken just at the onset of an attack  to try and abort or shorten an attack. Patients with frequent attacks may benefit from regular potassium tablets to maintain potassium levels and then top up tablets at the onset of an attack. We do not recommend patients take more than 10 tablets in a day and will need blood potassium monitoring when they first start treatment. 

Potassium-sparing diuretics

Patients who do not improve with acetazolamide and potassium supplementation can also trial potassium sparing diuretics to increase their potassium levels. We commonly use amiloride or spironolactone. We tend to avoid spironolactone in young men due to the incidence of gynaecomastia. Careful potassium monitoring is needed when initiating these treatments.

Other treatments for HyperPP

Thiazide Diuretics

If patients with HyperPP do not improve with acetazolamide we may add on a thiazide diuretic to help drop the potassium levels. Careful potassium monitoring is needed to ensure the potassium level does not drop too low as that may also trigger attacks. We commonly use bendroflumethiazide. Some patients have had very good effect in the past with hydrochlorothiazide but this is not easily available in the UK.