Paramyotonia Congenita and HypoKPP

Last month's Video Case Challenge (available on PracticalNeurology.com) presented a case of a 34-year old woman with muscle stiffness worsened by cold temperature and stress.

The demonstrated sign can be seen in:

1. Paramyotonia congenita (PMC)

2. Myotonia congenita (MC)

3. Hypokalemic periodic paralysis (HypoKPP)

4. Polymyositis

5. Oculopharyngeal muscular dystrophy

The correct answers are 1 (Paramyotonia congenita) and 3 (Hypokalemic periodic paralysis).

1. Eyelid myotonia is a feature of PMC and is sometimes observed in hypokalemic periodic paralysis (HypoKPP) but it is not a feature of myotonia congenita.

2. Worsening of myotonia with repetitive motion (paradoxical myotonia) is a feature of PMC.

3. Cold sensitivity is also an important feature of the disease and can be demonstrated neurophysiologically.

4. PMC is an autosomal dominant mutation of Sodium channels-Alpha subunit (SCN4A) on chromosome 17q23.3. Rarely paramyotonia is produced by Chloride channel mutation.

5. PMC is allelic to Hyperkalemic periodic paralysis and they can affect the same patient.

6. Mutated Na channels are dysfunctional and are abnormally inactivated, leading to continuous leak of Na into cell.

7. Symptoms usually appear in the first decade of life. During a crying episode, infants may have difficulty opening their eyes due to “exercise induced” myotonia.

8. Weakness is not prominent and creatine kinase (CK) is usually normal or slightly high.

9. Mexiletine 150-1000mg/day has shown to be effective in a double blind trial.1n

Aziz Shaibani, MD, FACP, FAAN, FANA is Director of Nerve & Muscle Center of Texas. He is also Clinical Professor of Medicine at Baylor College of Medicine in Houston, Texas. He is also an Adjunct Professor of Neurology at Kansas University Medical Center in Kansas City, Kansas.

1. Statland, JM. et al. Mexiletine for symptoms and signs of myotonia in non-dystrophic myotonia. JAMA 2012; 308(13): 1357-1365.