Myotonia

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Myotonia is a feature of some myotonic syndromes such as myotonia congenita and myotonic dystrophy and various other disorders.

Examination

Percussion myotonia is a prolonged muscle contraction lasting several seconds with a sustained dimple appearing on the skin. It is classically tested on the thenar eminence where the thumb may draw into sustained opposition with the fingers.

Normal individuals have percussion irritability where there is a faint transient dimple or ripple at the percussion site. Some normal individuals will also have percussion myoedema where there is a tiny lump at the percussion site.

Muscle contraction myotonia is commonly tested in the hands and eyes.

For the hands ask the patient to make a tight fist, hold it for 10 seconds, and ask them to open them as quickly as possible. In myotonia the fingers can't open quickly and the wrist flexes involuntarily because of a sustained "after contraction" or delayed relaxation of the wrist flexors.

The the eyes ask the patient to close them as tightly as possible for 5 seconds and then ask to open. The myotonic patient isn't able to open them right away.

Differential Diagnosis

Myotonic dystrophy
- DM type 1 (DMPK)
- DM type 2 (CNBP)
Non-dystrophic myotonias
- Chloride channelopathies (CLCN1)
  - Thomsen myotonia congenita
  - Becker myotonia congenita
- Sodium channelopathies (SCN4A)
  - Sodium channel myotonia
  - Paramyotonia congenita
  - Hyperkalaemic periodic paralysis
- Chondrodystrophic myotonia
Drug induced (statins, fibrates, cyclosporine, chloroquine, terbutaline, formoterol, propranolol)
Isolated electrical myotonia: Pompe Disease, Polymyositis, Myotubular congenital myopathy, hypothyroidism, malignant hyperpyrexia, severe denervation.