Fibromyalgia Look Alike – Myotonia Congenita

Myotonia Congenita is a rare neuromuscular disease manifest by “painful muscle cramps or myalgia as well as transient muscle weakness following myotonic stiffness”. Article describes 2 cases – one that got worse with pregnancy which is what Fibromyalgia also does. The transient weakness after cramping can be easily missed though EMG does pick up the problem.

J Clin Neurol. 2015 Apr;11(2):188-91. doi: 10.3988/jcn.2015.11.2.188.
The Overlap between Fibromyalgia Syndrome and Myotonia Congenita.
Nam TS(1), Choi SY(2), Park DJ(3), Lee SS(3), Kim YO(4), Kim MK(5).


  • muscle disorder
  • sarcolemmal hyperexcitability with mutations in chloride ion channel
  • 2 forms – autosomal-dominant myotonia and recessive generalized myotonia
  • “painful muscle cramps or myalgia as well as transient muscle weakness following myotonic stiffness”
  • pain and stiffness “worsened by emotional stress, cold exposure,
    sleep disturbance, dietary habit, excessive physical activity, and pregnancy.”
  • Needle EMG and genetic testing confirm

Case 1 – 31 yr old typical FM except:

  • “had a history of transient muscle weakness lasting for periods of no  more than several seconds in her legs when she suddenly  had to walk at a pedestrian crossing or a bus stop.”
  • Moderate  muscular hypertrophy was seen on exam.
  • positive EMG and genetics despite negative Fhx
  • used  sodium channel blockers (lacosamide or carbamazepine) and amitriptyline.

Case 2 – 37 yr old male with widespread pains and depression except:

  • Male (my comment)
  • Pain 8/10
  • “cold-water testing aggravated the muscle stiffness and cramps in his forearm.”
  • transient muscle weakness in childhood only
  • sister episodic transient muscle weakness
  • No meds successful

Comment – tough one.  Presence of  family history, muscle cramps followed by transient weakness, FM in a male, muscular hypertrophy, pain more severe and not necessarily responsive to treatments all are hints for Myotonia Congenita. EMG done for muscle cramps could confirm it. I wrote about how myotonic dystrophy type II can be confused with fibromyalgia:

Fibromyalgia (FM) Tragic Misdiagnosis – Myotonic Dystrophy Type 2 and Cervical Spinal Stenosis – New Treatments as Result



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