Myotonia Congenita Can Be Mistaken as Paroxysmal Kinesigenic Dyskinesia

Paroxysmal kinesigenic dyskinesia (PKD) is commonly thought to be first described by Kertez.1 However, the typical clinical features of PKD were previously described by Shuzo Kure in a Japanese medical journal in 1892, as reviewed by Kato et al.2 The case described a 23-year-old male who had involuntary movement attacks with onset at 10 years of age that gradually increased in frequency. The attacks were triggered by sudden movement and started in the legs, sometimes spread to the body, and were very brief. They were preceded by a sensory aura, and the patient learned how to inhibit or stunt the attacks. He never lost consciousness, and abnormal neurological signs were not observed.2 These descriptions are in perfect agreement with our current definition of PKD.3 However, Kure misdiagnosed the case as an “atypical case of Thomsen disease [myotonia congenita (MC)],” despite noting that there were no longlasting muscle contractions during the attacks and there was an absence of percussion myotonia.2 Herein, we describe a case of MC referred as PKD (Figure 1). MC and PKD are similar in that both are characterized by attacks of involuntary movement triggered by sudden movement and lasting only a brief duration. We will discuss the essentials for differential diagnosis between PKD and MC. The pediatrics department referred a 19-year-old man (III-2) with possible paroxysmal movement disorder. Since 12 years old, he noted difficulty with initiating movement and getting stiff at the start of voluntary movements. For example, when getting on the bus, he had difficulty with the first couple of steps because of his feet getting curled up and becoming stiff, which got better after several steps. His hand use was similarly impaired and would become stiff upon first grip. These problems worsened after a period of prolonged rest but did not occur in the middle of active exercise. The patient was conscripted to the military after 7 years of stable conditions. In the army, he was observed as having the same problems, which interfered with his military training, and he was asked to seek a medical opinion. At first, he went to the pediatrician treating his elder brother (III-1), who had been diagnosed with MC due to a difficulty relaxing his hand grip since the age of 13. The patient’s brother (III-1) had myotonia when gripping his hand and percussion myotonia in the thenar muscles and tongue. The presence of myotonic discharge was confirmed using electromyography (EMG), and a diagnosis of MC was made without genetic analysis. The pediatrician considered the patient’s clinical history to be different from his brother’s and referred the proband (III-2) to us, believing he might have a different disease. After a careful interview, it was determined that the involuntary movement attacks occurred simultaneously with immediate movement initiation, did not spread to other body parts, always consisted of stiffening without chorea, and were consistently triggered after a period of prolonged rest. Upon neurological examination, his cranial nerve, sensory, and motor systems were normal, apart from grip myotonia in both hands. He did not have percussion myotonia or muscular hypertrophy. In the clinic, a prolonged resting period was required before sudden standing and walking to induce muscle stiffening in the legs. This stiffening was localized to the legs and hindered his first several steps https://doi.org/10.14802/jmd.17056 / J Mov Disord 2018;11(1):49-51 pISSN 2005-940X / eISSN 2093-4939