Introduction The Myotonic Dystrophy Health Index (MDHI) is a disease‐specific, patient‐reported outcome measure. The objective of this study was to translate, evaluate, and validate a Japanese version of the MDHI (MDHI‐J). Methods We utilized forward and backward translations and qualitative interviews with 11 myotonic dystrophy type 1 (DM1) participants. We subsequently tested the internal consistency, test–retest reliability, concurrent validity against muscle strength, and 3 quality‐of‐life measures, and the known‐groups validity of the MDHI‐J with 60 adult patients. Results The MDHI‐J was found to be culturally appropriate, comprehensive, and clinically relevant. The MDHI‐J and its subscales had high internal consistency (mean Cronbach's α = 0.91), test–retest reliability (intraclass coefficient 0.678–0.915), and concurrent validity (Spearman's ρ − 0.869 to 0.904). MDHI‐J scores were strongly associated with employment, duration of symptoms, and modified Rankin Scale. Discussion The MDHI‐J is suitable and valid to measure patient‐reported disease burden in adult Japanese patients with DM1. Muscle Nerve 59:577–577, 2019

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We report a case with hereditary hypokalemic paralysis type 2 in whom thyrotoxicosis aggravated his attacks of paralysis. He experienced paralytic attacks several times a year from 24 years of age and was clinically diagnosed with hypokalemic periodic paralysis. At 49 years of age, a laboratory examination showed normal thyroid function. At 57 years of age, transient thyrotoxicosis was accompanied with an increase of the frequency and severity of attacks. Gene analysis revealed a missense mutation (c.2015G>A, p.R672H) in SCN4A, a known pathogenic mutation for hypokalemic paralysis type 2. This case highlights the importance of checking thyroid function when the frequency and severity of attacks are increased in patients with periodic paralysis.