INTRODUCTION: Mutations of the voltage-gated sodium channel gene (SCN4A), which 
encodes Nav1.4, cause nondystrophic myotonia that occasionally is associated 
with severe apnea and laryngospasm. There are case reports of nondystrophic 
myotonia due to mutations in the C-terminal tail (CTerm) of Nav1.4, but the 
functional analysis is scarce.
METHODS: We present two families with nondystrophic myotonia harboring a novel 
heterozygous mutation (E1702del) and a known heterozygous mutation (E1702K).
RESULTS: The proband with E1702K exhibited repeated rhabdomyolysis, and the 
daughter showed laryngospasm and cyanosis. Functional analysis of the two 
mutations as well as another known heterozygous mutation (T1700_E1703del), all 
located on EF hand-like motif in CTerm, was conducted with whole-cell recording 
of heterologously expressed channel. All mutations displayed impaired fast 
inactivation.
DISCUSSION: The CTerm of Nav1.4 is vital for regulating fast inactivation. The 
study highlights the importance of accumulating pathological mutations of Nav1.4 
and their functional analysis data.