Abstract: Provided herein a single-domain antibodies (sdAbs) that bind to voltage-gated sodium channel (Nav)1.4 or Nav1.5 and polynucleotides encoding the sdAb. Also provided herein are expression cassettes, vectors and host cells including polynucleotides N encoding the sdAb, and pharmaceutical composition including the sdAb, and methods of use thereof. The methods include the detection and/or capture Nav1.4 or Nav1.5 in a sample, the detection of a disease or condition in a subject, and the treatment of cardiac arrhythmia, myotonia, sudden cardiac death syndrome or cancer in a subject using sdAbs that bind Nav 1.4 or Nav 1.5.