myotonia vs tonus

myotonia and tonus both are nouns.

Word	Noun	Adjective	Verb	Adverb
myotonia	Yes	No	No	No
tonus	Yes	No	No	No

As nouns, tonus is a hypernym of myotonia; that is, tonus is a word with a broader meaning than myotonia:

myotonia: abnormally long muscular contractions; slow relaxation of a muscle after a contraction
tonus: the elastic tension of living muscles, arteries, etc. that facilitate response to stimuli

Other hypernyms of myotonia include tone, tonicity.

myotonia (noun)	tonus (noun)
abnormally long muscular contractions; slow relaxation of a muscle after a contraction	the elastic tension of living muscles, arteries, etc. that facilitate response to stimuli