opisthotonos vs muscle spasm

opisthotonos and muscle spasm both are nouns.

Word	Noun	Adjective	Verb	Adverb
opisthotonos	Yes	No	No	No
muscle spasm	Yes	No	No	No

As nouns, muscle spasm is a hypernym of opisthotonos; that is, muscle spasm is a word with a broader meaning than opisthotonos:

opisthotonos: severe spasm in which the back arches and the head bends back and heels flex toward the back
muscle spasm: a painful and involuntary muscular contraction

Other hypernyms of opisthotonos include cramp, spasm.

opisthotonos (noun)	muscle spasm (noun)
severe spasm in which the back arches and the head bends back and heels flex toward the back	a painful and involuntary muscular contraction