opisthotonos vs spasm

opisthotonos and spasm both are nouns.

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

As nouns, spasm is a hypernym of opisthotonos; that is, 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
spasm: a painful and involuntary muscular contraction

Other hypernyms of opisthotonos include cramp, muscle spasm.

opisthotonos (noun)	spasm (noun)
severe spasm in which the back arches and the head bends back and heels flex toward the back	(pathology) sudden constriction of a hollow organ (as a blood vessel)
	a painful and involuntary muscular contraction