sinus vs fistula

sinus and fistula both are nouns.

Word	Noun	Adjective	Verb	Adverb
sinus	Yes	No	No	No
fistula	Yes	No	No	No

As nouns, sinus and fistula are synonyms defined as:

sinus and fistula: an abnormal passage leading from a suppurating cavity to the body surface

sinus (noun)	fistula (noun)
a wide channel containing blood; does not have the coating of an ordinary blood vessel	an abnormal passage leading from a suppurating cavity to the body surface
any of various air-filled cavities especially in the bones of the skull	a chronic inflammation of the withers of a horse
an abnormal passage leading from a suppurating cavity to the body surface