English nouns ending in -ma are derived from neuter Latin nouns of Greek origin.
edema / oedema
The Latin genitive singular ending is -atis, & the Latin nominative plural ending (when there is one) is -ata
As English nouns, some of these are treated as mass nouns or uncountable nouns without a plural in some (possibly even most) contexts.
edema / oedema
However, in other contexts, it may be necessary to distinguish between different types of edema (caused by different factors) or different types of derma or plasma or parenchyma (resulting from different origins or with different physical structures or properties). In these latter contexts, a careful writer will use a phrase like "types of" (before the word) to make the distinctions, but it is also possible to make the word plural.
In forming the plural, one option is to treat the word as Latin & use the Latin plural ending:
edemata / oedemata
This option may be looked upon as erudite, scholarly, or other less generous descriptive adjectives.
However, if the words are truly accepted as English words, then the other option (& the one I prefer) is to use the standard English plural ending:
edemas / oedemas
This option is more casual, colloquial, & reader-friendly.
At least that's my opinion. In the Latin names of plants & animals, there are many more such endings similarly declined (-nema, -paegma, -stemma, -zema, -stelma, -stroma, -phragma, -uma).
Botanical Latin: History, Grammar, Syntax, Terminology and Vocabulary, by William T. Stearn (Hafner Publ. Co., 1966), p. 82