English to Russian Science Medical (general) дерматология

Cutis verticis gyrata may thus be classified as primary (*essential and nonessential*) or secondary. The term primary *essential* is used in cases with no other associated abnormalities, and primary *nonessential* refers to cases associated with mental retardation, cerebral palsy, epilepsy, schizophrenia, neurologic abnormalities, deafness, ophthalmologic abnormalities, or a combination of any of these.

http://www.actasdermo.org/en/primary-nonessential-cutis-vert...

С первичной и вторичной формой все просто. А как перевести essential и nonessential? В публикациях на русском складчатая пахидермия классифицируется только на первичную и вторичную форму. Заранее благодарю.