anonychia anonychia anonychia is the absence of fingernails or toenails, an anomaly which may be the result of a congenital ectodermal defect, ichthyosis, severe infection, severe allergic contact dermatitis, self-inflicted trauma, raynaud phenomenon, lichen planus, epidermolysis bullosa, or severe exfoliative diseases.: 784 anonychiaspecialtydermatology congenital form this is rare and is usually due to mutations in the r-spondin 4 (rspo4) gene which is located on the short arm of chromosome 20 (20p13). clinically it is manifest by the absence (anonychia) or hypoplasia (hyponychia) of finger- or toenails.