Trachyonychia, or "20-nail dystrophy" (TND), refers to brittle, rough, and thin nails, often described as being sandpaper-like. Trachyonychia was first described by Alkiewicz in 1950 and later named TND by Hazelrigg in 1977. Trachyonychia may affect 1-20 nails, and severity may vary between different nails.

Clinically, trachyonychia is divided into 2 types: shiny and opaque. In the shiny type, there is ridging and pitting, but nails retain their natural luster. In the opaque type, which is generally more severe, nails are rough, brittle, and thin with ridging caused by superficial striations. In both types, there may be thickened cuticles, onychoschizia, and koilonychia.

Trachyonychia may affect all age groups. It most commonly affects children and is typically idiopathic. In children, it usually affects all 20 nails. Trachyonychia is reported in 4% of patients with alopecia areata. Other dermatologic disease associations are psoriasis, lichen planus, and, less commonly, atopic dermatitis, ichthyosis vulgaris, vitiligo, and pemphigus vulgaris. Systemic disease associations include amyloidosis, sarcoidosis, primary biliary cirrhosis, and hematologic disease. Drugs such as chemotherapeutic agents may cause trachyonychia.

Histopathology of trachyonychia typically shows spongiosis with inflammatory cell exocytosis into the nail epithelia. There may be findings of nail matrix lichen planus or psoriasis in some cases.