median nail dystrophy median nail dystrophy median nail dystrophy (also known as "dystrophia unguis mediana canaliformis," "median canaliform dystrophy of heller,": 657 and "solenonychia") consists of longitudinal splitting or canal formation in the midline of the nail, a split which often resembles a fir tree, occurring at the cuticle and proceeding outward as the nail grows.: 788 median nail dystrophyspecialtydermatology thumbs, which are the most commonly involved, usually show an enlarged lunula resulting probably from repeated pressure applied on the base of the nail.: 657