Median nail dystrophy (dystrophia unguium mediana canaliformis or Heller dystrophy) consists of a longitudinal split defect in the middle of the nail plate running from the cuticle to the free edge. Often, short cracks extend laterally in a proximal direction from the split creating a herring-bone or fir-tree appearance. It may be associated with an enlarged lunula. Symmetrical involvement of both thumbnails is most common, though involvement of other fingers and toes has been observed.

The origin of median nail dystrophy is unknown in most patients, though certain cases have been associated with oral isotretinoin therapy. Familial cases have been reported. In approximately 20% of patients, there is known antecedent trauma. Spontaneous regression may be observed, but recurrences are common.