Traumatisms, tumors and inflammatory disorders were revealed to be the most common causes in this largest case series of longitudinal nail splitting to date. Rarer causes included congenital and systemic disorders. Half of the cases were attributable to tumors from which one third were malignant, mainly melanoma. When facing a monodactylic split nail, ruling out benign as well as malignant tumors before concluding to traumatic etiology is advised.

• Researchers assessed the clinical as well as histopathological features of 56 SN obtained at the nail consultation of the dermatology department at Saint Pierre University Hospital in Brussels, between 1997 and 2019.

• In 56 patients (34 women and 22 men) with a median age of 44.2 years, 3.2 times more frequent involvement of fingernails than toenails, especially the thumb, was evident.

• Frequent etiologies included tumors (45.6%), inflammatory diseases (26.3%) and traumas (19.3%), and rarer ones were congenital (5.3%) and systemic disorders (3.5%).

• Results of histopathological slide review corroborated that split nail occurs due to alteration of the nail matrix integrity, resulting either from matrix stretching by an underlying tumor or from impairment of the keratinization process by inflammatory diseases, melanocytic tumors and Bowen's disease.

• This is the first study to gather nail disorders causing split nail with their clinical pathological correlation.