This condition is known as medial canaliform dystrophy of
the nail. Note the horizontal grooves in the nail proximal to the split in an
inverted fir tree pattern. Note also there is no damage to the cuticle and the
posterior nail fold. A similar fir tree pattern can be seen in habit dystrophy
of the nail usually the thumb, where the index finger nail is used to pick at
the cuticle damaging the underlying nail matrix. This is known as a habit tic
deformity. However median canaliform dystrophy has the central nail canal shown
here. Some cases are familial. The thumbs are always involved. Cases have also
occurred with oral isotretinoin therapy in cystic acne but heal after the drug
is ceased. Other injury to the nail matrix can also cause it but most cases
have no history of trauma. Treatment is clear nail polish to strengthen the
nail and keeping it out of water which tends to soften keratin.
