Type B Digital Mucous Cysts Can Present with Cuticle Thickening as a Clinical Clue

Type B digital mucous cysts (DMCs) exert pressure over the nail matrix, leading to longitudinal nail plate groove. In the authors' experience, focal hyperkeratosis of the proximal nail fold (PNF) in type B DMC is frequently observed. We analyzed clinical images in the literature to assess how often this feature was present, if noted in the figure caption, and aimed for clinical description of this finding.

We performed a search of DMC images from PubMed literature and nail atlases. Consensus for DMC type, presence of PNF hyperkeratosis, hyperkeratosis described in the figure caption, and etiology of hyperkeratosis was performed.

Of available images, 33.9% were determined to be type B DMC; of these, 53.8% had presence of PNF hyperkeratosis. There was consensus that a majority (61.9%) of the hyperkeratotic findings were due to cuticle thickening, and only one figure caption mentioned PNF hyperkeratosis.

Type B DMCs often feature focal hyperkeratosis at the PNF. A review of type B DMC images revealed that this hyperkeratosis was mostly due to cuticle thickening, suggesting a reaction of the eponychium in response to the mass effect within the PNF. Bringing this characteristic to the attention of practicing physicians may aid in clinical diagnosis.