After the teledermatology consultation, an x-ray was recommended. The x-ray showed an elongated irregular radiopaque mass projecting from the anterior medial aspect of the midshaft of the distal phalanx of the great toe (Picture 3). With these findings, subungual exostosis was suspected, and she was referred to orthopedic surgery for excision of the lesion. Histopathology showed a stack of trabecular bone with a fibrocartilaginous cap, confirming the diagnosis of subungual exostosis.
Courtesy Dr. Puong To, Southern California Medical Group
Subungual exostosis is a benign osteocartilaginous tumor, first described by Dupuytren in 1874. These lesions are rare and are seen mainly in children and young adults. Females appear to be affected more often than males.1 In a systematic review by DaCambra and colleagues, 55% of the cases occur in patients aged younger than 18 years, and the hallux was the most commonly affected digit, though any finger or toe can be affected.2 There are reported case of congenital multiple exostosis delineated to translocation t(X;6)(q22;q13-14).3
The exact cause of these lesions is unknown, but there are multiple theories, which include a reactive process secondary to trauma, infection, or genetic causes. Pathologic examination of the lesions shows an osseous center covered by a fibrocartilaginous cap. There is proliferation of spindle cells that generate cartilage, which later forms trabecular bone.4
On physical examination, subungual exostosis appear like a firm, fixed nodule with a hyperkeratotic smooth surface at the distal end of the nail bed, that slowly grows and can distort and lift up the nail. Dermoscopy features of these lesions include vascular ectasia, hyperkeratosis, onycholysis, and ulceration.
Dr. Catalina Matiz
The differential diagnosis of subungual growths includes osteochondromas, which can present in a similar way but are rarer. Pathologic examination is usually required to differentiate between both lesions.5 In exostoses, bone is formed directly from fibrous tissue, whereas in osteochondromas they derive from enchondral ossification.6 The cartilaginous cap of this lesion is what helps to differentiate it in histopathology. In subungual exostosis, the cap is composed of fibrocartilage, while in osteochondromas it is made of hyaline cartilage similar to what is seen in normal growing epiphysis.5 Subungual exostosis can be confused with pyogenic granulomas and verruca, and often are treated as such, which delays appropriate surgical management.
Firm, slow-growing tumors in the fingers or toes of children should raise suspicion for underlying bony lesions like subungual exostosis and osteochondromas. X-rays of the lesion should be performed in order to clarify the diagnosis. Referral to orthopedic surgery is needed for definitive surgical management.
Dr. Matiz is a pediatric dermatologist at Southern California Permanente Medical Group, San Diego.
References
1. Zhang W et al. JAAD Case Rep. 2020 Jun 1;6(8):725-6.
2. DaCambra MP et al. Clin Orthop Relat Res. 2014 Apr;472(4):1251-9.
3. Torlazzi C et al. Int J Cancer. 2006;118:1972-6.
4. Calonje E et al. McKee’s pathology of the skin: With clinical correlations. (4th ed.) Philadelphia: Elsevier/Saunders, 2012.
5. Lee SK et al. Foot Ankle Int. 2007 May;28(5):595-601.
6. Mavrogenis A et al. Orthopedics. 2008 Oct;31(10).