Case of the week #7: Osteomyelitis of subungual exostosis in a 15yo boy.

Master V is a 15 year old skeletally immature adolescent with a six month history of toenail deformity, nail ulceration and pain. He was otherwise fit and well, with no history of diabetes or immunosuppression. He saw a podiatrist who initially suggested footwear changes and was concerned about tight gastrocnemius and EHL over-activation. Master V then returned to see the podiatrist with a moderate amount of fluid under the nail, and the podiatrist debrided the nail ulceration but no samples or imaging were taken at the time. An x-ray was then ordered after debridement which demonstrated a subungual exostosis arising from the distal phalanx of the great toe. Review with Master V's General Practitioner raised alarm about ulceration and erythema around the toenail, and he was sent to Dr Touzell for urgent surgical opinion.

On examination, Master V was systemically well and afebrile. The great toenail demonstrated previous evidence of debridement with a wedge excised from the nail. The surrounding skin was moderately cellulitic. No abscess was obvious and no pus was able to be exuded. He was neurovascularly intact with normal sensation and capillary refill, and dorsalis pedis and posterior tibial pulses present.

Master V's x-ray demonstrated a moderate sized subungual exostosis (see figure 1).

Figure 1: X-ray demonstrating subungual exostosis arising from dorsomedial border of distal phalanx.

Due to the chronic nature of Master V's condition and repeated nail debridements, Dr Touzell was concerned about the potential for osteomyelitis. An MRI scan was arranged urgently which demonstrated increased signal on PD weighted images suggestive of osteomyelitis (see figure 2). A blood test was performed which yielded a CRP of 0.7 (normal <5) and ESR of 3 (normal <11) which was reassuring that Master V was not systemically unwell from his infection.

Figure 2: PD weighted MRI scan demonstrated increased signal in the distal phalanx of the great toe consistent with osteomyelitis.

Surgery was arranged urgently. Master V underwent removal of the nail where a large subungual exostosis was noted to be eroding 30% of the nail bed. The exostosis was removed and multiple samples, including the exostosis, were sent for microscopy and culture as well as histopathology examination with a pathologist experienced in musculoskeletal tumours to exclude malignancy. The nail bed was repaired directly with 2mm of advancement of the distal skin for coverage. Complete excision of the exostosis was confirmed using Image Intensifier in theatre (figure 3).

Figure 3: Image Intensifier image demonstrating removal of the subungual exostosis.

Master V was commenced on intravenous cefazolin 2g tds. All intra-operative samples, including bone samples, grew heavy growth of staphylococcus aureus sensitive to cefazolin and histopathology reported chronic changes consistent with infection and no evidenece of malignancy. A referral to an Infectious Disease Physician was made and advice of four weeks of intravenous antibiotics followed by four weeks of oral clyndamycin was given. A PICC line was inserted the following day and Hospital in the Home arranged for ongoing antibioitc care. Master V was discharged four days post-operatively.

Master V tolerated his antibiotics without side effects and the PICC line was removed after four weeks. Three months after surgery, there is no evidence of recurrent infection and the toenail is growing back well. Master V is back wearing normal shoes, playing limited sport (due to the COVID-19 pandemic) and has no pain.

Key learning points:

1) Subungual exostosis are a rare, but important cause of nail deformity in the foot and hand, particularly in adolescents during their pre-pubescent growth spurt when the exostosis can grow rapidly.

2) Evidence of toenail erosion should be investigated with an x-ray prior to debridement where possible due to the risk of underlying bony deformity or tumour.

3) The clinician needs to be wary of osteomyelitis in patients with a history of chronic ulceration, and urgent orthopaedic referral is appropriate.

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