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A 76-year-old man presented with a 2-month history of swelling and tenderness of the left thumb and thenar. The patient had type 2 diabetes mellitus. He had been bitten by a dog on the left hand 1 year previously, and the wound had healed without treatment. He was initially diagnosed with non-infectious tenosynovitis and received steroid injections repeatedly (Fig. 1a). Thereafter, open diagnostic-drainage was performed, and the presence of ‘rice bodies’ was visually noted in the hand (Fig. 1b). Based on the pathological finding of granuloma and positive specimen culture for Mycobacterium intracellulare, he was diagnosed with tenosynovitis due to Mycobacterium avium complex (MAC). While his symptoms initially improved by isoniazid, rifampicin, and ethambutol, the redness and tenderness around the left wrist gradually worsened at 6 months after the first operation. Then therapeutic-drainage was performed again, and the regimen was changed to clarithromycin, rifampicin, ethambutol, and sitafloxacin after the introduction to our department. After 1 year, however, a nodule developed around the metacarpophalangeal joint, associated with an intense uptake on 18F-fluorodeoxyglucose positron emission tomography/computed tomography (Fig. 1c), implying the residual inflammation. Therapeutic-drainage was performed again (Fig. 1d), and he is now in remission under antimicrobial chemotherapy.

Fig. 1
figure 1

a Swelling and tenderness of the left thumb and thenar before the first operation (red arrow). b ‘Rice bodies’ were observed during the first open drainage performed for treating tenosynovitis. c 18F-fluorodeoxyglucose positron emission tomography/computed tomography showed intense uptake around the metacarpophalangeal joint, before the third drainage. d Intraoperative findings at the third drainage showed synovial thickening and synovial fluid retention. By this time, the ‘rice bodies’ had resolved (red arrow)

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MAC tenosynovitis is a refractory infectious disease, reported more commonly in Asians [1–3]. Most patients have an injury history and often require multiple operations as in this case [1–4]. The presence of ‘rice bodies’ is a characteristic intraoperative finding as well as tuberculosis [3, 5]. Although the appropriate duration of chemotherapy is unclear, past studies recommended a 1–2-year treatment period [4]. When seeing cases present with refractory tenosynovitis, MAC tenosynovitis should be considered in the differential diagnosis.