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Mycobacterium chelonae osteomyelitis presenting as a mycobacterial spindle cell pseudotumor

We report a case of an 88-year-old man with osteomyelitis of the right ankle, with histopathology demonstrating a Mycobacterium spindle cell pseudotumor. The Mycobacterium contained in this spindle cell pseudotumor was Mycobacterium chelonae. M. chelonae spindle cell pseudotumors are rare and have only been reported twice previously in the literature. Similarly, M. chelonae presenting as the pathogen in bone infection is rare. Due to this unusual presentation of M. chelonae, the antibiotic rationale was based largely on case reports and consisted of imipenem, clarithromycin, and linezolid. Antibiotic complications were experienced by the patient. Despite a renally adjusted dose of imipenem, the patient experienced imipenem toxicity and his antibiotics were modified to tigecycline and clarithromycin. Although his symptoms were clinically resolving, the patient sadly passed away before completing treatment.

the patient's estimated glomerular filtration rate of 35), clarithromycin 500 mg twice a day, and co-trimoxazole 960 mg twice a day. Imipenem is the preferred carbapenem in M. chelonae infection. [2] Following receipt of the Reference Laboratory sensitivities (reported sensitivity to amikacin, clarithromycin, and linezolid with resistance to cefoxitin, co-trimoxazole, ciprofloxacin, moxifloxacin, and doxycycline), the patient was switched to imipenem, clarithromycin, and linezolid. Previous studies have found that clarithromycin in combination with linezolid is clinically effective in immunocompromised patients with cutaneous M. chelonae. [7] The patient was sent home with outpatient antimicrobial therapy and regular blood monitoring.
Several complications were encountered with this choice. Most significantly, despite a renally adjusted dose of imipenem, the patient was re-admitted 12 days after starting treatment with confusion. Mild diffuse encephalopathy was diagnosed on electroencephalogram but no seizure activity. These symptoms did not reoccur after imipenem was stopped. Second, the patient began to experience myelosuppression secondary to linezolid (after 12 days of treatment).
Following his readmission, the patient was switched to a combination of tigecycline and clarithromycin. Tigecycline has been reported as improving outcomes as part of a multidrug regimen to treat M. chelonae. [8] There is no firm evidence upon which to base the duration of treatment in cases of osteomyelitic M. chelonae. Previous case reports have documented mycobacterial spindle cell pseudotumor caused by Mycobacterium intracellulare that took 2 years of antibiotic treatment to resolve. [9] The literature suggests that along with antibiotics, surgical intervention is vital to treating M. chelonae bone infection. However, given the patient's extensive medical history, he was not believed to be fit for surgery, and therefore, debridement of the bone was not performed.
Clinically, the inflammation of the ankle had begun to subside before readmission, and the patient's symptoms continued to improve with tigecycline and clarithromycin [ Figure 3] although the patient's inflammatory markers remained static.
After 16 days on this combination of antibiotics, the patient became hypotensive and tachycardic and sadly passed away, with the cause of death found to be a combination of restrictive cardiomyopathy and multiorgan failure . The exact nature of the underlying immunosuppressive pathology remains unknown.

dIScuSSIon
Mycobacterial spindle cell pseudotumors are benign lesions that present histologically as spindle-shaped histiocytes containing acid-fast bacilli. They are most commonly caused by Mycobacterium avium and Mycobacterium tuberculosis [5] and usually present in skin, lungs, eyes, and soft tissue. [1] The case presented here is unusual in two respects. The Mycobacterium spindle cell pseudotumor was formed by M. chelonae, a finding that has only been reported twice previously in the literature. [5] Second, M. chelonae presenting in bone has only been reported in nine cases in the literature. [1][2][3] Spindle cell pseudotumors are almost always associated with immunodeficiency: a review of Mycobacterium pseudotumor cases found that of the 23 cases of Mycobacterium spindle cell pseudotumors in the literature, 21 patients had immunosuppression either as a result of HIV/AIDs, immunosuppressive therapy, or diabetes. [5] Given the patient's history of melanoma, bone metastases were among the initial differential diagnoses. The literature reports that spindle cell pseudotumors are often mistakenly diagnosed radiologically as malignant lesions, for example, sarcomas, schwannomas or histiocytomas, until biopsy confirms otherwise. [9] Carbapenems are known to lower seizure thresholds and can cause confusion. Imipenem is primarily renally excreted, and therefore, dose adjustment in kidney failure is recommended to limit side effects. In this case, however, side effects occurred despite renal adjustment of the imipenem dose, which resolved on discontinuation of the treatment.

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The authors certify that they have obtained all appropriate patient consent forms. In the form the patient(s) has/have given his/her/their consent for his/her/their images and other clinical information to be reported in the journal. The patients understand that their names and initials will not be published and due efforts will be made to conceal their identity, but anonymity cannot be guaranteed.

Financial support and sponsorship
Nil.

Conflicts of interest
There are no conflicts of interest.