Presentation of a Rare Isolated Soft Tissue Nontuberculous Mycobacterial Infection with MRI Characterization

Background: Nontuberculous mycobacteria (NTM), also known as atypical or environmental mycobacteria, was first described in the early 20th century. Although these bacteria are related to Mycobacterium tuberculosis, they are a distinct group of mycobacteria. Like Mycobacterium tuberculosis, NTM most commonly affects the lungs, however extra-pulmonary infections involving soft tissue, skin, and lymph nodes can also occur. These extra-pulmonary infections are rare and are primarily acquired secondary to skin breaks following trauma, minor surgeries and cosmetic procedures. Due to the rarity of NTM soft tissue infections the diagnosis is challenging and often delayed. Patients who develop NTM infections are typically immunocompromised, as the rate of clinical infection is significantly low. The incidence of skin and soft tissue infections (SSTI) by NTM however has increased threefold over a thirty-year period, which has attracted attention in the medical community. Case Presentation: A 78-year-old male pecan farmer presented with a left lateral knee mass that progressively worsened over a four-year period. He described the initial lesion as a small knot and denied any trauma or skin abrasions to the affected area prior to developing the nodule. The patient had no history of immunodeficiency and he denied pain, redness, or drainage of the nodule. He also denied fevers, chills, night sweats, chronic cough, or unintentional weight-loss. A T2 weighted axial MRI demonstrated a 5.3 x 7.4 cm well circumscribed, sharply marginated, and encapsulated perifascial soft tissue collection with mixed high and low signal that was mildly heterogeneous along the iliotibial fascial band. These imaging manifestations suggested either a soft tissue tumor or an infectious process. After failing conservative treatment with antibiotics, the patient was referred to an orthopedic surgeon for excision of the mass. An encapsulated cyst-like mass was excised circumferentially around the IT band and was sent to pathology. The gross specimen was described as a 3-cm diameter smooth walled cyst that was partially filled with soft material. Cut sections revealed caseating granulomatous inflammation. Special stain was positive for acid-fast bacilli consistent with mycobacteria. The specimen was negative for atypia, neoplasm, fungi, or polarizable crystals. Cultures did not grow any organisms and the TB gold assay and T spot test were negative for tuberculosis. This left a nontuberculous mycobacterial soft tissue infection as the diagnosis of exclusion. Treatment for SSTI NTM includes a multi-drug antibiotic regimen for at least 6 months based on susceptibility testing. Surgical intervention is indicated if antibiotic treatment is not effective. For this patient, surgical resection was the definitive treatment. Conclusion: This case illustrates a unique presentation of a rare isolated chronic NTM soft tissue infection of the lateral knee in a patient with no known history of trauma or immunodeficiency. Knowledge and clinical suspicion of this pathogen along with MRI findings can aid in the accurate and timely diagnosis of NTM soft tissue infections.