Hip Pain and Fever During a Nadir Period: Who Came First?

Pyomyositis is a rare bacterial infection of the skeletal muscle that often presents with insidious symptoms, thus making the diagnosis challenging. It is categorized into primary and secondary (usually traumatic) variants, mainly occurring in tropical regions and occasionally in temperate climates, with predisposing factors including immunosuppression. Staphylococcus aureus is the most common pathogen. A 39-year-old woman with a history of breast cancer underwent a mastectomy followed by chemotherapy. After her second chemotherapy cycle, she developed fever, odynophagia, vomiting, and right hip pain (considered to be related to muscle tension due to functional exercises). Fever and hip pain progressively worsened and the other symptoms resolved. On the 12th day after chemotherapy, she received intramuscular diclofenac injections due to severe hip pain. Physical examination revealed tenderness in her right hip and signs of inflammation on her thigh and buttock. Laboratory tests showed elevation of inflammatory markers and mild kidney and liver dysfunction. A CT scan revealed an intramuscular collection in her right gluteal region (~45 x 70 mm), with adjacent fat densification. Attempts to drain the collection initially failed, but a later ultrasound-guided procedure was successful and pus was collected for bacterial culture, which identified methicillin-susceptible Staphylococcus aureus (MSSA). Antibiotic treatment was adjusted to target SA with flucloxacillin and the patient's condition improved. Subsequent imaging showed a resolving collection (<10 mm). The patient continued antibiotic treatment for six weeks, maintaining clinical improvement, normal inflammatory parameters, and apyrexia. Adjuvant chemotherapy was discontinued due to the risk of infection recurrence associated with the multiloculated collection caused by SA. The patient remained asymptomatic four months after hospitalization. An MRI then showed only a residual T2 hyperintensity in the deeper region of the right buttock, with no visible collections. The nadir period refers to the time after each chemotherapy cycle when the risk of neutropenia and subsequent infection is the highest, typically occurring between 7 and 14 days after each cycle. In this case, the intramuscular injection occurred 12 days after the second cycle of chemotherapy. It is most likely that this served as the entry point for the pyomyositis agent (MSSA) during a period of transient neutropenia.