Mycobacterium Tuberculosis Pyomyositis in an Infant
A previously healthy 9 month old boy, who was up to date on immunizations, was found to have an area of swelling and firmness on his right thigh noted by mom while bathing him. The induration increased over the following week, but there was no associated fever, pain, or history of trauma. Parents recalled routine immunizations given in both thighs three months ago. Children in Dubai receive Bacille Calmette-Guérin (BCG) vaccine in the deltoid at birth; followed by diphtheria/tetanus/acellular pertussis (DTaP), inactivated polio (IPV), hemophilus influenza B (HiB) and hepatitis B (HepB) vaccines given at 2-months, 4-months and 6-months in the thighs. His initial blood work revealed a CRP of 30 mg/L (normal: <6 mg/L); ESR 29 mm/hr (normal: <10 mm/hr) and normal complete blood count, AST, ALT and alkaline phosphatase.\nAn ultrasound of the right thigh showed a 6 cm × 4 cm deep abscess in its anterolateral aspect. The abscess was drained under general anesthesia and 15 ml of purulent material was evacuated; he was discharged home on oral cefuroxime pending culture results. Gram stain of abscess fluid showed gram positive cocci and gram negative rods; bacterial cultures remained negative. He completed 10 days of oral cefuroxime post-drainage without any improvement in thigh induration or swelling; hence he was referred for pediatric infectious diseases consultation.\nOn examination in the pediatric infectious disease clinic, he was a very well-appearing infant with normal vital signs, weight at 50% ile and length at 90% ile for age. The anterolateral right thigh was indurated and swollen, without any overlying erythema or tenderness, and a 3 cm incisional scar was noted to be healing well. He did not bear weight on his legs when placed in standing position, and did not crawl. There was no systemic lymphadenopathy, and the remainder of his physical examination was normal. An MRI scan with contrast obtained after abscess drainage and completion of oral antibiotic therapy showed extensive inflammatory granulation tissue involving the right lateral thigh compartment, without any evidence of underlying femoral osteomyelitis. The involved muscles, especially vastus intermedius and vastus lateralis were noted to have undergone significant fatty replacement [Figures and ].\nMycobacterial PCR on the abscess fluid was reported as positive; this PCR detects Mycobacterium tuberculosis (M. TB), Mycobacterium avium (M. avium), Mycobacterium bovis (M. bovis), and Mycobacterium microti (M. microti). Due to his unusual clinical presentation, and hence concerns about a false-positive test, the TB PCR was repeated on the sample fluid and was again highly positive. In addition, Ziehl-Neelsen and auramine-rhodamine staining of the pus sample demonstrated a large number of acid fast bacilli (AFB). After 6 weeks, the mycobacterial culture grew a pan-sensitive isolate of M. TB.\nThis was a previously healthy child who was born in Dubai, United Arab Emirates. He had received the BCG vaccine on his deltoid at birth, a routine practice in this region. He had travelled within the Middle East; there were no family pets; and no unpasteurized milk consumption was reported. He had no known exposures to tuberculosis or human immunodeficiency virus (HIV); and he had demonstrated no systemic symptoms i.e., fever, night sweats or weight loss prior to his presentation to the pediatrician for this complaint. Tuberculin skin test (TST) was not done; and a TB interferon gamma release assay (IGRA) was negative. A chest X-ray was normal. TSTs placed on all household contacts were negative.\nHe was empirically started on 4-drug anti-TB therapy with isoniazid, rifampin, ethambutol, pyrazinamide and pyridoxine pending AFB culture results, and followed clinically. He tolerated the medications very well; within one month of initiating therapy he started to bear weight on his legs, the thigh induration improved, and his ESR and CRP normalized. Once culture/sensitivity data were available, and after 2 months of receiving 4-drug therapy, he was continued on isoniazid and rifampin for the remainder of his treatment. A repeat MRI obtained after 6 months of starting anti-TB therapy showed significant resolution of the prior inflammatory changes; however, a residual 8 mm cystic area with an enhancing wall was noted between the vastus medialis and vastus lateralis muscles. Hence the anti-TB therapy was continued for an additional three months. An MRI obtained at the conclusion of 9 months of treatment showed no active inflammation, and treatment was stopped. On follow-up, one year after completing his treatment, he remains a very active two-year old with normal gait and physical activity; without evidence of disease recurrence.