HISTORY: He was admited for acute severe lower back pain with nocturnal exacerbation and fever (39.1°C) with onset 3 days before. No medical history was recorded. Weight (20kg) – Height (109cm).

CLINICAL: Physical examination showed elective lumbo-sacral pain, para-vertebral muscular contracture but no sciatic nerve irritation nor neurological deficiency. Blood samples revealed hyperleucocytosis (11 200/ml), inflammatory syndrome (CRP>4md/dl) but exhaustive microbiological samples, blood cultures, viral serology and all rheumatologic lab tests yielded negative.

ICONOGRAPHY: Plain X-rays of the spine showed no abnormality. Bilateral L5 spondylolysis with discrete fatty infiltrate of the right L5-S1 foramen was observed on CT (Fig1).

We did perform a BONE SCAN including SPECT. Planar WB-scan demonstrated a focus of bone-uptake at the right lateral side of L5 (Fig2).

SPECT revealed maximal enhancement at the right L5-S1 facet joint with extension to right pedicle of L5 (Fig3 et 4).

NMR study showed oedema with Gd-enhancement of the right L5-S1 facet joint as well as the postero-lateral paraspinal soft tissues (L4 to S1 levels), the right L5 spondylolysis and the right L5-S1 foramen (Fig5) suggesting an inflammatory/septic pathology.

CONCLUSION: Septic arthritis of the right L5-S1 lumbar facet joint with extension to paraspinal soft tissues of adjacent levels, intervertebral foramen and to isthmic lysis. Despite absence of pathogenic agent on bacteriological culture, an empirical antibiotherapy (vancomycine/azactam) was started with favorable outcome and regression of oedema on the 6-week-control NMR study.

TAKE HOME MESSAGE: Septic arthritis of a lumbar facet joint is an extremely rare condition, especially in paediatrics (only 2 other cases have been reported in the litterature). The source is most often hematogenous from bacteraemia (mostly staph. aureus). As clinical presentation is similar to spondylodiscitis, imaging is essential to diagnosis. When readily accessible NMR is the method of choice for a precise and sensitive diagnosis as well as assessment of soft paraspinal, epidural, foraminal extension. BONE SCAN may help to detect bone extension. BONE SPECT is especially accurate to determine facet joint, pedicular or laminar involvement and can even be performed in pediatric patients as young as five.