A woman with pain and weakness in both legs

A 49 year old woman was admitted with a four month history of progressive pain and weakness in both of her legs. Physical examination showed bilateral tenderness of the hips and sacroiliac joints, along with a decline in muscle strength and tension of the lower limbs. Magnetic resonance imaging was requested (fig 1⇓). No lytic bone lesions were detected by radiography. Plasma cells made up less than 2% of nucleated cells in a bone marrow aspirate and biopsy sample, and there was no evidence of a plasma cell neoplasm. No monoclonal proteins were found on electrophoresis or immunofixation of the serum or urine. She did not have anaemia, renal insufficiency, or hypercalcaemia. Phenotyping of peripheral blood lymphocytes showed that the proportion of CD16⁺CD56⁺CD3⁻ natural killer cells (5.34%) was low compared with the reference values in our laboratory (14.91%, standard deviation 4.87% in 68 healthy controls). No disturbance in the CD4 to CD8 ratio was noted.1