Infrared spectroscopy: shedding light on synovitis in patients with rheumatoid arthritis

Objectives. It is difficult to determine the extent of synovial involvement early in the course of rheumatoid arthritis. A spectroscopic technique was used to characterize the synovium of the small finger joints in both early and late rheumatoid arthritis. This synovium was also compared against normal joints. Methods. Near‐infrared spectroscopy assesses the absorption of near‐infrared light by specific joints, giving a characteristic ‘fingerprint’ of the properties of the underlying tissues. Triple measurements by infrared spectroscopy were taken at the bilateral second and third metacarpophalangeal joints. Multivariate analysis was applied. Results. Analysis was able to demonstrate relationships between the specific sources of spectral variation and joint tenderness or swelling as well as radiographic damage. Further use of multivariate analysis allowed recognition of the spectral patterns seen in early disease vs late rheumatoid arthritis and correct classification of over 74% of the joints. Conclusions. The spectral regions where differences occurred were in the absorption bands related to tissue oxygenation status, allowing the provocative implication that this technique could be detecting ischaemic changes within the joint. Near‐infrared spectroscopy may thus be able to provide us with some information about the biochemical changes associated with synovitis.