Objective: To establish by means of ultrasound and magnetic resonance imaging (MRI) the role of tenosynovitis and arthritis in determining the "sausage-like" aspect of finger dactylitis and to compare the results of the 2 examinations.
Methods: Twelve dactylitic fingers and their corresponding normal contralateral fingers belonging to 10 patients who met the Amor criteria for the diagnosis of seronegative spondylarthropathy (SpA) were studied by ultrasonography and MRI.
Results: MRI revealed a significant increase in the volar bone-to-skin distance in dactylitic fingers with respect to that of the normal contralateral fingers (P < 0.001). This increase was due to distension of the flexor synovial sheaths (P < 0.00001) by fluid collection. Peritendinous soft tissues were not involved, since these were found to be significantly thicker in the normal fingers (P < 0.05). Of the 36 joints of the 12 dactylitic fingers, only 1 showed capsule distension. Using MRI as the "gold standard," ultrasonography showed a 100% sensitivity and specificity for flexor tenosynovitis, but lacked sensitivity for joint involvement because it failed to reveal joint capsule distension in the only joint involved. Similarly, physical examination showed a 100% sensitivity and specificity for flexor sheath involvement.
Conclusion: Dactylitis is due to flexor tenosynovitis. Enlargement of the finger joint capsule is not an indispensable condition for the "sausage-like" feature. Physical examination is a sufficient method for the diagnosis of dactylitis.