A numeric index based on spatial frequency for the tortuosity of retinal vessels and its application to plus disease in retinopathy of prematurity

Background: In retinopathy of prematurity (ROP), tortuosity of vessels near the posterior pole of the fundus is an important clinical sign, yet clinicians have difficulty estimating how tortuous the vessels are. Methods: The authors have devised an objective, numeric index of retinal blood vessel tortuosity that is especially sensitive to the structural changes in vessels that occur in ROP, but it is not particularly sensitive to non-ROP changes. Computer software is used to calculate the index from fundus images; the quality of these images is typical of photographs or videocassette frames that the authors record in the premature nursery. Results: The index reliably segregates tortuous vessels from nontortuous ones and separates eyes that reach ROP treatment threshold from eyes that do not. Conclusions: The index forms an objective measure of the ROP disease state. Its calculation requires only segments of vessels and thus, is potentially adaptable to imaging systems that automatically extract vessel portions from fundus images.