Standardized A-scan and B-scan in vivo evaluation and measurement of the retinochoroidal layer

Abstract

Ever since standardized A-scan was introduced as a method for echographic screening of the posterior eye segment in the mid 1960’s, it has been used regularly for evaluation and measurement of the retinal and choroidal layers throughout the entire fundus. Over the years the normal thickness range of these layers and the critical measuring values that help distinguish between normal and abnormally thick layers were established for different portions of the eye. In the macular region, retinochoroidal thickness of more than 1.7 mm, or a thickness that is at least 0.2 mm greater than the thickness of the retinochoroidal layer in the macula of the normal fellow eye, should be considered abnormal. The same values hold true for the regions of the vortex veins. In other fundus areas a thickness of more than 1.2 mm is abnormal unless a similar thickness (within 0.2 mm) is found in the corresponding fundus region of a normal fellow eye. Naturally, even values well within the normal range must be considered abnormal if the same area in a normal fellow eye is more than 0.2 mm thinner.

Today such A-scan evaluation is routine for every intraocular clinical examination. It not only helps to detect thin fundus lesions such as choroidal nevi, macular edema or retinal hemorrhages, but also helps distinguish between different flat or shallow lesions on the basis of different reflectivities and distribution. Choroidal hyperemia, for instance, causes diffuse regular thickening of the choroidal layer with sustained high reflectivity. Inflammatory infiltration of the choroid in endophthalmitis, by contrast, produces irregular (nodular) thickening of the retina and choroid with decreased reflectivity. Accuracy in measuring retino-choroidal layers by the standardized A-scan method ranges from ± 0.05 to ± 0.1 mm depending on how extensive and how regular the thickening happens to be. The use of low measuring sensitivities and peak-to-peak measurements is essential for achieving this high measuring accuracy.