OCT has become instrumental in helping ODs image and diagnose myriad clinical conditions, including optic neuropathies. But the technology isn’t perfect. Research shows the precision of the retinal nerve fiber layer (RNFL) measurement can be affected by a number of clinical factors such as patient age, image quality, globe axial length, head tilt and fundus torsion. Most recently, researchers found that correcting for the disc-foveal angle is necessary to accurately determine the RNFL thickness profile in superior oblique palsy (SOP) eyes with large amounts of torsion.

The study evaluated 44 eyes of 22 patients with unilateral congenital SOP and 42 eyes of 42 normal controls. The team used fovea-to-disc (FoDi) OCT to measure RNFL thickness and calculate the position of the fovea relative to the optic disc. They then turned off the FoDi functionality and re-evaluated each OCT image to determine RNFL values according to the disc-foveal angle.

The investigators discovered that the average disc-foveal angles were -10.85±6.60° in the affected eyes and -10.71±6.63° in the fellow eyes of SOP patients. They note that these values were significantly greater than that of control subjects.

They also learned that there was no significant difference in RNFL sector values between SOP and control eyes with FoDi and that the measured inferotemporal thickness was thinner in SOP eyes without FoDi than in SOP eyes with FoDi (129.7±20.5µm vs. 144.6±17.8µm, respectively).

They add that differences in RNFL thicknesses with and without FoDi in SOP eyes in the superotemporal and inferotemporal sectors were significantly more than the RNFL thickness differences with and without FoDi in control eyes (0.30±6.57µm and 8.52±10.40µm, respectively). This, they note, carries implications for glaucoma detection and progression.

“Improper fovea alignment affects sectoral RNFL thickness measurement in SOP eyes and should be corrected when possible,” the study authors conclude.

Akbari M, Nikdel M, Moghimi S, et al. Effect of foveal location on retinal nerve fiber layer thickness profile in superior oblique palsy eyes. J Glaucoma. August 23, 2019. [Epub ahead of print].