When tracking optic nerve changes with your OCT, you should obtain minimum rim width measurements at the same time of day, according to Laura Pardon, OD, who presented her research findings at the Academy of Optometry meeting in Orlando last month. Her study revealed that minimum rim width is decreased at 7pm relative to 7am, and the time of day at which repeat scans are obtained influences the measurement’s repeatability. According to Dr. Pardon, it's important to understand changes in optic nerve and structure throughout the day if practitioners are to differentiate normal individuals from those with disease.

The study analyzed 10 healthy participants (mean age of 29.5 ± 4.8 years) who presented for OCT scan sessions at 7am, 7pm and one of seven randomly selected times (7am, 9am, 11am, 1pm, 3pm, 5pm, 7pm) on the same day. At each session, the researchers acquired 24-line radial and 12-degree circular scans centered on the optic nerve head (ONH). The team used radial scans to calculate global values for the optic minimum rim width and Bruch’s membrane opening (BMO) height. They also used circular scans to calculate global RNFL and choroid thicknesses. 

Minimum rim width demonstrated a mean difference of -12.2±16.9µm between the 7am and 7pm sessions and -1.5±5.9µm for scans within the same session; mean minimum rim width was significantly reduced at 7pm relative to 7am (362.5µm vs. 374.7µm). The range of repeatability values was narrower for BMO height, RNFL thickness and choroid thickness, which suggested to the team that it is not as critical to obtain these measurements at the same time of day when managing patients longitudinally.

“The reason that we are seeing this change in minimum rim width throughout the day remains unknown,” Dr. Pardon said during her lecture. “However, it may have important implications for glaucoma pathophysiology.”