Correlation between OCT measurement of the retinal nerve fibre layer thickness and RGC loss after optic nerve section in the adult rat

Abstract

Purpose: To correlate the measurements of the retinal nerve fibre layer (RNFL) thickness obtained with Optic Coherence Tomography (OCT) with Retinal Ganglion Cell (RGC) survival after optic nerve (ON) section.

Methods: The left ON of adult albino rats was sectioned at its exit from the eye. Three, 7 and 12 days, 1 and 2 months later the optic disk (OD) and peripapillary retina was analyzed using spectral-domain OCT (Spectralis, Heidelberg Engineering). As control 8 rats were analyzed. Two image acquisition protocols were used: circular scan (1000 µm diameter) and volume raster pattern (61 horizontal sections equally spaced), both centred in the OD. Manual delineation of the RNFL was used for both scanning methods to obtain a mean RNFL thickness for the circular scan and for the 3 concentric sectors (400, 1200 and 2400 µm in diameter) of the volume scan. Right after the OCT, retinas were dissected as whole mounts and immunoreacted for Brn3a and phosphorylated neurofilament high molecular weight subunit protein (pNFH) to identify RGCs and their intra-retinal axons, respectively. The total number of RGCs was automatically quantified using automatic counting methods.

Results: RNFL thickness in control retinas was 38±3 µm in the circular scan and 46±15, 40±7 and 33±3 µm, respectively, in the central, medial and peripheral sectors of the volume scan. In experimental animals, RNFL loss was first observed at 12 days in the circular scan (5% decrease), and progressed up to 2 months (50% decrease) after ON section. The volume scan showed an increased thickness of the RNFL in the central and medial sectors 3 (63±11 and 47±6 µm) and 7 days (67±14 and 45±7 µm) after ON section, respectively. Significant RNFL thinning was observed: in the central sector at 2 months (19±13 µm), in the medial sector at 1 (26±4 µm) and 2 months (24±5 µm) and in the peripheral sector at all time periods between 12 days (27±3 µm) to 2 months (20±3 µm) after ON section. The percentages of Brn3a+ RGCs decreased to approximately 71, 53, 17, 5 and 3% at 3, 7 and 12 days, 1 and 2 months after ON section, respectively. The aspect of the pNFH+ RGC axons did not change qualitatively in the first 12 days, but showed decreased axonal densities at 1 and 2 months after ON section.

Conclusions: RGC death precedes the decrease in thickness of the RNFL after ON section.

Keywords: 531 ganglion cells • 695 retinal degenerations: cell biology • 552 imaging methods (CT, FA, ICG, MRI, OCT, RTA, SLO, ultrasound)