SD-OCT Imaging of the Retina and Optic Nerve in Normal and Glaucomatous Cats

Abstract

Purpose: : To evaluate the feasibility of using spectral domain OCT (SD-OCT) to obtain reproducible retinal nerve fiber layer (RNFL ) thickness measurements in a feline model of primary congenital glaucoma(PCG).

Methods: : Cirrus SD-OCT (Carl Zeiss Meditec Inc., Dublin, CA) was used to acquire optic nerve cube scans on at least 2 occasions (5-30 days apart) from 24 eyes of 12 normal cats and 16 eyes from 9 cats with PCG. Cats ranged in age from 0.5-6.2 yrs. Cats were anesthetized with ketamine/xylazine and custom fitted plano contact lenses applied to prevent corneal drying. During each session, at least 3- 5 separate scans were acquired per eye. Scans with signal strength <8/10; areas of signal drop-out, or errors in the automated segmentation of retinal layers were excluded from analysis. Differences between groups were evaluated by student's t-test or Mann-Whitney test.

Results: : Contact lens application increased mean signal strength values from 8.9 to 9.6/10 in PCG cats and from 8.9 to 9.9/10 in normal cats and facilitated more rapid collection of high quality scans. In PCG cats, 20% of scans were excluded, while in normal cats only 1.3% of scans were excluded from analysis. Segmentation errors were more pronounced in PCG cats: the inner limiting membrane segmentation line was accurately placed in scans from normal cats but not in all scans from PCG cats and the RPE layer segmentation was more variable in PCG cats. Scans of PCG cats clearly demonstrated dramatic ONH cupping, with posterior displacement of the lamina cribrosa that persisted after normalization of IOP. There was no significant difference in mean RNFL thickness between right and left eyes in either group. Mean RNFL thickness values in adult PCG cats (54.12 ± 2.46 µm(n=6)) were significantly less than in normal cats (62.81± 1.18 µm (n=10); p=0.0047) There was a trend towards decrease in RNFL thickness with age in glaucomatous but not in normal cats. Thickness values were highly reproducible. Intra-session CoV for normal cats =0.02 (OU) and for PCG cats(n=6) CoV = 0.107(OD) and 0.15 (OS). There was no significant inter-session difference between values obtained in two sessions 7 days apart (10 cats, 4 scans/cat) (p>0.70).

Conclusions: : It is possible to obtain high quality SD-OCT images in normal and glaucomatous, anesthetized cats. The ability to derive reproducible retinal thickness measurements in vivo will facilitate longitudinal assessment of structure-function correlations in this spontaneous animal model of PCG.

Keywords: imaging/image analysis: non-clinical • imaging methods (CT, FA, ICG, MRI, OCT, RTA, SLO, ultrasound) • nerve fiber layer