Changes in retinal layer thickness with maturation in the dog: an in vivo spectral domain - optical coherence tomography imaging study

Interspecies Retinal Diversity and Optic Nerve Anatomy in Odontocetes

Throughout evolution, odontocete vision has had to readapt to the aquatic environment, which has had far-reaching effects on ocular anatomy and neurology. The most prominent features include the iris with an operculum, a well-developed choroid, the presence of giant ganglion cells in the retina, and the hemispherical shape of the thick eyecup. In the present study, the optic nerve and the retina were comparatively studied in Odontoceti (Cuvier's beaked whale, common bottlenose dolphin, false killer whale, long-finned pilot whale, Risso's dolphin, striped dolphin), the semi-aquatic common hippopotamus, and the fully terrestrial bovine. Cross-sections of the tissue were treated with histological and immunohistochemical techniques. Substantial differences were seen between the odontocetes and the reference species as well as within the cetaceans. The morphological structure of the optic nerve mainly appeared species specific, while the density of retinal ganglion cells was significantly higher in the terrestrial bovine than in the cetaceans. However, some typical characteristics of the cetacean retina were absent: the giant ganglion cells and the high retinal thickness. Immunohistochemical research showed varying degrees of neurofilament 200 expression in the retinal ganglion cells, while calretinin was only expressed in those of the common bottlenose dolphin and bovine.

Optical Coherence Tomography: A Review of Current Applications in Veterinary Ophthalmology

Normative standards for healthy animal structures have been established by optical coherence tomography (OCT). OCT has been used in animal studies to characterize more precisely ocular lesions, identify the origin of the affected layer, and eventually provide a curative treatment. To acquire a high image resolution, several challenges must be overcome when performing an OCT scan on animals. Sedation or general anesthesia is usually necessary in OCT image acquisition to alleviate motion during image acquisition. Mydriasis, eye position and movements, head position, and corneal hydration must also be managed during the OCT analysis.

Morphological and Morphometric Analysis of Canine Choroidal Layers Using Spectral Domain Optical Coherence Tomography

The choroid, a multifunctional tissue, has been the focus of research interest for many scientists. Its morphology and morphometry facilitate an understanding of pathological processes within both the choroid and retina. This study aimed to determine the choroidal layer thicknesses in healthy, mixed-breed mesocephalic dogs, both male (M) and female (F), using spectral domain optical coherence tomography (SD-OCT) with radial, cross-sectional, and linear scans. The dogs were divided into two groups based on age: middle-aged (MA) and senior (SN). Thicknesses of choroidal layers, namely RPE-Bruch's membrane-choriocapillaris complex (RPE-BmCc) with tapetum lucidum in the tapetal fundus, the medium-sized vessel layer (MSVL), and the large vessel layer with lamina suprachoroidea (LVLS), as well as whole choroidal thickness (WCT), were measured manually using the caliper function integrated into the OCT software. Measurement was performed dorsally and ventrally at a distance of 5000-6000 μm temporally and nasally at a distance of 4000-7000 μm to the optic disc on enhanced depth scans. The measurements were conducted temporally and nasally in both the tapetal (temporal tapetal: TempT, nasal tapetal: NasT) and nontapetal (temporal nontapetal: TempNT, nasal nontapetal: NasNT) fundus. The ratio of the MSVL thickness to the LVLS thickness for each region was calculated. In all examined dogs, the RPE-BmCc in the dorsal (D) region and MSVL in the Tt region were significantly thicker than those in the other regions. The MSVL was thinner in the ventral (V) region than in the D, TempT, TempNT and NasT regions. The MSVL was significantly thinner in the NasNT region than in the D region. LVLS thickness and WCT were significantly greater in the D and TempT regions than those in the other regions and significantly lesser in the V region than those in the other regions. The MSVL-to-LVLS thickness ratio did not differ between the age groups. Our results reveal that the choroidal thickness profile does not depend on age. Our findings can be used to document the emergence and development of various choroidal diseases in dogs in the future.

An unusual inherited electroretinogram feature with an exaggerated negative component in dogs

OBJECTIVES

To assess an inherited abnormal negative response electroretinogram (NRE) that originated in a family of Papillon dogs.

ANIMALS STUDIED

Thirty-eight dogs (Papillons, or Papillon cross Beagles or Beagles).

PROCEDURES

Dogs underwent routine ophthalmic examination and a detailed dark-adapted, light-adapted and On-Off electroretinographic study. Vision was assessed using a four-choice exit device. Spectral-domain optical coherence tomography (SD-OCT) was performed on a subset of dogs. Two affected males were outcrossed to investigate the mode of inheritance of the phenotype.

RESULTS

The affected dogs had an increased underlying negative component to the ERG. This was most pronounced in the light-adapted ERG, resulting in a reduced b-wave and an exaggerated photopic negative response (PhNR). Changes were more pronounced with stronger flashes. Similarly, the On-response of the On-Off ERG had a reduced b-wave and a large post-b-wave negative component. The dark-adapted ERG had a significant increase in the scotopic threshold response (STR) and a significant reduction in the b:a-wave ratio. Significant changes could be detected at 2 months of age but became more pronounced with age. Vision testing using a four-choice device showed affected dogs had reduced visual performance under the brightest light condition. There was no evidence of a degenerative process in the affected dogs up to 8.5 years of age. Test breeding results suggested the NRE phenotype had an autosomal dominant mode of inheritance.

CONCLUSIONS

We describe an inherited ERG phenotype in Papillon dogs characterized by an underlying negative component affecting both dark- and light-adapted ERG responses.

Retrospective evaluation of pre-surgical electroretinography results in a mixed-breed canine population presented for cataract removal surgery

OBJECTIVE

Electroretinography (ERG) is used prior to cataract removal surgery to assess retinal function. We aimed to replicate and improve upon previous studies by performing a full ECVO protocol and by examining the retina post-surgery in all patients.

ANIMALS STUDIED

One hundred twenty-seven eyes from 67 dogs were included in the study.

PROCEDURES

A full ECVO protocol electroretinography, which includes extensive rod and cone analysis, was performed on all dogs presenting for cataract surgery.

RESULTS

Our main findings were that amplitudes, but not implicit times of rod responses decreased with advanced cataracts. Amplitudes of the single flash rod and rod flicker responses were significantly lower in eyes with mature cataracts, and the former also decreased in hypermature cataracts. Cone flicker amplitude responses were also significantly lower in eyes with mature and hypermature cataracts. However, mixed single flash rod-cone and cone responses, with the exception of the mixed rod-cone a-wave amplitude in eyes with hypermature cataracts, were unaffected by cataract stage. The b-wave amplitude of the scotopic, mixed rod-cone, and photopic cone responses were affected by age and decreased by an average of 2.9, 7.5, and 1.5 μV/year, retrospectively (p < 0.01).

CONCLUSIONS

Lower ERG amplitudes in canine cataract patients may result from aging or the presence of advanced cataracts and may not indicate the presence of retinal disease.

Outer retinal thickness and visibility of the choriocapillaris in four distinct retinal regions imaged with spectral domain optical coherence tomography in dogs and cats

Purpose

To evaluate the outer retinal band thickness and choriocapillaris (CC) visibility in four distinct retinal regions in dogs and cats imaged with spectral domain optical coherence tomography (SD‐OCT). To attempt delineation of a fovea‐like region in canine and feline SD‐OCT scans, aided by the identification of outer retinal thickness differences between retinal regions.

Methods

Spectralis® HRA + OCT SD‐OCT scans from healthy, anesthetized dogs (n = 10) and cats (n = 12) were analyzed. Scanlines on which the CC was identifiable were counted and CC visibility was scored. Outer nuclear layer (ONL) thickness and the distances from external limiting membrane (ELM) to retinal pigment epithelium/Bruch's membrane complex (RPE/BM) and ELM to CC were measured in the area centralis (AC), a visually identified fovea‐like region, and in regions superior and inferior to the optic nerve head (ONH). Measurements were analyzed using a multilevel regression.

Results

The CC was visible in over 90% of scanlines from dogs and cats. The ONL was consistently thinnest in the fovea‐like region. The outer retina (ELM‐RPE and ELM‐CC) was thickest within the AC compared with superior and inferior to the ONH in dogs and cats (p < .001 for all comparisons).

Conclusions

The CC appears a valid, albeit less than ideal outer retinal boundary marker in tapetal species. The AC can be objectively differentiated from the surrounding retina on SD‐OCT images of dogs and cats; a fovea‐like region was identified in dogs and its presence was suggested in cats. These findings allow targeted imaging and image evaluation of these regions of retinal specialization.

Morphometrical analysis of the canine choroid in relation to age and sex using spectral domain optical coherence tomography

This study determined the choroidal thickness of senior (SN, n = 24) and middle-aged (MA, n = 17) healthy, mixed-breed mesocephalic dogs, both males (M) and females (F), using spectral domain optical coherence tomography (SD-OCT). The dogs were divided into two groups for examination: MA dogs (4-7 years old; 6 M, 11 F) and SN dogs (8-13 years old; 12 M, 12 F). Choroidal thickness of the dogs was investigated using SD-OCT radial and linear scans. The software of the device allowed determination of the exact measurement location on the choroid. Measurements of the choroid were taken manually using the SD-OCT calliper function at distances of 5,000-6,000 μm (dorsal and ventral) and 4,000-7,000 μm (nasal and temporal) from the optic disc. Mean (µm ± SD) (MA, SN) dorsal (188 ± 28, 184 ± 33), ventral (116 ± 23, 111 ± 16), temporal (152 ± 31, 151 ± 26), and nasal (135 ± 27, 132 ± 18) choroidal thicknesses demonstrated significant differences (P < 0.02-0.001) between all areas within each group. The choroid was thickest in the dorsal region and thinnest in the ventral region. There were no significant differences based on age. Mean (µm ± SD) (M, F) dorsal (181 ± 32, 190 ± 30), ventral (117 ± 16, 11 ± 21), temporal (150 ± 26, 153 ± 30), and nasal (128 ± 20, 138 ± 23) choroidal thicknesses demonstrated significant differences (P < 0.05) between dorsal and nasal regions. The choroidal thickness in dogs depends on the area assessed independently of their age and sex.