Comparison of longitudinal in vivo measurements of retinal nerve fiber layer thickness and retinal ganglion cell density after optic nerve transection in rat

Neuroinflammation and gliosis in the injured and contralateral retinas after unilateral optic nerve crush

The main purpose of this study is to analyze the effects of unilateral optic nerve crush in the gene expression of pro- and anti-inflammatory mediators, and gliosis markers in injured and contralateral retinas. Retinas from intact, unilaterally optic nerve injured or sham-operated C57BL/6J mice were analyzed 1, 3, 9 and 30 days after the surgery (n = 5/group and time point) and the relative expression of TGF-β1, IL-1β, TNF-α, Iba1, AQP4, GFAP, MHCII, and TSPO was analyzed in injured and contralateral using qPCR. The results indicated that compared with intact retinas, sham-operated animals showed an early (day 1) upregulation of IL-1β, TNF-α and TSPO and a late (day 30) upregulation of TNF-α. In sham-contralateral retinas, TNF-α and TSPO mRNA expression were upregulated and day 30 while GFAP, Iba1, AQP4 and MHCII downregulated at day 9. Compared with sham-operated animals, in retinas affected by optic nerve crush GFAP and TSPO upregulated at day 1 and TNF-α, Iba1, AQP4 and MHCII at day 3. In the crushed-contralateral retinas, TGF-β1, TNF-α, Iba1 and MHCII were upregulated at day 1. TSPO was upregulated up to day 30 whereas TGF-β1 and Iba1 downregulated after day 9. In conclusion, both sham surgery and optic nerve crush changed the profile of inflammatory and gliosis markers in the injured and contralateral retinas, changes that were more pronounced for optic nerve crush when compared to sham.

Mechanisms implicated in the contralateral effect in the central nervous system after unilateral injury: focus on the visual system

The retina, as part of the central nervous system is an ideal model to study the response of neurons to injury and disease and to test new treatments. During the last decade is becoming clear that unilateral lesions in bilateral areas of the central nervous system trigger an inflammatory response in the contralateral uninjured site. This effect has been better studied in the visual system where, as a rule, one retina is used as experimental and the other as control. Contralateral retinas in unilateral models of retinal injury show neuronal degeneration and glial activation. The mechanisms by which this adverse response in the central nervous system occurs are discussed in this review, focusing primarily on the visual system.

Establishing the ground squirrel as a superb model for retinal ganglion cell disorders and optic neuropathies

Retinal ganglion cell (RGC) death occurs after optic nerve injury due to acute trauma or chronic degenerative conditions such as optic neuropathies (e.g., glaucoma). Currently, there are no effective therapies to prevent permanent vision loss resulting from RGC death, underlining the need for research on the pathogenesis of RGC disorders. Modeling human RGC/optic nerve diseases in non-human primates is ideal because of their similarity to humans, but has practical limitations including high cost and ethical considerations. In addition, many retinal degenerative disorders are age-related making the study in primate models prohibitively slow. For these reasons, mice and rats are commonly used to model RGC injuries. However, as nocturnal mammals, these rodents have retinal structures that differ from primates - possessing less than one-tenth of the RGCs found in the primate retina. Here we report the diurnal thirteen-lined ground squirrel (TLGS) as an alternative model. Compared to other rodent models, the number and distribution of RGCs in the TLGS retina are closer to primates. The TLGS retina possesses ~600,000 RGCs with the highest density along the equatorial retina matching the location of the highest cone density (visual streak). TLGS and primate retinas also share a similar interlocking pattern between RGC axons and astrocyte processes in the retina nerve fiber layer (RNFL). In addition, using TLGS we establish a new partial optic nerve injury model that precisely controls the extent of injury while sparing a portion of the retina as an ideal internal control for investigating the pathophysiology of axon degeneration and RGC death. Moreover, in vivo optical coherence tomography (OCT) imaging and ex vivo microscopic examinations of the retina in optic nerve injured TLGS confirm RGC loss precedes proximal axon degeneration, recapitulating human pathology. Thus, the TLGS retina is an excellent model, for translational research in neurodegeneration and therapeutic neuroprotection.

Progression of optic atrophy in traumatic optic neuropathy: retrograde neuronal degeneration in humans

OBJECTIVE

We used optical coherence tomography (OCT) to document the time course of retrograde neuronal degeneration following indirect optic nerve injury.

METHODS

We retrospectively studied patients diagnosed with unilateral indirect traumatic optic neuropathy (TON). Patients with total or near-total optic atrophy were included. All patients underwent complete ophthalmological examinations, including OCT imaging, within 1 day and at 1, 2, 3, 4, 6, 8, 12, 24, and 48 weeks after trauma.

RESULTS

The mean thicknesses of the circumpapillary retinal nerve fiber layer (cpRNFL) and macular retinal ganglion cell-inner plexiform layer (mGCIPL) decreased significantly at 2 weeks after trauma (p = 0.027 and p = 0.043). Changes in mGCIPL thickness preceded changes in cpRNFL thickness. The rates of reduction in mGCIPL and cpRNFL thicknesses were greatest between 2 to 4 weeks and 4 to 6 weeks after trauma. The reduction in mGCIPL thickness then slowed, and stabilized at 12 weeks after trauma. The proportions of cpRNFL and mGCIPL losses at 2, 4, 6, 8, and 12 weeks compared to 24 weeks were 17.1, 33.7, 59.8, 77.9, and 87.9% and 30.0, 73.3, 76.1, 88.3, and 97.9%, respectively.

CONCLUSIONS

OCT revealed optic atrophy progression 2 weeks after trauma, which was most rapid from 2 to 6 weeks, and then gradually stabilized. Loss of retinal ganglion cell bodies and dendrites seemed to precede the axonal degeneration. Observations of morphological changes in retinal layers using OCT in TON patients improve our understanding of retrograde neuronal degeneration of the central nervous system.

Longitudinal outcomes of circumlimbal suture model-induced chronic ocular hypertension in Sprague-Dawley albino rats

PURPOSE

To characterise longitudinal structural and functional changes in albino Sprague-Dawley rats following circumlimbal suture ocular hypertension (OHT) induction.

METHODS

Ten-week-old rats (n = 24) underwent suture implantation around the limbal region in both eyes. On the next day, the suture was removed from one eye (control eyes) and left intact in the other eye (OHT eyes) of each animal. Intraocular pressure (IOP) was monitored weekly twice for the next 15 weeks. Optical coherence tomography (OCT) and electroretinogram (ERG) were measured at baseline and weeks 4, 8, 12, and 15, and eyes were then collected for histological assessment.

RESULTS

Sutured eyes (n = 12) developed IOP elevation of ~ 50% in the first 2 weeks that was sustained at ~ 25% above the control eye up to week 15 (p = 0.001). Animals with insufficient IOP elevation (n = 6), corneal changes (n = 3), and attrition (n = 3) were excluded from the analysis. OHT eyes developed significant retinal nerve fibre layer (RNFL) thinning (week 4: - 19 ± 14%, p = 0.10; week 8: - 17 ± 12%, p = 0.04; week 12: - 16 ± 10%, p = 0.04, relative to baseline) and reduction in retinal ganglion cell (RGC) density (- 32 ± 26%, p = 0.02). At week 15, both inner (9 ± 7%, p = 0.01) and outer retinal layer thicknesses (6.0 ± 5%, p = 0.001) showed a mild increase in thicknesses. The positive scotopic threshold response (- 28 ± 25%, p = 0.04) and a-wave were significantly reduced at week 12 (- 35 ± 21%; p = 0.04), whereas b-wave was not significantly affected (week 12: - 18 ± 27%, p = 0.24).

CONCLUSION

The circumlimbal suture model produced a chronic, moderate IOP elevation in an albino strain that led to RNFL thinning and reduced RGC density along with the reductions in ganglion and photoreceptoral cell functions. There was a small thickening in both outer and inner retinal layers.

Activation of adenosine A3 receptor protects retinal ganglion cells from degeneration induced by ocular hypertension

Glaucoma is a progressive chronic retinal degenerative disease and a leading cause of global irreversible blindness. This disease is characterized by optic nerve damage and retinal ganglion cell (RGC) death. The current treatments available target the lowering of intraocular pressure (IOP), the main risk factor for disease onset and development. However, in some patients, vision loss progresses despite successful IOP control, indicating that new and effective treatments are needed, such as those targeting the neuroprotection of RGCs. Adenosine A₃ receptor (A₃R) activation confers protection to RGCs following an excitotoxic stimulus. In this work, we investigated whether the activation of A₃R could also afford protection to RGCs in the laser-induced ocular hypertension (OHT) model, a well-characterized animal model of glaucoma. The intravitreal injection of 2-Cl-IB-MECA, a selective A₃R agonist, abolished the alterations induced by OHT in the negative and positive components of scotopic threshold response (STR) without changing a- and b-wave amplitudes both in scotopic and photopic conditions. Moreover, the treatment of OHT eyes with the A₃R agonist promoted the survival of RGCs, attenuated the impairment in retrograde axonal transport, and improved the structure of the optic nerve. Taking into consideration the beneficial effects afforded by 2-Cl-IB-MECA, we can envisage that A₃R activation can be considered a good therapeutic strategy to protect RGCs from glaucomatous damage.

The effect of retrobulbar anesthesia for cataract surgery on the retinal nerve fiber layer thickness

OBJECTIVE

The aim of this study was to evaluate the effect retrobulbar and topical anesthesia on optic nerve by measuring retinal nerve fiber layer (RNFL) thickness before and after phacoemulsification cataract surgery using optical coherence tomography (OCT).

METHODS

A total of 126 eyes of 108 patients were included in the study. Retrobulbar anesthesia was used in 61 eyes of 53 patients, and topical anesthesia was used in 65 eyes of 55 patients. OCT scans were done 1 week before the surgery and 4 weeks after surgery, and the results were evaluated.

RESULTS

The average RNFL thickness increased postoperatively in both groups (p < 0.001 for topical anesthesia group and p = 0.001 for retrobulbar anesthesia group). The preoperative and postoperative average and quadrant RNFL thickness and the change in RNFL thickness were not significantly different between the two anesthesia groups (p > 0.05).

CONCLUSION

Our results suggest that retrobulbar anesthesia has no unfavorable effect on the retinal nerve fiber layer.

Did you choose appropriate tracer for retrograde tracing of retinal ganglion cells? The differences between cholera toxin subunit B and Fluorogold

Cholera toxin subunit B (CTB) and Fluorogold(FG) are two widely utilized retrograde tracers to assess the number and function of retinal ganglion cells (RGCs). However, the relative advantages and disadvantages of these tracers remain unclear, which may lead to their inappropriate application. In this study, we compared these tracers by separately injecting the tracer into the superior Colliculi (SC) in rats, one or 2 weeks later, the rats were sacrificed, and their retinas, brains, and optic nerves were collected. From the first to second week, FG displayed a greater number of labeled RGCs and a larger diffusion area in the SC than CTB; The number of CTB labeled RGCs and the diffusion area of CTB in the SC increased significantly, but there was no distinction between FG; Furthermore, CTB exhibited more labeled RGC neurites and longer neurites than FG, but no difference was evident between the same trace; The optic nerves labeled using CTB were much clearer than those labeled using FG. In conclusion, both CTB and FG can be used for the retrograde labeling of RGCs in rats at 1 or 2 weeks. FG achieves retrograde labeling of a greater number of RGCs than CTB, whereas CTB better delineates the morphology of RGCs. Furthermore, CTB seems more suitable for retrograde labeling of some small, non-image forming nuclei in the brain to which certain RGC subtypes project their axons.

Response of the Retinal Nerve Fiber Layer Reflectance and Thickness to Optic Nerve Crush

Purpose

To study the effects of acute optic nerve damage on the reflectance of the retinal nerve fiber layer (RNFL) and to compare the time courses of changes of RNFL reflectance and thickness.

Methods

A rat model of optic nerve crush (ONC) was compared with previously studied normal retinas. The reflectance and thickness of the RNFL were studied at 1 to 5 weeks after ONC. Reflectance spectra from 400 to 830 nm were measured for eyes with ONC, their contralateral untreated eyes, and eyes with sham surgery. Directional reflectance was studied by varying the angle of light incidence. RNFL thickness was measured by confocal microscopy.

Results

After ONC, the RNFL reflectance remained directional. At 1 week, RNFL reflectance decreased significantly at all wavelengths (P < 0.001), whereas there was no significant change in RNFL thickness (P = 0.739). At 2 weeks, both RNFL reflectance and thickness decreased significantly, and by 5 weeks they declined to approximately 40% and 30%, respectively, of the normal values. Although RNFL reflectance decreased at all wavelengths, there was a greater reduction at short wavelengths. Spectral shape at long wavelengths was similar to the normal. Some of these changes were also found in the contralateral untreated eyes, but none of these changes were found in eyes with sham surgery.

Conclusions

Decrease of RNFL reflectance after ONC occurs prior to thinning of the RNFL and the decrease is more prominent at short wavelengths. Direct measurement of RNFL reflectance, especially at short wavelengths, may provide early detection of axonal damage.

Ocular fundus pulsations within the posterior rat eye: Chorioscleral motion and response to elevated intraocular pressure

A multi-functional optical coherence tomography (OCT) approach is presented to determine ocular fundus pulsations as an axial displacement between the retina and the chorioscleral complex in the albino rat eye. By combining optical coherence elastography and OCT angiography (OCTA), we measure subtle deformations in the nanometer range within the eye and simultaneously map retinal and choroidal perfusion. The conventional OCT reflectivity contrast serves as a backbone to segment the retina and to define several slabs which are subsequently used for quantitative ocular pulsation measurements as well as for a qualitative exploration of the multi-functional OCT image data. The proposed concept is applied in healthy albino rats as well as in rats under acute elevation of the intraocular pressure (IOP). The evaluation of this experiment revealed an increased pulsatility and deformation between the retinal and chorioscleral complex while increasing the IOP level from 15 mmHg to 65 mmHg. At IOP levels exceeding 65 mmHg, the pulsatility decreased significantly and retinal as well as choroidal perfusion vanished in OCTA. Furthermore, the evaluation of the multi-parametric experiment revealed a spatial correlation between fundus pulsatility and choroidal blood flow. This indicates that the assessed pulsatility may be a valuable parameter describing the choroidal perfusion.

Shared and Differential Retinal Responses against Optic Nerve Injury and Ocular Hypertension

Glaucoma, one of the leading causes of blindness worldwide, affects primarily retinal ganglion cells (RGCs) and their axons. The pathophysiology of glaucoma is not fully understood, but it is currently believed that damage to RGC axons at the optic nerve head plays a major role. Rodent models to study glaucoma include those that mimic either ocular hypertension or optic nerve injury. Here we review the anatomical loss of the general population of RGCs (that express Brn3a; Brn3a⁺RGCs) and of the intrinsically photosensitive RGCs (that express melanopsin; m⁺RGCs) after chronic (LP-OHT) or acute (A-OHT) ocular hypertension and after complete intraorbital optic nerve transection (ONT) or crush (ONC). Our studies show that all of these insults trigger RGC death. Compared to Brn3a⁺RGCs, m⁺RGCs are more resilient to ONT, ONC, and A-OHT but not to LP-OHT. There are differences in the course of RGC loss both between these RGC types and among injuries. An important difference between the damage caused by ocular hypertension or optic nerve injury appears in the outer retina. Both axotomy and LP-OHT induce selective loss of RGCs but LP-OHT also induces a protracted loss of cone photoreceptors. This review outlines our current understanding of the anatomical changes occurring in rodent models of glaucoma and discusses the advantages of each one and their translational value.

Assessing retinal ganglion cell damage

Retinal ganglion cell (RGC) loss is the hallmark of optic neuropathies, including glaucoma, where damage to RGC axons occurs at the level of the optic nerve head. In experimental glaucoma, damage is assessed at the axon level (in the retinal nerve fibre layer and optic nerve head) or at the soma level (in the retina). In clinical glaucoma where measurements are generally limited to non-invasive techniques, structural measurements of the retinal nerve fibre layer and optic nerve head, or functional measurements with perimetry provide surrogate estimates of RGC integrity. These surrogate measurements, while clinically useful, are several levels removed from estimating actual RGC loss. Advances in imaging, labelling techniques, and transgenic medicine are making enormous strides in experimental glaucoma, providing knowledge on the pathophysiology of glaucoma, its progression and testing new therapeutic avenues. Advances are also being made in functional imaging of RGCs. Future efforts will now be directed towards translating these advances to clinical care.

Posterior rat eye during acute intraocular pressure elevation studied using polarization sensitive optical coherence tomography

Polarization sensitive optical coherence tomography (PS-OCT) operating at 840 nm with axial resolution of 3.8 µm in tissue was used for investigating the posterior rat eye during an acute intraocular pressure (IOP) increase experiment. IOP was elevated in the eyes of anesthetized Sprague Dawley rats by cannulation of the anterior chamber. Three dimensional PS-OCT data sets were acquired at IOP levels between 14 mmHg and 105 mmHg. Maps of scleral birefringence, retinal nerve fiber layer (RNFL) retardation and relative RNFL/retina reflectivity were generated in the peripapillary area and quantitatively analyzed. All investigated parameters showed a substantial correlation with IOP. In the low IOP range of 14-45 mmHg only scleral birefringence showed statistically significant correlation. The polarization changes observed in the PS-OCT imaging study presented in this work suggest that birefringence of the sclera may be a promising IOP-related parameter to investigate.

Evaluating retinal ganglion cell loss and dysfunction

Retinal ganglion cells (RGC) bear the sole responsibility of propagating visual stimuli to the brain. Their axons, which make up the optic nerve, project from the retina to the brain through the lamina cribrosa and in rodents, decussate almost entirely at the optic chiasm before synapsing at the superior colliculus. For many traumatic and degenerative ocular conditions, the dysfunction and/or loss of RGC is the primary determinant of visual loss and are the measurable endpoints in current research into experimental therapies. To actually measure these endpoints in rodent models, techniques must ascertain both the quantity of surviving RGC and their functional capacity. Quantification techniques include phenotypic markers of RGC, retrogradely transported fluorophores and morphological measurements of retinal thickness whereas functional assessments include electroretinography (flash and pattern) and visual evoked potential. The importance of the accuracy and reliability of these techniques cannot be understated, nor can the relationship between RGC death and dysfunction. The existence of up to 30 types of RGC complicates the measuring process, particularly as these may respond differently to disease and treatment. Since the above techniques may selectively identify and ignore particular subpopulations, their appropriateness as measures of RGC survival and function may be further limited. This review discusses the above techniques in the context of their subtype specificity.

Bilateral retinal microglial response to unilateral optic nerve transection in rats

When retinal ganglion cells undergo apoptosis after optic nerve (ON) injury, microglial cells proliferate and promptly clear the degenerated debris in the ipsilateral retina. However, microglial changes in the contralateral retina have not been fully elucidated. This study characterized the long-term bilateral retinal microglial responses after unilateral ON transection. We analyzed the time course of proliferation and morphology changes of microglial cells, between 3 days and 12 weeks post ON transection, of undisturbed and reactive microglia in bilateral retinas of adult Fischer rats with unilateral ON transection. Microglia in retinas without ON transection were distributed homogeneously and possessed a highly ramified morphology, as judged by immunohistochemistry for ionized calcium-binding adapter molecule 1 (Iba1). After ON transection, microglia density in the ipsilateral retina increased gradually from 3 days to 2 weeks, and decreased from 3 weeks to 12 weeks, along with dramatic inverted alteration of process branch points of microglia in the ganglion cell layer (GCL). Transformation of ramified microglia into ameboid-like macrophages with few branching processes was observed in the ipsilateral retina from 1 week to 3 weeks. Though an increase in microglial density was weak in the contralateral retina and could only be statistically detected in the central retina, the morphological alteration over time was obvious and similar to that of the ipsilateral retina. In the inner plexiform layer (IPL), cell density and morphological changes of microglia in both the ipsilateral and contralateral retina were not prominent. These findings indicates that, though proliferation of microglial cells is weak in the contralateral retina after unilateral ON transection, conspicuous alterations in microglial morphology occur bilaterally. These suggest that using the contralateral retina as a control in studies of retinal degeneration should be considered with caution.

Relating Retinal Ganglion Cell Function and Retinal Nerve Fiber Layer (RNFL) Retardance to Progressive Loss of RNFL Thickness and Optic Nerve Axons in Experimental Glaucoma

PURPOSE

To relate changes in retinal function and retinal nerve fiber layer (RNFL) retardance to loss of RNFL thickness and optic nerve axon counts in a nonhuman primate (NHP) model of experimental glaucoma (EG).

METHODS

Bilateral longitudinal measurements of peripapillary RNFL thickness (spectral-domain optical coherence tomography, SDOCT; Spectralis), retardance (GDxVCC), and multifocal electroretinography (mfERG; VERIS) were performed in 39 NHP at baseline (BL; median, 5 recordings; range, 3-10) and weekly after induction of unilateral EG by laser photocoagulation of the trabecular meshwork. Multifocal ERG responses were high-pass filtered (>75 Hz) to measure high- and low-frequency component (HFC and LFC) amplitudes, including LFC features N1, P1, and N2. High-frequency component amplitudes are known to specifically reflect retinal ganglion cell (RGC) function. Complete (100%) axon counts of orbital optic nerves were obtained in 31/39 NHP.

RESULTS

Postlaser follow-up was 10.4 ± 7.9 months; mean and peak IOP were 18 ± 5 and 41 ± 11 mm Hg in EG eyes, 11 ± 2 and 18 ± 6 mm Hg in control (CTL) eyes. At the final available time point, RNFL thickness had decreased from BL by 14 ± 14%, retardance by 20 ± 11%, and the mfERG HFC by 30 ± 17% (P < 0.0001 each). Longitudinal changes in retardance and HFC were linearly related to RNFL thickness change (R2 = 0.51, P < 0.0001 and R2 = 0.22, P = 0.002, respectively); LFC N2 was weakly related but N1 or P2 (N1: R2 = 0.07, P = 0.11; P1: R2 = 0.04, P = 0.24; N2: R2 = 0.13, P = 0.02). At zero change from BL for RNFL thickness (Y-intercept), retardance was reduced by 11% (95% confidence interval [CI]: -15.3% to -6.8%) and HFC by 21.5% (95% CI: -28.7% to -14.3%). Relative loss of RNFL thickness, retardance, and HFC (EG:CTL) were each related to axon loss (R2 = 0.66, P < 0.0001; R2 = 0.42, P < 0.0001; R2 = 0.42, P < 0.0001, respectively), but only retardance and HFC were significantly reduced at zero relative axon loss (Y-intercept; retardance: -9.4%, 95% CI: -15.5% to -3.4%; HFC: -10.9%, 95% CI: -18.6% to -3.2%; RNFL thickness: +1.8%, 95% CI: -4.9% to +5.4%).

CONCLUSIONS

Retinal nerve fiber layer retardance and RGC function exhibit progressive loss from baseline before any loss of RNFL thickness or orbital optic nerve axons occurs in NHP EG. These in vivo measures might serve as potential biomarkers of early-stage glaucomatous damage preceding axon loss and RGC death.

In vivo imaging methods to assess glaucomatous optic neuropathy

The goal of this review is to summarize the most common imaging methods currently applied for in vivo assessment of ocular structure in animal models of experimental glaucoma with an emphasis on translational relevance to clinical studies of the human disease. The most common techniques in current use include optical coherence tomography and scanning laser ophthalmoscopy. In reviewing the application of these and other imaging modalities to study glaucomatous optic neuropathy, this article is organized into three major sections: 1) imaging the optic nerve head, 2) imaging the retinal nerve fiber layer and 3) imaging retinal ganglion cell soma and dendrites. The article concludes with a brief section on possible future directions.

Retinal nerve fiber layer and ganglion cell complex thicknesses measured with spectral-domain optical coherence tomography in eyes with no light perception due to nonglaucomatous optic neuropathy

PURPOSE

To measure retinal nerve fiber layer thickness (RNFLT) and ganglion cell complex thickness (GCCT) in eyes with no light perception due to nonglaucomatous optic neuropathy using spectral-domain optical coherence tomography.

METHODS

Fourteen eyes of 14 patients (9 women, 5 men; mean age 56.0 ± 16.6 (standard deviation) years] with no light perception due to optic neuropathy were recruited to this retrospective study. Only clinically stable eyes were included. Eyes were imaged at least 6 months after the onset of the disease. Five patients lost light perception due to traumatic optic neuropathy, four patients had ischemic optic neuropathy, two patients had optic neuritis, two patients had compressive optic neuropathy, and one patient had optic nerve atrophy. Global and quadrant RNFLTs were measured with the Cirrus HD-optical coherence tomography (OCT) system; global and hemisphere GCCTs were measured by spectral-domain OCT (RTVue OCT system). Only reliable OCT images were used for further analysis.

RESULTS

Reliable RNFL images were obtained in 12 eyes, and reliable GCC images were obtained in 11 eyes. Global, superior, temporal, inferior, and nasal RNFLT were 57.5 ± 6.7, 60.6 ± 7.6, 54.1 ± 11.2, 59.7 ± 9.5, and 55.6 ± 7.4 µm, respectively. Global, superior, and inferior GCC thicknesses were 68.8 ± 9.6, 70.7 ± 12.2, and 67.8 ± 8.8 µm, respectively.

CONCLUSIONS

A considerable proportion of RNFL and GCC remained in eyes with no light perception vision. Clinicians should take this into account when evaluating the severity of optic neuropathy from OCT-measured RNFLT and GCCT.

Comparison of retinal nerve fiber layer thickness in vivo and axonal transport after chronic intraocular pressure elevation in young versus older rats

PURPOSE

To compare in young and old rats longitudinal measurements of retinal nerve fiber layer thickness (RNFLT) and axonal transport 3-weeks after chronic IOP elevation.

METHOD

IOP was elevated unilaterally in 2- and 9.5-month-old Brown-Norway rats by intracameral injections of magnetic microbeads. RNFLT was measured by spectral domain optical coherence tomography. Anterograde axonal transport was assessed from confocal scanning laser ophthalmolscopy of superior colliculi (SC) after bilateral intravitreal injections of cholera toxin-B-488. Optic nerve sections were graded for damage.

RESULTS

Mean IOP was elevated in both groups (young 37, old 38 mmHg, p = 0.95). RNFL in young rats exhibited 10% thickening at 1-week (50.9±8.1 µm, p<0.05) vs. baseline (46.4±2.4 µm), then 7% thinning at 2-weeks (43.0±7.2 µm, p>0.05) and 3-weeks (43.5±4.4 µm, p>0.05), representing 20% loss of dynamic range. RNFLT in old rats showed no significant change at 1-week (44.9±4.1 µm) vs. baseline (49.2±5.3 µm), but progression to 22% thinning at 2-weeks (38.0±3.7 µm, p<0.01) and 3-weeks (40.0±6.6 µm, p<0.05), representing 59% loss of dynamic range. Relative SC fluorescence intensity was reduced in both groups (p<0.001), representing 77-80% loss of dynamic range and a severe transport deficit. Optic nerves showed 75-95% damage (p<0.001). There was greater RNFL thinning in old rats (p<0.05), despite equivalent IOP insult, transport deficit and nerve damage between age groups (all p>0.05).

CONCLUSION

Chronic IOP elevation resulted in severely disrupted axonal transport and optic nerve axon damage in all rats, associated with mild RNFL loss in young rats but a moderate RNFL loss in old rats despite the similar IOP insult. Hence, the glaucomatous injury response within the RNFL depends on age.