The non-human primate experimental glaucoma model

The Magnitude of Hypotony and Time Course of Intraocular Pressure Recovery Following Anterior Chamber Cannulation in Nonhuman Primates

Purpose

To determine the magnitude of ocular hypotony and the length of recovery time to 6 and 10 mm Hg IOP following anterior chamber (AC) cannulation.

Methods

Bilateral IOP was recorded 500 times per second via telemetry immediately before, during, and immediately after AC cannulation with a 27-G needle in 10 different sessions at least 2 weeks apart in four male rhesus macaques (nonhuman primates; NHPs) aged 3- to 6-years old. Bilateral IOP was recorded continuously using a proven telemetry system while the NHPs were under general anesthesia during IOP transducer calibration experiments involving manometric control of IOP via AC cannulation, then continuously after the AC needles were removed until IOP recovered to precannulation levels. The change in IOP from baseline to AC cannulation was tested using the signed-rank test. The times necessary for IOP to recover to 6 and 10 mm Hg, respectively, were calculated.

Results

Average precannulation IOP was 11.5 mm Hg and significantly decreased to an average of 2.3 mm Hg immediately following AC needle removal (P = 0.0156). On average, IOP recovered from 2.3 to 6 and 10 mm Hg in 32.4 and 63.7 minutes, respectively. Recovery times of IOP were not affected by repeated AC cannulations every 2 weeks.

Conclusions

Generally, IOP recovers relatively quickly after repeated AC cannulation, and did not result in extended duration hypotony. It is important to consider hypotony in animal experiments and clinical procedures involving AC cannulation and paracentesis when consideration of IOP or its effects is important.

Lamina cribrosa in glaucoma

PURPOSE OF REVIEW

This article presents, summarizes, and interprets the most recent advances in the study and understanding of the lamina cribrosa in glaucoma, in the context of previous work.

RECENT FINDINGS

The lamina is an active living structure that responds to strain by changing morphology at the micro-scale and macro-scale in glaucoma. Changes in lamina cribrosa morphology in glaucoma include posteriorization of the laminar insertion into the sclera, increased cupping or depth of the lamina cribrosa, and the development of focal lamina cribrosa defects. These lamina cribrosa changes are associated with disk hemorrhages and visual field damage, and are detectable with clinical imaging techniques such as optical coherence tomography. Glaucomatous changes in the lamina cribrosa are thought to be driven by cellular processes mediated by focal cyclical mechanical strain. Strain is eye specific and mediated by intraocular pressure, cerebrospinal fluid pressure, scleral and lamina cribrosa morphology, and structural stiffness; deleterious lamina cribrosa strains can occur at all levels of mean intraocular pressure.

SUMMARY

Laminar morphology is ever changing in health and disease, and recent studies have identified several promising morphological changes that are indicative of glaucoma susceptibility, onset, and progression.

Mapping in-vivo optic nerve head strains caused by intraocular and intracranial pressures

Although it is well documented that abnormal levels of either intraocular (IOP) or intracranial pressure (ICP) can lead to potentially blinding conditions, such as glaucoma and papilledema, little is known about how the pressures actually affect the eye. Even less is known about potential interplay between their effects, namely how the level of one pressure might alter the effects of the other. Our goal was to measure in-vivo the pressure-induced stretch and compression of the lamina cribrosa due to acute changes of IOP and ICP. The lamina cribrosa is a structure within the optic nerve head, in the back of the eye. It is important because it is in the lamina cribrosa that the pressure-induced deformations are believed to initiate damage to neural tissues leading to blindness. An eye of a rhesus macaque monkey was imaged in-vivo with optical coherence tomography while IOP and ICP were controlled through cannulas in the anterior chamber and lateral ventricle, respectively. The image volumes were analyzed with a newly developed digital image correlation technique. The effects of both pressures were highly localized, nonlinear and non-monotonic, with strong interactions. Pressure variations from the baseline normal levels caused substantial stretch and compression of the neural tissues in the posterior pole, sometimes exceeding 20%. Chronic exposure to such high levels of biomechanical insult would likely lead to neural tissue damage and loss of vision. Our results demonstrate the power of digital image correlation technique based on non-invasive imaging technologies to help understand how pressures induce biomechanical insults and lead to vision problems.

Comparative Anatomy of the Trabecular Meshwork, the Optic Nerve Head and the Inner Retina in Rodent and Primate Models Used for Glaucoma Research

Glaucoma is a heterogeneous group of ocular disorders with a multi-faceted etiology. Although numerous studies on glaucoma using different animal models have been published, it is unwise to simply generalize the results of one model to all glaucomatous situations because of the differences in the anatomy and morphology of animal eyes in comparison with humans’. In this review, we highlight the differences in the trabecular meshwork (TM) tissue, lamina cribrosa (LC) region, optic nerve head (ONH) and the inner layer of the retina in mice, rats and monkeys. In comparison with humans, non-human primates show TM, retina and ONH that are anatomically almost identical. The rat model shows many similarities in the aqueous outflow pathway compared to humans. The mouse ONH lacks collagenous LC, and this finding is observed across different mouse strains. The tissue structure of the ONH in rodents is similar to that in humans, although the blood supply shows differences. The number of cells in the ganglion layer depends on the rodent strain. Despite some differences from humans, rodents are a good choice for studying different types of glaucoma, and the modeling method should be selected based on the experimental needs and the hypothesis being tested.

The Connective Tissue Components of Optic Nerve Head Cupping in Monkey Experimental Glaucoma Part 1: Global Change

PURPOSE

To characterize optic nerve head (ONH) connective tissue change within 21 monkey experimental glaucoma (EG) eyes, so as to identify its principal components.

METHODS

Animals were imaged three to five times at baseline then every 2 weeks following chronic unilateral IOP elevation, and euthanized early through end-stage confocal scanning laser tomographic change. Optic nerve heads were serial-sectioned, three-dimensionally (3D) reconstructed, delineated, and quantified. Overall EG versus control eye differences were assessed by general estimating equations (GEE). Significant, animal-specific, EG eye change was required to exceed the maximum physiologic intereye differences in six healthy animals.

RESULTS

Overall EG eye change was significant (P < 0.0026) and animal-specific EG eye change most frequent, for five phenomena (number of EG eyes and range of animal-specific change): posterior laminar deformation (21, -29 to -437 μm), laminar thickening (11, 20-73 μm) and thinning (3, -23 to -31 μm), scleral canal expansion (17, 20-139 μm), outward anterior (16, -16 to -124 μm) and posterior (17, -22 to -279 μm) laminar insertion migration, and peripapillary scleral bowing (11, 21-77 μm). Experimental glaucoma versus control eye laminar thickness differences were bimodal in behavior, being thickened in most EG eyes demonstrating the least deformation and less thickened or thinned in most EG eyes demonstrating the greatest deformation.

CONCLUSIONS

Our postmortem studies retrospectively identify five connective tissue components of ONH "cupping" in monkey EG which serve as targets for longitudinally staging and phenotyping ONH connective tissue alteration within all forms of monkey and human optic neuropathy.

Expansions of the neurovascular scleral canal and contained optic nerve occur early in the hypertonic saline rat experimental glaucoma model

PURPOSE

To characterize early optic nerve head (ONH) structural change in rat experimental glaucoma (EG).

METHODS

Unilateral intraocular pressure (IOP) elevation was induced in Brown Norway rats by hypertonic saline injection into the episcleral veins and animals were sacrificed 4 weeks later by perfusion fixation. Optic nerve cross-sections were graded from 1 (normal) to 5 (extensive injury) by 5 masked observers. ONHs with peripapillary retina and sclera were embedded, serial sectioned, 3-D reconstructed, delineated, and quantified. Overall and animal-specific EG versus Control eye ONH parameter differences were assessed globally and regionally by linear mixed effect models with significance criteria adjusted for multiple comparisons.

RESULTS

Expansions of the optic nerve and surrounding anterior scleral canal opening achieved statistical significance overall (p < 0.0022), and in 7 of 8 EG eyes (p < 0.005). In at least 5 EG eyes, significant expansions (p < 0.005) in Bruch's membrane opening (BMO) (range 3-10%), the anterior and posterior scleral canal openings (8-21% and 5-21%, respectively), and the optic nerve at the anterior and posterior scleral canal openings (11-30% and 8-41%, respectively) were detected. Optic nerve expansion was greatest within the superior and inferior quadrants. Optic nerve expansion at the posterior scleral canal opening was significantly correlated to optic nerve damage (R = 0.768, p = 0.042).

CONCLUSION

In the rat ONH, the optic nerve and surrounding BMO and neurovascular scleral canal expand early in their response to chronic experimental IOP elevation. These findings provide phenotypic landmarks and imaging targets for detecting the development of experimental glaucomatous optic neuropathy in the rat eye.