Chronic ocular hypertension alters local retinal responsiveness

Prophylactic anti-glaucoma therapy in dogs with primary glaucoma: A practitioner survey of current medical protocols

AIM

To examine the use of prophylactic anti-glaucoma medications in the normotensive fellow eye in dogs with unilateral overt primary glaucoma by veterinary ophthalmology clinicians.

METHODS

A survey of veterinary ophthalmology clinicians was distributed over two international list serves servicing veterinary ophthalmologists, trainees, and individuals whose practice consisted primarily of ophthalmic patients. The survey was developed following analysis of historical and currently available medical options for control of intraocular pressure and for neuroprotection.

RESULTS

Responses from 199 veterinary ophthalmology clinicians were evaluated. While a large variety of topical anti-hypertensive drugs and protocols were used, the most commonly used medications were aqueous humor production suppressors such as dorzolamide 2.0% ophthalmic solution, timolol 0.5% ophthalmic solution, and a combination product containing both drugs. Latanoprost 0.005% ophthalmic solution was used infrequently for prophylaxis by comparison. The majority of respondents do not use concurrent anti-inflammatory medications (61.22%), although a sizeable minority used prednisolone acetate, dexamethasone, or ketorolac as prophylactic treatment. Systemically administered ocular anti-hypertensive agents were rarely used. Only 40% of respondents used neuroprotectant agents; the most commonly prescribed were the calcium channel blocker amlodipine and the nutraceutical Ocu-Glo™. Recommended intervals between re-examination by the clinician ranged from one month to one year, with most re-evaluations occurring every 3 to 6 months. The majority of respondents recommended more frequent assessments of IOP at intervals between once monthly and once every 3 months.

CONCLUSIONS

Data analysis of medical therapy for the normotensive fellow eye of dogs previously diagnosed with primary glaucoma suggests that there is a great need for well-designed, prospective, controlled, multi-center studies to determine which protocols have the greatest efficacy in delaying an overt attack in the previously normotensive eye in dogs with a genetic predisposition to glaucoma. Prospective studies utilizing a carbonic anhydrase inhibitor such as dorzolamide and a prostaglandin analogue such as latanoprost would be reasonable as these two drugs are widely used in the treatment of overt glaucoma and would allow for an exploration of the impact of different mechanisms of action of lowering IOP on the pathophysiology of primary glaucoma.

Development of inner retinal function, evidenced by the pattern electroretinogram, in the rat

Though the rat is increasingly used as an animal model in ophthalmic research, including the study of glaucoma, little is known about age-related changes in its inner retinal function. The aim of this study was to evaluate these changes in the rat during the first 18 weeks of life. The pattern electroretinogram (PERG) was used to monitor inner retinal activity in 16 developing rats. In each animal, recordings were conducted at ages 3, 5, 7, 11, 14 and 18 weeks to assess age-related changes in function. Signals were evoked by five stimuli of progressively increasing check width (subtending 82-1312 arc minutes of visual angle) that were projected directly onto the fundus through a specially modified ophthalmoscope which allowed visual and manual control of stimulus quality. Poor signal:noise ratio prevented signal analysis at age 3 weeks. Subsequently, PERG amplitude increased significantly, up to 242% (depending on stimulus check width), during weeks 5-11. After peaking at 11 weeks, signal amplitude declined moderately. Signal latency mirrored that of amplitude, decreasing during the first 11 weeks, and then increasing steadily. Latency was not affected by stimulus check width. Age was highly correlated with P1 latency (R(2)=0.80) and moderately correlated with N2 latency (R(2)=0.52). Therefore, we propose that studies of inner retinal diseases (such as glaucoma) in the rat model should use age-matched controls, as electrophysiological results may be confounded by age-related changes. The rat PERG undergoes many of the age-related changes that have been reported in humans, and thus may serve as an animal model to study development of inner retinal function.

Pattern electroretinography in a rat model of ocular hypertension: functional evidence for early detection of inner retinal damage

With the increasing use of the rat as an animal model for glaucoma and for the evaluation of neuroprotective treatments, there is a need for a sensitive test of retinal ganglion cell (RGC) function in this species. The aims of this study were to detect functional abnormalities of the inner retina in a rat model of high intraocular pressure (IOP) using the pattern electroretinogram (PERG), and to correlate them with morphometric analysis of RGC survival and the functional integrity of the inner retina. Unilateral ocular hypertension was induced in 17 Lewis rats through laser photocoagulation. Pattern ERGs were recorded prior to lasering and 3 weeks later, using a series of shifting patterns of decreasing spatial frequency projected directly onto the animals' fundus. IOP was measured at the same intervals, and the number of surviving RGCs estimated. Low amplitude PERG signals could be recorded in response to a narrow grating of 0.368 cycles per degree (cpd), and increased with stimulus size. Lasering caused mean (+/-s.d.) IOP to increase significantly from 18.3+/-4.5 (baseline) to 29.8+/-8.8 mmHg within 3 weeks (p<0.0001). At this time, PERG amplitudes were significantly reduced (p<0.05), declining an average of 45% compared to the normotensive, control eyes. No outer retinal damage was observed, but the mean number of RGCs decreased significantly (p<0.001), from 2 525.0+/-372.4 to 1 542.8+/-333.8 cells per mm2. This decrease in RGC number was significantly (p=0.03) correlated the decrease in PERG amplitude. The correlation between functional integrity of the inner retina and the rat PERG was further demonstrated by intravitreal tetrodotoxin injections, which temporarily abolished the PERG but did not affect outer retinal activity, reflected in the flash ERG. The evidence for early functional deficits, combined with tonometry and documentation of correlated ganglion cells loss, confirms the sensitivity of this diagnostic tool and the validity and importance of this animal model in glaucoma research.

T-cell-based vaccination for morphological and functional neuroprotection in a rat model of chronically elevated intraocular pressure

Acute or chronic glaucoma is often associated with an increase in intraocular pressure (IOP). In many patients, however, therapeutic pressure reduction does not halt disease progression. Neuroprotection has been proposed as a complementary therapeutic approach. We previously demonstrated effective T-cell-based neuroprotection in experimental animals vaccinated with the synthetic copolymer glatiramer acetate (copolymer-1, Cop-1), a weak agonist of self-antigens. This study was undertaken to test different routes and modes of vaccination with Cop-1 as treatment modalities for protection against retinal ganglion cell (RGC) death caused by chronic elevation of IOP in rats, and to determine whether anatomical neuroprotection is accompanied by functional neuroprotection. In a chronic model of unilaterally high IOP, Cop-1 vaccination, with or without an adjuvant, protected rats against IOP-induced loss of RGCs by eliciting a systemic T-cell-mediated response capable of cross-reacting with self-antigens residing in the eye. In rats deprived of T cells, Cop-1 (unlike treatment with alpha2-adrenoreceptor agonists) was not protective of RGCs, substantiating the contention that its beneficial effect is not conferred directly but is T-cell-mediated. Pattern electroretinography provided evidence of functional protection. Thus, vaccination with adjuvant-free Cop-1 can protect RGCs from the consequences of elevated IOP in rats. This protection is manifested both morphologically and functionally. These findings can be readily implemented for the development of a therapeutic vaccination to arrest the progression of glaucoma.

Progressive changes in ophthalmic blood velocities in Beagles with primary open angle glaucoma

OBJECTIVE

To measure changes in the ocular and orbital blood flow velocities by color Doppler imaging (CDI) in beagles with primary open angle glaucoma as the disease progressed from early to advanced stages.

METHODS

CDI measurements were performed periodically on 13 glaucomatous Beagles during the nontreated mild, moderate and advanced stages of POAG over the course of 4 years. CDI was performed with the dogs lightly anesthetized (butorphanol 0.1 mg/kg IV, acepromazine maleate 0.02 mg/kg IV, and atropine sulfate 0.05 mg/kg) while the CD transducer was placed directly on the cornea anesthetized with 0.5% tetracaine hydrochloride. Intraocular pressure (IOP) by pneumatonography or TonoPen XL, heart rate and mean arterial blood pressure were measured at the beginning, middle and end of each study. The ophthalmic vessels examined included: external ophthalmic arteries and veins, long and short posterior ciliary arteries, anterior ciliary arteries and veins, primary retinal arteries, and vortex veins. Recordings of each vessel included peak systolic velocity (PSV), end diastolic velocity (EDV) and time averaged velocity (TAV), and when possible the resistive index (RI) and pulsatility index (PI) were computed.

RESULTS

CDI abnormalities were present before intraocular pressure exceeded the normal range. As the animals aged, and the glaucoma progressed with higher levels of IOP, significant changes occurred in nearly all vessels, and generally included a major increase in RI (P < 0.001) and an increase in the PI (P < 0.001). Mean arterial blood pressure (105 +/- 18 mmHg) and heart rate (118 +/- 33/min) remained reasonably constant. The IOP gradually increased as the disease progressed (early and normotensive: 19.4 +/- 3.9 mmHg; moderate: 29.7 +/- 2 mmHg; and advanced: 44.5 +/- 6 mmHg). The ocular veins seemed most influenced early on in the disease. Late in the disease, ocular venous blood flow could not be consistently demonstrated. An increase in the PI of ocular veins occurred in the moderately and severely affected glaucomatous Beagles. As the IOP increased, there were trends of increasing resistive index and pulsatility index in most arteries, and periods of marked decreased velocities of the vortex and external ophthalmic veins in severe cases.

CONCLUSION

CDI measurements in Beagles with primary open angle glaucoma during the course of 4 years indicate easily measurable and repeatable progressive blood flow abnormalities before the elevation of IOP and, thereafter, with gradually increased levels of IOP.

Endothelin 1 levels in the aqueous humor of dogs with glaucoma

PURPOSE

Endothelin 1 is a small peptide that is involved in regulation of intraocular pressure and modulation of ocular circulation. To investigate the role of endothelin 1 in canine glaucoma, the authors measured aqueous humor levels of endothelin 1 in healthy dogs and in dogs with hypertensive glaucoma.

METHODS

Aqueous humor samples were obtained with general anesthesia from the eyes of healthy dogs (n = 5) and dogs with hypertensive glaucoma (n = 10). Measurements were made by enzyme immunoassay for endothelin 1.

RESULTS

The endothelin 1 aqueous humor range was 1.12 - 3.63 pg/mL for healthy dogs and 1.97 - 14.97 pg/mL for glaucomatous dogs. The healthy and glaucomatous canine endothelin 1 aqueous levels (mean +/- SD) were 2.33 +/- 0.90 and 8.11 +/- 5.03 pg/mL, respectively. A two-way analysis of variance indicated that this difference was significant (P = 0.0084). The effect of age on endothelin 1 levels was not significant (P = 0.6283). The large variability found within the glaucomatous group could be explained by the degree of damage of the retina (P = 0.0006). There was no significant correlation between intraocular pressure and endothelin 1 aqueous humor levels within the glaucomatous group (P = 0.29).

CONCLUSIONS

The aqueous humor of dogs with hypertensive glaucoma contains significantly higher levels of endothelin 1 than that of healthy dogs.

Comparative Doppler imaging of the ophthalmic vasculature in normal Beagles and Beagles with inherited primary open-angle glaucoma

The objective of this study was to compare orbital and ocular vasculature velocity, measured by Doppler imaging, in normal Beagles and Beagles with inherited primary open-angle glaucoma. Eight normal Beagles and 13 Beagles with different stages of primary open-angle glaucoma were evaluated twice with a 2-4-week period between measurements. Doppler imaging was performed with the dogs anesthetized, and the Doppler transducer applied directly on the corneal surface. The majority of the orbital vasculature (external ethmoidal artery; internal ophthalmic artery and vein; and external ophthalmic artery and vein) and ocular blood vessels (anterior ciliary artery and veins; long posterior ciliary arteries; short posterior ciliary arteries; primary retinal arteries; and the vortex veins) were identified and Doppler blood velocity parameters were determined. The glaucomatous dogs demonstrated significant differences in the Doppler velocity parameters of several orbital vessels (external ethmoidal, external ophthalmic, and internal ophthalmic arteries), and several ocular vessels (anterior ciliary, short posterior ciliary, and long posterior ciliary arteries). These differences included decreased blood velocities, and increased pulsatility and resistive indexes. The Doppler blood flow velocities of the primary retinal arteries were unchanged between the normal and glaucomatous dogs. In the glaucomatous dogs, the Doppler imaging suggests increased vascular resistance downstream in both the orbital and ocular vasculature. These blood velocity parameter changes may be primary or secondary, and may offer therapeutic opportunities to increase perfusion, prolong the retina and optic nerve head function, and maintain vision in the canine glaucomas.

Flicker perimetry using a luminance threshold strategy at frequencies from 5-25 Hz in glaucoma, ocular hypertension and normal controls

A prototype automated flicker perimeter was used to investigate flicker luminance thresholds in glaucoma patients (n = 11), ocular hypertensives (high risk n = 10, low and intermediate risk n = 21) and normal controls (n = 10). Differential light sensitivity was measured for one eye of each subject using a flickering stimulus at frequencies of 5, 10, 15, 20 and 25 Hz at ten test locations of 1 degree in the arcuate and nasal regions of the central 50 degrees of the visual field using a 4-2 dB staircase. The glaucoma patients had significantly reduced sensitivities for all frequencies (p < 0.001) with minimum sensitivity at 15 Hz. The 'high risk' ocular hypertensives had significantly reduced sensitivities at 10 and 15 Hz (p < 0.02). At present the ocular hypertensive patients in this study show no abnormalities demonstrable by conventional automated static perimetry. These findings may represent the earliest of perimetric deficits in patients with glaucomatous optic neuropathy.