Assessment of retinal hemodynamics with the Canon laser blood flowmeter after a single dose of 2 % dorzolamide hydrochloride eyedrops

CLINICAL STUDIES

Assessment of retinal hemodynamics with the Canon laser blood flowmeter after a single dose of 2% dorzolamide hydrochloride eyedrops Dana Faingold, *t MD; Chris Hudson, *t PhD; John Flanagan, *t PhD; Kit Guan, * OD; Mustafa Rawji, * MSc; Yvonne M. Buys, *t MD, FRCSC; Graham E. Trope,*t MB, PhD, FRCSC ABSTRACT • RESUME Background: Oorzolamide hydrochloride is a carbonic anhydrase inhibitor that

reduces intraocular pressure (lOP) by decreasing the production of aqueous humour in the ciliary body. Theoretically, topical use of this agent has the potential to directly affect retinal vasculature through local induced acidosis. We performed a study to determine whether there are changes in retinal arteriole hemodynamics, as assessed with the Canon laser blood flowmeter, in healthy subjects following topical administration of dorzolamide. Methods: We recruited 17 healthy volunteers, nine men and eight women aged 25 to 55 years (mean 31.4 [standard deviation (SO) 9.88] years). The inclusion criteria were Snellen visual acuity of 20/30 or better, normal anterior eye examination, lOP of 21 mm Hg or less, and a normal fundus appearance. One eye of each subject was randomly assigned to receive a drop of 2% dorzolamide. The contralateral eye of I0 of the subjects received a placebo drop (artificial tears). Before and I hour after drop administration, we obtained blood flow measurements from an inferotemporal arteriole approximately I disc diameter from the optic nerve head rim using the Canon laser blood flowmeter, model I00. The lOP was measured by means of Goldmann applanation tonometry before and I hour after drop administration. Results: The mean lOP was significantly reduced in the dorzolamide-treated eyes, from 14.4 mm Hg (SO 2.94 mm Hg) to 11.7 mm Hg (SO 2.50 mm Hg) (p < 0.001). The lOP was also reduced in the placebo group ( 15.6 mm Hg [SO 3.41 mm Hg] vs. 14.6 mm Hg [SO 3.28 mm Hg]), but the difference was not significant. There was no significant difference in mean arteriole diameter, mean blood velocity or mean blood flow after drug administration in the dorzolamide-treated eyes.

From the Departments of Ophthalmology, *Toronto Western Hospital, University Health Network, and tFaculty of Medicine, University of Toronto, Toronto, Ont.

Reprint requests to: Dr. Graham E. Trope, Edith Cavell Wing 7-044, The Toronto Hospital, Western Division, 399 Bathurst St., Toronto ON MST 2S8; fax (416) 603-5333, [email protected]

Originally received July 7, 2003

This article has been peer-reviewed.

Accepted for publication Nov. 17, 2003

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Retinal hemodynamics and dorzolamide-Faingold et al

Interpretation: Our results indicate that a single topical application of dorzolamide in healthy subjects has no effect on retinal arteriole diameter, blood velocity or blood flow, as measured with the Canon laser blood flowmeter. Longer-term studies of retinal hemodynamics in patients with glaucoma are warranted as evolving treatments aim to improve ocular blood flow as well as reduce lOP.

Contexte : Le chlorhydrate de dorzolamide est un inhibiteur de !'anhydrase carbonique, qui reduit Ia pression intraoculaire (PIO) en abaissant Ia production de l'humeur aqueuse dans le corps ciliaire. En principe, !'application topique de cet agent peut affecter directement le systeme vasculaire de Ia retine en induisant une acidose locale. Nous avons cherche a etablir s'il y avait eu modification de l'hemodynamique des arterioles retiniennes, telle qu'evaluee a l'aide du debitmetre sanguin Canon au laser, chez des sujets en sante apres application topique de dorzolamide. Methodes : Nous avons recrute 17 volontaires en sante, neuf hommes et huit femmes de 25 a 55 ans (moyenne de 31,4 ans [ecart type (ET) 9,88 ans]). Les criteres d'inclusion comprenaient une acuite visuelle d'au mains 20/30 a l'echelle de Snellen, un examen oculaire anterieur normal, une PIO de 21 mm Hg ou mains et un fond d'ceil d'apparence normale. Chez chaque patient, un ceil au hasard a rec;u une goutte de dorzolamide a 2 %. L'ceil contralateral de I0 patients a rec;u une goutte de placebo (larmes artificielles).Avant !'instillation et une heure apres, nous avons mesure, avec le debitmetre sanguin Canon au laser, modele I00, le debit sanguin dans une arteriole infero-temporale ayant approximativement le diametre d'une papille a partir du bord de Ia papille optique. La PIO a ete mesuree selon Ia tonometrie par aplanation de Goldmann avant et une heure apres !'instillation. Resultats : La PIO moyenne a ete abaissee de fac;on significative dans les yeux traites a Ia dorzolamide, passant de 14,4 mm Hg (ET 2,94 mm Hg) a 11,7 mm Hg (ET 2,50 mm Hg) (p < 0,00 I). La PIO a aussi ete abaissee dans le groupe placebo (I 5,6 mm Hg [ET 3,41 mm Hg] c. 14,6 mm Hg [ET 3,28 mm Hg]), mais Ia difference n'etait pas significative. II n'y avait pas de difference significative quant au diametre moyen de !'arteriole, a Ia velocite ou au debit sanguin moyen apres !'instillation du medicament dans l'ceil traite a Ia dorzolamide. Interpretation : Nous en concluons que Ia simple application topique de dorzolamide chez les sujets en sante n'affecte pas le diametre des arterioles retiniennes, ni Ia velocite ni debit sanguin, mesures par le debitmetre sanguin Canon au laser. II faudra poursuivre des etudes a long terme de l'hemodynamique de Ia retine chez les patients atteints de glaucome au fur et a mesure de !'evolution des traitements visant a ameliorer le debit sanguin de meme qu'a reduire Ia PIO.

ntraocular pressure (lOP) is considered to be an important risk factor in the pathogenesis of glaucoma.1 However, visual field loss may occur in patients with normal lOP. Other factors, especially perfusion, have also been implicated as potential risk factors for glaucoma. 2•3 Noninvasive hemodynamic studies in patients with glaucoma have suggested an impairment of blood flow in some cases. 4·5 Dorzolamide hydrochloride is a carbonic anhydrase

I

inhibitor for topical use that reduces lOP by decreasing the production of aqueous humour in the ciliary body. Topically applied dorzolamide has been shown to penetrate to the posterior segment of the eye, binding to ocular pigment. 6 Theoretically, topical use of this agent has the potential to directly affect retinal vasculature through local induced acidosis. Harris and colleagues7 used colour Doppler imaging and fluorescein angiography to describe acceleration of the retinal

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arteriovenous transit time after dorzolamide administration in patients with normal-tension glaucoma. They also found acceleration of the superior temporal arteriovenous passage time in patients with primary open-angle glaucoma. 8 Martinez and associates 9 also used colour Doppler techniques after dorzolamide treatment and reported improvement in peak systolic velocity in the central retinal artery in glaucomatous eyes along with improvement in end diastolic velocity in the ophthalmic and central retinal artery of normal and glaucomatous eyes. Bergstrand and coworkers, 10 using a combination of colour Doppler and scanning laser ophthalmoscopy, found no measurable vascular effect with dorzolamide in patients with previously untreated glaucoma. The different vascular responses reported may be due to the measurement techniques used, since the various methods assess different ocular and periocular vascular beds. We performed a study to determine whether there are changes in retinal blood flow, as assessed with the Canon laser blood flowmeter (Canon Inc., Tokyo), in healthy subjects following topical administration of dorzolamide. METHODS

We performed the sample size calculation using the quantitative test procedure with the largest standardized effect size, i.e., retinal arteriole blood flow. The standardized effect size represents the standard deviation (SD) of the test procedure divided by the required difference in means for a significant effect to be detected. Guan and colleagues 11 found a group mean retinal blood flow of 8.8 IlL/min and a group mean coefficient of repeatability of 2.6 !.!L/min in a group of 20 healthy subjects. For a change in group mean retinal blood flow to be significant, it must exceed the baseline value by 30% (2.6/8.8). The group mean arteriolar blood flow before drug administration in the current study was 10.7 (SD 3.69) IlL/min. Therefore, the standardized effect size was 3.69/(10.7 x 30%) == 1.15. With a two-tailed a of 0.05 and a~ of 0.10 (i.e., power of 90% ), the required sample size was 16. We recruited 17 clinically healthy subjects, nine men and eight women aged 25 to 55 years (mean 31.4 [SD 9.88] years). The inclusion criteria were Snellen visual acuity of 20/30 or better, normal anterior eye examination, lOP of 21 mm Hg or less, and a normal fundus appearance (dilation achieved with one drop of 1% tropicamide). Subjects with a high refractive error

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(±8.00 dioptres or greater), a history of eye disease, surgery or injury, a history of cardiovascular disease or sulfa allergy were excluded from the study, as were those who were taking orally administered medications and pregnant subjects. One eye of each subject was randomly assigned to receive a drop of 2% dorzolamide hydrochloride. The contralateral eye of 10 of the subjects received a placebo drop (artificial tears). We measured retinal blood flow before and 1 hour after administration of the drop using the Canon laser blood flowmeter, model 100. This instrument consists of two lasers that simultaneously determine the centreline blood velocity (in millimetres per second) and diameter (in micrometres) of retinal arterioles and venules in a noninvasive manner. It calculates blood flow in microlitres per minute. The first laser, a helium-neon laser (543 nm), measures the arteriolar or venular diameter. Light from the second diode laser (675 nm) is reflected off the blood cells, creating a Doppler effect that is used to measure the velocity of the blood cells. To ensure that the incident laser beam remains centred on the target retinal arteriole or venule, an eye tracking system is used. Focus is achieved by displaying a real-time image on an adjacent video screen. We obtained baseline measurements from an inferotemporal arteriole approximately 1 disc diameter from the edge of the optic nerve. Six sequential measurements were performed. An image of the retinal measurement site was captured for future reference. We measured lOP before and 1 hour after administration of the drop using Goldmann applanation tonometry. Axial length was measured by means of A-scan ultrasonography. We compared vessel diameter, blood velocity and blood flow measurements after drug or placebo administration to baseline values using paired, twotailed t tests. In addition, the data for the 10 subjects who received placebo in the contralateral eye were analysed for between-eye differences. The procedures were reviewed and approved by the Research Ethics Board of the University Health Network, Toronto, and all subjects signed informed consent before participating in the study. All procedures conformed to the tenets of the Declaration of Helsinki. RESULTS

The group mean lOP was reduced after treatment in the dorzolamide-treated eyes, from 14.4 mm Hg (SD

Retinal hemodynamics and dorzolamide-Faingold et al

Table 1-Mean retinal arteriole diameter, blood velocity and blood flow before and after administration of one drop of 2% dorzolamide hydrochloride or placebo Group; mean (and standard deviation) Dorzolamide (n

Placebo (n

lh

p value

Baseline

lh

p value

112.7 (13.30)

110.6 (12.86)

0.14

I03.4 (I 0.25)

I06.5 ( 16.14)

0.36

34.6 (8.1 0)

36.6 (8.21)

0.27

34.1 ( 13.13)

32.4 ( 12.11)

0.50

10.7 (3.69)

10.8 (3.24)

0.85

8.9 (4.62)

8.8 (3.87)

0.85

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Fig. 2-Change in mean retinal arteriole blood velocity versus mean blood velocity before and after administration of dorzolamide (+) or placebo (•).

2.94 rom Hg) to 11.7 rom Hg (SD 2.50 rom Hg) (p < 0.001). In the eyes that received placebo the group mean lOP was 15.6 rom Hg (SD 3.41 rom Hg)

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at baseline and 14.6 rom Hg (SD 3.28 rom Hg) after administration, a nonsignificant difference. There were no significant between-eye differences in lOP at baseline. Following drop administration, there was a significant difference in lOP between the dorzolamidetreated eyes and the placebo-treated eyes (p = 0.02) . There was no significant change from baseline in any of the three hemodynamic measures examined in the dorzolamide-treated eyes or the eyes that received placebo (Table 1, Figs. 1, 2 and 3). There were no significant between-eye differences in the hemodynamic measures at baseline or following drop administration. INTERPRETATION

The Canon laser blood flowmeter has the potential to reliably measure retinal blood flow in a single major retinal arteriole or venule (i.e., diameter of 100 Jliil). Eye movements, tear film breakup and eye

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Retinal hemodynamics and dorzolamide-Faingold et al tracking errors may affect the measurement. Canon laser blood flowmeter measurements were validated in our laboratory in a previous study. 11 We did not find any significant change in retinal hemodynamics after administration of dorzolamide in healthy subjects. These findings are in concordance with previous studies evaluating the effect of dorzolamide on blood flow in healthy subjects. Grunwald and associates, 12 using laser Doppler velocimetry, found that a single drop of dorzolamide did not cause a significant change in retinal blood flow in healthy subjects. Pillunat and coworkers 13 reported that capillary optic nerve blood flow, as measured with scanning laser Doppler flowmetry, did not change after short-term dorzolamide treatment in healthy subjects. Our study is unique since the Canon laser blood flowmeter quantifies blood flow rather than a single measure, such as blood velocity, whose effect on blood flow may depend on changes in vessel diameter. In our study, mean lOP was reduced by 19% in the dorzolamide-treated eyes. The group mean arteriole diameter diminished, and the group mean arteriole velocity increased slightly; however, neither change was statistically significant. Retinal blood flow was unchanged. These results are likely due to autoregulation of retinal circulation in healthy subjects, with constant local blood flow being maintained in the presence of increased ocular perfusion pressure associated with reduction in lOP. We studied retinal hemodynamics after a single drop of dorzolamide. A longer period of treatment may lead to increased posterior segment drug penetration. In a glaucoma population, where the autoregulation capacity of the retinal vasculature is thought to be impaired, 14•15 changes in retinal blood flow measures may reflect changes in ocular perfusion pressure. Longer-term studies of retinal hemodynamics in patients with glaucoma are warranted as evolving treatments aim to improve ocular blood flow as well as reduce lOP. REFERENCES

1. Kass MA, Heuer DK, Higginbotham EJ, Johnson CA, Keltner JL, Miller JP, et al, for the Ocular Hypertension Treatment Study Group. The Ocular Hypertension

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Treatment Study. Arch Ophthalmol2002;120:701-13. 2. Drance SM. Glaucoma: a look beyond intraocular pressure. Am J Ophthalmol1997;123(6):817-9. 3. Broadway DC, Drance SM. Glaucoma and vasospasm. Br J Ophthalmol1998;82:862-70. 4. Piltz-Seymour JR, Grunwald JE, Hariprasad SM, Dupont J. Optic nerve blood flow is diminished in eyes of primary open-angle glaucoma suspects. Am J Ophthalmol 2001; 132(1):63-9. 5. Nicolela MT, Hnik P, Drance SM. Scanning laser Doppler flowmeter study of retinal and optic disk blood flow in glaucomatous patients. Am J Ophthalmol 1996;122: 775-83. 6. Sugrue MF. Pharmacological and ocular hypotensive properties of topical carbonic anhydrase inhibitors. Prog Retin Eye Res 2000;19(1):87-112. 7. Harris A, Arend 0, Kagemann L, Garrett M, Chung HS, Martin B. Dorzolarnide, visual function and ocular hemodynamics in normal tension glaucoma. J Ocul Pharmacal Ther 1999;15:189-97. 8. Harris A, Jonescu-Cuypers CP, Kagemann L, Nowacki EA, Garzozi H, Cole C, et al. Effect of dorzolamide timolol combination versus timolol 0.5% on ocular blood flow in patients with primary open-angle glaucoma. Am J Ophthalmol2001;132:490-5. 9. Martinez A, Gonzalez F, Capeans C, Perez R, Salorio SM. Dorzolarnide effect on ocular blood flow. Invest Ophthalmol Vis Sci 1999;40:1270-5. 10. Bergstrand IC, Heijl A, Harris A. Dorzolarnide and ocular blood flow in previously untreated glaucoma patients: a controlled double masked study. Acta Ophthalmol Scand 2002;80: 176-82. 11. Guan K, Hudson C, Flanagan JG. Variability and repeatability of retinal blood flow measurements using the Canon Laser Blood Flowmeter. Microvasc Res 2003;65(3): 145-51. 12. Grunwald JE, Mathur S, DuPont J. Effects of dorzolamide hydrochloride on the retinal circulation. Acta Ophthalmol Scand 1997;75(3):236-8. 13. Pillunat LE, BohmAG, Koller AU, Schmidt KG, Klemm M, Richard G. Effect of topical dorzolarnide on optic nerve head blood flow. Graefes Arch Clin Exp Ophthalmoll999;237:495-500. 14. Grunwald JE, Riva CE, Stone RA, Keates EU, Petrig BL. Retinal autoregulation in open angle glaucoma. Ophthalmology 1984;91(12):1690-4. 15. Flammer J, Haefliger IO, Orgiil S, Resink T. Vascular dysregulation: A principal risk factor for glaucomatous damage? J Glaucoma 1999;8:212-9. Key words: blood flow, intraocular pressure, dorzolamide hydrochloride