Influence of topical dorzolamide on retinal branch vessel diameter

Edgar Nagel ¹, Walthard Vilser ², Gabriele Fuhrmann², Ines M. Lanzl³, Gabriele E. Lang ⁴

¹ Augenarztpraxis Rudolstadt, Anton-Sommer-Str. 55, D-07407 Rudolstadt,

phone: ++49 3672 411471

² Technische Universität Ilmenau, G.- Kirchhoff-Str. 1, D-98684 Ilmenau,

phone: ++49 3677 691329

³ Augenklinik und Poliklinik der Technischen Universität München, Ismaninger Str.22,

D-81675 München, phone: 0049 731 50201

⁴ Augenklinik der Universität Ulm, Oberer Eselsberg, D-89081 Ulm,

phone: 0049 731 50201

Commercial relations: none

Corresponding author:

Dr. med. E. Nagel, Augenarztpraxis Rudolstadt, Anton-Sommer-Str. 55, D-07407 Rudolstadt

Germany

Abstract:

Background:

The topical carboanhydrase inhibitor dorzolamide lowers intraocular pressure (IOP). Retinal branch vessel diameter plays an important role in regulation of retinal microcirculation. The reaction of retinal branch vessels after application of dorzolamide is investigated in this study.

Methods:

Arterial and venous retinal vessel diameter were measured by the Retinal-Vessel-Analyzer (RVA) in 12 primary open angle glaucoma (POAG) and normal tension glaucoma (NTG) patients. Retinal vessel diameters were evaluated before applying dorzolamide for the first time, after 4 weeks of therapy 2 and 4 hours after administration of the drug. IOP and systemic blood pressure were recorded.

Results:

There was no significant change in arterial or venous diameter in either the POAG or NTG group after either first time use orafter four weeks of therapy.

Discussion:

Dorzolamide does not influence the diameter of retinal branch vessels as measured by RVA in glaucoma patients. In contrast to normal young subjects who demonstrated arterial dilation after single use of dorzolamide in previous studies older glaucoma patients did not show such an effect in this study. This might be due to impaired regulatory mechanisms encountered in glaucoma patients as well as in the older population, who may also suffer simulataneously from cardiovascular disease. Other studies found a change in retinal microcirculation after dorzolamide use. This could represent an effect within the retinal capillary bed which was not measured in our study. Further research is warranted to define the clinical reaction of retinal vessels to provocative stimuli as well as the effect of pharmaceutical drugs on that reaction.

Introduction:

Diagnosis and therapy of glaucoma was primarily focused on IOP and its lowering until recently. Newer epidemiological studies could prove, that up to 60% of patients presenting with a glaucomatous field defect did have no or only slightly elevated IOP [1]. Vascular and genetic factors are suspected to influence the disease as well as IOP. Thus the role of ocular perfusion for glaucoma and its progression is gaining increased clinical interest.

Because of these new aspects of the pathophysiology of glaucoma, quite a few efforts were undertaken to assess the effect of topical antiglaucomatous drugs on the intraocular vascular situation. A possible scenario could be a successful IOP lowering with no improvement or even worsening of the ocular perfusion due to vasoconstriction and systemic or ocular blood pressure lowering.

For the last 3 years dorzolamide has been available commercially [2]. The drug inhibits carboanhydrase, a key enzyme for aqueous production in the ciliary body. Animal studies demonstrated a good penetration of the substance into the anterior chamber, ciliary body and retina [3]. An effect of the enzymatic blockade of the carboanhydrase inhibitor (CAI) consists in increasing CO2 concentration and subsequent pH lowering. Thus it could be possible, that a topical CAI does not only lower IOP by means of decreasing aqueous production but additionally leads to a vasodilation of retinal and optic nerve head vessels, caused by an increase of the local CO2 concentration or rather a local drop in pH.

The retinal vessel analysis with the RVA allows the assessment of the diameter of retinal branch vessels with an unsurpassed reproducibility (1-3%) [4]. Our study investigates whether topical administration of dorzolamide induces changes in retinal vessel diameter.

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Patients and methods:

1. Glaucoma patients:

The effect of Trusopt was evaluated in 12 volunteer glaucoma patients. They were studied before and after their first single application and after continuous t.i.d. use of the drug for 4 weeks. 9 female and 3 male patients with a mean age of 62 years (ranging from 47 to 73 years) were examined.

Inclusion criteria were: primary open angle glaucoma or low tension glaucoma requiring medical treatment in at least one eye; indication for monotherapy with dorzolamide eye drops either as first choice drug or as a switch over from another topical medication after completion of a 4 weeks wash-out phase.

Exclusion criteria were: other ocular disease (except for refraction anomalies); visual acuity less than 20/40; astigmatism higher than 1.0 dpt; contact lens wear within 24 hours of examination;

topical medication with possible vasoactive effect ( e.g. vasoconstricitve agents); acute infectious disease; pregnancy or lactation period.

The same randomly assigned eye was tested in each person during the whole course of the study.

2. Test protocol:

Every patient underwent extensive ophthalmologic examination (history, visual acuity, objective refractometry, computerized static perimetry, Octopus 101, program G2, slit lamp examination, binocular funduscopy, Goldmann applanation tonometry).

Pupil dilation was achieved using tropicamide 0.5% (Mydrum). A baseline measurement of the vessel diameter was performed with the RVA for 5 minutes. Immediately afterwards the IOP was measured and Trusopt applied topically to the study eye. 120 and 240 minutes later a repeat vessel diameter assessment with five minutes study duration and an IOP measurement were performed.

During RVA recordings ECG was monitored and every minute blood pressure was taken by the Riva Rocci method.

The examination was repeated after 4 weeks of continuous t.i.d. topical use of dorzolamide. No perimetry, visual acuity and refractometry were performed on the repeat exams. For each individual however the same time of the day was chosen for the repeat examination, to rule out possible influences due to the diurnal rhythm. All examinations were performed by the same observer in order to eliminate possible interobserver bias.

3. Data collection:

During the examination at least 1 retinal branch vessel segment was measured on-line and the mean vessel diameter stored digitally with its time and place dependent characteristics. At the same time a fundus image was stored on a video tape. This tape served as the basis for later off-line measurements of other vessel segments. In all cases the on-line measurement was performed on the branch vein and the off- line measurement on the corresponding arterial segment. Due to individual vessel architecture the measurement area was located between 1 to 3 disc diameters from the disc margins. The measured segment ranged from 1.0 to 1.5 mm in length. The RVA stores information on measurement location. This enables later measurements in the same location.

The mean vessel diameter of the examined vessel segment during the 5 min duration of the exam was analyzed. Since there is a large difference in vessel diameters depending on the vessel architecture of each individual we used the vessel diameter change in percent from the baseline as our measurement. For all obtained data the confidence interval is presented.

4. Statistical analysis:

The two tailed paired t-test was used to determine significant differences between sessions.

5. Ethics and consent:

The study design has been reviewed and approved by the ethics committee of the Landesärztekammer Thüringen and has therefore been performed in accordance with the ethical standards laid down in the 1964 Declaration of Helsinki. All study subjects gave their informed consent prior to their inclusion in the study.

Results:

1. IOP

Dorzolamide significantly lowered IOP in the observed glaucoma patients. After 4 weeks of continued t.i.d. use of dorzolamide a lower IOP compared to baseline was found before the first application of the drop for the study day. This level was further lowered 2 and 4 hours after application of the drop within the study. (see tab.1 and fig.1).

2. Mean vessel diameter:

No significant change in either arterial or venous retinal vessel diameter was found.(see fig.2).

3. Systemic blood pressure and ocular perfusion pressure

Systemic blood pressure showed a slight decrease. This decrease was significant in glaucoma patients on the first examination day. One case was excluded. This glaucoma patient developed a hypertensive crisis with an increase of systolic blood pressure by 45mm Hg to 245mmHg at the 4 week interval. After exclusion of this case no significant change was observed within the patient group.

The retinal perfusion pressure showed a tendency to increase after application of dorzolamide in both measurement set-ups within the glaucoma patient group. This increase was not statistically significant in either group.

Discussion:

Dorzolamide significantly lowers IOP. The amount of IOP lowering is dependent on the initial IOP level. It is remarkable that after continuous 4 week t.i.d. application of the drug a persisting IOP lowering effect could be observed overnight and after renewed application of the drop in the morning a further lowering effect could be found. This further effect brought the IOP to an even lower level than the one achieved with the first single drop application. We assume that this is due to an accumulation of the drug in ocular tissues.

The mean vessel diameters in the group of glaucoma patients did not change significantly either after the first dose of dorzolamide or after 4 weeks of continuous t.i.d. therapy. Retinal branch vessels reactions to dorzolamide application have so far been only investigated by scanning-laser fluorescein angiography [6]. This study did not find a change in vessel diameter after application of the drug in normal tension glaucoma patients. However a significant decrease in arterio-venous passage time (mean of 18%) and an increase in capillary blood velocity in the retinal and epipapillary capillaries by 16% and 15% respectively could be observed. No significant change in systemic blood pressure was measured in that study. Therefore the lowering of IOP by dorzolamide of 22% induced a considerable increase in retinal perfusion pressure. Most likely the decrease in retinal perfusion time and the increase in capillary velocity is due to that increase in perfusion pressure in retinal vessels with unchanged diameters. Systemic blood pressure changes can induce a direct effect on mean retinal arterial perfusion pressure. To date the reactions of systemic blood pressure and their consequences on ocular perfusion pressures probably are not receiving the deserved clinical attention. Phases of systemic hypotension during sleep in the early morning and a possible simultaneous increase in IOP can lead to repeat slight perfusion deficits and the subsequent known glaucomatous defects. A clinical study of changes in ocular hemodynamics in POAG patients could prove these interactions [7].

The measurement of capillary blood flow velocity after application of dorzolamide using the laser Doppler flowmeter showed a significant increase in study animals of 8.4% [10]. The same set-up in normal volunteers could only find a not statistically significant tendency for increased blood flow velocity in the capillaries of a magnitude of 1.9% [11]. An increase in capillary blood flow velocity necessitates dilation of the capillaries. This dilation of the capillaries could be explained by the inhibition of carboanhydrase found in the capillary walls [12] due to inhibition by dorzolamide and subsequent increase in tissue pCO2 and tissue pH. Regulation in retinal branch vessels diameter seems to be less dependent on CO2 concentration. This hypothesis is based on

studies of retinal vessel diameters after increase of blood CO2 by increasing the endexpiratory CO2 in the inhaled air [13]. Earlier RVA measurements such a set-up demonstrated a brief vasodilation followed by vasoconstriction.

A preliminary part of the study presented here consisted in examining the effect of dorzolamide on retinal branch vessel diameter in 12 normal volunteers. 8 of these volunteers were female and 4 male with a mean age of 32 years (ranging from 21 to 69 years). Exclusion criteria were known ocular diseases and diseases of the cardiovascular system (e.g. elevated systemic blood pressure, diabetes, Raynaud-syndrome); diseases of the respiratory system; acute infectious diseases; use of systemic medication with possible influence on perfusion, such as antihypertensives or blood thinning agents; topical medication with possible vasoactive effect (e.g. vasoconstricitve agents); pregnancy or lactation period.

The 12 volunters were studied in a double-blind cross-over fashion. On the first day either dorzolamide or placebo was randomly assigned and administered topically Vessel diameter was assessed before, immediately after and 4 hours after administration of the eye drop. After a wash out phase of at least 2 weeks the other medication was tested. During the whole study the same randomly chosen eye of each test person was examined with both substances. The placebo consisted of an eye drop with the same chemical specifics (pH, viscosity, preservative) as commercially available dorzolamide (Trusoptâ) The ingredients were provided by the manufacturer, Chibret Pharmazeutische GmbH, Lindenplatz 1, 85540 Haar, Germany.

The group of normal volunteers showed a significant arterial dilation (3.1mm, p<0.05) 4 hours after application of dorzolamide. The application of placebo did not cause a significant change in vessel diameter. This result prompted us to examine the glaucoma patients presented here.

When interpreting the individual reaction of our 12 normal volunteers however, it becomes obvious that the increase in vessel diameter is due to the reaction of 3 individuals (see fig.3, cases n41, n20, n46). The normal subjects demonstrating the most pronounced arterial dilation were the ones that also showed a significant drop in systemic blood pressure. We therefore suppose that the small arterial vessel dilation which was found in that group might represent an effect of systemic blood pressure rather than a drug induced one.

We acknowledge the fact that vessel reactions are the result of a multitude of influencing factors with most of them not taken into account (hormonal situation, activation of the sympathetic nerve system, blood glucose level, diurnal changes etc.) The interpretation of a specific vessel reaction to a specific stimulus (e.g. perfusion pressure changes) warrants extensive further research.

Dorzolamide did not show a change in mean retinal branch vessel diameter as measured by RVA in the presented glaucoma patients. The threshold for the method to detect changes is 2% for the mean within a group and 5% in an individual. The changes in vessel diameter in different locations along the same vessel and during the time course warrant further basic research before all aspects of the influence of drugs on these parameters can be determined.

In the disease model of glaucoma ocular hemodynamics seem to display an inadequate reaction to decreased perfusion pressure. Further investigations of the reaction of retinal branch vessels in normals and glaucoma patients should create a better insight into this entity. One such model could consist in examining the reactive dilation after artificial short term IOP increase. The increased insight into ocular regulatory reactions could lead to completely new therapeutic strategies for glaucoma patients.

The exact connections between mean vessel diameter, spatial and temporal changes of vessel diameters, systemic blood pressure and IOP as well as changes in those parameters are not yet completely understood and applicable. This is not only true for the individual case in a clinical setting but also for studies within the research community. It would be advisable to create and evaluate a model of interconnection of factors of the physiological perfusion status of ocular microcirculation. This model should be able to integrate findings resulting from different measurement methods.

References:

[1] Leibowitz HM (1980) The Framingham Eye Study Monograph. An ophthalmological and epidemiological study of cataract, glaucoma,diabetic retinopathy, macular degeneration and visual acuity in a general population of 2631 adults, 1973 – 1975. Survey of Ophthalmology 24: Suppl. 335-810

[2] Chibret (1996) Fachinformation Trusopt^â

[3] Sugrue MF (1996) The preclinical pharmacology of Dorzolamide hydrochloride, a topical carbonic anhydrase inhibitor. Journal of Ocular Pharmacology and Therapeutics 12: 363-376

[4] ] Vilser W (1997) Retinal Vessel Analyzer. Jahresband der 16. Jhtg. der Dt. Ges. f. Klinische Mikrozirkulation und Rheologie v. 30.-31. Okt. 1997 in Dresden (in press)

[5] Ulrich Ch (1980) Klinik und Praxis der Ophthalmodynamometrie, Ophthalmodynamographie, Temporalisdynamographie. Abhandlungen aus dem Gebiete der Augenheilkunde , Bd. 46, VEB Georg Thieme Verlag Leipzig

[6] Arend O, Harris A, Kagemann C, Martin BJ (1998) Circulatory effects on retinal vasculature of betaxolol versus dorzolamide in patients with normal tension glaucoma. IOVS 39: 1000

[7] Kanzow U, Pillunat LE, Böhm AG (1998) Änderung der Hämodynamik im Tagesverlauf beim primär chronischen Offenwinkelglaukom. Ophthalmologe 95: Suppl. 1 V41

[8] Barnes GE, Byron L, Dean T, Chandler ML (1998) Improvement of optic nerve head blood flow after one week b.i.d. topical CAI treatment with brinzolamid in the rabbit. IOVS 39: 262

[9] Mavrodis L, O´Brart DSP, Kohner EM (1998) Topical antiglaucoma medications. Do they have a direct vasoactive effect on the optic nerve head hemodynamics. IOVS 39: 898

[10] Terashima H, Suzuki K, Kato K, Sugai N (1996) Membrane-bound carbonic anhydrase activity in the rat corneal endothelium and retina. Jpn J Ophthalmol 40: 142-153

[11] Lang GE, Harris A, Vilser W, Tobis M, Kagemann I, Riemer Th, Lang GK (1998) A new method analyzing retinal vessel diameters sensitive to small PCO2 changes. IOVS 39: 268

[12] Spraul CW, Lang GE, Münch K, Bräuer-Burchardt Ch, Vilser W (1997) Local dependency of the reactivity of retinal branch vessels on oxygen provocation. IOVS 38: 780

Tables and legends:

Table 1: Intraocular pressure in mmHg (mean of each group, standard deviation, number n of subjects), significance level compared to baseline of each study day’s measurements

group	before application of drug	after 2h	after 4h	after 4 weeks. before application	after 2h	after 4h
normals placebo n = 12	11,8 (± 2,2)	10,6 (± 2,6) n.s.	10,8 (± 2,0) n.s.
normals dorzolamide n = 13	11,9 (± 1,8)	8,9 (± 2,1) p < 0,05	9,3 (± 1,9) p < 0,05
glaucoma patients n = 12	21,7 (± 4,4)	15,5 (± 4,1) p < 0,05	14,6 (± 2,9) p < 0,05	17,8 (± 2,7)	15,5 (± 2,4) p < 0,05	15,8 (± 3,3) p < 0,05

Table 2: Arterial vessel diameter in mm (mean of each group, standard deviation, number n of subjects), significance level compared to baseline of each study day’s measurements

group	before application of drug	after 2h	after 4h	after 4 weeks. before application	after 2h	after 4h
normals placebo n = 12	117,1 (± 18,4) n=11	118,1 (± 18,9) n=12 n.s.	119,6 (± 19,2) n=12 n.s.
normals dorzolamide n = 13	115,4 (± 19,5) n=13	116,3 (± 18,5) n=12 n. s.	118,5 (± 18,8) n = 10 p < 0,05
glaucoma patients n = 12	128,2 (± 15,6) n=12	128,1 (± 14,9) n=12 n. s.	126,8 (± 13,5) n=12 n. s.	129,8 (± 14,0) n=12	130,2 (± 13,7) n=12 n. s.	129,5 (± 12,4) n=12 n. s.

Table 3: Venous vessel diameter in mm (mean of each group, standard deviation, number n of subjects), significance level compared to baseline of each study day’s measurements

group	before application of drug	after 2h	after 4h	after 4 weeks. before application	after 2h	after 4h
normals placebo n = 12	161,2 (± 21,1) n=12	162,0 (± 21,8) n=12 n.s.	162,0 (± 21,8) n=12 n.s.
normals dorzolamide n = 13	159,4 (± 22,6) n=13	161,3 (± 21,4) n=13 n. s.	159,1 (± 19,8) n=13 n. s.
glaucoma patients n = 12	155,1 (± 13,9) n=12	157,4 (± 14,7) n=12 n. s.	158,6 (± 13,5) n=12 n. s.	156,5 (± 14,3) n=12	157,5 (± 15,9) n=12 n. s.	157,8 (± 13,7) n=12 n. s.