Effects of acetazolamide on choroidal blood flow

Carbonic anhydrase, its inhibitors and vascular function

It has been known for some time that Carbonic Anhydrase (CA, EC 4.2.1.1) plays a complex role in vascular function, and in the regulation of vascular tone. Clinically employed CA inhibitors (CAIs) are used primarily to lower intraocular pressure in glaucoma, and also to affect retinal blood flow and oxygen saturation. CAIs have been shown to dilate vessels and increase blood flow in both the cerebral and ocular vasculature. Similar effects of CAIs on vascular function have been observed in the liver, brain and kidney, while vessels in abdominal muscle and the stomach are unaffected. Most of the studies on the vascular effects of CAIs have been focused on the cerebral and ocular vasculatures, and in particular the retinal vasculature, where vasodilation of its vessels, after intravenous infusion of sulfonamide-based CAIs can be easily observed and measured from the fundus of the eye. The mechanism by which CAIs exert their effects on the vasculature is still unclear, but the classic sulfonamide-based inhibitors have been found to directly dilate isolated vessel segments when applied to the extracellular fluid. Modification of the structure of CAI compounds affects their efficacy and potency as vasodilators. CAIs of the coumarin type, which generally are less effective in inhibiting the catalytically dominant isoform hCA II and unable to accept NO, have comparable vasodilatory effects as the primary sulfonamides on pre-contracted retinal arteriolar vessel segments, providing insights into which CA isoforms are involved. Alterations of the lipophilicity of CAI compounds affect their potency as vasodilators, and CAIs that are membrane impermeant do not act as vasodilators of isolated vessel segments. Experiments with CAIs, that shed light on the role of CA in the regulation of vascular tone of vessels, will be discussed in this review. The role of CA in vascular function will be discussed, with specific emphasis on findings with the effects of CA inhibitors (CAI).

The effect of acetazolamide on the retinal and choroidal vasculature of the macula and the optic disc using OCT angiography

INTRODUCTION AND OBJECTIVES

To investigate the effect of acetazolamide (AZ) on the retinal and choroidal ocular microvasculature in the macula and radial peripapillary capillaries (RPC) of the optic disc with OCT Angiography (OCTA).

MATERIALS AND METHODS

Nine-month observational cross-sectional study. Forty-five eyes from 45 healthy participants who underwent cataract surgery were recruited. Macular retina and choriocapillaris vessel density (VD) and RPC VD in the optic disc area were compared before and 60min after 250mg acetazolamide per os. Intraocular pressure (IOP) and systemic blood pressure (BP) were also measured before each scan.

RESULTS

Mean age was 73.1±6.9 years. VDs in the superficial (SCP) and deep (DCP) capillary plexus of the retina and the choriocapillaris (CC) in the macular area showed no significant change (P>.5, for all parameters). VD in the RPC showed no significant change with AZ (P>.5, for all parameters). Foveal and parafoveal thickness increased from 248.98 (±23.89) to 250.33 (±23.74) and from 311.62 (±16.53) to 311.98 (±16.38) (P<.001 and P=.046), respectively. IOP decreased from 13.2 (±3.0) mmHg to 11.8 (±3.2) mmHg (P<.001), while systolic and diastolic BP decreased from 144.8 (±21.8) to 137.7 (±19.0) and from 80.0 (±12.7) to 76.2 (±11.7) (P=.021 and P=.030), respectively.

CONCLUSIONS

OCTA imaging did not reveal any significant changes in the VD of the optic disc or the retinal and choroidal VD in the macula with oral AZ one hour after its administration in otherwise healthy participants who underwent cataract surgery.

Topical carbonic anhydrase inhibitors and glaucoma in 2021: where do we stand?

Carbonic anhydrase inhibitors (CAIs) have been used for many decades in the treatment of glaucoma. Systemic CAIs were an early treatment option to lower intraocular pressure by reducing aqueous humour production; however, frequent side effects including polyuria and paresthesia contributed to the eventual development of topical CAIs. As topical drug development evolved over time, prostaglandin analogues and beta-blockers have become the gold standard of glaucoma therapies. Although prescribed less often than other classes of topical glaucoma therapies, topical CAIs continue to be used in combination therapies with beta-blockers and alpha agonists. Topical CAIs have also been demonstrated to alter biomarkers of ocular haemodynamics, which have relevance in glaucoma. The purpose of this review is to review and summarise the current state of topical CAI prescribing trends, known efficacy and suggested mechanisms and potential influence on ocular haemodynamics for the future of glaucoma management.

Dorzolamide/Timolol Fixed Combination: Learning from the Past and Looking Toward the Future

The key clinical attributes of preserved dorzolamide/timolol fixed combination (DTFC) and the emerging potential of preservative-free (PF) DTFC are reviewed with published evidence and clinical experience. The indications and role of DTFC in current glaucoma management are critically discussed. Preserved DTFC became the first intraocular pressure (IOP)-lowering fixed combination (FC) approved by the US Food and Drug Administration (FDA) and remains one of most commonly used medications worldwide. The pharmacological properties of DTFC reflect those of its two time-tested constituents, i.e., the carbonic anhydrase inhibitor dorzolamide and the non-selective beta-blocker timolol. In regulatory studies DTFC lowers IOP on average by 9 mmHg (32.7%) at peak and by 7.7 mmHg (27%) at trough. In trials DTFC shows equivalence to unfixed concomitant therapy, but in real-life practice it may prove superior owing to enhanced convenience, elimination of the washout effect from the second drop, improved tolerability, and better adherence. PF DTFC became the first PF FC approved, first in unit-dose pipettes, and more recently in a multidose format. Cumulative evidence has confirmed that PF DTFC is at least equivalent in efficacy to preserved DTFC and provides a tangible clinical benefit to patients with glaucoma suffering from ocular surface disease by improving tolerability and adherence. Finally, we identify areas that warrant further investigation with preserved and PF DTFC

Evaluation of the effects on choroidal thickness of bimatoprost 0.03% versus a brinzolamide 1.0%/timolol maleate 0.5% fixed combination

OBJECTIVE

To investigate the effects of two different medical treatment options on choroidal thickness (CT) in cases of open-angle glaucoma (OAG).

METHODS

Sixty-seven eyes newly diagnosed with OAG and 52 healthy eyes constituting the control group were included in the study. Glaucomatous eyes were randomly divided into two subgroups; Group I was started on bimatoprost 0.03% and Group II on a brinzolamide 1.0%/timolol maleate 0.5% fixed combination (BTFC). Intraocular pressure (IOP), ocular pulse amplitude (OPA) and subfoveal CT measurements were performed in all eyes in the study before treatment and on weeks 2, 4 and 8 after treatment.

RESULTS

Mean initial IOP values in groups I and II and the control group were 25.5 ± 4.7, 25.1 ± 5.2 and 16.1 ± 2.9 mmHg, mean OPA values were 3.7 ± 1, 3.6 ± 1.4 and 2.4 ± 0.6 mmHg and mean CT values were 269.4 ± 83, 264.5 ± 84.4 and 320.1 ± 56.6 μm, respectively. Eight weeks after treatment, mean IOP values in Groups I and II and the control group were 18.3 ± 2.6, 18.1 ± 3.4 and 15.7 ± 2.9 mmHg, mean OPA values were 2.9 ± 1.2, 2.8 ± 1.5 and 2.3 ± 0.8 mmHg and mean CT values were 290.2 ± 87.3, 271.8 ± 82.5 and 319.3 ± 56.8 μm, respectively. No significant difference was determined in terms of the decrease in IOP and OPA obtained after treatment in Group I and Group II. However, a significant difference was observed between the two groups in terms of choroidal thickening after treatment.

CONCLUSION

The use of topical ocular hypotensive medication in eyes with OAG results in an increase in CT. This increase is relatively greater with bimatoprost 0.03% therapy compared to BTFC.

Evaluation of Choroidal Thickness in Patients with Pseudoexfoliation Syndrome and Pseudoexfoliation Glaucoma

Purpose. To compare the macular and peripapillary choroidal thickness in eyes with pseudoexfoliation (PEX) syndrome and PEX glaucoma with the normal eyes of healthy controls. Materials and Methods. In this prospective study, 30 eyes of 30 patients with PEX syndrome, 28 eyes of 28 patients with PEX glaucoma, and 30 eyes of 30 age-matched healthy subjects were enrolled. Choroidal thicknesses in the macular and peripapillary areas were measured by using spectral domain optical coherence tomography. Results. Gender, age, and axial length did not significantly differ between the groups (all, p > 0.05). The mean values of choroidal thickness in the macular and peripapillary areas (except the superior quadrant) in the patients with PEX syndrome and PEX glaucoma were lower compared with controls (all p < 0.05). The mean values of the macular and peripapillary choroidal thickness in the PEX glaucoma group were lower compared with PEX syndrome group; however this difference was not significant. Conclusions. The findings of this study revealed that macular and peripapillary choroidal thicknesses were decreased in PEX syndrome and PEX glaucoma cases. The role of choroid in the development of glaucomatous damage in patients with PEX syndrome remains unclear.

Evaluation of retinal nerve fiber layer thickness and choroidal thickness in pseudoexfoliative glaucoma and pseudoexfoliative syndrome

OBJECTIVES

To compare retinal nerve fiber layer thickness (RNFLT) and choroidal thickness (ChT) measurements in eyes with pseudoexfoliative (PEX) glaucoma, PEX syndrome and healthy control eyes.

METHODS

Eighteen patients with PEX glaucoma in one eye and PEX syndrome in the fellow eye were included. The right eyes of thirty-nine age- and sex-matched healthy subjects were included as control group. All participants underwent a detailed biomicroscopic and funduscopic examination. RNFLT and ChT measurements were performed with a commercially available spectral-domain optical coherence tomography (SD-OCT). ChT measurements were performed by using enhanced depth imaging (EDI) mode. Patients with PEX underwent diurnal IOP measurements with 4-hour intervals before inclusion in the study. RNFLT results included the average measurement and 6 quadrants (temporal, inferotemporal, inferonasal, nasal, superonasal and supero-temporal). ChT measurements were performed in the subfoveal region and around the fovea (500µm and 1500 µm nasal and temporal to the fovea), as well as around the optic disc (average peripapillary and eight quadrants in the peripapillary region (temporal, inferotemporal, inferior, inferonasal, nasal, superonasal, superior, supero-temporal)).

RESULTS

RNFLT in all quadrants and average thickness were significantly lower in PEX glaucoma eyes compared to PEX syndrome eyes and healthy control eyes (p<0.001 for both). RNFLT comparisons between PEX syndrome and healthy control eyes did not show a significant difference (p>0.05) except the inferotemporal quadrant. ChT measurements were similar between groups (p>0.05).

CONCLUSIONS

Thinning of the RNFL in association with unchanged ChT may mean that the presence of PEX material is a much more significant risk factor than choroidal changes in the progression of PEX syndrome to PEX glaucoma.

Enhanced depth imaging-optical coherence tomography of the choroid in moderate and severe primary angle-closure glaucoma

PURPOSE

To compare the choroidal thickness (CT) in eyes with moderate and severe primary angle-closure glaucoma (PACG) with the eyes of healthy controls.

METHODS

Fifty-nine patients (59 eyes) with PACG and 56 age-matched normal subjects underwent macular CT scanning using enhanced depth imaging-optical coherence tomography. The subjects with PACG were further classified as having moderate [-12 dB ≤ visual field mean deviation (VF-MD) ≤ -6 dB] or severe (VF-MD < -12 dB) glaucoma. The average CT of the PACG eyes at each location was compared to that of normal eyes.

RESULTS

The CT was significantly increased in moderate and severe PACG eyes compared with the normal eyes at all nine macular locations, with the exception of 1 and 3 mm superior to the fovea. The mean CT under the fovea was 292.0 ± 48.2 μm in the moderate PACG eyes, 277.1 ± 58.3 μm in the severe PACG eyes and 249.9 ± 72.1 μm in the normal eyes, respectively. There were no significant differences between the macular CTs in the eyes with moderate PACG and those in severe PACG eyes (all p > 0.05). Factors associated with a thinner choroid were ageing (p < 0.001), female (p = 0.036) and increasing axial length (p = 0.021). Neither the VF-MD nor pattern standard deviation was found to be associated with CT.

CONCLUSIONS

The CT is greater in patients with PACG, but it does not differ between moderate and severe PACG, suggesting a lack of relationship between CT and the progression of glaucoma based on EDI-OCT measurements.

New insights into carbonic anhydrase inhibition, vasodilation, and treatment of hypertensive-related diseases

Carbonic anhydrase (CA) and its inhibitors are relevant to many physiological processes and diseases. The enzyme is differentially expressed throughout the body, in concentration and subcellular location, and as 13 catalytically active isoforms. Blood vessels contain small amounts of CA, but the enzyme's role in vascular physiology and blood pressure regulation is uncertain. However, considerable recent evidence points to vasodilation by CA inhibitors. CA inhibition in vascular smooth muscle, endothelium, heart, blood cells, and nervous system could all contribute. It is equally plausible that other targets besides CA for all known CA inhibitors may account for their vascular effects. I will review this knowledge and important remaining gaps relating to treatment of hypertensive-related diseases with potent sulfonamide inhibitors, such as acetazolamide; but also the possibility that CA inhibition by thiazides and loop diuretics, although generally weaker, may have antihypertensive effects beyond their inhibition of renal sodium transporters.

Pharmacotherapy of glaucoma

Glaucoma is a group of diseases involving the optic nerve and associated structures, which is characterized by progressive visual field loss and typical changes of the optic nerve head (ONH). The only known treatment of the disease is reduction of intraocular pressure (IOP), which has been shown to reduce glaucoma progression in a variety of large-scale clinical trials. Nowadays, a relatively wide array of topical antiglaucoma drugs is available, including prostaglandin analogues, carbonic anhydrase inhibitors, beta-receptor antagonists, adrenergic agonists, and parasympathomimetics. In clinical routine, this allows for individualized treatment taking risk factors, efficacy, and safety into account. A major challenge is related to adherence to therapy. Sustained release devices may help minimize this problem but are not yet available for clinical routine use. Another hope arises from non-IOP-related treatment concepts. In recent years, much knowledge has been gained regarding the molecular mechanisms that underlie the disease process in glaucoma. This also strengthens the hope that glaucoma therapy beyond IOP lowering will become available. Implementing this concept with clinical trials remains, however, a challenge.

Diurnal variation in choroidal thickness in relation to sex, axial length, and baseline choroidal thickness in healthy Korean subjects

PURPOSE

To investigate diurnal variations in choroidal thickness (CT) in relation to various factors in healthy Korean subjects using spectral-domain optical coherence tomography.

METHODS

A prospective study was conducted on 100 healthy volunteers who each underwent measurements of CT in the same subfoveal area using an enhanced depth imaging technique with spectral-domain optical coherence tomography at 8 AM, 11 AM, 2 PM, and 5 PM. Possible correlations between the diurnal variation of CT and other factors, such as sex, axial length (AL), baseline CT (8 AM), blood pressure, and intraocular pressure, were evaluated.

RESULTS

One hundred volunteers with a mean age of 30.1 years were scanned. A significant pattern of diurnal variation was observed, with a mean CT of 278.28 ± 91.78 μm at 8 AM, 271.57 ± 89.08 μm at 11 AM, 266.39 ± 86.18 μm at 2 PM, and 264.92 ± 87.10 μm at 5 PM in a right eye. Right eye and left eye had a similar trend of diurnal cycle. Based on sex, AL, and baseline CT, the pattern of diurnal variation was greater and the amplitude significantly was greater in men, AL ≤ 24 mm and baseline CT ≥ 300 μm (men vs. women: P = 0.048, AL ≤ 24 mm vs. AL > 24 mm: P = 0.036, baseline CT ≥ 300 μm vs. baseline CT < 200 μm: P = 0.002, baseline CT ≥ 300 μm vs. 200 μm ≤ baseline CT < 300 μm: P = 0.008). There were no significant correlations between the diurnal variation of CT and systolic blood pressure, diastolic blood pressure, or intraocular pressure.

CONCLUSION

In men as well as in those with a shorter AL and thick baseline CT, a greater pattern of diurnal variation with significantly greater amplitude was observed.

Exercise-induced acute changes in systolic blood pressure do not alter choroidal thickness as measured by a portable spectral-domain optical coherence tomography device

PURPOSE

To measure choroidal thickness in patients manifesting an acute change in systemic arterial blood pressure using a portable spectral-domain optical coherence tomography device (iVue).

METHODS

Fifteen patients (15 eyes) undergoing cardiac exercise stress testing were scanned using a portable spectral-domain optical coherence tomography system (iVue). Two scan protocols were used: cross line scan for measuring choroidal thickness and the retina map scan to measure retinal thickness. Each patient was scanned before and within 3 minutes after the stress test. Blood pressure was measured at the same time as the acquisition of the scans. Choroidal thickness was measured from the posterior edge of the retinal pigment epithelium to the choroid-sclera junction at 500-μm intervals up to 1,000 μm temporal and nasal to the fovea. Retinal thickness was measured by an automated software. All choroidal thickness measurements were performed by two independent observers.

RESULTS

Fifteen patients (15 eyes) with a mean age of 60.6 (±10.4 years) were scanned. There was a significant increase in systolic but not diastolic pressure after stress testing (P < 0.05). The mean choroidal thickness measurements showed no significant difference before and after exercise stress testing (P > 0.05). In addition, there was no significant difference in retinal thickness before and after stress testing measurements (P > 0.05).

CONCLUSION

There was no change in choroidal thickness or retinal thickness, despite an acute change in the systemic systolic blood pressure induced by exercise.

Effects of antiglaucoma drugs on blood flow of optic nerve heads and related structures

An association between glaucoma development or progression and compromised ocular blood flow has been postulated as a result of population-based studies and prospective cohort studies. Blood flow in the optic nerve head (ONH) is of primary importance in the pathogenesis of glaucoma. The potential to modify the blood flow in the ONH and its related structures has been reported in various agents, including topical antiglaucoma drugs and systemic drugs such as calcium channel antagonists, which are reviewed in this manuscript. Clinical implications of the improvement in ocular blood flow on the treatment of glaucomatous optic neuropathy require further investigation.

The effect of acetazolamide on different ocular vascular beds

PURPOSE

To assess the effect of acetazolamide (AZ) on different ocular vascular beds.

METHODS

In a prospective study, 32 healthy volunteers (16 male, 16 female) with a mean age of 23.9 ± 3.3 years (20-39 years) were included. Before and after intravenous administration of 1,000 mg AZ (single dose), ocular microcirculation parameters were measured every 20 min for 2 h. Retinal vessel diameters (VD) were measured by the retina vessel analyzer, blood flow (BF) in the neuroretinal rim by the laser doppler flowmeter according to Riva, and the parapapillary retinal BF by the scanning laser Doppler flowmeter. Additionally, the Langham ocular blood flow system was used to determine the ocular pulse amplitude (OPA) and the pulsatile ocular blood flow (pOBF). The measurements were correlated with systemic blood pressure (BP), ocular perfusion pressure (OPP), capillary base excess parameters and serum AZ levels.

RESULTS

Arterial and venous VD were significantly increased by about 4-5% each. Papillary BF increased significantly about 40%. Parapapillary retinal flow dropped significantly about 19% (120 min). OPA and pOBF showed no statistically significant changes. BP showed no significant changes, and OPP was significantly increased. There were no correlations with pH or systemic perfusion parameters.

CONCLUSIONS

AZ leads to a dilatation of retinal VD, to an increase of BF in the optic nerve head, and to a decrease of parapapillary retinal BF. The different BF changes in different vascular beds might be due to different regulatory mechanisms, steal effects, or different distributions of the carbonic anhydrase.

Use of colour Doppler imaging in ocular blood flow research

The main objective of this report is to encourage consistent quality of testing and reporting within and between centres that use colour Doppler imaging (CDI) for assessment of retrobulbar blood flow. The intention of this review is to standardize methods in CDI assessment that are used widely, but not to exclude other approaches or additional tests that individual laboratories may choose or continue to use.

Contrast sensitivity, ocular blood flow and their potential role in assessing ischaemic retinal disease

PURPOSE

To examine the definition, evaluation methodology, association to ocular blood flow and potential clinical value of contrast sensitivity (CS) testing in clinical and research settings, focusing in patients with ischemic retinal disease.

METHODS

A review of the medical literature focusing on CS and ocular blood flow in ischemic retinal disease.

RESULTS

CS may be more sensitive than other methods at detecting subtle defects or improvements in primarily central retinal ganglion cell function early on in a disease process. CS testing attempts to provide spatial detection differences which are not directly assessed with standard visual acuity chart testing. Analyzing all studies that have assessed both CS change and ocular blood flow, it is apparent that both choroidal circulation and retinal circulation may have an important role in influencing CS.

CONCLUSION

The concept that CS is directly influenced by ocular blood flow is supported by reviewing the studies involving both. Although the studies in the literature have not established a direct cause and effect relationship per se, the literature review makes it logical to assume that changes in retinal and choroidal blood flow influence CS. This raises the possibility that a subjective visual characteristic, specifically CS, may be able to be evaluated more objectively by studying blood flow. It appears appropriate to study the relationship between blood flow and CS more extensively to develop improved ways of measuring various aspects of blood flow to the eye and to best quantify early changes in visual function.

Central serous chorioretinopathy: a pathogenetic model

Despite numerous studies describing predominantly its demography and clinical course, many aspects of central serous chorioretinopathy (CSCR) remain unclear. Perhaps the major impediment to finding an effective therapy is the difficulty of performing studies with large enough cohorts, which has meant that clinicians have focused more on therapy than on a deeper understanding of the pathogenesis of the disease. Hypotheses on the pathogenesis of CSCR have ranged from a basic alteration in the choroid to an involvement of the retinal pigment epithelium (RPE). Starting from evidence that affected subjects often present a personality prone to stress with altered pituitary–hypothalamic axis response (HPA) and that they have higher levels of serum and urinary cortisol and catecholamines than healthy subjects, we hypothesize a cascade of events that may lead to CSCR through hypercoagulability and augmented platelet aggregation. In particular we investigated the role of tissue plasminogen activator, increasing plasminogen activator inhibitor 1 (PAI-1), and plasmin-α2- plasmin inhibitor complexes. We reviewed the different therapeutic approaches, including adrenergic antagonists, carbonic anhydrase inhibitors, mifepristone, ketoconazole, laser photocoagulation, intravitreal injection of bevacizumab, and photodynamic therapy with verteporfin (PDT) and our model of pathogenesis seems to be in agreement with the clinical effects obtained from these treatments. In accord with our thesis, we began to treat a group of patients affected by CSCR with low-dose aspirin (75–100 mg), because of its effectiveness in other vascular diseases and its low ocular and general toxicity with prolonged use. The formulation of a causative model of CSCR enables us to understand how the therapeutic approach cannot be based on a generalized therapy but should be individualized for each patient, and that sometimes a combined strategy of treatment is required. Moreover a complete knowledge of the disease will help to identify patients prone to the most persistent forms of CSCR, and thus help to find a treatment.

Ocular hemodynamic effects of nitrovasodilators in healthy subjects

Nitric oxide (NO) plays a key role in the regulation of ocular blood flow and may be an interesting therapeutic target in ocular ischemic disease. In the present study, we hypothesized that NO-releasing drugs may increase blood flow to the head of the optic nerve and also in the choroid. The study employed a randomized, placebo-controlled, double blind, four-way crossover design. On separate study days, 12 healthy subjects received infusions of nitroglycerin, isosorbide dinitrate, sodium nitroprusside, or placebo. All three study drugs reduced the mean arterial pressure (MAP) and ocular perfusion pressure (OPP) (P < 0.001). None of the administered drugs increased the ocular hemodynamic variables. By contrast, vascular resistance decreased dose dependently during administration of the study drugs (P < 0.001). These results indicate that systemic administration of NO-donor drugs is associated with a decrease in vascular resistance in the ocular vasculature. However, because these drugs also reduce blood pressure, they do not improve perfusion to the posterior eye pole.

Acetazolamide influences ocular pulse amplitude

PURPOSE

To investigate the effect of systemic acetazolamide on ocular pulse amplitude (OPA) and on intraocular pressure (IOP).

METHODS

We determined OPA, IOP, mean arterial blood pressure and heart rate (HR) in 17 patients suffering from hypertensive primary open-angle glaucoma (htPOAG) and 22 healthy controls. Measurements were taken before and 2 h after oral intake of 500 mg of acetazolamide.

RESULTS

Mean OPA decreased 20.24% (SEM 2.95%) from 2.76 mmHg (SEM 0.16 mmHg) to 2.14 mmHg (SEM 0.13 mmHg) 2 h after oral administration of 500-mg acetazolamide. IOP dropped by 2.98 mmHg (SEM 0.28 mmHg), corresponding to a 17.19% (SEM 1.56%) reduction. The decrease in IOP correlated with the decrease of OPA (r(2) = 0.4, P < 0.0001).

CONCLUSIONS

Ocular pulse amplitude measurements are influenced by oral administration of acetazolamide. There was a decrease in the pulsatile component of choroidal blood flow and in IOP in both htPOAG and healthy controls. This needs to be taken into account for measurement of OPA with dynamic contour tonometry.

A comparative analysis of the effects of the fixed combination of timolol and dorzolamide versus latanoprost plus timolol on ocular hemodynamics and visual function in patients with primary open-angle glaucoma

AIMS

The aim of this study was to assess the effects of fixed combination of timolol and dorzolamide and latanoprost plus timolol on retinal, choroidal, and retrobulbar hemodynamics and visual function in primary open-angle glaucoma (OAG) subjects.

METHODS

Sixteen (16) OAG patients (age, 63.5 +/- 10.8 years; 9 male) were evaluated in a randomized, crossover, double-blind study design after 4 weeks of treatment of latanoprost with timolol and fixed combination of timolol and dorzolamide. After randomization, 9 right eyes and 7 left eyes were included in the hemodynamic portion of the study. Measurements included: adverse events check, visual acuity, contrast sensitivity, blood pressure, heart rate, intraocular pressure (IOP), and fundus examination. Ocular blood flow was assessed using confocal scanning laser Doppler flowmetry, color Doppler imaging, and scanning laser ophthalmoscopy.

RESULTS

Both therapies were effective at lowering IOP, whereas there was no statistically significant difference between latanoprost plus timolol and the fixed combination of timolol and dorzolamide (13.9% and 12.2% reduction, respectively; P = 0.5533). Fixed combination of timolol and dorzolamide significantly increased central retinal artery end diastolic blood flow velocity (P = 0.0168) and lowered resistance to flow (P = 0.0279). Temporal posterior ciliary artery peak systolic and end diastolic velocities were significantly increased with the fixed combination of timolol and dorzolamide (P = 0.0125 and 0.0238, respectively). Latanoprost plus timolol had no significant effects on ocular blood flow during 4 weeks of treatment. There were no statistically significant differences in adverse events, blood pressure, heart rate, visual acuity, contrast sensitivity scanning laser ophthalmoscopy, or Heidelberg Retinal Flowmeter for any treatment period.

CONCLUSIONS

Fixed combination of timolol and dorzolamide therapy might increase blood flow in OAG patients while attaining a similar IOP reduction compared to latanoprost plus timolol. Visual function, however, was not different in this short-term comparison between the two treatments.

Effect of Indomethacin on the Hypercapnia-Associated Vasodilation of the Optic Nerve Head Vessels: An Experimental Study in Miniature Pigs

PURPOSE

To evaluate whether prostaglandins are mediators of the hypercapnia-associated vasodilation of the optic nerve head vessels.

METHODS

We measured the PO(2) at intervascular areas of the optic disc in 9 anaesthetized miniature pigs using oxygen-sensitive microelectrodes placed at <50 mum from the optic disc. PO(2) was measured continuously under normoxia, hyperoxia (breathing of 100% O(2)), carbogen breathing (95% O(2), 5% CO(2)), and hypercapnia (40% increase in inhaled CO(2)). Similar measurements under these conditions were also done after intravenous administration of the prostaglandin inhibitor indomethacin.

RESULTS

Before the injection of indomethacin, we observed a slight increase in the optic disc PO(2) during hypercapnia (DeltaPO(2) = 2.0 +/- 1.7 mm Hg; p < 0.001; n = 18) or hyperoxia (DeltaPO(2) = 3.4 +/- 1.6 mm Hg; p < 0.001; n = 23), but a much more important increase during carbogen breathing (DeltaPO(2) = 12.0 +/- 5.1 mm Hg; p < 0.001; n = 23). After the injection of indomethacin, the increase in the optic disc PO(2) was similar during hyperoxia (DeltaPO(2) = 5.6 +/- 2.2 mm Hg; p < 0.001; n = 9) or carbogen breathing (DeltaPO(2) = 5.8 +/- 3.2 mm Hg; p < 0.001; n = 9), while in hypercapnia the variation of the optic disc PO(2) was minimal (DeltaPO(2) = 0.5 +/- 1.9 mm Hg; p > 0.1; n = 6).

CONCLUSIONS

Indomethacin inhibits the vasodilatory effect of increased systemic PaCO(2) on the optic nerve head vessels, leading to a similar moderate increase in the optic disc PO(2) during carbogen breathing as in hyperoxia. Indomethacin also inhibits the increase in the optic disc PO(2) seen during hypercapnia. Those results indicate that prostaglandins are mediators of the hypercapnia-associated vasodilation of the optic nerve head vessels.

Optic nerve oxygenation

The oxygen tension of the optic nerve is regulated by the intraocular pressure and systemic blood pressure, the resistance in the blood vessels and oxygen consumption of the tissue. The oxygen tension is autoregulated and moderate changes in intraocular pressure or blood pressure do not affect the optic nerve oxygen tension. If the intraocular pressure is increased above 40 mmHg or the ocular perfusion pressure decreased below 50 mmHg the autoregulation is overwhelmed and the optic nerve becomes hypoxic. A disturbance in oxidative metabolism in the cytochromes of the optic nerve can be seen at similar levels of perfusion pressure. The levels of perfusion pressure that lead to optic nerve hypoxia in the laboratory correspond remarkably well to the levels that increase the risk of glaucomatous optic nerve atrophy in human glaucoma patients. The risk for progressive optic nerve atrophy in human glaucoma patients is six times higher at a perfusion pressure of 30 mmHg, which corresponds to a level where the optic nerve is hypoxic in experimental animals, as compared to perfusion pressure levels above 50 mmHg where the optic nerve is normoxic. Medical intervention can affect optic nerve oxygen tension. Lowering the intraocular pressure tends to increase the optic nerve oxygen tension, even though this effect may be masked by the autoregulation when the optic nerve oxygen tension and perfusion pressure is in the normal range. Carbonic anhydrase inhibitors increase the optic nerve oxygen tension through a mechanism of vasodilatation and lowering of the intraocular pressure. Carbonic anhydrase inhibition reduces the removal of CO2 from the tissue and the CO2 accumulation induces vasodilatation resulting in increased blood flow and improved oxygen supply. This effect is inhibited by the cyclo-oxygenase inhibitor, indomethacin, which indicates that prostaglandin metabolism plays a role. Laboratory studies suggest that carbonic anhydrase inhibitors might be useful for medical treatment of optic nerve and retinal ischemia, potentially in diseases such as glaucoma and diabetic retinopathy. However, clinical trials and needed to test this hypotheses.

Indomethacin lowers optic nerve oxygen tension and reduces the effect of carbonic anhydrase inhibition and carbon dioxide breathing

BACKGROUND/AIMS

Prostaglandins are important in blood flow regulation. Carbon dioxide (CO(2)) breathing and carbonic anhydrase inhibition increase the oxygen tension in the retina and optic nerve. To study the mechanism of this effect and the role of cyclo-oxygenase in the regulation of optic nerve oxygen tension (ONPO(2)), the authors investigated how indomethacin affects ONPO(2) and the ONPO(2) increases caused by CO(2) breathing and carbonic anhydrase inhibition in the pig.

METHODS

Optic nerve oxygen tension was measured in 11 pigs with a polarographic oxygen electrode. The tip of the electrode was placed 0.5 mm above the optic disc. The effects of indomethacin, CO(2) breathing (3%) before and after indomethacin treatment, and carbonic anhydrase inhibition with or without indomethacin treatment were investigated.

RESULTS

Administration of 300 mg indomethacin decreased optic nerve oxygen tension significantly. Carbonic anhydrase inhibition and CO(2) breathing increased ONPO(2) significantly. After indomethacin had been given, the rise in ONPO(2) caused by CO(2) breathing and carbonic anhydrase inhibition was significantly reduced.

CONCLUSION

Systemic administration of indomethacin decreases the optic nerve oxygen tension; this is probably the result of decreased blood flow through vasoconstriction of vessels in the optic nerve. Additionally, indomethacin diminishes the ONPO(2) increasing effect of CO(2) breathing and carbonic anhydrase inhibition, thus affecting the reactivity of vessels in the optic nerve.

The effects of antiglaucoma and systemic medications on ocular blood flow

Based on the body of evidence implicating ocular blood flow disturbances in the pathogenesis of glaucoma, there is great interest in the investigation of the effects of antiglaucoma drugs and systemic medications on the various ocular vascular beds. The primary aim of this article was to review the current data available on the effects of antiglaucoma drugs and systemic medications on ocular blood flow. We performed a literature search in November 2002, which consisted of a textword search in MEDLINE for the years 1968-2002. The results of this review suggest that there is a severe lack of well-designed long-term studies investigating the effects of antiglaucoma and systemic medications on ocular blood flow in glaucomatous patients. However, among the 136 articles dealing with the effect of antiglaucoma drugs on ocular blood flow, only 36 (26.5%) investigated the effects of medications on glaucoma patients. Among these 36 articles, only 3 (8.3%) were long-term studies, and only 16 (44.4%) were double-masked, randomized, prospective trials. Among the 33 articles describing the effects of systemic medications on ocular blood flow, only 11 (33.3%) investigated glaucoma patients, of which only one (9.1%) was a double-masked, randomized, prospective trial. Based on this preliminary data, we would intimate that few antiglaucoma medications have the potential to directly improve ocular blood flow. Unoprostone appears to have a reproducible antiendothelin-1 effect, betaxolol may exert a calcium-channel blocker action, apraclonidine consistently leads to anterior segment vasoconstriction, and carbonic anhydrase inhibitors seem to accelerate the retinal circulation. Longitudinal, prospective, randomized trials are needed to investigate the effects of vasoactive substances with no hypotensive effect on the progression of glaucoma.

The impact of ocular blood flow in glaucoma

Two principal theories for the pathogenesis of glaucomatous optic neuropathy (GON) have been described--a mechanical and a vascular theory. Both have been defended by various research groups over the past 150 years. According to the mechanical theory, increased intraocular pressure (IOP) causes stretching of the laminar beams and damage to retinal ganglion cell axons. The vascular theory of glaucoma considers GON as a consequence of insufficient blood supply due to either increased IOP or other risk factors reducing ocular blood flow (OBF). A number of conditions such as congenital glaucoma, angle-closure glaucoma or secondary glaucomas clearly show that increased IOP is sufficient to lead to GON. However, a number of observations such as the existence of normal-tension glaucoma cannot be satisfactorily explained by a pressure theory alone. Indeed, the vast majority of published studies dealing with blood flow report a reduced ocular perfusion in glaucoma patients compared with normal subjects. The fact that the reduction of OBF often precedes the damage and blood flow can also be reduced in other parts of the body of glaucoma patients, indicate that the hemodynamic alterations may at least partially be primary. The major cause of this reduction is not atherosclerosis, but rather a vascular dysregulation, leading to both low perfusion pressure and insufficient autoregulation. This in turn may lead to unstable ocular perfusion and thereby to ischemia and reperfusion damage. This review discusses the potential role of OBF in glaucoma and how a disturbance of OBF could increase the optic nerve's sensitivity to IOP.

Dorzolamide and ocular blood flow in previously untreated glaucoma patients: a controlled double-masked study

PURPOSE

The aim of this study was to investigate the haemodynamic effects of topical dorzolamide treatment in eyes with newly detected and previously untreated glaucoma.

METHODS

Forty-seven patients with open angle glaucoma were consecutively randomized to dorzolamide versus placebo TID during a 6-week double-masked treatment trial. Ocular blood flow was investigated at baseline and on therapy using colour Doppler imaging of the retrobulbar vessels and scanning laser ophthalmoscope fluorescein angiograms of the retinal circulation.

RESULTS

None of the flow parameters, retrobulbar or retinal, changed significantly on therapy when the results were analysed with the Bonferroni method. Analysis with non-simultaneous tests also failed to reveal any significant changes either in retrobulbar flow velocities in the central retinal artery, ophthalmic artery or in the short posterior ciliary arteries, or in the retinal parameters (arm-retina time, arteriovenous passage time, mean dye velocity or macular capillary velocity), while capillary velocities at the optic disc decreased significantly in the dorzolamide group (P = 0.03). Intraocular pressure reduction was significantly more pronounced in the dorzolamide group (P = 0.002), with - 4.8 +/- 2.9 mmHg (P < 0.0001) versus -1.8 +/- 3.0 mmHg in the placebo group (P = 0.006).

CONCLUSIONS

The present study indicated no measurable vascular effects from topical dorzolamide treatment in previously untreated glaucoma eyes.

The impact of glaucoma medication on parameters of ocular perfusion

Whereas intraocular pressure is considered a major risk factor in glaucoma, growing evidence now indicates that ocular ischemia plays a major role too. By virtue of this and because many existing medications are able to interact with vasculature, altering ocular blood flow, it is essential that current and future medications for glaucoma be evaluated for their effect on ocular circulation. The authors review published papers examining the effect of topical and some systemic medications on ocular blood flow, focusing mostly on data from the human eye. The authors provide a comprehensive review on the effect of subclasses of medications (eg, carbonic anhydrase inhibitors, beta-blockers, alpha-adrenergic agonists, and prostaglandin analogues on optic nerve head, and on retinal, choroidal, and retrobulbar circulation. The various claims for enhancements or reduction of ocular circulation within each class of medication are reviewed and evaluated.

Acetazolamide-induced cerebral and ocular vasodilation in humans is independent of nitric oxide

Acetazolamide, a carbonic anhydrase inhibitor, is used orally in the treatment of primary and secondary open-angle glaucoma and induces ocular and cerebral vasodilation. Several in vitro studies have shown that carbonic anhydrase pharmacology and the L-arginine-nitric oxide (NO) pathway are closely related. We investigated the role of NO in acetazolamide-induced vasodilation on cerebral and ocular vessels in 12 healthy subjects in the presence or absence of NG-monomethyl-L-arginine (L-NMMA), a NO synthase inhibitor, and in the presence or absence of L-arginine, the precursor of NO. Acetazolamide was administered after pretreatment with either L-NMMA or placebo and either L-arginine or placebo. Pulsatile choroidal blood flow was assessed with laser interferometric measurement of fundus pulsation. In addition, mean blood flow velocity (MFV) in the middle cerebral artery (MCA) and ophthalmic artery (OA) was measured with Doppler sonography. Acetazolamide increased ocular fundus pulsation amplitude (FPA; +27%, P < 0.001) and MFV in the MCA (+38%, P < 0.001) and in the OA (+19%, P = 0.003). Administration of L-NMMA alone reduced FPA (-21%, P < 0.001) and MFV in the MCA (-11%, P = 0. 030) but did not change MFV in the OA. All hemodynamic effects of L-NMMA were reversed by L-arginine. However, neither L-NMMA nor L-arginine altered acetazolamide-induced changes in cerebral or ocular hemodynamic parameters. The present data indicate that acetazolamide-induced hemodynamic changes are not mediated by NO. Which mediators other than NO are involved in the hemodynamic effects as induced by carbonic anhydrase inhibitors remains to be elucidated.