Partial coherence interferometry: a novel approach to biometry in cataract surgery

PURPOSE

To compare biometry performed by an enhanced version of dual beam partial coherence interferometry and applanation ultrasound in a prospective study of 85 cataract eyes to improve refractive outcome of cataract surgery due to a more accurate calculation of intraocular lens power.

METHODS

The SRK II formula using ultrasound biometry data was employed. Three months after surgery, partial coherence interferometry biometry was repeated and refractive outcome was determined. Preoperative partial coherence interferometry biometry data were used to determine the refractive power of the intraocular lenses retrospectively and to calculate the possible refractive outcome.

RESULTS

Precision of partial coherence interferometry biometry was more than 10 times better than that of ultrasound. Therefore, the possible mean absolute error for postoperative refraction achieved with partial coherence interferometry biometry was 0.49 diopters (compared with 0.67 diopters with ultrasound biometry), resulting in an improvement of 27%. Axial eye length measured with the two techniques differed by a mean of 460 microm. The difference in lens thickness measured with partial coherence interferometry and ultrasound significantly correlated with cataract grade. A mean shortening of 120 microm of axial eye length following cataract surgery was also detected by partial coherence interferometry.

CONCLUSIONS

The enhanced version of partial coherence interferometry offers biometry with unprecedented precision (<10 microm) and resolution (approximately 12 microm), therefore improving the refractive outcome in cataract surgery. This noninvasive technique provides a high degree of comfort for the patient, with no need for local anesthesia or pupil dilation and minimized risk of corneal infection.

Eye elongation during accommodation in humans: differences between emmetropes and myopes

PURPOSE

The pathophysiology and pathogenesis of myopia are still a matter of controversy. Exaggerated longitudinal eye growth is assumed to play an important role in the development of myopia. A significant correlation between refraction and amount of near-work has been reported. However, current knowledge of changes of axial eye length with accommodation is limited because clinical ultrasound biometry does not provide the precision and resolution required to thoroughly investigate these phenomena.

METHODS

Partial coherence interferometry (PCI), a noninvasive biometric technique, uses laser light with short coherence length in combination with interferometry to achieve precision in the micrometer to submicrometer range and resolution of 10 microm. In the present study this technique was used to investigate axial eye length changes in 11 emmetropic and 12 myopic eyes during monocular fixation at the far and near point. In 7 subjects, the contralateral eye has also been measured to investigate interocular differences in eye elongation.

RESULTS

All investigated eyes elongated during accommodation. This elongation was more pronounced in emmetropes than in myopes (P < 0.001). Mean accommodation-induced eye elongations of 12.7 microm (range, 8.6-19.2 microm) and 5.2 microm (range, 2.1-9.5 microm), corresponding to a dioptric change of approximately -0.036 D and -0.015 D, were obtained for emmetropes and myopes. No significant difference in accommodative amplitudes between groups (5.1 +/- 1.2 D [range, 3.8-7.1 D] versus 4.1 +/- 2.0 D [range, 1.0-7.1 D]; P = 0.14) was detected. No significant interocular difference in accommodation-induced eye elongation was revealed (P = 0.86). Also, a mean backward movement of the posterior lens pole of 38 microm (range, 9-107 microm) was observed in both study groups.

CONCLUSIONS

The detected eye elongation can be explained by the accommodation-induced contraction of the ciliary muscle, which results in forward and inward pulling of the choroid, thus decreasing the circumference of the sclera, and leads to an elongation of the axial eye length. Finally, it was demonstrated that PCI, in contrast to clinical ultrasound, is capable of characterizing eye length changes during accommodation in humans.

Accurate determination of effective lens position and lens-capsule distance with 4 intraocular lenses

PURPOSE

To measure effective lens position (ELP) of 4 intraocular lenses (IOLs) using high precision and high resolution dual-beam partial coherence interferometry (PCI) and to assess the tendency of these IOLs to produce a lens-capsule distance (LCD), a possible risk factor for posterior capsule opacification.

SETTING

Department of Ophthalmology, Vienna General Hospital; Institute of Medical Physics, University of Vienna, Austria.

METHODS

In a retrospective study, PCI was used to measure ELP and LCD in 139 pseudophakic eyes of 110 patients with 4 IOLs: acrylic 3-piece IOL (AcrySof MA60BM); silicone 3-piece IOL without a capsular tension ring (PhacoFlex SI30) and with a capsular tension ring (PhacoFlex SI30 and Morcher Type 14); silicone plate-haptic IOL (Staar AA4203VF); and a hydrogel plate-haptic IOL (logel 1103).

RESULTS

The ELP and LCD were determined with a precision of approximately 3 to 4 microns. An LCD was detected in 21% eyes with the AcrySof, 20% of eyes with the SI30 without a capsular tension ring, 10% of eyes with a capsular tension ring, 21% of eyes with the Staar, and 17% of eyes with the logel. The LCDs detected by PCI, but not by slitlamp examination, were significantly smaller than those detected by both.

CONCLUSION

The amount of LCD detected by PCI was approximately the same with all IOL types (approximately 20%) except the PhacoFlex SI30 with a capsular tension ring (10%).

High precision biometry of pseudophakic eyes using partial coherence interferometry

PURPOSE

To investigate the applicability of the scanning version of dual-beam partial coherence interferometry (PCI) for measuring the anterior segment and axial length of pseudophakic eyes in a clinical setting and to determine the achievable precision with this biometry technique.

SETTING

Department of Ophthalmology, Vienna General Hospital, and Institute of Medical Physics, University of Vienna, Austria.

METHODS

Partial coherence interferometry was performed in 39 pseudophakic eyes of 39 patients after implantation of a foldable acrylic intraocular lens (IOL).

RESULTS

Effective lens position (ELP), IOL thickness and lens-capsule distance (LCD) were determined with a precision of 2 to 3 microns; corneal thickness and axial eye length, with a precision of 0.8 and 5.0 microns, respectively. The mean ELP of the IOL was 4.093 mm +/- 0.290 (SD). In 7 eyes (18%), a positive LCD of 68 +/- 40 microns was detected with PCI. Mean corneal thickness was 526.4 +/- 31.5 microns; mean IOL thickness, 791.5 +/- 40.2 microns; and mean axial length, 23.388 +/- 0.824 mm.

CONCLUSION

The scanning version of PCI enables high precision (< or = 5 microns) and high resolution (approximately 12 microns) biometry of pseudophakic eyes that is better than conventional ultrasound by a factor of more than 20. For the first time, positive LCD, a possible risk factor for posterior capsule opacification, could be detected and quantified. Furthermore, this technique offers a high degree of comfort for the patient since it is a noncontact method with no need for local anesthesia or pupil dilation and has a reduced risk of corneal infection.

Anterior capsulotomy created by radiofrequency endodiathermy and continuous curvilinear posterior capsulorhexis in a patient with intumescent cataract and primary capsular fibrosis

A 62-year-old woman with intumescent cataract and primary posterior capsular fibrosis was operated on using two different techniques to create continuous curvilinear capsule openings in the same eye. The biomicroscopic appearance of the edge after continuous tear and radiofrequency endodiathermy capsulotomy were compared.

Noninvasive investigations of the normal ocular circulation in humans

PURPOSE

To compare the reproducibility of laser interferometric measurements of fundus pulsation, pneumatonometric measurement of pulse amplitude (PA) and pulsatile ocular blood flow (POBF), and Doppler ultrasonic measurements of blood flow velocity in the ophthalmic artery (OA) and the posterior ciliary arteries (PCAs) and to investigate the association of the results obtained with these methods and to characterize ocular hemodynamics during Valsalva maneuver and isometric handgrip.

METHODS

All studies were performed in healthy subjects. Fundus pulsation (n = 48), POBF (n = 24), and blood flow velocities (n = 24) were measured at baseline and during the Valsalva maneuver and isometric handgrip. Intraclass correlation coefficients were calculated for test/retest variability, for short-term variability, and for interobserver variability.

RESULTS

Intraclass correlation coefficients were between 0.95 and 0.98 for fundus pulsation measurements, between 0.54 and 0.76 for pneumatonometric measurements, between 0.44 and 0.88 for Doppler sonographic measurements in the OA and between 0.32 and 0.60 in the PCAs. There was a high degree of association between pneumatonometric parameters and fundus pulsation amplitude. Valsalva maneuver significantly reduced fundus pulsations, PA, and POBF, whereas isometric handgripping did not change these parameters.

CONCLUSIONS

The reproducibility of fundus pulsation measurements was excellent. Reproducibility of pneumatonometric parameters and ultrasonographic measurements in the OA was satisfactory. Reproducibility of ultrasonographic measurements in the PCAs was low. Data obtained during the autonomic stimuli experiments argue against the sole use of systems to measure pulsatile blood flow, if no additional data on flow pulsatility are available. At least in cases when changes in blood pressure are likely to occur the ratio of pulsatile to nonpulsatile blood flow might not be constant and POBF might not be an adequate measure of total ocular blood flow.

Effects of acetazolamide on choroidal blood flow

BACKGROUND AND PURPOSE

The acetazolamide provocation test is commonly used to study cerebrovascular vasomotor reactivity. On the basis of the effect of a carbonic anhydrase inhibitor in the central nervous system, we hypothesized that acetazolamide may also increase blood flow in the human choroid.

METHODS

In a placebo-controlled, randomized, double-blind, three-way crossover design, acetazolamide (500 mg or 1000 mg i.v.) or placebo was administered to nine healthy subjects. The effect of acetazolamide was studied at 15-minute intervals for 90 minutes. Pulsatile choroidal blood flow was assessed with laser interferometric measurement of fundus pulsation. In addition, mean blood flow velocity and resistive index in the ophthalmic artery were measured with Doppler sonography. In a second study in six healthy subjects, we assessed the effect of acetazolamide (1000 mg i.v.) on intraocular pressure.

RESULTS

Acetazolamide increased fundus pulsation amplitude in a dose-dependent manner (1000 mg: +33%; 500 mg: +20%; P<0.001, ANOVA). The effect of acetazolamide on MFV (1000 mg: +18%; 500 mg: +8%; P=0.003, ANOVA) and RI (1000 mg: -4%; 500 mg: -2%; P=0.006, ANOVA) was less pronounced but also significant. Acetazolamide did not induce any changes in systemic hemodynamic parameters but significantly decreased intraocular pressure (1000 mg: -37%; P<0.0001).

CONCLUSIONS

The present data show for the first time that intravenously administered acetazolamide increases choroidal blood flow in humans. This phenomenon therefore indicates that the acetazolamide provocation test may qualify as a tool to investigate ocular vasomotor reactivity in a variety of ocular diseases. Moreover, the increase in choroidal blood flow after carbonic anhydrase inhibition can be expected to contribute to the therapeutic efficacy of carbonic anhydrase inhibitors in glaucoma.

Postoperative inflammation after clear corneal and sclerocorneal incisions

PURPOSE

To compare two standard small incision techniques for cataract surgery--clear corneal incision and sclerocorneal incision--with regard to postoperative disturbance of the blood-aqueous barrier (BAB).

SETTING

Department of Ophthalmology, University of Vienna, Austria.

METHODS

This prospective comparative study comprised 108 eyes with senile cataract. A clear corneal tunnel incision was made in 53 eyes and a sclerocorneal incision with tunnel preparation in 55. All other surgical procedures and postoperative medication were standardized. Postoperative BAB disruption was measured with a laser flare-cell meter (Kowa FC 1000) in an undilated pupil on 1, 3, 7, 14, 28, 60, and 90 days postoperatively. The values were compared using a two-sided t-test.

RESULTS

In both groups, flare and cell values were highest on the first postoperative day. They gradually declined but without recovery of preoperative flare values on day 28. At no time was there a noticeable between-group difference in mean cell and flare values except for flare at day 14 (P = .04).

CONCLUSION

Postoperative trauma, measured by the BAB disturbance, was equally low after clear corneal and sclerocorneal incisions.

Age dependence of choroidal blood flow

OBJECTIVE

To investigate the age dependence of choroidal blood flow.

DESIGN

A cross-sectional study.

SETTING

Department of Clinical Pharmacology, Vienna University.

PARTICIPANTS

A total of 130 healthy volunteers between the ages of 19 and 83 years.

MEASUREMENTS

Fundus pulsation amplitude (FPA) with a recently developed laser interferometric method, mean arterial pressure (MAP) with an automated oscillometric device, intraocular pressure (IOP) with an applanation tonometer, and ocular perfusion pressure (OPP) as calculated from MAP and IOP.

RESULTS

There was a significant correlation of FPA with age r = -0.242 (P = .005). MAP, IOP, and OPP showed a significant positive correlation with age. Multiple regression analysis showed that FPA is associated with age but not with MAP, IOP, or OPP.

CONCLUSION

Choroidal blood flow is reduced in older subjects, which argues in favor of an increase in ocular vascular resistance with age. This may be a risk factor in the development of common ocular diseases such as age-related macular degeneration or glaucoma.

Topical fundus pulsation measurements in age-related macular degeneration

BACKGROUND

The purpose of the present study was to investigate regional fundus pulsations in age-related macular degeneration (AMD) patients with subretinal neovascular membranes.

METHODS

Local fundus pulsation amplitudes (FPAs) were measured in 12 patients with AMD with classic neovascular membranes. Measurements were performed directly on the membrane and adjacent to the membrane. FPAs were assessed with a recently developed laser interferometric method. FPA measurements were performed in 12 healthy subjects at similar posterior pole locations.

RESULTS

In AMD patients FPAs were consistently lower when measured directly on the neovascular membrane ("inside") than at measurement sites around the membrane ("outside"). The difference in FPA was 26 +/- 3% (mean +/- SEM, range 13-40%, P < 0.0001). In healthy subjects, however, FPAs were significantly higher at the measurement points corresponding to "inside" points (15 +/- 4%, P < 0.0006).

CONCLUSIONS

We have shown that FPAs are reduced at classic neovascular membranes in patients with AMD. The mechanism behind this finding remains unclear. Hence, future studies have to ascertain whether this observation is associated with changes in fundus layers or with local choroidal perfusion abnormalities.

Dual beam optical coherence tomography: signal identification for ophthalmologic diagnosis

The dual beam version of optical coherence topography can be used for noninvasive, high-resolution imaging of the human eye fundus, enabling in vivo visualization of retinal morphology as well as accurate quantification of the thickness profiles of its layers. Interferometric fundus signals-optical A-scans-and retinal tomograms of patients with glaucoma, diabetic retinopathy, and age-related macular degeneration are compared with those of healthy, normal subjects to elucidate the origin of the signal peaks detected and to investigate and interpret the retinal microstructures contained in the cross-sectional images. © 1998 Society of Photo-Optical Instrumentation Engineers.

Reversal of endothelin-1-induced ocular hemodynamic effects by low-dose nifedipine in humans

BACKGROUND AND PURPOSE

There is evidence that calcium channel blockers may be useful in patients with normal tension glaucoma and vasospastic reactions. We therefore hypothesized that calcium channel blockers may increase ocular blood flow and that there may be a functional antagonism between endothelin-1 (ET-1) and calcium channel blockers in the ocular vasculature.

METHODS

This was a randomized, double-blind, three-way crossover study with respect to ET-1 infusions (placebo, 2 ng/kg/min ET-1, and 4 ng/kg/min ET-1) and a randomized double-blind study in two parallel groups with respect to nifedipine (placebo or 5 mg nifedipine). Ocular hemodynamics in the 12 healthy subjects participating in the study was assessed by laser interferometric measurement of fundus pulsation amplitude (FPA) in the optic disc and two-dimensional scanning laser Doppler flowmetry in the optic disc.

RESULTS

ET-1 caused a dose-dependent decrease in FPA and flow. With a dose of 4 ng/kg/min a decrease of -18% +/- 5% (p < 0.001) and -17% +/- 5% (p = 0.023) on FPA and flow, respectively, were observed. This effect was completely reversed by nifedipine compared with placebo (FPA, p < 0.001; flow, p = 0.011). However, nifedipine did not affect ocular hemodynamics after placebo infusion.

CONCLUSIONS

These results show that nifedipine does not increase optic nerve head blood flow during baseline conditions but reverses ET-1-induced constriction in ocular vasculature at doses that do not affect systemic hemodynamics. This supports the close relation of the therapeutic effect of calcium channel blockers in patients with normal tension glaucoma to the endothelin system. Moreover, the present study provides a strong rationale for a study of low dose nifedipine as a supplementary medication in glaucoma patients.

Short-term oral pentoxifylline use increases choroidal blood flow in patients with age-related macular degeneration

OBJECTIVE

To study the ocular hemodynamic effects of a 3-month oral treatment with pentoxifylline in patients with nonexudative age-related macular degeneration.

DESIGN

Double-blind, placebo-controlled, randomized, parallel group study.

SETTING

Outpatient clinic of the Department of Ophthalmology, Vienna University, Vienna, Austria, that specializes in age-related macular degeneration.

METHODS

Forty patients with age-related macular degeneration received pentoxifylline (400 mg 3 times a day orally, n=20) or placebo (n=20) for 3 months. Retinal blood flow was assessed by scanning laser Doppler flowmetry and pulsatile choroidal blood flow was assessed by laser interferometric measurement of fundus pulsation amplitude.

MAIN OUTCOME MEASURES

Changes in retinal blood flow and fundus pulsation amplitude.

RESULTS

Four patients receiving pentoxifylline and 3 patients receiving placebo discontinued medication because of nausea. In the remaining subjects, the use of pentoxifylline increased ocular fundus pulsation amplitude (P<.001 vs placebo and baseline). The maximum increase was 28% after 3 months. In contrast, retinal blood flow was not changed by the use of pentoxifylline.

CONCLUSIONS

A 3-month course of oral pentoxifylline treatment increases choroidal but not retinal blood flow in patients with age-related macular degeneration. These data strongly support the concept that pentoxifylline might be useful in the treatment of age-related macular degeneration. Long-term clinical outcome trials are now warranted to test this hypothesis.

Investigation of dispersion effects in ocular media by multiple wavelength partial coherence interferometry

We report on quantitative measurements of group refractive indices and group dispersion in water and in human ocular media such as the cornea, the aqueous humor, the lens, artificial intraocular lenses, as well as a total value averaged over the media along the axial eye length of normal subjects and pseudophakic patients in vivo using dual beam partial coherence interferometry. Different optical thickness values due to the dispersion of the cornea are demonstrated using two spectrally displaced light sources. The displacement can be used to indirectly calculate the group dispersion of the human cornea in the spectral region between 810 nm and 860 nm. If the object under investigation is dispersive, resolution is limited due to a broadening of the detected signals. This broadening increases with group dispersion, i.e., the extent to which the group refractive index of the medium varies with wavelength and thickness of the tissue under investigation as well as with the spectral bandwidth of the light source. Measurements of the group dispersion in the cornea, lens and vitreous of pseudophakic and normal human eyes, show that the cornea and the lens are more dispersive than water-by a factor of about 5 and 2, respectively-in the investigated spectral region. The cornea is approximately threefold more dispersive than the human crystalline lens, the aqueous humor is less dispersive than water and the group dispersion of all ocular components together, averaged over the axial length of normal and pseudophakic eyes, was only slightly higher compared to that of water. Since the highly dispersive cornea and lens together have only a thickness of about one sixth of that of the axial eye length, it seems that their contribution to the group dispersive effect along the whole axial eye length is only small.

Role of NO in the O2 and CO2 responsiveness of cerebral and ocular circulation in humans

It is well known that changes in PCO2 or PO2 strongly influence cerebral and ocular blood flow. However, the mediators of these changes have not yet been completely identified. There is evidence from animal studies that NO may play a role in hypercapnia-induced vasodilation and that NO synthase inhibition modulates the response to hyperoxia in the choroid. Hence we have studied the effect of NO synthase inhibition by NG-monomethyl-L-arginine (L-NMMA, 3 mg/kg over 5 min as a bolus followed by a continuous infusion of 30 micrograms.kg-1.min-1) on the changes of cerebral and ocular hemodynamic parameters elicited by hypercapnia and hyperoxia in healthy young subjects. Mean flow velocities in the middle cerebral artery and the ophthalmic artery were measured with Doppler ultrasound, and ocular fundus pulsation amplitude, which estimates pulsatile choroidal blood flow, was measured with laser interferometry Administration of L-NMMA reduced ocular fundus pulsation. (-19%, P < 0.005) but only slightly reduced mean flow velocities in the larger arteries. Hypercapnia (PCO2 = 48 mmHg) significantly increased mean flow velocities in the middle cerebral artery (+26%, P < 0.01) and fundus pulsation amplitude (+16%, P < 0.005) but did not change mean flow velocity in the ophthalmic artery. The response to hypercapnia in the middle cerebral artery (P < 0.05) and in the choroid (P < 0.05) was significantly blunted by L-NMMA. On the contrary, L-NMMA did not affect hyperoxia-induced (PO2 = 530 mmHg) hemodynamic changes. The hemodynamic effects of L-NMMA (at baseline and during hypercapnia) were reversed by coadministration of L-arginine. The present study supports the concept that NO has a role in hypercapnia induced vasodilation in humans.

Renal and ocular hemodynamic effects of insulin

There is evidence that the vasodilator action of insulin is mediated by the release of nitric oxide (NO). We hypothesized that euglycemic hyperinsulinemia might increase renal and ocular blood flow, and that the vasodilator capacity of insulin might be NO-dependent. Euglycemic insulin clamps were performed in 10 healthy subjects. Sixty minutes after the start of insulin administration, an intravenous coinfusion of N-monomethyl-L-arginine (L-NMMA), an inhibitor of NO synthase, or of norepinephrine (NE), an endothelium-independent vasoconstrictor, was started. Renal plasma flow was measured by para-aminohippurate (PAH) clearance method. Ocular hemodynamics were assessed by laser interferometric measurement of fundus pulsations and Doppler sonographic measurement of blood flow velocity in the ophthalmic artery. Renal plasma flow and ocular fundus pulsations were increased by insulin. L-NMMA almost completely abolished the vasodilative effects of insulin, whereas the effects of combined infusion of insulin and NE were approximately the sum of the hemodynamic changes induced by each agent alone. The results show that during euglycemic hyperinsulinemia, renal and ocular blood flow are increased, which may be mediated either by a local vasodilator effect or a systemic increase in flow. The hemodynamic effects of insulin in the kidney and the eye are at least partially dependent on NO synthesis. Because the insulin plasma levels we obtained are in the high physiological range, it may be assumed that insulin plays a role in renal and ocular blood flow regulation.

Effects of changes in intraocular pressure on human ocular haemodynamics

PURPOSE

Myogenic autoregulation is the ability of a vascular bed to maintain blood flow despite changes in perfusion pressure. Ocular perfusion pressure is defined as the difference between ocular arterial pressure and ocular venous pressure, the latter dependent on intraocular pressure (IOP). The aim of the present study was to investigate the effect of moderate increases in IOP on ocular haemodynamics.

METHODS

Changes in IOP (+ 10 mmHg, +20 mmHg) were induced by a suction cup in 10 healthy subjects. Ocular fundus pulsations in the macula and the optic disc were measured by laser interferometry; blood flow velocities in the central retinal artery (CRA) and in the ophthalmic artery (OA) were measured by Doppler sonography.

RESULTS

Changes in IOP caused a significant reduction in fundus pulsations, which was more pronounced in the macula (at +10 mmHg: -9 +/- 2%, p < 0.01; at +20 mmHg: -19 +/- 3%, p < 0.001) than in the optic disc (at +10 mmHg: -5 +/- 2% (ns); at +20 mmHg: -9 +/- 3%, p < 0.01). Mean flow velocity in the CRA was reduced by -5 +/- 3% at +10 mmHg (ns) and by -14 +/- 5% at +20 mmHg (p < 0.005), resistive index was increased by +4 +/- 1% at +10 mmHg (p < 0.05) and by +6 +/- 2% at +20 mmHg (p < 0.01). In contrast, a rise in IOP did not affect blood flow parameters in the OA.

CONCLUSIONS

Our results from fundus pulsation measurements indicate that choroidal blood flow decreases when IOP is increased. The Doppler sonographic findings in the CRA indicate reduced blood flow velocity in this artery during raised IOP.

Biometric investigation of changes in the anterior eye segment during accommodation

Non-invasive biometry of the anterior structures of the human eye can be performed with unprecedented precision of 8-10 microns and a resolution of approximately 9 microns by partial coherence interferometry, which has the potential to assess the effect of cycloplegia on the ocular components of the anterior eye segment, to further improve the precision to 1-2 microns by the use of these agents and to quantify the amount of residual accommodations in different states of cycloplegia. In addition, the anterior chamber depth, the thickness of the crystalline lens, their changes during accommodation, as well as the movement of the anterior and posterior lens pole during accommodation can be quantified objectively and accurately to investigate the mechanism of accommodation.

Effects of losartan on cerebral and ocular circulation in healthy subjects

AIMS

The introduction of specific inhibitors of AT1 receptors, such as losartan, has enabled the investigation of the renin-angiotensin-system (RAS) in humans in vivo. We studied the role of the RAS in the cerebral and ocular circulation in healthy subjects. Haemodynamic effects of orally administered losartan were investigated with non-invasive methods.

METHODS

In a placebo-controlled randomized, double-blind two way crossover design losartan (100 mg orally) or placebo was administered in 10 healthy subjects. The effect of losartan was studied at hourly intervals for 8 h. In addition, the effect of losartan on haemodynamic changes induced by exogenous angiotensin II (Ang II) was assessed. Blood flow velocities in the ophthalmic and the middle cerebral artery (OA, MCA) were measured with Doppler sonography. Pulsatile choroidal blood flow was estimated with laser interferometric measurement of fundus pulsation.

RESULTS

Losartan significantly increased fundus pulsation amplitude (+11%, 95% CI: 5 to 16% P<0.0001), tended to increase resistive index in the ophthalmic artery (+12%, 95% CI: 0 to 23%) and tended to decrease mean arterial pressure (-15%, 95% CI: -23 to -1%). Ang II induced effects on cerebral, ocular and systemic haemodynamics were prevented by preceding administration of losartan.

CONCLUSIONS

The present data suggest that Ang II is not a major determinant of cerebral and ocular blood flow in vivo. The observed changes in cerebral and ocular haemodynamic parameters after losartan administration reflect effects on systemic blood pressure.

Ocular and systemic reactivity to isoprenaline in patients with insulin-dependent diabetes mellitus

There is experimental evidence of decreased beta-adrenergic myocardial sensitivity in patients with insulin-dependent diabetes mellitus (IDDM). In the present study we hypothesized that the ocular response to isoprenaline, as a consequence of increased arterial vessel rigidity, might also be blunted in patients with IDDM. We therefore compared the correlation between systemic pulse pressure amplitude (PPA) and fundus pulsation amplitude (FPA) during intravenous isoprenaline administration in 11 otherwise healthy IDDM patients and 11 healthy control subjects. Ocular fundus pulsations were measured by a recently developed laser interferometric method. Isoprenaline increased PPA in both study groups in a dose-dependent way, but the response was significantly less in IDDM patients (at 0.8 microgram min-1: +38% in control subjects, +27% in IDDM patients, P < 0.05 between groups). Moreover, a dose-dependent increase in FPA was observed, which again was more pronounced in healthy subjects (at 0.8 microgram min-1: +45% in controls, +17% in IDDM patients, P < 0.005 between groups). The regression line between PPA and FPA was very close to the 45 degrees line in healthy subjects, whereas it was significantly flattened in IDDM patients. In conclusion, linear regression between PPA and FPA during isoprenaline suggests arterial stiffening in patients with IDDM. Hence, comparison of systemic PPA and FPA during isoprenaline provocation may be a useful method of estimating changes in arterial capacitance in patients with diabetes mellitus.

Effects of endothelin-1 (ET-1) on ocular hemodynamics

PURPOSE

There is evidence from in vitro and animal data that endothelin-1 (ET-1) plays an important role in ocular blood flow. The aim of the present study was to investigate the effect of systemic ET-1 administration on ocular circulation in healthy subjects.

METHODS

In a double blind, placebo-controlled, randomized, 2-way cross over study in 10 healthy male subjects, we administered stepwise, increasing doses of ET-1 (0 (saline), 1.25, 2.5 and 5.0 ng/kg/minutes; 20 minutes per dose (level) or placebo. Blood flow velocity in the ophthalmic artery as well as ocular fundus pulsations in the macula and the optic disc, and systemic hemodynamic parameters were measured.

RESULTS

ET-1 dose-dependently reduced fundus pulsations in the macula (maximum effect -12 +/- 2% versus baseline; p < 0.001 versus baseline and placebo) and the optic disc (maximum effect: -19 +/- 5% versus baseline; p < 0.001 versus baseline and placebo), but did not affect blood flow velocity in the ophthalmic artery or systemic hemodynamics.

CONCLUSIONS

Endothelin-1 reduces pulsatile blood flow in the choroid and the optic disc at doses which do not affect systemic hemodynamics or flow velocity in the ophthalmic artery. These results indicate that ocular circulation is particularly sensitive to changes in local ET-1 concentration and confirms the hypothesis that ET-1 may play a role in ocular vascular diseases.

Submicrometer precision biometry of the anterior segment of the human eye

PURPOSE

To demonstrate the feasibility of measuring the anterior structures of the human eye by partial coherence interferometry and to determine its precision for eyes under normal and cycloplegic conditions.

METHODS

The dual-beam version of partial coherence interferometry, a recently developed noninvasive optical ranging technique, enables high resolution measurements of several intraocular distances with unprecedented precision. A modified, more sensitive scanning version of this technique was used to assess the central and peripheral corneal thickness, the anterior chamber depth, and the lens thickness of 20 healthy, emmetropic to moderately myopic eyes. Furthermore the anterior structures of three eyes were measured under cycloplegia (1% cyclopentolate) to investigate the influence on the precision of this technique after suppression of residual accommodations.

RESULTS

The mean geometric precision (standard deviation) of the measurement of the central corneal thickness was 0.29 micron (range, 0.22 micron to 0.38 micron) and 0.43 micron (range, 0.27 micron to 0.56 micron) for the peripheral corneal thickness at a distance 2 mm from its apex. The precision for measuring the anterior chamber depth and the lens thickness for fixation at infinity was 8.7 microns (range, 3.9 microns to 16.8 microns) and 8.9 microns (rang, 2.9 microns to 14.4 microns) for noncycloplegic eyes and 1.9 microns (range, 1.7 microns to 2 microns) and 1.4 microns (range, 0.7 micron to 1.8 microns) for cycloplegic eyes, respectively.

CONCLUSIONS

The dual-beam partial coherence interferometry enables fast, noninvasive, submicrometer precision biometry of the anterior segment of the eye. The precision of determining the anterior chamber depth and the lens thickness is more than one order of magnitude better than that of the currently used ultrasound and optical techniques, and it can be improved by a factor of 5 by using cycloplegia.

Dark-adapted thresholds in children with histories of mild retinopathy of prematurity

PURPOSE

To test the hypothesis that rod-mediated visual thresholds at 10 degrees eccentricity are elevated in children with histories of mild retinopathy of prematurity (ROP).

METHODS

Dark-adapted thresholds for detection of 50 msec, 2 degrees diameter spots at a 10 degrees eccentric site, and at a peripheral reference site, 30 degrees eccentric, were measured in 20 children with a history of mild ROP and known courses of refractive development. Ten myopic control subjects also were tested. The thresholds of the ROP and control subjects were compared.

RESULTS

Six of the subjects with ROP had elevated thresholds at the 10 degrees site. High myopia had been present since age 18 months or younger in each of the six. The thresholds of all other subjects with ROP, whose courses of refractive development had been toward emmetropia, and the control subjects with myopia were normal. In subjects with ROP, the association of early, persistent high myopia and an elevated threshold at 10 degrees was significant (chi 2 = 20; P < 0.01). Among the subjects with ROP, refractive error and axial length were correlated.

CONCLUSIONS

ROP or factors causing ROP appear to alter rod-mediated retinal function. The association of abnormal rod-mediated sensitivity and refractive development suggests that rod-mediated retinal function is involved in the regulation of eye growth in children with a history of mild ROP.

Nitric oxide synthase inhibition in the histamine headache model

Histamine has been widely used experimentally to induce headache in healthy subjects and migraine in migraineurs. There is evidence that the vascular effects of histamine are at least partially mediated by nitric oxide (NO). Hence we hypothesized that subjective symptoms and hemodynamic effects of histamine could be reduced by systemic NO-synthase inhibition. We therefore studied the effect of pretreatment with N-monomethyl-L-arginine (L-NMMA), a competitive inhibitor of NO-synthase, or placebo on headache, flush and discomfort scores during histamine infusion. Additionally, blood flow velocities in the middle cerebral and the ophthalmic artery and ocular fundus pulsations were measured. Whereas L-NMMA blunted the effect of histamine in the ophthalmic artery and the ocular circulation, NO-synthase inhibition did not mitigate subjective symptoms. Histamine did not affect mean blood flow velocities in the middle cerebral artery. Hence, we conclude that NO-synthase inhibition reduces the histamine-induced vascular effects in the ocular circulation, but is not sufficient to attenuate or abort the subjective symptoms provoked by histamine infusion.