Reproducibility and sensitivity of scanning laser Doppler flowmetry during graded changes in PO2

Assessment of optic disk blood flow in patients with open-angle glaucoma

PURPOSE

To characterize optic disk blood flow in patients with open-angle glaucoma compared with age-matched healthy control subjects.

METHODS

In this prospective cross-sectional study, 90 eyes of 90 patients with open-angle glaucoma and 61 eyes of 61 age-matched healthy control subjects were evaluated. Flow in the optic disk cup and the neuroretinal rim were assessed with scanning laser Doppler flowmetry. Fundus pulsation amplitude in the cup and the macula were assessed with laser interferometry. Visual field mean deviation was measured with the Humphrey 30 to 2 program.

RESULTS

Flow in the neuroretinal rim (-18%, P =.002), and in the cup (-46%, P <.001) and fundus pulsation amplitude in the cup (-33%, P <.001) and in the macula (-24%, P <.001) were significantly lower in patients with open-angle glaucoma compared with healthy control subjects. A significant association between blood flow measurements in the cup and fundus pulsation amplitudes in the cup was observed in both study cohorts. A significant association was also observed between the mean defect from visual field testing and ocular hemodynamic parameters.

CONCLUSIONS

Reduced optic disk perfusion in patients with open-angle glaucoma is evidenced from two independent methods in the present study. Moreover, our data indicate that reduced ocular blood flow in these patients is linked to visual field changes. It remains to be established whether compromised optic disk and choroidal blood flow contributes to optic disk damage in glaucomatous eyes or is a secondary functional phenomenon.

High-altitude retinopathy and retinal vascular dysregulation

PURPOSE

(a) To show that high-altitude retinopathy (HAR) is common at high altitudes even in well-acclimatised climbers and that it should not be regarded as part of the spectrum of benign mountain sickness but rather as a clinical sign with a separate aetiology. (b) To test the hypothesis that HAR could be interpreted as a clinical expression of 'ocular vascular dysregulation'.

METHODS

Both eyes of the 8 mountaineers of the First Vienna Himalayan Expedition in May/June 1996 were examined 2 weeks before departure to and 2 weeks after descent from a high altitude. Retinal blood flow was measured in the right eyes of 7 climbers, using the Heidelberg Retina Flowmeter (HRF).

RESULTS

Two of the 8 climbers had bilateral retinal haemorrhage after the expedition. In 5 climbers chronic hypoxic exposure caused an increase in retinal blood flow between +18% and +96%, and in 2 climbers a decrease in retinal blood flow between -21% and -31%. The 2 climbers (climbers 1 and 2) with bilateral retinal haemorrhage showed a significant increase in HRF parameters.

CONCLUSIONS

HAR may be a clinical sign of mountaineers with a tendency towards ocular vascular dysregulation. The pronounced increase in all haemodynamic parameters in the 2 climbers with retinal haemorrhage combined with a dilated epipapillary network 2 weeks after the exposure reflects a retinal vessel configuration, as might be expected at high altitudes under acute hypoxic stress. An inadequate autoregulatory response of the retinal circulation under conditions of chronic hypoxia may play an important part in the pathogenesis of HAR.

Ocular haemodynamics and colour contrast sensitivity in patients with type 1 diabetes

BACKGROUND

There is evidence that altered ocular blood flow is involved in the development and progression of diabetic retinopathy. However, the nature of these perfusion abnormalities is still a matter of controversy. Ocular haemodynamics were characterised with two recently introduced methods.

METHODS

The cross sectional study was performed in 59 patients with type 1 diabetes with a diabetes duration between 12 and 17 years and an age less than 32 years and a group of 25 age matched healthy controls. Scanning laser Doppler flowmetry and laser interferometric measurement of fundus pulsation amplitude were used to assess retinal and pulsatile choroidal blood flow, respectively. In addition, colour contrast sensitivity along the tritan axis was determined.

RESULTS

Fundus pulsation amplitude, but not retinal blood flow, increased with the progression of diabetic retinopathy. Retinal blood flow was influenced by plasma glucose levels (r = 0.32), whereas fundus pulsation amplitude was associated with HbA(1c) (r = 0.30). In addition, a negative correlation between the colour contrast sensitivity along the tritan axis and retinal blood flow was observed.

CONCLUSIONS

The present study indicates that pulsatile choroidal blood flow increases with the progression of diabetic retinopathy. Increased retinal blood flow appears to be related to loss of colour sensitivity in patents with type 1 diabetes.

The effect of brimonidine tartrate on retinal blood flow in patients with ocular hypertension

PURPOSE

To study the effects of topical brimonidine tartrate 0.2%, an alpha(2)-agonist ocular hypotensive drug, on retinal capillary blood flow in patients with ocular hypertension.

METHODS

The study was a double-masked, randomized, placebo-controlled trial set in a tertiary eye center. Ocular hypertensive patients with repeatable intraocular pressures greater than 21 mm Hg and normal visual fields and optic disks were consecutively recruited. After an eye examination, baseline retinal blood flow measurements were made with confocal scanning laser Doppler flowmetry in one study eye. Patients were then randomly assigned to receive either brimonidine or placebo (saline) twice daily for 8 weeks. Blood flow and intraocular pressure measurements were then repeated after 4 and 8 weeks.

RESULTS

Seventeen patients were randomly assigned to receive brimonidine, and 14 received placebo. One patient in each group failed to complete the study. The mean group differences in baseline age and intraocular pressure were not statistically significant (59. 23 [+/-10.24] and 52.23 [+/-16.46] years, respectively, and 24.84 [+/-2.08] and 24.56 [+/-2.85] mm Hg, respectively). Brimonidine reduced intraocular pressure by 17.90% and 16.17% at 4 and 8 weeks, respectively, with a significant difference in treatment effect compared with the placebo group (P <.007). The group difference in treatment effect in any of the three hemodynamic parameters velocity, volume, and flow was within 8% and not significantly different at 4 or 8 weeks (P.360). Based on a type I error of 0.05, our study had a power greater than or equal to 75% to detect group differences in treatment effect of greater than or equal to 15% to 20%.

CONCLUSIONS

Brimonidine reduces intraocular pressure without altering retinal capillary blood flow in patients with ocular hypertension.

A comparison between laser interferometric measurement of fundus pulsation and pneumotonometric measurement of pulsatile ocular blood flow. 2. Effects of changes in pCO2 and pO2 and of isoproterenol

PURPOSE

We have shown in the companion paper that, under baseline conditions, there is a high degree of association between laser interferometrically measured fundus pulsation amplitude (FPA) and pneumotonometrically measured pulse amplitude (PA) and pulsatile ocular blood flow (POBF). The present study investigated the effect of high pCO2, of high pO2 and of isoproterenol on POBF as assessed with laser interferometry and pneumotonometry.

METHODS

Pneumotonometry and laser interferometry were performed in young healthy subjects during breathing of 100% O2 (n = 10; hyperoxia) and of 5% CO2 + 95% air (n = 8; hypercapnia). In addition these parameters were studied during stepwise increasing doses of isoproterenol, a beta-receptor agonist (n = 8).

RESULTS

Inhalation of 5% CO2 + 95% air increased FPA (24 +/- 12%, p < 0.001), PA (26 +/- 13%, p < 0.001) and POBF (15 +/- 8%, p = 0.002). Inhalation of 100% O2 decreased FPA (-5 +/- 7%, p = 0.027), but did not change PA or POBF. The effect of 100% O2 inhalation on FPA in the optic disc was more pronounced (-11% to -20%) than in the macula. Isoproterenol caused a dose-dependent increase in FPA, PA and POBF (p < 0.001). The association between the induced changes in FPA and PA or POBF was highly significant.

CONCLUSIONS

The present study shows that FPA can be taken as a valid relative measure of pulsatile choroidal blood flow. Our results in the optic disc indicate that FPA at the neuroretinal rim and at the cup is influenced by retinal and choroidal circulation.

The effect of brimonidine tartrate on retinal blood flow in patients with ocular hypertension [corrected]

PURPOSE

To study the effects of topical brimonidine tartrate 0.2%, an alpha2-agonist ocular hypotensive drug, on retinal capillary blood flow in patients with ocular hypertension.

METHODS

The study was a double-masked, randomized, placebo-controlled trial set in a tertiary eye center. Ocular hypertensive patients with repeatable intraocular pressures greater than 21 mm Hg and normal visual fields and optic disks were consecutively recruited. After an eye examination, baseline retinal blood flow measurements were made with confocal scanning laser Doppler flowmetry in one study eye. Patients were then randomly assigned to receive either brimonidine or placebo (saline) twice daily for 8 weeks. Blood flow and intraocular pressure measurements were then repeated after 4 and 8 weeks.

RESULTS

Seventeen patients were randomly assigned to receive brimonidine, and 14 received placebo. One patient in each group failed to complete the study. The mean group differences in baseline age and intraocular pressure were not statistically significant (59.23 [+/-10.24] and 52.23 [+/-16.46] years, respectively, and 24.84 [+/-2.08] and 24.56 [+/-2.85] mm Hg, respectively). Brimonidine reduced intraocular pressure by 17.90% and 16.17% at 4 and 8 weeks, respectively, with a significant difference in treatment effect compared with the placebo group (P < .007). The group difference in treatment effect in any of the three hemodynamic parameters velocity, volume, and flow was within 8% and not significantly different at 4 or 8 weeks (P > .360). Based on a type I error of 0.05, our study had a power greater than or equal to 75% to detect group differences in treatment effect of greater than or equal to 15% to 20%.

CONCLUSIONS

Brimonidine reduces intraocular pressure without altering retinal capillary blood flow in patients with ocular hypertension.

Scanning laser ophthalmoscopy-based evaluation of epipapillary velocities: method and physiologic variability

PURPOSE

Scanning laser ophthalmoscopy is capable of producing high-resolution fluorescein angiograms. Measurements of capillary blood velocities in the fovea are well established. In this study, we used an identical technique to measure particle velocities in the superficial layers of the optic nerve head. We compared these data with simultaneous velocity measurements in the retinal and macular vasculature.

METHODS

Fluorescein angiograms were performed in 30 subjects (12 men, 18 women; mean age, 26 years; standard deviation [SD], 3 years). Off-line, the velocities of hypofluorescent particles through the microvasculature of epipapillary and macular vessels were measured by image-processing techniques. We also assessed the arteriovenous passage (AVP) time of the fluorescein dye front.

RESULTS

Epipapillary blood velocities ranged from 2.7 to 6.5 mm/sec (mean, 4.0 mm/sec; SD, 0.99 mm/sec) and differed significantly from macular capillary blood velocities (MCBVs), which ranged from 1.67 to 3.31 mm/sec (mean, 2.53 mm/sec; SD, 0.34 mm/sec). The AVP time in all subjects was 1.74 +/- 0.5 sec (mean +/- SD) and correlated with the MCBV (P < 0.05, r = -0.46). Epipapillary blood velocities showed no correlation with either AVP time or MCBV.

CONCLUSIONS

The scanning laser methodology, adapted to objectively assess velocities in the epipapillary vasculature, shows that these measurements are neither correlated with velocities in the perifoveal network, nor inversely correlated with overall retinal transit time. Epipapillary blood velocities were found to be substantially higher than those measured in macular capillaries. This result might be explained by the different anatomy of epipapillary vessels compared with macular capillaries.

Peripapillary retinal blood flow in normal tension glaucoma

AIMS

To determine if normal tension glaucoma (NTG) patients differ from age matched controls in blood flow to the peripapillary retina, as measured with confocal scanning laser Doppler flowmetry (cSLDF; "Heidelberg retinal flowmetry").

METHODS

12 NTG patients and 12 age matched controls were compared using (a) 10 x 10 pixel boxes (the instrument default sample size), taken from the nasal and temporal peripapillary retina, (b) the average from two of these boxes, and (c) every qualifying pixel within the peripapillary retina.

RESULTS

Patients and controls did not differ in blood flow measured using the default sample from a single 10 x 10 pixel box, placed in either the temporal or nasal peripapillary retina, or expressed as the average from these two boxes. However, in histograms using every pixel from the peripapillary retina, NTG patients displayed significantly higher percentages of minimal flow pixels (defined as less than one arbitrary unit of flow: 30% v 19%, p < 0.01), and significantly lower flow in the 25th, 50th, and 75th percentile flow pixel (each p < 0.05) than did age matched controls.

CONCLUSION

NTG is characterised by reduced blood flow in the peripapillary retina, a result suggesting that blood flow deficits accompany, and perhaps may contribute to, disease development in these patients.

Automatic full field analysis of perfusion images gained by scanning laser Doppler flowmetry

BACKGROUND

Scanning laser Doppler flowmetry (SLDF) enables the measurement of the laser Doppler frequency shift in retinal tissue. This process allows the quantification of retinal and optic nerve head perfusion in an area of 2.7 mm x 0.7 mm within 2 seconds and with a spatial resolution of 10 microns x 10 microns. Owing to the local heterogeneity of the retinal microcirculation itself and to heart associated pulsation the capillary retinal blood flow depends on location and time. Because of technical limitations measurements of flow are only valid in retinal points with adequate brightness and focus, and away from big vessels. To include the heart beat associated pulsation and the spatial heterogeneity of retinal blood flow into the evaluation of blood flow an algorithm was developed examining automatically the whole SLDF perfusion image.

AIM

To report intraobserver reliability and interobserver reliability of a new method for analysing automatically full field perfusion images.

METHOD

The base of blood flow calculation by the automatic full field perfusion image analyser (AFFPIA) was 16,384 intensity time curves of all pixels of the whole perfusion image gained by the SLDF. AFFPIA calculates the Doppler frequency shift and the haemodynamic variables flow, volume, and velocity of each pixel. The resulting perfusion image was processed with respect to (1) underexposed and overexposed pixels, (2) saccades, and (3) the retinal vessel tree. The rim area and the saccades were marked interactively by the operator. The capillaries and vessels of the retinal vessel tree were identified automatically by pattern analysis. Retinal vessels with a diameter greater than 30 microns, underexposed or overexposed areas, and saccades were excluded automatically. Based on the whole perfusion image total mean flow, total mean volume, total mean velocity, standard deviation, cumulative distribution curve of flow, and the capillary pulsation index were calculated automatically. Heart beat associated pulsation of capillary blood flow was estimated by plotting the mean capillary flow of each horizontal line against time. Intraobserver reliability was estimated by measuring 10 eyes of 10 subjects on five different days by one observer. Interobserver reliability of AFFPIA was evaluated by analysing 10 perfusion maps by five different operators. To find a baseline of retinal blood flow, perfusion maps of 67 eyes of normal subjects with a mean age of 40.4 (SD 15) years were evaluated by AFFPIA.

RESULTS

The coefficient of reliability of the intraobserver reproducibility of flow was 0.74. The coefficient of reliability of the interobserver reproducibility was 0.95. The juxtapapillary retinal capillary flow was temporally 484 (SD 125), nasally 450 (117); the rim area capillary flow was 443 (110). The mean capillary pulsation index of retinal flow was 0.56 (0.14).

CONCLUSION

Retinal blood flow evaluation by the AFFPIA increases significantly the interobserver reliability compared with conventional evaluation of 100 microns x 100 microns areas in SLDF images with the original Heidelberg retina flowmeter software. The intraobserver reliability of AFFPIA was in the same range as conventional evaluation.