The blood flow velocity and systolic acceleration measurements in the ophthalmic and central retinal arteries and veins, and in the short posterior ciliary arteries were performed in 100 orbits of 50 glaucoma patients.

The average age of glaucoma patients was 63.0 ± 11.2 years (range 37 to 92 years). Diagnosis of the patients was as follows: 28 patients with primary open-angle glaucoma, 7 with normal tension glaucoma, 5 with primary angle closure glaucoma and 10 with suspected glaucoma.

The systolic peak, the end-diastolic and the time-averaged velocities, the systolic acceleration and systolic acceleration time were measured in arteries, and resistivity (RI) and pulsatility (PI) indices were calculated according to the following equations: RI = peak systolic velocity minus end-diastolic velocity divided by peak systolic velocity and PI = peak systolic velocity minus end-diastolic velocity divided by time-averaged mean velocity. In veins, the maximum and minimum flow velocities were measured. For measurements, Ultramark-9 HDI ultrasound equipment with a 5-10 MHz linear probe and a 5 MHz sector probe was applied. Angle correction was performed (6-7).

Repeated velocity and systolic acceleration measurements were performed by the same operator within one session. The mean and standard deviation of these repeated measurements was calculated. The reproducibility of the measurement is characterised as a ‘coefficient of variation’ (COV) which is calculated as the standard deviation of the repeated measurements, for each orbit, averaged for all orbits, expressed as a percentage of the mean value of the parameter for all the orbits.

The study was approved by the Regional Committee of Science and Research Ethics of Semmelweis Medical University. Informed consent was obtained from all subjects.

The reproducibility coefficients (COV) of 9 blood flow parameters, determined for 3 arteries and 2 veins measured in glaucoma patients, are summarised in Tables 1 and 2.

Slightly different reproducibilities were found for measurements in the three different arteries: the reproducibility was best in the ophthalmic artery, less good in the central retinal artery, and least good in the ciliary arteries (Table 1). The velocity determinations proved to be less reproducible in the ophthalmic vein than in the central retinal vein or in the arteries.

The peak systolic velocity seems to be the most reproducible velocity parameter and the end-diastolic velocity determination is less reproducible (Table 1). The reproducibility of time-averaged velocity is intermediate between that for the other two velocity measurements (Table 1).

The reproducibility coefficient of the systolic acceleration was between 10.3%-17.6%, and that of the systolic acceleration time between 7.7% and 12.0% (Table 1).

By far the most reproducible flow parameter is the resistivity index while the pulsatility index had slightly worse reproducibility (Table 1).

The flow velocity determination was found to be more reproducible in the central retinal vein than in the ophthalmic vein (Table 2).

Table 1. Reproducibility of blood flow parameters in orbital arteries determined by means of repeated measurements in 100 glaucomatous eyes using color Doppler imaging. Table shows the average standard deviation as a percentage of the average value for each parameter (coefficient of variation: COV). Abbreviations: OA: Ophthalmic artery, CRA: Central retinal artery, NSPCA: Nasal short posterior ciliary artery, TSPCA: Temporal short posterior ciliary artery.

                              OA      CRA     NSPCA   TSPCA
--------------------------------------------------------------
Peak systolic velocity        6,4     9,3      9,6    13,7
End-diastolic velocity       10,5    14,9     18,8    16,9
Time-averaged velocity        7,9     9,9     13,6    14,0
Systolic acceleration        10,3    12,0     11,2    17,6
Systolic acceleration time    9,5     9,9     10,9     7,7
Resistivity index             2,9     4,8      3,2    10,4
Pulsatility index             6,7     9,2      7,0     9,4

Table 2. Reproducibility of blood flow velocity measurements in orbital veins determined by means of repeated measurements in 100 glaucomatous eyes using color Doppler imaging. Table shows the coefficient of variation (%) for each parameter.

                    Ophthalmic vein       Central retinal vein
----------------------------------------------------------------
Maximum velocity        14.3                     10.2
Minimum velocity        16.2                     11.7

Based on literature data, the diagnostic value and reproducibility of CDI velocity measurements are dependent on the vessel which is being examined. In case of orbital vessels, good reproducibility was reported (1, 4, 10).

Our results confirm that high reproducibility of color Doppler blood-velocity measurements in orbital vessels is obtainable. The resistivity index (RI) exhibited the highest reproducibility (COV between 2.9% and 10.4%). The pulsatility index (PI) exhibited somewhat less good reproducibility (COV: 6.7-9.4%) as it is recommended for use in the case of large vessels. The RI and PI parameters have the advantage that they require no angle correction even if the flow direction is not aligned with the ultrasonic beam.

Among velocity values, the peak systolic velocity measurements proved to be the most reliable (COV between 6.4% and 13.7%). The end-diastolic and time-averaged velocity determinations proved to be less accurate. Possible cause might be that the end-diastolic velocity is more influenced by the slight pressure elevation in the orbit caused by the contact of the ultrasound probe during the examination and the lower velocities in small ocular vessels might be close to the resolution for detection by the Doppler measuring instrument.

We found relatively good reproducibility during velocity measurements in ophthalmic and central retinal veins (COV between 10.2% and 16.2%). In our experience, a very small contact through the gel is sufficient to get a good quality ultrasound image of the orbit, and this contact does not greatly influence the velocity measurements.

We found that the reproducibility of the systolic acceleration time was also good (COV between 9.5% and 10.9%). The repeated measurements of systolic acceleration exhibited somewhat higher variability (COV: 10.3-17.6%). To the best of our knowledge, there is no published data concerning the reliability of systolic acceleration and acceleration time measurements in orbital vessels.

Concerning the reproducibility, we found slight differences between different vessels. The easiest case is the imaging of the central retinal artery and vein. The imaging of the ophthalmic artery deep in the orbit is a little more difficult because of the ultrasound shadowing effect of the optic nerve, but in our case it exhibited the highest reproducibility. We preferred to use a 5 MHz probe for the examination of the ophthalmic artery as it made the imaging of this vessel deep in the orbit easier. The most difficult measurements are in the short ciliary arteries as their number might be as much as 20 and they have a great variation in their course. We however in our results found only slight differences in variability between the various arteries which might be accidental, or it might be a consequence of our careful examination protocol.

In conclusion, our results have demonstrated high intra-observer reproducibility of flow velocity, systolic acceleration and resistivity index measurements in the ophthalmic and central retinal arteries and veins; this might give the potential for clinical and follow-up studies. The resistivity index was found to be the most reliable flow parameter. The measurement of peak systolic and maximum velocities, systolic acceleration and systolic acceleration time can also be performed with good reproducibility. Less reliable is the determination of end-diastolic velocity in all vessels; thus for this parameter only relatively large changes would be demonstrable as pathologic alterations during a time-sequence of examinations.

The study was supported by a grant from the National Scientific Research Foundation of Hungary (OTKA T 017764).

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