Dynamic contour tonometry is a reliable method for intraocular pressure measurement, according to study by German researchers. Unlike Goldmann applanation tonometry, DCT is not influenced by central corneal thickness, and in clinical practice advantages from dynamic contour tonometry may be expected for cooperative patients, outpatients, and patients with sufficient bilateral ocular fixation. GAT measurements are more reliable in case of patients with inadequate cooperation, poor vision or nystagmus.

DCT (Swiss Microtechnology AG, Port, Switzerland) was recently introduced as a new method of IOP measurement, supposedly independent of corneal properties. The prospective study included 100 eyes without glaucoma, and included measurement of CCT, corneal curvature, and IOP using DCT and GAT. It analyzed the agreement and correlation of DCT and GAT and investigated the influence of central corneal thickness and corneal curvature; they also considered preferential patient groups for both methods.

They found a clear correlation between DCT and GAT (r=0.693; p<0.001). DCT generally resulted in higher IOP measurements (median difference + 1.8 mmHg, mean difference + 2.34 mmHg). Unlike DCT, GAT was remarkably affected by central corneal thickness, the study found, but neither method was significantly influenced by corneal curvature. Bland-Altman graphs showed remarkable disagreement between DCT and GAT, which could be partially explained by the influence of CCT on GAT. To obtain valid readings, DCT required a more extensive selection of patients than Goldmann applanation tonometry.

(J Glaucoma 2006;15:2-6)

Schneider, E, Grehn, F.

OCT3 Thickness May Differ

Researchers at the New England Eye Center took a look at normal macular thickness measurements in healthy patients using the latest commercially available optical coherence tomography (OCT) mapping software, version 3.0, from the Stratus OCT (OCT3). Thickness measurements, they report, may differ from earlier OCT technology.

Thirty-seven eyes from 37 individuals underwent the complete ophthalmologic examination. Foveal thickness (mean thickness in the central 1000-µm diameter area) and central foveal thickness (mean thickness at the point of intersection of six radial scans) on the OCT3 were 212 ±20 and 182 ±23 µm, respectively. Macular thickness measurements were thinnest at the center of the fovea, thickest within 3-mm diameter of the center, and diminished toward the periphery of the macula. The temporal quadrant was thinner than the nasal quadrant. Central foveal thickness was also manually determined as 170 ±18 µm, approximately 12 µm less than the value automatically obtained from the OCT3 software. There was no correlation between age and foveal thickness (p=0.80).

Mean foveal thickness measurements were 38 to 62 µm thicker than previously reported values, while mean central foveal thickness measurements were 20 to 49 µm thicker than previously published values. This discrepancy should be considered when interpreting OCT scans, the researchers say.

The researchers observed, "The macular thickness measurements for healthy eyes are higher than the values obtained using earlier versions" of the apparatus, and they conclude that as this particular tool becomes more widely available, "normative data will be important in interpreting pathological features of the macula."

They also comment that OCT "has emerged as a useful imaging technique by providing new high-resolution, cross-sectional information about various pathological features of the macula."

(Arch Ophthalmol. 2006;124:193-198)

Chan A, Duker JS, Ko TH, Fujimoto JG, Schuman JS.


Learning Effect for SWAP Patients with OHT, OAG with SAP

Test results from patients inexperienced in SWAP testing may require additional attention, according to a recent study. During the initial examinations, apparent field loss with short-wavelength automated perimetry in patients exhibiting normal field by standard automated perimetry may relate to inexperience with SWAP. Apparently deeper or wider field loss in the initial examinations with SWAP compared with that exhibited by SAP in open-angle glaucoma also may arise from inexperience in SWAP.

This experimental study looked at 35 patients (22 with ocular hypertension and 13 with OAG) who had previously undergone at least three threshold SAP visual field examinations with the Humphrey Field Analyzer, and nine patients with OHT who had not previously undertaken any form of perimetry, to document the magnitude of any learning effect for SWAP.

Each patient was tested on SWAP on five occasions, each separated by one week. At each visit, both eyes were examined using Program 24-2 of the HFA; the right eye was always examined before the left eye. The change over the five examinations, in each eye, of the visual field indices Mean Deviation (MD), Short-term Fluctuation (SF), Pattern Standard Deviation (PSD), and Corrected Pattern Standard Deviation. Change in each eye between the first and fifth visit in proportionate Mean Sensitivity (pMS) for the central annulus of stimulus locations was compared with that for the peripheral annulus thereby determining the influence of stimulus eccentricity on any alteration in sensitivity. Change between the first and fifth visit in the number and magnitude of the Pattern Deviation (PD) probability levels was associated with any alteration in sensitivity.

The MD, SF, and PSD each improved over the five examinations (each at p<0.001). The improvement in pMS between the first and fifth visits was greater for the peripheral annulus than for the central annulus by approximately twofold for the patients with OAG. Considerable variation was present between patients, within and between groups, and in the number of locations exhibiting an improving sensitivity between the first and fifth visit by one or more PD probability levels. 

(Ophthalmology 2006;113:206-215)

Wild JM, Kim LS, Pacey IE, Cunliffe IA.