High-frequency ultrasound biomicroscopy versus ultrasound and optical pachymetry for the measurement of corneal thickness

Ultrasound biomicroscopy in glaucoma assessment

Ultrasound biomicroscopy (UBM) is an important tool in the diagnosis, evaluation and follow up of glaucoma patients. Even if we are dealing with a primary angle closure glaucoma (PACG) or a primary open angle glaucoma (POAG) patient, the mechanism of angle closure can be revealed by performing an UBM. The device can help differentiate between the two types of glaucoma even in patients with opaque corneas when gonioscopy cannot be performed. Knowing the type of glaucoma is vital, especially regarding an individualized treatment, since each patient is unique and needs to be treated accordingly, in order to prevent glaucomatous optic neuropathy and visual field loss. Abbreviations: AC = anterior chamber, ICE = iridocorneal endothelial syndrome, IOP = intraocular pressure, NTG = normal tension glaucoma, PACG = primary angle closure glaucoma, PC = posterior chamber, PEX = pseudoexfoliation syndrome, POAG = primary open angle glaucoma, UBM = ultrasound biomicroscopy.

Comparability of corneal thickness and opacity depth assessed by OCT and UBM

OBJECTIVE

To compare the measurement of central thickness and depth of involvement of opacity-bearing corneas at different intensities (mild, moderate, and dense) using different instruments: Visante and Optovue OCTs, ultrasound biomicroscopy (UBM), and ultrasound pachymetry (central thickness).

METHODS

Sample of 102 eyes: 70 eyes (68.63 %) with corneal opacity; 32 eyes (31.37%) with normal corneas. Corneal opacity grading included mild (28 eyes, 40.00 %), moderate (27 eyes, 37.57 %), and dense (15 eyes, 21.43 %). Opacity intensity was graded and documented. Central corneal thickness was determined using Optovue and Visante OCTs, ultrasound pachymetry, and UBM VuMax (50 MHz), and depth of corneal opacity, using Optovue and Visante OCTs and UBM.

RESULTS

Total corneal thickness of the control group showed differences with a small correction factor using Optovue OCT (534.03 ± 39.88 μm), Visante OCT (523.72±38.70 μm), and ultrasound pachymetry (529.84 ± 39.76 μm), and were higher when compared to UBM (492.06 ± 37.93 μm). In mild opacity, depth measurements were the same with OCT Optovue and Visante and higher than those by UBM. In moderate opacities, there were no differences in measurements by different instruments. In dense opacities, OCT Optovue and Visante measurements were the same and higher than those by UBM.

CONCLUSIONS

Central corneal thickness measurements were considered higher in corneas with opacity than in normal corneas. We observed that the greatest thicknesses were measured in corneas with dense opacities. There was a difference between the measurements taken by different instruments, both in central thickness and in opacity depth in cases of mild and dense opacity, and no difference in moderate opacity.

Direct and Indirect Flap Measurements in Femtosecond Laser-Assisted In Situ Keratomileusis

PURPOSE

To compare direct and indirect LASIK flap thickness measurements using ultrasound and Scheimpflug technology.

METHODS

Eighty-two eyes treated with laser-assisted in situ keratomileusis refractive surgery using a femtosecond laser (IntraLase FS150) were prospectively included in the study. Flap thickness was set to 115 μm. Corneal flap thickness was measured using the direct method-ie, ultrasound pachymetry immediately after flap construction in the presence of cavitation bubbles-and indirect methods, with subtraction of intraoperative post-lift corneal thickness measured using ultrasound pachymetry (intrastroma) from preoperative central corneal thickness using ultrasound (Indirect-US) or Scheimpflug thinnest pachymetry (Indirect-Scheimpflug).

RESULTS

Mean flap thickness was overestimated using the indirect methods, Indirect-US and Indirect-Scheimpflug (122.6 ± 24.5 μm and 128.1 ± 26.1 μm, respectively; P < 0.0060 and P < 0.0001, respectively). There were no significant correlations between the direct and indirect methods. Indirect-Scheimpflug was significantly higher (P = 0.0122) than Indirect-US. The closest average flap thickness compared with the set parameter of 115 μm was that of the direct method (115.6 ± 8.6 μm; 95% confidence interval: -1.3 to 2.5; P = 0.5163). The direct method provided the lowest SD of all groups (SD: 8.64).

CONCLUSIONS

The direct method of flap thickness measurement was the most comparable to the set parameter compared with the indirect subtraction methods. Additional studies are needed to determine which method allows for the most accurate measurement of flap thickness.

Central corneal thickness determination in corneal edema using ultrasound pachymetry, a Scheimpflug camera, and anterior segment OCT

PURPOSE

The purpose of this study is to determine the influence of post-surgical corneal edema on the reliability and reproducibility of central corneal thickness (CCT) measurements by a Scheimpflug camera (Pentacam), ultrasound pachymetry (USP), and anterior-segment spectral-domain optical coherence tomography (AS-OCT).

METHODS

Thirty-two patients planned for cataract surgery (n = 16) or vitrectomy (n =  6) were included in a prospective study. The non-surgery eye was used as control. Two investigators acquired two measurements each, with the Pentacam (Oculus, Germany) and the AS-OCT (Heidelberg Engineering, Germany) in a randomized order, followed by USP (Tomey SP-100, Germany). CCT was evaluated using the apex value for Pentacam, the corneal apex cut in AS-OCT and averaging eight single measurements for USP. Coefficients of variation (COV) and intra-class correlation coefficients (ICC) were determined.

RESULTS

Post-surgery corneas showed a thickness of (investigators 1 and 2): Pentacam (615.9 ± 58.02 μm and 615.1 ± 60.17 μm), USP (601.4 ± 63.77 μm and 614.5 ± 70.91 μm), AS-OCT (608.8 ± 65.67 μm and 606.9 ± 64.41 μm) ,with no significant difference (ANOVA p > 0.99). The COVs (investigators 1 and 2) for control eyes were: Pentacam (0.78 ± 0.52 and 0.70 ± 0.76), USP (0.66 ± 0.29 and 0.98 ± 0.44), AS-OCT (0.59 ± 0.61 and 0.59 ± 0.40). The COVs (investigators 1 and 2) for post-surgical eyes were: Pentacam (0.98 ± 1.25 and 0.97 ± 0.73), USP (0.73 ± 0.64 and 1.35 ± 0.85), AS-OCT (1.34 ± 1.57 and 1.19 ± 1.18).The ICC was determined in post-surgery corneas (ICC > 0.96) and control corneas (ICC > 0.95).

CONCLUSION

USP measurements have the highest user dependence. Post-surgical corneal edema leads to higher intraobserver variability. All methods reached a high level of agreement in CCT determination in edematous as well as healthy corneas.

Comparison of central corneal thickness measurements using ultrasound pachymetry, ultrasound biomicroscopy, and the Artemis-2 VHF scanner in normal eyes

Purpose

To compare the precision of central corneal thickness (CCT) measurements taken with the handheld ultrasound pachymeter (USP), ultrasound biomicroscopy (UBM), and the Artemis-2 very high frequency ultrasound scanner (VHFUS) on normal subjects.

Design

Prospective study.

Methods

One eye from each of 61 normal subjects was randomly selected for this study. The measurements of the CCT were taken with the USP, VHFUS, and UBM. Results were compared statistically using repeated-measures analysis of variance (ANOVA), Pearson’s correlation coefficient, and limits of agreement.

Results

The average CCT (± standard deviation) was 530.1 ± 30.5 μm, 554.9 ± 31.7 μm, and 559.5 ± 30.7 μm for UBM, VHFUS, and USP respectively. The intraobserver repeatability analyses of variance are not significant for USP, UBM, and VHFUS. P-values were 0.17, 0.19, and 0.37 respectively. Repeated-measures ANOVA showed a significant difference between the three different methods of measuring CCT (P = 0.0001). The ANOVA test revealed no statistically significant difference between USP and VHFUS (P > 0.05), yet statistical significant differences with UBM versus USP and UBM versus VHFUS (P < 0.001). There were high correlations between the three instruments (P < 0.0001). The mean differences (and upper/lower limits of agreement) for CCT measurements were 29.4 ± 14.3 (2.7/56), 4.6 ± 8.6 (−14.7/23.8), and −24.8 ± 13.1 (−50.4/0.8) for USP versus UBM, USP versus VHFUS, and UBM versus VHFUS, respectively.

Conclusion

The UBM produces CCT measurements that vary significantly from those returned by the USP and the VHFUS, suggesting that the UBM may not be used interchangeably with either equipment for monitoring the CCT in the clinical setting.

Comparison of internal anterior chamber diameter measured with ultrabiomicroscopy with white-to-white distance measured using digital photography in aphakic eyes

PURPOSE

Measuring the anterior chamber diameter (AC) is important both for diagnostics and before ocular surgery. In clinical practice, horizontal AC diameter is usually estimated on the basis of measurements of corneal diameter (so-called white-to-white [WTW] distance). The usual method for selecting an appropriate size for an AC lens is to add 1 mm to WTW measurement. The purpose of this study was to compare AC diameter measured using ultrasound biomicroscopy (UBM) with WTW distance measured using digital photography in aphakic eyes.

METHODS

The data were gathered prospectively from a nonrandomized consecutive series of aphakic patients. The examined group consisted of 23 eyes of 23 adult patients, mean age 71.17 years. WTW diameter was measured in the 3 o'clock and 9 o'clock meridian on a digital photograph. Internal horizontal AC diameter measurements were performed in the 3 o'clock and 9 o'clock meridian with the Sonomed Ultrabiomicroscopy device with a 50-MHz probe.

RESULTS

Mean horizontal WTW diameter measured using digital photographs was 11.65+/-0.38 mm and mean horizontal internal AC diameter measured using UBM was 13.53+/-0.83 mm. A significant difference in mean WTW diameter measured using digital photography and UBM was found (Mann-Whitney U test, p<0.05). There was a significant correlation between these values (Spearman=0.70, p<0.001).

CONCLUSIONS

In order to obtain good and precise measurements of internal AC diameter, a method of direct visualization of the intraocular structures should be used. Despite the limits of a small case series, our results support the use of UBM technique for preoperative evaluation of the internal AC diameter and AC intraocular lens sizing in aphakic eyes.

Comparison of central corneal thickness as measured by non-contact specular microscopy and ultrasound pachymetry before and post LASIK

PURPOSE

To compare central corneal thickness (CCT) measurements made by ultrasound pachymetry and non-contact specular microscopy in eyes before and post laser in situ keratomileusis (LASIK).

METHODS

Forty eyes of 20 consecutive refractive surgery candidates were included in this study. The CCT in both eyes was measured before and 3 months after LASIK with a non-contact specular microscope and a ultrasound pachymeter.

RESULT

Both before and after LASIK, the difference of the mean CCT between non-contact specular microscopy and ultrasound pachymetry was statistically significant (P < 0.001 before surgery, and P = 0.02 after surgery). The difference between them was -14.41 microm and -3.69 microm, respectively. In both situations, non-contact specular microscopy and ultrasound pachymetry measurements were highly correlated (r = 0.959, before surgery, and r = 0.979 after LASIK). Bland-Altman analysis showed that either before or post LASIK, the 95% limits of agreement have a span of approximately 30 microm. Preoperatively, the mean standard deviation of repeated measurements was 4.26 microm by ultrasound pachymetry, and 4.02 microm by non-contact specular microscopy, whereas it was 3.83 microm by ultrasound pachymetry, and 3.89 microm by non-contact specular microscopy, postoperatively. No statistically significant differences were found between the mean standard deviations by these two devices in both situations (P = 0.561 before surgery, and P = 0.849 after LASIK).

CONCLUSIONS

Both before and after LASIK, non-contact specular microscopy is not in complete agreement with ultrasound pachymetry in measuring CCT. Both devices provided comparable repeatability of measurements.

Recent advances in ophthalmic anterior segment imaging: a new era for ophthalmic diagnosis?

Anterior segment imaging is a rapidly advancing field of ophthalmology. New imaging modalities, such as rotating Scheimpflug imaging (Pentacam-Scheimpflug) and anterior segment optical coherence tomography (Visante OCT and Slit-Lamp OCT), have recently become commercially available. These new modalities supplement the more established imaging devices of Orbscan scanning slit topography and ultrasound biomicroscopy (UBM). All devices promise quantitative information and qualitative imaging of the cornea and anterior chamber. They provide a quantitative angle estimation by calculating the angle between the iris surface and the posterior corneal surface. Direct angle visualisation is possible with the OCT devices and UBM; they provide images of the scleral spur, ciliary body, ciliary sulcus and even canal of Schlemm in some eyes. Pentacam-Scheimpflug can measure net corneal power, a feature particularly useful for cataract patients having undergone previous corneal surgery. Anterior segment OCT can measure corneal flap depth following LASIK and anterior chamber width prior to phakic intraocular lens implantation. The arrival of the new imaging devices may herald the dawn of a new era for ophthalmic diagnosis, particularly in view of the ease and non-contact nature of examination.

Photorefractive keratectomy in megalophthalmos anterior

PURPOSE

To evaluate the results of photorefractive keratectomy (PRK) for the correction of myopia and myopic astigmatism in megalophthalmos anterior.

METHODS

Four eyes of two brothers with megalophthalmos anterior were treated with PRK. In patient 1, best spectacle-corrected visual acuity (BSCVA) was 20/20 in both eyes with a refraction of -4.50 -4.50 x 180 degrees in the right eye and -3.75 -3.00 x 175 degrees in the left eye. In patient 2, BSCVA was 20/25 in both eyes with a refraction of -4.25 x 166 degrees in the right eye and +0.50 -4.00 x 175 degrees in the left eye.

RESULTS

Topographic map, slit-lamp, ultrasound biomicroscopy, and postoperative course (no progression), supported with vectorial analysis, demonstrated megalophthalmos anterior. During 24-month follow-up, mild haze was observed and BSCVA was maintained.

CONCLUSIONS

Myopia and astigmatism are often observed in this type of nonprogressive corneal dysgenesis. Based on this fact and our results, we recommend PRK in cases of megalophthalmos anterior.

Keratoconus staging: a computer-assisted ultrabiomicroscopic method compared with videokeratographic analysis

PURPOSE

The aim of this study was to introduce a new paradigm for keratoconus assessment, the keratoconus index (KI), generated from the ratio of peripheral corneal thickness (PCT) to the thinnest corneal thickness (TCT), and calculated by a computer-assisted procedure after ultrabiomicroscope (UBM) examination. Then we compared KI and the keratoconus severity index (KSI), obtained by videokeratography in patients with different stages of keratoconus.

METHODS

We studied 60 eyes with different forms of keratoconus using the TMS-3 autotopographer, provided with a keratoconus screening program (using Smolek-Klyce methods) and the commercial version of the ultrasound biomicroscope (Paradigm UBM Plus Model P45) equipped with a 50-MHz probe, which was provided with our computer-assisted program. The proportion test Z and the correlation coefficient R were applied to the outcomes.

RESULTS

The keratoconus severity index, KSI, obtained by color-coded videokeratographic maps, was in the range 95% to 32% (mean 52.22%). By means of UBM examination, we obtained 60 images and found values of TCT 0.278-0.592 mm and PCT 0.475-0.704 mm. Applying the computer-assisted method, we obtained values for KI of 1.112-2.159 (mean 1.428).

CONCLUSIONS

KI is correlated as well as KSI with the severity of the keratoconus (R = 0.76, P < 0.0001). It can be used as a similar parameter to measure the evolution of the disease, on the basis of corneal thickness rather than the curvature.

Anterior segment imaging: ultrasound biomicroscopy

Ultrasound biomicroscopy technology has become an indispensable tool in qualitative and quantitative assessment of the anterior segment. Advances in soft-ware design and algorithms will improve theoretical understanding of the pathophysiology of anterior segment disorders. Future applications of quantitative techniques will yield important information regarding mechanisms of angle closure, improving understanding of the dynamic functions of the iris,accommodation, presbyopia, and other aspects of anterior segment physiology and pathophysiology.

Comparison of Four Corneal Pachymetry Techniques in Corneal Refractive Surgery

PURPOSE

To assess the effectiveness of optical and ultrasonic pachymetry when examining corneas of patients before and after laser in situ keratomileusis (LASIK).

METHODS

We conducted a prospective study of 25 patients (50 eyes) who had LASIK for myopia or myopic astigmatism (mean spherical equivalent refraction -4.80 +/- 3.60 D; range -1.75 to -12.00 D). Corneal thickness was measured using 1) conventional ultrasonic pachymetry (20 MHz probe) in the central cornea, 2) ASL confocal microscopy (CMTF analysis), 3) Orbscan II Analyzer, and 4) ultrasound biomicroscopy (UBM) (50 MHz probe) preoperatively and 3 months postoperatively.

RESULTS

Preoperative pachymetry values measured with the Orbscan II Analyzer (551.3 +/- 27.3 microm) were almost identical to those measured with the 20 MHz ultrasonic probe (553.7 +/- 25.7 microm; t-test P = .652). The mean preoperative pachymetry for UBM was 566.7 +/- 38.8 microm and for CMTF was 553.2 +/- 15.5 microm. The optic and ultrasonic system measurements differed in operated eyes; 20 MHz ultrasonic pachymetry yielded an average 478.7 +/- 23.5 microm and the UBM, 482.4 +/- 40.3 microm. These measurements differed from the Orbscan II (Tukey test; P = .023) measurement of 461.1 +/- 37.5 microm but not from the confocal microscopy measurement of 475.2 +/- 41.8 microm. UBM registered greater values that were more variable than those obtained with ultrasonic pachymetry.

CONCLUSIONS

In non-operated eyes, corneal thickness was similar using conventional ultrasound, Orbscan II, or confocal techniques, with ultrasound biomicroscopy giving thicker readings.

Ferrara Intrastromal Corneal Ring Segments for Severe Keratoconus

PURPOSE

Intrastromal Ferrara ring segments were inserted in eyes with severe keratoconus to evaluate safety and efficacy of this procedure.

METHODS

Intrastromal Ferrara ring segments were placed in 36 eyes of 35 patients with severe keratoconus, who were evaluated after 12 months of follow-up. All patients had highly disabling visual acuity, contact lens intolerance, and a previous indication for penetrating keratoplasty. Statistical analysis included preoperative and postoperative uncorrected visual acuity, best spectacle-corrected visual acuity, manifest refraction, anterior chamber depth, keratometry, and pachymetry.

RESULTS

No patient had a loss of visual acuity. Uncorrected visual acuity improved in 28 eyes (77.78%) and best spectacle-corrected visual acuity improved in 29 eyes (80.56%). Spherical equivalent refraction decreased from -7.29 +/- 3.12 D to -4.80 +/- 3.04 D at 12 months after Ferrara intrastromal ring segment implantation. Corneal topography and ultrasound biomicroscopy showed corneal flattening, demonstrated by thinning of the central cornea and a reduction in anterior chamber depth. Segment decentration occurred in one eye (2.7%), asymmetric positioning of the segments in two eyes (5%), inadequate depth in two eyes (5%), migration of the segments in two eyes (5%), segment extrusion in five eyes (13.8%), conjunctivitis in one eye (2.7%), bacterial keratitis in one eye (2.7%), and hydrops in one eye (2.7%).

CONCLUSIONS

With these early results, Ferrara intrastromal ring segments appear to be an alternative for the treatment of severe keratoconus.

Measuring central corneal thickness with ultrasound pachymetry

PURPOSE

To determine how many ultrasound pachymetry measurements are required to obtain a reasonable estimate of central corneal thickness.

METHODS

Five central corneal thickness measurements were obtained from each eye of 101 patients with normal corneas. The means of fewer than five readings were compared with the mean of five values for each eye to determine the incremental clinical value of additional measurements.

RESULTS

Compared with the mean of five values, fewer than three measurements per eye provided a mean central corneal thickness value with inadequate confidence. Conversely, means of more than three readings provided little additional confidence over means of three values.

CONCLUSIONS

Central corneal thickness by ultrasound pachymetry can be adequately assessed in the majority of eyes by taking three measurements per eye.

Corneal endothelial cell density and pachymetry measured by contact and noncontact specular microscopy

PURPOSE

To determine the endothelial cell density and thickness of normal human and postkeratoplasty corneas with contact specular microscopy and to compare these measurements with those obtained by noncontact specular microscopy.

SETTING

Department of Ophthalmology, University of Erlangen-Nürnberg, Erlangen, Germany.

METHODS

The central corneal endothelial cell density and thickness were determined in 65 healthy eyes of 39 patients with a mean age of 71 years +/- 12 (SD) and in 50 corneal grafts of 41 patients with a mean age 53 +/- 17 years using noncontact (Topcon SP-2000P, Topcon Corp.) and contact (EM-1000, Tomey) specular microscopes. Appropriate conversion factors were used for accurate cell count comparison.

RESULTS

The mean cell count of the normal corneas was 2445 +/- 425 cells/mm(2) measured by noncontact specular microscopy and 2471 +/- 393 cells/mm(2) measured by contact specular microscopy (P =.70). After penetrating keratoplasty, the mean cell density was 1610 +/- 499 cells/mm(2) and 1584 +/- 469 cells/mm(2), respectively (P =.88). Significantly lower thickness was measured with the noncontact specular microscope than by contact pachymetry in normal eyes (543 +/- 46 micro m and 642 +/- 42 micro m, respectively) and postkeratoplasty eyes (538 +/- 61 micro m and 627 +/- 48 micro m, respectively) (P <.0001).

CONCLUSION

To determine endothelial cell density, contact and noncontact specular microscopy may be used interchangeably. However, for the combined measurement of endothelial cell density and pachymetry, the use of the same specular microscope is recommended for long-term patient follow-up.

Early Onset Ectasia Following Laser in situ Keratomileusus: Case Report and Literature Review

PURPOSE

Laser in situ keratomileusis (LASIK) has been associated with the development of postoperative corneal ectasia. We present a case of early onset ectasia after LASIK, review known risk factors in development, and discuss possible strategies for prevention.

METHODS

A 39-year-old man underwent bilateral LASIK for moderate myopia. Preoperative cycloplegic refractions were -9.00 + 0.25 x 140 degrees OD and -7.75 sphere OS. Corneal topography demonstrated mild inferior steepening bilaterally although definite evidence of keratoconus by either the Klyce/Maeda and Smolek/Klyce keratoconus screening tests was not present. Following the creation of flaps with 160-microm plates, ablations of 93 microm OD and 80 microm OS were performed, estimated to leave residual stromal beds of at least 314 microm OD and 330 microm OS.

RESULTS

On the first postoperative day, uncorrected visual acuities were 20/400 OD and 20/40 OS. On the fifth postoperative day, the patient's uncorrected visual acuity was 20/400 OD, and 20/300 OS. Corneal topography of the right eye showed profound inferior steepening with an apical corneal power in excess of 57 D; topography of the left eye showed mild inferior steepening. Eighteen months after surgery best corrected visual acuity was 20/40 OD and 20/30 OS with rigid gas permeable contact lenses.

CONCLUSIONS

This case highlights the need for a high index of suspicion when one notes an asymmetric bow-tie pattern on preoperative LASIK corneal topography, despite seemingly safe estimates of residual stromal bed thickness.

Scanning-slit and specular microscopic pachymetry in comparison with ultrasonic determination of corneal thickness

PURPOSE

To determine the central corneal thickness values in healthy eyes with the recently developed Orbscan scanning-slit system, contact and noncontact specular microscopic pachymetry and compare the results to conventional ultrasonic pachymetry.

METHODS

In the following sequence, Orbscan, Topcon SP-2000P noncontact specular microscope, AL-1000 ultrasound, and Tomey contact specular microscope were used to record thickness values. Thirty-four healthy right corneas of 34 healthy subjects were investigated.

RESULTS

Orbscan pachymetry correlated significantly with ultrasound (r = 0.64, p < 0.001), contact (r = 0.45, p < 0.001), and noncontact specular microscopy (r = 0.72, p < 0.001). Likewise, the Topcon SP-2000P noncontact specular microscopy pachymetry disclosed similar statistical results compared with ultrasound (r = 0.88, p < 0.001), and contact specular microscopy pachymetry (r = 0.76, p < 0.001). The mean central corneal thickness results were significantly higher ( p < or = 0.01) than ultrasonic values (580 +/- 43 microm) using the contact specular microscope (640 +/- 43 microm) or Orbscan system (602 +/- 59 microm) but were significantly lower ( p < 0.001) using the noncontact specular microscope (547 +/- 49 microm).

CONCLUSIONS

The results indicate that the devices tested cannot be simply used interchangeably. For long-term patient follow-up, one specific instrument is recommended. Recently developed pachymetry machines are especially helpful when additional corneal data such as thickness profile, elevation maps, anterior chamber depth, and endothelial morphology are required.

Corneal thickness measurements with contact and noncontact specular microscopic and ultrasonic pachymetry

PURPOSE

To evaluate the central corneal thickness values in normal and postkeratoplasty corneas with the new Topcon SP-2000P noncontact specular microscopic, contact specular microscopic, and the "common standard" ultrasonic pachymetry.

METHODS

Central corneal thickness was determined in 119 eyes of 81 patients (73 normal eyes of 44 patients and 46 eyes after penetrating keratoplasty) first with a noncontact specular microscopic (Topcon SP-2000P; Topcon Corporation, Tokyo, Japan), then an ultrasonic (AL-1000; Tomey, Erlangen, Germany), and finally with a contact specular microscopic (EM-1000; Tomey, Erlangen, Germany) pachymetry two times each by the same investigator.

RESULTS

Reliability of the central corneal measurements was equally high both in normal and in postkeratoplasty corneas with all of the instruments (Cronbach alpha = 0.99). Noncontact specular microscopic corneal thickness determination correlated significantly both with ultrasonic (r =.86, P <.0001) and contact specular microscopic pachymetry (r =.62, P <.0001). The ultrasonic pachymetry correlated well with the Tomey pachymetry (r =.69, P <.0001). The Topcon normal mean central corneal thickness value (542 +/- 46 microm) was 28 +/- 4 microm lower (P <.0001) compared with the ultrasonic data (570 +/- 42 microm), which was 68 +/- 1 microm lower (P <.0001) compared with Tomey thickness (638 +/- 43 microm).

CONCLUSIONS

Central corneal thickness measurements with noncontact specular microscopic, contact specular microscopic, and ultrasonic pachymetry demonstrate that each of the instruments is reliable but cannot be simply used interchangeably.

Predicting sulcus size using ocular measurements

PURPOSE

To predict sulcus size using ocular measurements.

SETTING

Michel Pop Clinics, Montreal, Quebec, Canada.

METHODS

Forty-three eyes were evaluated using several techniques. Ultrasound biomicroscopy (UBM) echograms were taken to measure the anterior chamber depth (ACD), sulcus size, and central corneal thickness. The limbus size was measured with a caliper. Axial length, ACD, and pachymetry were measured by contact ultrasonography. Refraction and corneal power were also evaluated.

RESULTS

The coefficient of linear regression was 0.05 between the limbus and the sulcus size (P =.78), 0.76 between ultrasonography and UBM ACD measurements (P <.001), and 0.69 between ultrasonography and UBM pachymetry (P <.001). Paired t tests showed that ultrasound and UBM ACD measurements were not statistically different (P =.70) but that ultrasound and UBM pachymetry measurements were (P <.001). The sulcus versus limbus difference was 0.6 mm for myopia and 0.3 mm for hyperopia. A backward elimination multiple regression performed with all measures to predict sulcus size resulted in the following formula: Sulcus size = 18.9 - 0.023 x sphere + 0.15 x mean keratometry (R = 0.49; P =.005; statistical power = 0.89; standard error of estimate = 0.5 mm).

CONCLUSION

Traditional estimation of sulcus size through limbal measurement is inadequate because limbus size alone cannot predict sulcus size. A general formula using the sphere and the mean corneal power can help predict sulcus size. Corneal power was significantly and negatively correlated with sulcus and limbus size as well as sphere. The standard error of sulcus measurement by UBM was 0.4 mm.

The thickness of the aging human lens obtained from corrected Scheimpflug images

Commonly, measurements of lens thickness are performed using A-scan ultrasonography or slitlamp Scheimpflug photography. Both techniques have their drawbacks in the study of presbyopia: ultrasonography requires the velocity of sound in the lens which may change with age, whereas Scheimpflug photography requires knowing the refractive index of the lens to enable correction of the photographs for the distortion due to the refraction of the cornea and lens. By combining Scheimpflug photography and axial optical eye-length measurements, we were able to individually correct the Scheimpflug images for distortion and calculate the refractive index and thickness of the human lens. Lens thickness of 90 subjects ranging in age between 16 and 65 years was measured, and an average increase of 24 microm/year was found. This value is consistent with ultrasonographic measurements assuming an age-independent velocity of sound in the lens of 1641 m/s. The posterior lens surface recedes from the cornea with age, and this backward movement does not differ significantly from the forward movement of the anterior lens surface.

Use of high-frequency ultrasound imaging to improve delineation of anterior uveal melanoma for proton irradiation

The aim of this study was to evaluate high-frequency ultrasound imaging (HFUI) as an aid in localizing anterior margins of tumours of the eye for proton therapy. Proton irradiation of ocular melanoma requires an accurate assessment of all tumour margins. The tumour is marked surgically by suturing to the sclera four or five tantalum rings on the borders of the tumour defined by transillumination. In order to evaluate the clinical usefulness of high-frequency ultrasound imaging, four and five rings were surgically placed in a patient with an iris/ciliary body melanoma and in a patient with ciliochoroidal melanoma using transillumination to localize the tumour margins. Subsequently margins were verified by HFUI. In the first patient, the distances between the rings and the limbus were measured using calipers during surgery and were compared with HFUI measurements and measurements from planning software. The distances were comparable within 0.5 mm. In the second patient the treatment was planned in two different ways using EYEPLAN software. In the first scenario the shape of the tumour and its relation to the rings were obtained from the surgeon's mapping, the fundus drawing using a transilluminating point light source, and the HFUI. In the second scenario, the shape of the tumour was deduced from the ring positions only. It was observed that the maximum difference between the tumour edge as seen on high-frequency ultrasound images and the rings was 2.6 mm. The tumour volume was underestimated by 39% when tumour shape was obtained from ring positions only. During the past year we have utilized HFUI in 18 patients having tumours involving the anterior segment of the eye, among which four were treated with proton therapy. In conclusion, we believe that high-frequency ultrasound imaging provides additional information with respect to the location of tumour margins in ciliary body and anterior uveal melanoma. Occult extension of the tumour within the ciliary body or posterior iris may not be appreciated by transillumination alone.

Quantitative assessment of the anterior segment using ultrasound biomicroscopy

The development of ocular imaging devices has progressed rapidly during the past 10 years. Ultrasound biomicroscopy has revolutionized the evaluation of the anterior segment of the eye. The qualitative information gathered using this technology has contributed to our understanding of the pathophysiology of angle-closure glaucoma, pigmentary glaucoma, and a variety of other anterior segment disorders. However, the area of quantitative analysis of ultrasound biomicroscopic images remains largely to be developed. This review describes the role of ultrasound biomicroscopy in the measurement of the anatomic structures and their configurations within the anterior segment. Included are previously published and established methods as well as methods in their early stages of development. Application of quantitative image analysis techniques should yield significant information about mechanisms of appositional angle closure, dynamic functions of the iris, accommodation, and presbyopia.