Contact A-scan Research Articles

A comparison of visual acuities obtained following biometry measurements by contact and immersion A-scan techniques

Objectives: This study aimed to obtain and compare ocular biometry measurements in terms of axial length, anterior chamber depth, and lens thickness by contact and immersion ultrasound techniques of A-scan and compare postoperative results in terms of visual acuity and spherical refractive error obtained by each of these techniques. Materials and Methods: This study was a prospective cohort and a comparative study that was done on 188 eyes under evaluation for cataract surgery. Out of these, 94 eyes were evaluated for biometry measurements using the immersion technique, and the rest were evaluated using the contact technique. Intraocular lens (IOL) powers were calculated using each of the above biometry parameters, and postoperative visual outcomes were obtained in terms of visual acuity and spherical refractive error. Results of demographic, clinical, and biochemical characteristics were expressed as range, mean, and median. For qualitative data, the Chi-square test and for comparison, Students’ unpaired t-tests were used. TAQhe statistical software SPSS Statistics was used to analyze the data. Microsoft Office Word and Excel 2019 were used to generate graphs and tables. P<0.05 was considered as significant. Results: Visual acuity was significantly better in the patients who underwent IOL implantation whose power was calculated by the immersion method on the 1st postoperative day. However, there was no significant difference in the final corrected visual acuity. Spherical refractive error at the end of 1 month was significantly greater in patients who underwent IOL power calculation by the contact method as compared to the immersion method. Conclusion: The immersion method of A-scan gives a lesser refractive error postoperatively and is a more precise method of biometry, though there is no difference in the final visual acuity obtained by either of these methods.

Influence of measurement differences of anterior chamber depth and axial length on lens thickness evaluation in cataract patients: a comparison of two tests

BackgroundThe purpose of this study was to investigate the agreement of lens thickness (LT) measurements made by contact A-scan ultrasonography and Lenstar LS900 as well as the influence of anterior chamber depth (ACD) and axial length (AL) measurement differences on LT measurement in cataract patients in the two techniques.Methods1247 cataract patients (1247 eyes) participated in this retrospective cross-sectional study. Ocular biometric measurements were performed with Lenstar LS900 and A-scan ultrasonography respectively, and the measured results of AL, ACD and LT were compared using Pearson correlation coefficients (r) and Bland-Altman analyses.ResultsBland-Altman analyses showed poor agreement between the A-scan ultrasonography and Lenstar LS900 in measuring AL and ACD. The average difference of LT was 0.01 mm; the consistency limit was − 0.86 mm, 0.88 mm; and 95.27% of datapoints were within the 95% consistency limit. The consistency of LT measurements between the two techniques was poor for those subjects whose ACD or AL values were beyond the 95% consistency limit. Among the subjects whose AL or ACD values measured by A-scan ultrasonography were greater than those measured by Lenstar LS900, 93.33% of them were within the 95% consistency limit, suggesting that the consistency of LT measurement between the two techniques was poor. Of patients whose ACD or AL measured by A-scan ultrasonography were smaller than that of Lenstar LS900, 96.01% of them were within the 95% consistency limit.ConclusionsThere was good agreement of the LT measurements between A-scan ultrasonography and Lenstar LS900, except for the axis deviating from the apparent axis during A-scan ultrasonography. If this error can be avoided, A-scan ultrasonography can replace Lenstar LS900 in LT measurement in cataract patients.

Accuracy of axial length measurements from immersion B-scan ultrasonography in highly myopic eyes.

To evaluate the accuracy of axial length (AL) measurements obtained from immersion B-scan ultrasonography (immersion B-scan) for intraocular lens (IOL) power calculation in patients with high myopia and cataracts. Immersion B-scan, contact A-scan ultrasonography (contact A-scan), and the IOLMaster were used to preoperatively measure the AL in 102 eyes from 102 patients who underwent phacoemulsification and IOL implantation. Patients were divided into two groups according to the AL: one containing patients with 22 mm≤AL<26 mm(group A) and the other containing patients with AL≥26 mm (group B). The mean error (ME) was calculated from the difference between the AL measurement methods predicted refractive error and the actual postoperative refractive error. In group A, ALs measured by immersion B-scan (23.48±1.15) didn't differ significantly from those measured by the IOLMaster (23.52±1.17) or from those by contact A-scan (23.38±1.20). In the same group, the standard deviation (SD) of the mean error (ME) of immersion B-scan (-0.090±0.397 D) didn't differ significantly from those of IOLMaster (-0.095±0.411 D) and contact A-scan (-0.099±0.425 D). In group B, ALs measured by immersion B-scan (27.97±2.21 mm) didn't differ significantly from those of the IOLMaster (27.86±2.18 mm), but longer than those measured by Contact A-scan (27.75±2.23 mm, P=0.009). In the same group, the standard deviation (SD) of the mean error (ME) of immersion B-scan (-0.635±0.157 D) didn't differ significantly from those of the IOLMaster (-0.679±0.359 D), but differed significantly from those of contact A-scan (-0.953±1.713 D, P=0.028). Immersion B-scan exhibits measurement accuracy comparable to that of the IOLMaster, and is thus a good alternative in measuring AL in eyes with high myopia when the IOLMaster can't be used, and it is more accurate than the contact A-scan.

Postoperative hyperopic shift after cataract surgery in silicone oil‐filled eyes

Vitrectomy with silicone oil (SO) instillation has been widely used in complicated vitreoretinal diseases to achieve retina stability. Because progressive lens opacity is common after vitrectomy and SO infusion, the removal of SO and cataract extraction with intra-ocular lens (IOL) implantation may be performed in the same setting. The refractive outcomes of cataract surgery with simultaneous SO removal have shown acceptable results. However, the refractive outcome after cataract surgery is less predictable in some complicated cases in which intravitreal SO is retained to achieve a long-term tamponade effect. We investigate the refractive outcome of cataract surgery in 21 SO-filled eyes with complicated vitreoretinal diseases and compare the results with those in 42 vitrectomized eyes without SO instillation. The SO used in our study was 5000 centistokes. Axial length (AL) measurements were performed using contact A-scan echography (Sonomed A-2500, New York, NY, USA). To adjust the falsely elongated AL due to the low sound velocity in SO, the following formula was used: AL = anterior chamber depth + lens thickness + vitreous length *0.68. IOL power was calculated using the SRK-T formula. In all SO-filled eyes, we generally chose a more myopic calculated target refraction when deciding the IOL power to compensate for the possible hyperopic shift due to a higher refractive index of SO (Stefansson et al. 1988; Smith et al. 1990; Hotta & Sugitani 2005). Foldable acrylic IOLs (AcrySof SA60AT for power ≧6 D or AcrySof MA60MA for power<6 D; Alcon Surgery) were implanted into the capsular bag in all eyes. Intergroup comparison was shown in Table 1. The refractive outcomes in the SO-filled group were more variable (1.07 ± 2.26 D versus −0.57 ± 1.15 D) and showed a significant tendency of hyperopic shift (+2.89 ± 2.15 D versus + 0.04 ± 1.0 D, p < 0.001). Eyes in the SO-filled group had a higher proportion of cases undergoing scleral buckling procedure, a shorter anterior chamber depth (ACD) and a longer adjusted AL in average. In one eye in the SO-filled group, the refraction changed from +3.25 D after cataract surgery to 0 D after SO removal 3 years later. The uncertainty of postoperative refraction after cataract surgery in SO-filled eyes is due to several perioperative factors. In our study, the SO-filled group had significantly longer average AL and shorter average ACD, which might be related to a higher rate of scleral buckling procedure performed in those eyes. A longer AL could augment the SO-induced acoustic signal attenuating effect and make the measurement less reliable (Ghoraba et al. 2002). Previous studies found that a hyperopic shift in refraction would be induced when vitreous was replaced with SO through vitrectomy in phakic or pseudophakic eyes (Stefansson et al. 1988; Smith et al. 1990; Hotta & Sugitani 2005). This tendency of SO-induced hyperopic shift was also noted in our study. However, the amount of hyperopic shift showed great variation among different studies. The posterior convex curvature of the IOL is thought to be related to the amount of hyperopic shift (Hotta & Sugitani 2005). Incomplete SO filling in the vitreous cavity causing changes in curvature and contact area of SO-fluid interface in different head positions may also result in variability in refractions and AL measurement (Dick et al. 1997). In conclusion, the refractive results of cataract surgery in SO-filled eyes were generally more variable than in simply vitrectomized eyes. Because a mean hyperopic shift is about 3.0 D in our study, we suggest choosing −2.5 to − 3.0 D target refraction, which may partially compensate for the SO-induced hyperopic shift, if SO is intended to be left for a long-term tamponade effect.

Analysis of intraocular lens power calculation for cataract patients with high myopia and posterior scleral staphyloma

Background The accurate calculation of intraocular lens (IOL) power is essential for attaining the desired refractive outcome after cataract surgery,especially for patients with high myopia and posterior scleral staphyloma.Objective This study was to evaluate the clinical feasibility of IOL Master compared with contact A-scan in cataract patients with high myopia and posterior scleral staphyloma,then compare the accuracy of different IOL power calculation formulas.Methods This was a prospective case control clinical research.Fourty-one eyes with age-related cataract of 28 patients underwent phacoemulsification with monofocal foldable IOL implantation in Tianjin Medical University Eye Hospital were involved,who were all high myopia with posterior scleral staphyloma.Preoperative measurement was measured with IOL Master as well as with contact A-scan and manual keratometry.IOL power was calculated according to the SRK-Ⅱ,SRK-T,Haigis,Hoffer Q,Holladay 1 formulas.The refractive outcome was followed-up 3 months after operation.Results The difference was significant between the 2 methods in axial length (AL) and anterior chamber depth (ACD) measurement (P =0.005,0.000) ; In corneal curvature measurement,there was no significant difference between them (P =0.398).When mean absolute refractive error (MAE) was divided by ±1.00 D,The SRK/T and Haigis formula performed better than other formulas measured by IOL Master;The Holladay 1,Hoffer Q and Haigis formula performed better than other formulas measured by contact A-scan combined with manual keratometry,respectively.Conclusions For cataract patients with high myopia and posterior scleral staphyloma,SRK/T and Haigis formula were recommended when employing IOL Master; whereas when using contact A-scan combined with manual keratometry,we prefer Holladay 1,Hoffer Q or Haigis formula. Key words: High myopia; Posterior scleral staphyloma; Axial length; IOL Master; Contact A-scan

Comparison of globe axial length measures using contact versus immersion ultrasound

The technique of contact A-scan ultrasound is thought to commonly result in significantly shorter globe axial length (AL) measurements than immersion technique, which could significantly impact refractive outcomes. We have not experienced this phenomenon and report our contact vs. immersion ultrasound globe AL measurements.

Axial length measurement techniques in pediatric eyes with cataract

Globe axial length (AL) in children is commonly measured using either contact or immersion technique. Office measurement of AL can be difficult in young children and infants and must often be done under anesthesia in an eye that is unable to cooperate with precise fixation and centration. Contact A-scan measurements yield shorter AL, on average, than immersion A-scan measurements in pediatric eyes. This difference is mainly the result of the anterior chamber depth rather than the lens thickness value. During intraocular lens power calculation, if globe axial length is measured by the contact technique, it will result in the use of an average 1-D stronger IOL power than is actually required. This can lead to induced myopia in the postoperative refraction. In our studied patients, there was a significant difference in prediction error between contact A-scan biometry and immersion A-scan biometry. The immersion A-scan technique is recommended for pediatric IOL power calculation. We also provide a review of biometry in pediatric eyes. The overall mean AL of pediatric cataractous eyes is significantly different than the mean AL of non cataractous eyes. More importantly, the standard deviation is higher in eyes with cataract than in those without. Three phases of eye growth in children have been documented: A rapid, postnatal phase from birth to 6months of age, followed by a slower, infantile phase from 6 to 18months of age, and finally a slow, juvenile phase from 18months forward. In our study, girls had shorter ALs than boys and African-American subjects had longer ALs than Caucasians. Eyes with unilateral cataract had shorter ALs than eyes with bilateral cataract during the earlier years, but had longer ALs during later childhood.

Prediction error after pediatric cataract surgery with intraocular lens implantation: Contact versus immersion A-scan biometry

To evaluate the accuracy of pediatric intraocular lens (IOL) calculations performed using contact and immersion A-scan biometry. Storm Eye Institute, Charleston, South Carolina, USA. Evaluation of diagnostic test or technology. Data from a prospective study of pediatric eyes that had in-the-bag implantation of an AcrySof SN60WF IOL and had refraction results available from 14 days to 3 months postoperatively were retrospectively analyzed. The contact and immersion A-scan biometry techniques were performed in each eye and compared. The mean age at surgery of the 22 patients (22 eyes) was 4.8 years ± 4.1 (SD). The mean prediction error was +0.4 ± 0.7 diopter (D) in the contact group and -0.4 ± 0.8 D in the immersion group (P < .001) and the mean absolute prediction error, 0.7 ± 0.4 D and 0.7 ± 0.6 D, respectively (P=.694). The absolute prediction error was less than 0.5 D in 5 eyes (23%) using the contact technique and 11 eyes (50%) using the immersion technique. The mean postoperative spherical equivalent was +2.9 ± 2.5 D, which was significantly different from the mean predicted refraction for contact A-scan (3.3 ± 2.8 D; P=.010) but not immersion A-scan (2.5 ± 2.5 D; P=.065). There was a significant difference in prediction error between postoperative refractive results obtained with contact biometry and immersion A-scan biometry in children. Based on the results, the immersion A-scan technique is recommended for pediatric IOL power calculation.

Predicted Refraction Versus Refraction Outcome in Cataract Surgery After Trabeculectomy

To determine whether the manifest refraction after cataract surgery in eyes that had undergone previous trabeculectomy was different from the predicted refraction. University-based tertiary glaucoma service and 3 glaucoma private practices. Retrospective, interventional, case-control study. The medical records of patients with glaucoma and medically treated glaucoma suspects who underwent cataract extraction between January 1, 2004 and February 11, 2006 were reviewed. The axial length was measured by contact A-scan ultrasonography. The study group included 27 eyes of 25 patients who underwent phacoemulsification with intraocular lens implantation after trabeculectomy, whereas the control group included 52 eyes of 49 glaucoma patients or medically treated glaucoma suspects who underwent phacoemulsification with intraocular lens implantation only. The main outcome measure was the difference between the postphacoemulsification manifest refraction and predicted refraction between the 2 groups. The difference between the mean final refraction and mean predicted refraction in the study group (-0.852+/-1.056 D) and the control group (-0.501+/-0.542 D) was not statistically significant (P=0.115). The mean intraocular pressure in the study group decreased significantly after trabeculectomy (25.9+/-8.9 to 9.6+/-4.3 mm Hg, P<0.001), and increased significantly after phacoemulsification (9.6+/-4.3 to 12.5+/-5.3, P=0.001). Lower prephacoemulsification intraocular pressure was weakly correlated with a myopic shift in final refraction (r=0.269, P=0.017). Though numerous variables can potentially influence the refractive outcome in cataract extraction after trabeculectomy, our study shows that the refractive outcome in these cases remained reasonably predictable. However, lower prephacoemulsification intraocular pressure was weakly correlated with a myopic shift in final refraction.

Accuracy of IOL calculations in children: A comparison of immersion versus contact A-scan biometery

To evaluate the accuracy of pediatric IOL calculations performed under general anesthesia by using immersion A-scan biometry and to compare the results to those obtained using contact A-scan biometry. A retrospective review of 203 consecutive cases of pediatric cataract extraction with primary IOL implantation within the capsular bag; mean patient age was 5.52 +/- 4.19 years (range, 18 days to 18 years). Axial length measurements were performed with the contact A-scan biometry in 138 eyes, whereas immersion technique biometry was used in the remaining consecutive 65 eyes. Preoperative predictive target refraction was compared with the refractive result obtained at the 2-month postoperative visit. The mean of the absolute value lens prediction error for all eyes was 1.08 +/- 0.93 D from the desired postoperative refractive result. Lens prediction error for the contact A-scan subgroup was 1.11 +/- 0.90 D, whereas the immersion A-scan subgroup was less at 1.03 +/- 0.98 D. This result was not statistically significant (p = 0.6442). Statistically significant correlations were found between increased lens prediction error and age at time of surgery or corneal curvature. This retrospective, noncomparative pilot study showed no significant difference in IOL prediction error when comparing the postoperative refractive results obtained with immersion versus contact A-scan biometry in pediatric IOL calculations.

Accuracy of intraocular lens calculations in children: A comparison of immersion A-scan biometry versus applanation A-scan biometry

Introduction: Advances in pediatric cataract surgery have resulted in an increased use of IOL implants. It is speculated that the most accurate axial length measurement is obtained using the immersion A-scan technique. This study evaluates the accuracy of pediatric IOL calculations performed using immersion A-scan biometry and compares the results to those obtained using applanation A-scan biometry. Methods: Retrospective review of 214 consecutive cases of pediatric cataract extraction with IOL implantation. Axial length measurements were performed using contact A-scan biometry in the first 101 eyes, while immersion technique biometry was used in the remaining 113 eyes. Preoperative predictive data were then compared with the refractive results obtained at the 2 month postoperative visit. Results: Median age at time of surgery was 4.7 years, (range, 22 days to 18 years). Even though overall accuracy of PE was good, significant variability did occur. Discussion: In a previous study Neely et al reported the results after using applanation biometry on 101 pediatric patients; since then, the only parameter that has changed was the shift from applanation to immersion biometry. Therefore we aim to compare both techniques in regards to the predictive error. Conclusion: The immersion technique provides reliable predictive error when used in IOL implant calculations. Randomized prospective study evaluating predictive IOL calculations is needed to verify which technique is most accurate in the pediatric age group.

Non‐invasive phakometric measurement of corneal and crystalline lens alignment in human eyes

We describe a non-invasive phakometric method for determining corneal axis rotation relative to the visual axis (beta) together with crystalline lens axis tilt (alpha) and decentration (d) relative to the corneal axis. This does not require corneal contact A-scan ultrasonography for the measurement of intraocular surface separations. Theoretical inherent errors of the method, evaluated by ray tracing through schematic eyes incorporating the full range of human ocular component variations, were found to be larger than the measurement errors (beta < 0.67 degrees , alpha < 0.72 degrees and d < 0.08 mm) observed in nine human eyes with known ocular component dimensions. Intersubject variations (mean +/- S.D.: beta = 6.2 +/- 3.4 degrees temporal, alpha = 0.2 +/- 1.8 degrees temporal and d = 0.1 +/- 0.1 mm temporal) and repeatability (1.96 x S.D. of difference between repeat readings: beta +/- 2.0 degrees , alpha +/- 1.8 degrees and d +/- 0.2 mm) were studied by measuring the left eyes of 45 subjects (aged 18-42 years, 29 females and 16 males, 15 Caucasians, 29 Indian Asians, one African, refractive error range -7.25 to +1.25 D mean spherical equivalent) on two occasions.

Comparative estimation of anterior chamber depth by ultrasonography, Orbscan II, and IOLMaster

Purpose: To assess the degree of agreement of anterior chamber depth (ACD) measurements by 2 optical devices (Orbscan II and IOLMaster) and compare them with contact ultrasound A-scan. Setting: Department of Ophthalmology at Hull Royal Infirmary, England. Methods: In this prospective study of 81 eyes of 41 consecutive patients, ACD estimation was done by 3 methods—scanning slit topography (Orbscan II), partial coherence interferometry (IOLMaster), and contact ultrasound A-scan in that order. The same observer performed all measurements. Repeated measures Analysis of Variance (ANOVA) was used to analyze difference between ACD measurements by the 3 methods. Difference in measurements between methods was assessed using the paired t test. Results: The mean ACD (±SD) by the 3 methods was 3.32 mm (0.60), 3.33 mm (0.61), and 2.87 mm (0.55), respectively. There was a statistically significant difference between measurements recorded by the 3 methods ( P<.01). Mean contact A-scan measurements were 0.40 mm and 0.43 mm lower than by Orbscan II and IOLMaster respectively ( P<.01). Conclusion: Applanation ultrasound gives consistently lower measurements for ACD compared to Orbscan II and IOLMaster. Although a high degree of agreement between Orbscan II and IOLMaster was noted, further studies are needed to assess the interchangeability of measurements in clinical practice.

The Dimensions of Eyes with Chronic Angle-closure Glaucoma

The measurement of eyes with primary angle-closure glaucoma has been comprehensively documented in people of European ethnic origin. In comparison, few biometric studies have examined the relationship of one intraocular structure to another in people of Oriental or African ethnic background with primary angle-closure glaucoma. To determine the ocular characteristics of people with this ethnic background suffering from chronic angleclosure glacoma, both eyes of 46 patients with chronic angle-closure glaucoma were measured by contact A-scan ultrasonography, and the measurements were compared with those found in two groups of 23 matched normals. The correlation between the measurements obtained in the right and left eyes in each group was statistically significant (p < 0.001). Although the mean axial length was less (22.43 mm vs. 23.17 and 23.25 mm, p = 0.0001), the mean anterior chamber depth shallower (2.48 mm vs. 2.80 and 2.81 mm, p < 0.001). and the mean "relative lens position'' more anterior in eyes with chronic angle-closure glaucoma compared with normal, the mean lens thickness in all three groups was similar (4.73 mm). These measurements are significantly different from those reported in other ethnic groups. In comparison with the previously reported findings in Europeans, an anterior lens position without significant lens enlargement is responsible for the crowded anterior segment in our patients with chronic angle-closure glaucoma.

Echography in Ophthalmology

In this second edition, the author expresses a dual purpose: to better inform the referring ophthalmologist of the indications for ophthalmic echography and to better educate the practicing ophthalmic echographer in the performance of examination techniques. The author has performed ophthalmic ultrasonography since 1964, using both immersion and contact A-scan and B-scan systems. This relatively short book (81 pages) is based on Poujol's current philosophy of using contact B-scan, supplemented by an A-scan transducer (with a logarithmic amplifier), to detect, differentiate, and measure intraocular and orbital lesions. Unfortunately... the information provided is neither complete nor presented systematically enough. Chapter 1 offers a brief description of the physics of ultrasound. In addition, it explains several display methods (ie, A-mode, B-mode, C-mode and M-mode), as well as the different types of amplifiers used. Chapter 2 discusses instrument settings that may be adjusted by the echographer. It also offers the author's recommendations

A combined system of contact A-scan and B-scan.

A combined system of contact A-scan and B-scan.