Indian Journal of Clinical and Experimental Ophthalmology

Print ISSN: 2395-1443

Online ISSN: 2395-1451

CODEN : IJCEKF

Indian Journal of Clinical and Experimental Ophthalmology (IJCEO) is open access, a peer-reviewed medical journal, published quarterly, online, and in print, by the  Innovative Education and Scientific Research Foundation (IESRF) since 2015. To fulfill our aim of rapid dissemination of knowledge, we publish articles ‘Ahead of Print’ on acceptance. In addition, the journal allows free more...


  • Article highlights
  • Article tables
  • Article images

Article statistics

Viewed: 73

PDF Downloaded: 75


Damor, Bhojak, Shah, and Mahant: Ocular surface topographic evaluation in patients of astigmatism in normal and pathologic eyes


Introduction

The anterior cornea works as the major refracting surface of the eye. The integrity of its topography is essential for good vision and depends upon the anatomical and physiological aspects of the cornea. The shape of human cornea is not simply spherical, but is described as a prolate ellipse1 and is commonly astigmatic.1, 2 The surface of the cornea can be disrupted by many pathological processes which can cause major changes in topography and affect the visual performance. The surface topography of the cornea can be disrupted by 3 basic pathological processes. These include abnormalities of the epithelium, degradation of the stroma and external compression from lid or orbital masses.

Keratometry is the measurement of anterior surface of corneal curvature in its steepest and flattest axis by various methods. There are many advances in measurement of keratometric values.

Manual keratometry is gold standard method which usually measures central 2-3 mm of cornea. Many modern topography systems display simulated readings of min-K and max-K, and they have the added advantage of determining the shape of the cornea and providing much information with minimum patient discomfort.

Astigmatism (more than 0.5 diopters) is a commonly encountered refractive error, accounting for about aprx. 13 per cent of the refractive errors of the human eye.3, 4 It is commonly encountered clinically, with prevalence rates up to 30% or higher depending on the age or ethnic groups.5, 6 Astigmatism of more than 0.5 D is common among older adults, and the prevalence increases with age. In general, regular astigmatism is common; irregular astigmatism has been considered an uncommon refractive error. However, with the advent of computerized video keratography, the prevalence of some patterns definable as irregular may be as high as 40%, and significant irregularity may reside in the posterior corneal surface.7, 8, 9 Astigmatism can be divided into congenital and acquired categories.

The occurrence of irregular astigmatism varies from natural to surgically induced causes. Examples of natural causes include primary irregular astigmatism and secondary irregular astigmatism caused by various corneal pathologies associated with elevated lesions, such as keratoconus or Salzmann’s nodular degeneration.10 Examples of surgically induced astigmatism include pterygium removal, cataract extraction, lamellar and penetrating keratoplasty, myopic keratomileusis, radial and astigmatic keratectomy, PRK, and laser in situ keratomileusis (LASIK). Other causes of irregular astigmatism include corneal trauma and infection.11

Ocular astigmatism can occur as a result of unequal curvature along the two principal meridian of the anterior cornea (known as corneal astigmatism) and /or it may be due to the posterior cornea, unequal curvatures of the front and back surfaces of the crystalline lens, decentration or tilting of the lens or unequal refractive indices across the crystalline lens (known as internal or noncorneal astigmatism). The combination of the corneal and the internal astigmatism gives the eye’s total astigmatism (that is, total astigmatism equals corneal astigmatism plus internal astigmatism).4 Corneal astigmatism is often classified according to the axis of astigmatism as being either with-the-rule (WTR), oblique or against-the-rule (ATR).

The keratometer uses optical doubling techniques to precisely measure radii of curvature of a small paracentral area. Computerized videokeratoscopy is the current gold standard for measuring corneal topography. Modern videokeratoscopes provide an accurate representation of the anterior corneal surface to allow assessment of corneal irregularities in keratoconus, pellucid marginal degeneration,12 preoperative cataract workup - IOL calculation/TORIC lens implantation, refractive surgery preoperative assessment and post-op evaluation, evaluating pathologies like coneal dystrophies,scars, pterigium,recurrent erosions and contact lens fitting and evaluating the corneal contribution to optical aberrations of the whole eye.13 Three types of systems are currently used to measure corneal topography, and they are categorized as Placido based, elevation based, and interferometric.

The aim of this study is to assess the type of astigmatism in patients having visually significant astigmatism of ≥1D in both the eyes using automated keratometer, Bausch and Lomb manual keratometer and topography, to assess the various causes of corneal astigmatism, to find out the reliability and repeatability of various methods of keratometry in measuring the keratometric values, to find out incidence of bilateral astigmatism.

Materials and Methods

A detailed cross sectional study of 200 eyes of 100 patients attending outpatient department was done at tertiary eye care centre over period of 2 year.

Detailed clinical history was taken than all the patients were subjected to detailed ophthalmic examination like BCVA, slit lamp examination, IOP measurement, direct and indirect ophthalmoscopy for fundus examination to rule out any posterior segment pathology. Written consent was taken and patients were subjected to keratometric and topographic evaluation by Bausch & Lomb manual keratometer, autorefractometer and oculus topographer.

Inclusion criteria

  1. Patients diagnosed with bilateral astigmatism of ≥1D on subjective correction of refractive error.

  2. Patients suffering from suspected keratoconus, pellucid marginal degeneration, blunt trauma, corneal opacities or scar with regular surface, pterygium and with no clinically detected pathology found.

Exclusion criteria

We have excluded Patients suffering from ocular surface disorders e.g. severe dry eye where keratometry is not possible. Pediatric patients ≤ 10 years due to lack of their co-operation accurate for examination. Patients with unilateral astigmatism of ≥ 1D on subjective correction of refractive error.

Before every measurement, each Instrument is calibrated before use. Patient is explained the procedure and seated comfortably in front of the instrument with chin on chin rest & head against head rest. For Bausch and Lomb keratometer, Mires are focused in the centre of cornea. Keratometry is measurement of curvature of the anterior surface of cornea across a fixed chord length, usually 2 - 3 mm, which lies within the optical spherical zone of cornea. Mires are adjusted as shown in Figure 1.

Figure 1
https://typeset-prod-media-server.s3.amazonaws.com/article_uploads/1e121d98-39d7-4858-846c-bb9d4c3041cf/image/3b43f01e-7de8-4218-9b82-63b57057a699-uimage.png

The plus signs of central & left images were superimposed using horizontal measuring control to measure curvature in horizontal meridian. The minus signs of central & upper images were coincided with the help of vertical measuring control to measure curvature in vertical meridian. In presence of oblique astigmatism, two plus signs will not be aligned. Entire instrument is rotated till they are aligned. Recordings on both the measuring knobs were noted as horizontal and vertical keratometry values of cornea. Three readings for each patient were noted.

In automated keratometer, Modern version of scheiner principle is accomplished with infrared light emitting diodes (IR-LEDs) that are optically presented in substitution for the apertures in a scheiner disc for readings. The topography of the cornea is revealed by the shape taken by the reflected pattern. A computer provides the necessary analysis, typically determining the position and height of several thousand points across the cornea. The topographical map can be represented in a number of graphical formats, such as a sagittal map, which color-codes the steepness of curvature according to its dioptric value. Three consecutive keratometric values are noted down for every patient.

All the data values were then subjected to analysis of variance (ANOVA) test to check for the variability of measured values by all the three methods. Post hock analysis was done via Tukey HSD method to rule out any significant difference between individual groups.

Observations and Result

A total of 850 patients attending the outpatient department have been screened and total 200 eyes of 100 patients having astigmatism and fulfilling the inclusion and exclusion criteria have been included over period of 2 year.

These 100 patients were investigated for keratometric and topographic evaluation by Bausch and Lomb manual keratometer, autorefractometer and oculus topographer.

There were 52% of males and 48% of females in our study of astigmatism consisting of 100 patients. In our study almost 50% of patients were in age group of 41-60 years and around 30% of patients were in age group of 21-40 years with overall average age of 45 years.

Table 1

Age in years

No. of patients

11-20

5(5%)

21-30

13(13%)

31-40

17(17%)

41-50

24(24%)

51-60

24(24%)

61-70

16(16%)

71-80

1(1%)

Table 2

Total no. of patients screened

850

Patients with bilateral astigmatism

100

Thus the incidence of bilateral astigmatism in our study comes out to be 11.76%.

Type of astigmatism

Out of 200 eyes of 100 patients having astigmatism, 185(92.5%) eyes had regular astigmatism and 15(7.5%) eyes had irregular astigmatism. Out of 15 eyes having irregular astigmatism 8 were right eye and 7 were left eye.

Figure 2
https://s3-us-west-2.amazonaws.com/typeset-prod-media-server/88800ce6-31b5-4791-9826-db5f1204a380image2.png

Cause of astigmatism

Out of 200 eyes having astigmatism of ≥1D, pterygium was found in 16(8%) eyes, keratoconus was found in 7(3.5%) eyes, keratoconus suspect was found in 4(2%) eyes, pellucid marginal degeneration was found in 4(2%) eyes, corneal degeneration was found in 12(6%) eyes, corneal opacity was found in 5(2.5) eyes, corneal scar was found in 3(1.5%) eyes and eyes with no clinically detected pathological cause were 149(74.5%).

Table 3

Cause of Astigmatism

Number of eyes involved

Pterygium

16

Keratoconus

7

Keratoconus suspect

4

Pellucid Marginal Degeneration

4

Corneal Degeneration

12

Corneal Opacity

5

Corneal Scar

3

No Clinically Detected Pathology

149

Figure 3
https://s3-us-west-2.amazonaws.com/typeset-prod-media-server/88800ce6-31b5-4791-9826-db5f1204a380image3.png

Status of lens

In our study, 122 eyes had clear lens, 53 eyes had immature cataract, 24 eyes were pseudophakic and one eye was aphakic.

Visual acuity (logMAR)

In our study consisting of 200 eyes, mean visual acuity was logMAR 0.263 with maximum visual acuity of logMAR 0.0 and minimum visual acuity of logMAR 1.7.

Figure 4
https://typeset-prod-media-server.s3.amazonaws.com/article_uploads/1e121d98-39d7-4858-846c-bb9d4c3041cf/image/e8aaf453-8d26-4bb1-b810-0c3582097dbf-uimage.png

Descriptive statistical analysis

All the data in our study was subjected to statistical analysis. Cronbach’s alpha ≥0.9 suggests good reliability of data. In our study, Cronbach’s alpha is 0.982 which suggests that our data is reliable.

Table 4

Cronbach's alpha

Cronbach's alpha based on standardized items

0.982

0.983

Both right eye and left eye of 100 patients were subjected to calculation of flat meridian of astigmatism (K1) and steep meridian of astigmatism (K2). Both K1 and K2 were calculated by three machines: Oculus topographer, Bausch and Lomb keratometer and autorefractometer. Three readings were taken for each instrument. Difference between Mean values of 3 readings of each instrument was taken.

Table 5

Bausch and Lomb manual keratometer

N

Mean

Std. Deviation

Std. Error

95% Confidence interval for mean

Minimum

Maximum

Lower Bound

Upper Bound

2.00

100

44.6300

2.21436

.22144

44.1906

45.0694

40.25

54.75

3.00

100

44.6500

2.23409

.22341

44.2067

45.0933

40.25

55.00

Total

300

44.6275

2.21253

.12774

44.3761

44.8789

40.00

55.00

Table 6

Auto refractometer

N

Mean

Std. Deviation

Std. Error

95% Confidence Interval for Mean

Minimum

Maximum

Lower Bound

Upper Bound

1.00

100

44.6225

2.25168

.22517

44.1757

45.0693

40.00

55.00

2.00

100

44.6625

2.22797

.22280

44.2204

45.1046

40.25

54.75

3.00

100

44.6875

2.23278

.22328

44.2445

45.1305

40.00

54.75

Total

300

44.6575

2.23017

.12876

44.4041

44.9109

40.00

55.00

Table 7

Topographer

N

Mean

Std. Deviation

Std. Error

95% Confidence Interval for Mean

Minimum

Maximum

Lower Bound

Upper Bound

1.00

100

44.6580

2.25882

.22588

44.2098

45.1062

40.20

54.90

2.00

100

44.6530

2.26142

.22614

44.2043

45.1017

40.00

54.80

3.00

100

44.6610

2.24877

.22488

44.2148

45.1072

40.10

54.80

Total

300

44.6573

2.24879

.12983

44.4018

44.9128

40.00

54.90

So the difference in mean in manual keratometer was 0.0475, in autorefractometer was 0.065, and in topographer was 0.008.

This shows topography has more repeatability in calculating right eye K1 than other two methods.

Table 8

Bausch and Lomb manual keratometer

N

Mean

Std. Deviation

Std. Error

95% Confidence Interval for Mean

Minimum

Maximum

Lower Bound

Upper Bound

1.00

100

46.2675

2.40193

.24019

45.7909

46.7441

42.00

58.00

2.00

100

46.2475

2.38061

.23806

45.7751

46.7199

42.00

57.75

3.00

100

46.2600

2.40053

.24005

45.7837

46.7363

42.00

57.75

Total

300

46.2583

2.38637

.13778

45.9872

46.5295

42.00

58.00

Table 9

Auto refractometer

N

Mean

Std. Deviation

Std. Error

95% Confidence Interval for Mean

Minimum

Maximum

Lower Bound

Upper Bound

1.00

100

46.2675

2.33313

.23331

45.8046

46.7304

42.00

57.75

2.00

100

46.2625

2.35095

.23510

45.7960

46.7290

42.00

57.75

3.00

100

46.2775

2.32110

.23211

45.8169

46.7381

42.00

57.50

Total

300

46.2692

2.32728

.13437

46.0047

46.5336

42.00

57.75

Table 10

Topographer

N

Mean

Std. Deviation

Std. Error

95% Confidence Interval for Mean

Minimum

Maximum

Lower Bound

Upper Bound

1.00

100

46.4050

2.42330

.24233

45.9242

46.8858

42.10

58.20

2.00

100

46.3860

2.41862

.24186

45.9061

46.8659

42.00

58.10

3.00

100

46.3900

2.42702

.24270

45.9084

46.8716

42.10

58.20

Total

300

46.3937

2.41488

.13942

46.1193

46.6680

42.00

58.20

So the difference in mean in manual keratometer was 0.02, in autorefractometer was 0.01, and in topographer was 0.01. This shows topography and autorefractometer has more repeatability in calculating right eye K2 than manual keratometer. On calculating K1 of left eye by.

Table 11

Bausch and Lomb manual keratometer

N

Mean

Std. Deviation

Std. Error

95% Confidence Interval for Mean

Minimum

Maximum

Lower Bound

Upper Bound

1.00

100

44.4500

1.90228

.19023

44.0725

44.8275

39.00

50.25

2.00

100

44.4500

1.88696

.18870

44.0756

44.8244

39.00

50.50

3.00

100

44.4925

1.89479

.18948

44.1165

44.8685

39.25

50.75

Total

300

44.4642

1.88845

.10903

44.2496

44.6787

39.00

50.75

Table 12

Auto refractometer

N

Mean

Std. Deviation

Std. Error

95% Confidence Interval for Mean

Minimum

Maximum

Lower Bound

Upper Bound

1.00

100

44.4500

1.89597

.18960

44.0738

44.8262

39.00

50.50

2.00

100

44.4549

1.90173

.19017

44.0776

44.8322

38.75

50.25

3.00

100

44.4775

1.90431

.19043

44.0996

44.8554

39.00

50.50

Total

300

44.4608

1.89434

.10937

44.2456

44.6760

38.75

50.50

Table 13

Topographer

N

Mean

Std. Deviation

Std. Error

95% Confidence Interval for Mean

Minimum

Maximum

Lower Bound

Upper Bound

1.00

100

44.4730

1.90453

.19045

44.0951

44.8509

38.90

50.60

2.00

100

44.4710

1.89733

.18973

44.0945

44.8475

38.90

50.50

3.00

100

44.4700

1.89254

.18925

44.0945

44.8455

39.00

50.60

Total

300

44.4713

1.89178

.10922

44.2564

44.6863

38.90

50.60

So the difference in mean in manual keratometer was 0.0425, in autorefractometer was 0.0275, and in topographer was 0.003.

This shows topography has more repeatability in calculating Left Eye K1 than manual keratometer and autorefractometer.

Table 14

Bausch and Lomb manual keratometer

N

Mean

Std. Deviation

Std. Error

95% Confidence Interval for Mean

Minimum

Maximum

Lower Bound

Upper Bound

1.00

100

46.1875

2.27397

.22740

45.7363

46.6387

42.00

57.25

2.00

100

46.1675

2.27360

.22736

45.7164

46.6186

41.75

57.00

3.00

100

46.1675

2.27776

.22778

45.7155

46.6195

42.00

57.00

Total

300

46.1742

2.26751

.13091

45.9165

46.4318

41.75

57.25

Table 15

Auto refractometer

N

Mean

Std. Deviation

Std. Error

95% Confidence Interval for Mean

Minimum

Maximum

Lower Bound

Upper Bound

1.00

100

46.2000

2.24874

.22487

45.7538

46.6462

42.00

56.75

2.00

100

46.1800

2.28883

.22888

45.7258

46.6342

42.00

56.75

3.00

100

46.1925

2.23490

.22349

45.7490

46.6360

42.00

56.50

Total

300

46.1908

2.25006

.12991

45.9352

46.4465

42.00

56.75

Table 16

Topographer

N

Mean

Std. Deviation

Std. Error

95% Confidence Interval for Mean

Minimum

Maximum

Lower Bound

Upper Bound

1.00

100

46.2960

2.31228

.23123

45.8372

46.7548

42.10

57.30

2.00

100

46.2780

2.31253

.23125

45.8191

46.7369

42.00

57.30

3.00

100

46.2710

2.32306

.23231

45.8101

46.7319

42.00

57.30

Total

300

46.2817

2.30823

.13327

46.0194

46.5439

42.00

57.30

So the difference in mean in manual keratometer was 0.02, in autorefractometer was 0.02, and in topographer was 0.02.

This shows all 3 methods had equal repeatability in calculating K2 of left eye. Analysis of variance test was performed for the data and F value and P value were calculated. In all the readings, P value is ≥0.9, which shows that the null hypothesis is accepted and the keratometry readings by all the methods are reliable. ANOVA test (F value) was performed in all the 3 methods and it showed that there is no variability in the keratometry values of all the methods which proved all 3 methods are reliable. Post hock analysis was done via Tukey HSD method which suggested no significant difference between individual groups.

Discussion

We used autorefractometer which calculated total astigmatism and Bausch and Lomb manual keratometer and oculus topographer which calculated corneal astigmatism. In our study, we calculated astigmatism between age group 16-79 years and its incidence was 11.76% in which majority of age groups was between 41-60 years followed by 21-40 age groups. We found similar spread of astigmatic errors in normal eyes, but the pathological eyes showed asymmetric refractive errors.

The genetic contribution to astigmatism is low, with environmental factors being the major contributors. Down syndrome has been associated with significant ocular abnormalities. Astigmatism (greater than 0.5 D) was the most common refractive error found in the Down’s population and severe astigmatism (greater than 3 D) was found in aprx. 20 per cent of the children. There have been numerous reports of how certain eyelid pathologies can cause corneal distortions and changes in corneal astigmatism. External irregularities of the cornea and eyelids including chalazion, ptosis and unusually tight lids may produce corneal distortions that may lead to monocular diplopia. In our study, various pathologies which led to corneal astigmatism were pterygium, corneal degeneration, corneal opacity, keratoconus, and PMCD etc.

In our study, we found oculus topographer having maximum reproducibility followed by autorefractometer and then manual keratometer.14, 15 We also observed keratometer showed more repeatability in calculating values for normal corneas while topographer is better for better judgement of pathologies affecting corneal curvature.

Our study has some limitations like the sample size is small, larger sample size would help more detailed analysis. Pediatric patients were excluded which can affect the incidence rate of astigmatism which is more prevalent in pediatric age group. Younger age group patients are less in our study.

Conclusion

In our study 92.5% of eyes had regular astigmatism while 7.5% of eyes had Irregular astigmatism. Various causes of corneal astigmatism were pterygium, keratoconus, PMCD, corneal degeneration, corneal opacity in that order. No significant clinical cause was found in 74.5% of eyes. Maximum repeatability was found with oculus topographer followed by autorefractometer and Bausch and Lomb manual keratometer.

Source of Funding

None.

Source of Interest

None.

References

1 

RB Mandell The Enigma of the Corneal ContourEye Contact Lens199218426773

2 

PR Keller PP Saarloos Perspectives on corneal topography: a review of videokeratoscopyClin Exp Optom1997801183010.1111/j.1444-0938.1997.tb04843.x

3 

J Porter A Guirao IG Cox DR Williams Monochromatic aberrations of the human eye in a large populationJ Opt Soc Am A2001188179310.1364/josaa.18.001793

4 

SA Read MJ Collins LG Carney A review of astigmatism and its possible genesisClin Exp Optom200790151910.1111/j.1444-0938.2007.00112.x

5 

SM Saw PP Goh A Cheng A Shankar DT Tan L Ellwein Ethnicityspecific prevalences of refractive errors vary in Asian children in neighbouring Malaysia and SingaporeBr J Ophthalmol2006901012305

6 

RN Kleinstein Refractive Error and Ethnicity in ChildrenArch Ophthalmol200312181141710.1001/archopht.121.8.1141

7 

NA Alpins Treatment of irregular astigmatismJ Cataract Refractive Surg19982456344610.1016/s0886-3350(98)80258-7

8 

SJ Bogan GO Waring O Ibrahim C Drews L Curtis Classification of normal corneal topography based on computer-assisted videokeratographyArch Ophthalmol199010879459

9 

T Oshika A Tomidokoro K Maruo T Tokunaga N Miyata Quantitative evaluation of irregular astigmatism by fourier series harmonic analysis of videokeratography dataInvest Ophthalmol Vis Sci19983957059

10 

CJ Rapuano Excimer Laser Phototherapeutic KeratectomyInt Ophthalmol Clin19963641273610.1097/00004397-199603640-00017

11 

GET Fernandez MG Serrano Early clinical experience using custom excimer laser ablations to treat irregular astigmatismJ Cataract Refractive Surg2000261014425010.1016/s0886-3350(00)00565-4

12 

RB Mandell A guide to videokeratographyInt Contact Lens Clin19962362052810.1016/s0892-8967(96)00103-4

13 

A Guirao P Artal Corneal wave aberration from videokeratography: accuracy and limitations of the procedureJ Opt Soc Am A20001769556510.1364/josaa.17.000955

14 

S Mehravaran S Asgari S Bigdeli A Shahnazi H Hashemi Keratometry with five different techniques: a study of device repeatability and inter-device agreementInt Ophthalmol20143448697510.1007/s10792-013-9895-3

15 

SB Hannush Reproducibility of Normal Corneal Power Measurements With a Keratometer, Photokeratoscope, and Video Imaging SystemArch Ophthalmol1990108453910.1001/archopht.1990.01070060087055



jats-html.xsl

© This is an open access article distributed under the terms of the Creative Commons Attribution License - Attribution 4.0 International (CC BY 4.0). which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.


Article type

Original Article


Article page

38-44


Authors Details

Vijay Damor, Ashish Bhojak, Harshit Shah, Anupamaben Mahant


Article History

Received : 15-08-2020

Accepted : 02-09-2020

Available online : 30-03-2021


Article Metrics


View Article As

 


Downlaod Files