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Year : 2013 | Volume : 1 | Issue : 1 | Page : 12 - 16  


Original Articles
Correlation between Heidelberg Retinal Tomography (HRT II) Parameters and Visual Field Indices in Primary Open Angle Glaucoma Patients

Renu Shukla Dubey1, B.K. Nayak2, Nisheeta Agarwal3

1 Sr Resident, Malla Reddy Insititute of Medical Sciences, Hyderabad, 2HOD and 3 Consultant Ophthalmologist, P.D.Hinduja National Hospital, Mumbai.

 

Abstract

Background: Glaucoma is a common cause of visual impairment and blindness. The Heidelberg Retinal Tomogram (HRT) evaluates the structural details of the optic disc and has been widely investigated as a research tool in imaging of the ONH. The correlation between the perimetry and HRT data in Caucasian eyes has been reported previously. Objective: To correlate the HRT parameters and visual field indices in Indian eyes including established glaucoma and glaucoma suspect patients. Methods: Eighty-three eyes of 50 patients having primary open angle glaucoma underwent automated Humphrey perimetry (30-2) and confocal scanning laser ophthalmoscopy (HRT II). The global visual field indices and the HRT II parameters were correlated. Results: The average MD of the study group was –3.70 ± 3.99dB (range 1.11 to –19.54). The correlations between global parameters by HRT and global visual field indices were found to be significant for rim area, rim volume, height variation contour, mean RNFL thickness and found RNFL cross sectional area. The strongest correlation was between rim area and PSD (r = 0.32, p = 0.0029). Conclusion: There was a good correlation between global visual field indices and HRT II parameters with highest correlation coefficient of 0.32. Since we had several parameters correlating well, we can say that in early glaucomatous damage HRT II is as sensitive as perimetry and HRT II could be a useful tool in evaluation and diagnosis of glaucoma.

 KEYWORDS: Glaucoma, Heidelberg Retinal Tomography, visual field indices

Corresponding Author: Dr. Renu Shukla Dubey, 7-1-215/C/1/B, Dudey Nivas, Balkampet, Hyderabad-500016. Email id : drrenushukla@hotmail.com

 

Introduction

Glaucoma is a common cause of visual impairment and blindness. Primary open angle glaucoma (POAG) is a chronic progressive optic neuropathy with characteristic optic nerve head (ONH) findings and visual field loss with various risk factors including raised intraocular pressure (IOP).

It has been widely accepted that changes in the ONH may precede the detectable field loss in early glaucoma. [1-6] Controlling the IOP at an early stage in glaucoma has been shown to slow down/stop progression of the disease. [7, 8]

Standard automated perimetry (SAP), which tests the functional aspect of the optic nerve fibres, has good diagnostic precision for glaucoma [9, 10] but is not specific for glaucoma. The Heidelberg Retinal Tomogram (HRT) evaluates the structural details of the optic disc and has been widely investigated as a research tool in imaging of the ONH. [11, 12] Several studies have demonstrated that the HRT may be used to differentiate between normal and glaucomatous eyes. [13-17] The correlation between the perimetry and HRT data in Caucasian eyes has been reported previously. [18-25]

We investigated the correlation between HRT parameters and visual field indices in Indian eyes.

 

Material and Methods

Eighty-three eyes of 50 consecutive patients with POAG, established glaucoma as well as glaucoma suspect who attended the outpatient department at P.D.Hinduja Hospital were included in the study. All patients underwent a complete ophthalmological examination including abbreviated medical history, full ocular history, vision, slitlamp biomicroscopy, applanation tonometry, gonioscopy, and dilated fundus examination. Automated achromatic perimetry (Humphrey perimetry 30-2 full threshold) and confocal scanning laser tomography of the optic disc using HRT II. (Software version 1.4.1.0, Heidelberg Engineering, Heidelberg, Germany) were also performed. The sample size was worked out to be 75 using a error of 0.05 (5%) and b error of 0.2 (80% power).

The inclusion criteria were, age ³ 40 years, refractive error, spherical £ ±5.0 dioptre (D), cylinder £ ±3.0 D, IOP ³ 21 mmHg (measured on two or more occasions) AND/OR disc changes suggestive of glaucoma (CDR ³ 0.5/difference in CDR³0.2 of both eyes, open angles on gonioscopy, ≥ 2 reliable visual fields and good quality HRT images. The exclusion criteria were optic nerve / retinal disease that cause visual field defects, history of ocular trauma and intraocular surgery.

The patients were diagnosed as “established glaucoma” who had the abnormal visual fields according to the ‘Anderson’s criteria’ [26]

The patients were diagnosed as “glaucoma suspects” [27] who had normal visual field but either had a IOP³21 or CDR³0.5/difference in CDR³0.2 of both eyes.

Humphrey field analyzer:

Achromatic automated perimetry was performed using 30-2 Full threshold strategy. A glaucomatous field loss was defined as, (i) A cluster of 3 or more non-edge points with p<5% on pattern deviation plot with one point having p<1% AND (ii) Glaucoma hemifield test (GHT) outside normal limits. The reliability parameters for visual fields were fixation losses <20%, false positive and false negative responses <25%. Patients showing ≥ 2 reliable visual fields were included in the study.

Confocal scanning laser ophthalmoscope:

The HRT uses a diode laser (wavelength 670nm) to scan the retinal surface in three dimensions. A topographic image is usually taken as a series of 32 confocal images at 32 consecutive focal planes, each consisting of 256 x 256 pixels.    

Laser scanning tomography was performed with HRT on the same day as perimetry. Images were obtained by one of the two trained technicians. Before each measurement, the subject’s corneal curvature radius was entered into the software. The patient’s face was then gently placed onto the head-and-chin rest of the HRT, and imaging was performed at the 1.5-cm imaging head–eye distance recommended in the instruction manual, as the subject viewed a distant fixation target. Three 10° field images were obtained for each eye through (3mm diameter pupil.

The mean topography of the three images was generated and the contour line was drawn to outline the disc margin along the inner edge of the peripapillary scleral ring of Elschnig. Mean images with a mean SD of the height measurements >50 µm were excluded from analysis.

The HRT software automatically calculates several stereometric parameters: cup area, disc area, cup disc area ratio, cup volume, rim volume, cup shape measure, maximum cup depth, height contour, mean RNFL height, RNFL cross sectional area globally and in the six segments of optic disc.

HRT classifies a given eye as Normal, Borderline or Abnormal based on a discriminant function elaborated by Mikelberg et al. [28]

Data analysis:

The visual field indices and HRT parameters were entered in the excel sheet. Pearson’s correlation coefficient was used to assess the correlation between mean deviation (MD), pattern standard deviation (PSD) and each of HRT parameters. A statistically significant association was taken when the p value was £ 0.05.

 

Table 1: Age demography

Age

No. of patients

40-50

23

51-60

13

61-70

11

71-80

3

 

Results:

The study enrolled eighty-three eyes of 50 patients with established glaucoma and glaucoma suspects. The age of the patients ranged from 40-73 years, mean 53.18+9.34 years. The age demography is shown in table 1. The male: female ratio was 32:18.

 

Table 2: Distribution based on MD values

MD

No. of eyes

> - 6 (mild)

65

-6 to -12 (moderate)

15

< -12 (severe)

3

 

Glaucoma suspect eyes = 53, glaucoma eyes = 30. Mean IOP was 20.43± 4.68 mmHg. Based on MD values, 65 eyes had mild damage (MD > 6 db), 15 eyes had moderate damage (MD 6 to –12) and 3 eyes had severe damage (MD < -12) as shown in table 2.

The mean global visual field indices and global topographical parameters by HRT are shown in Table 3. The average MD of the study group was –3.70 ± 3.99dB (range 1.11 to –19.54).

The correlations between global parameters by HRT and global visual field indices were performed and found to be significant for rim area, rim volume, height variation contour, mean RNFL thickness and found RNFL cross sectional area as shown in table 4.

Rim area (figure 1), rim volume (figure 2) and RNFL cross sectional area (figure 3) were correlated to both MD and PSD.

Height variation contour and mean RNFL thickness were correlated to MD.

 

Table 3 : shows the mean HRT and visual field parameters.

 

Mean

SD

MD

-3.707590361

3.993331206

PSD

3.861445783

2.470793855

Cup area

1.093012048

0.463314003

Rim area

1.285650602

0.290192301

CDR

0.443746988

0.153441245

RDR

0.556253012

0.153441245

Disc area

2.378686747

0.43726224

Cup volume

0.375168675

0.264347093

Rim volume

0.283445783

0.116861316

mean cup depth

0.341048193

0.129245362

max cup depth

0.751903614

0.220929721

ht variation contour

0.345

0.096445612

Cup shape measure

-0.089060241

0.079335249

mean RNFL

0.213879518

0.069522482

RNFL csa

1.15060241

0.342865968

*MD-mean deviation, PSD-pattern standard deviation, CDR-cup disc ratio

RDR-rim disc ratio, RNFL-retinal nerve fibre layer, csa-cross sectional area.

 

The strongest correlation was between rim area and PSD (r = 0.32, p = 0.0029) also shown in figure 4.

 

DISCUSSION

 

The Humphrey perimetry is presently the gold standard in the diagnosis of glaucoma but as reported earlier clinically detectable glaucomatous changes in the optic disc may precede the onset of white on white visual field defects. [1-6] Presence of even an early field defect on standard automated perimetry is considered moderate glaucoma. [29]

The introduction of HRT is helpful for evaluating and monitoring the optic disc quantitatively. The HRT provides us with quantitative optic nerve head parameters that have been shown to be highly reproducible. [12, 30] A number of previous studies have examined the correlation between the HRT parameters and visual field indices. [18-25] They included cup area, CDR, rim area, rim volume, cup shape measure, RNFL thickness, and RNFL cross section area. The correlations were better for MD than PSD/CPSD.

 

TABLE 4: Correlation of HRT parameters and visual field indices

            

MD

PSD

 

R

r

Cup area

- 0.0714

0.0835  

Rim area

0.2667*

0.3233*  

CDR

- 0.1410

0.1783  

RDR

0.1410

- 0.1783  

Disc area

0.1013

- 0.1262  

Cup volume

- 0.0776

0.1164  

Rim volume

0.2342*

0.2534*

Mean cup

depth

- 0.0494

0.1335  

Max cup

depth

0.0275

0.0768

Ht variation

contour

0.2432*

- 0.1904

Cup shape

measure

- 0.1807

0.1762  

Mean RNFL

0.2221*

- 0.1813

RNFL csa

0.2634*

0.2371*

 *p< 0.05 , CDR – cup disc ratio, RDR- rim disc ratio, RNFL- retinal nerve fibre layer, csa- cross sectional area

 

In our study involving Indian eyes, there was statistically significant correlation between several structural ONH parameters and the visual field indices. The strongest correlation was found between rim area and PSD. Similar results have been reported by Iester et al. who showed the rim area to be an important predictor of MD and CPSD. [19] More correlations were found between MD and HRT parameters as shown in table 5.

Tole et al. [22] found all the significant correlations disappeared when analysis was confined to the glaucoma patients with MDs of < -10 dB. They supposed that it could be due to the statistical effect of reducing the numbers from 106 eyes to 61 eyes. However, relations between topographic parameters and MD were found with smaller sample size (47) in other studies. [19]

Our study included only glaucoma (established and suspects) patients. As mentioned before, the correlations were better for combined groups because of the larger range of values. Thus even though there were several parameters correlating significantly, we found lower correlation coefficient as compared to other studies (maximum r = 0.3233).

The visual field defects in our study were less severe as compared to other studies. Mean MD in our study was –3.70

 

Table 5 : Comparision of HRT and visual field parameters in various studies

 

Reference

N

Average MD

(dB)

Parameters

HRT --   Visual field

Correlation

Coefficient

Iester et al.36

294

- 4.0

Rim area       MD                                                                                            

                     CPSD

CSM           MD

                     CPSD

0.44

-0.48

-0.43

0.38

Brigatti et al.37

46

- 4.8

CSM           MD

                     CPSD

-0.65

0.55

Eid et al.35

125

- 7.5

RNFLT       MD

CDAR           MD

0.49

0.41

Teesalu et al.38

77

- 3.5

CSM             MD

RNFLT         MD

-0.65

0.62

Tsai et al.40

34

- 3

Rim area       MD

0.47

Lan et al.42

62

- 5.77

Rim area       MD

0.32

Our study

83

- 3.7

Rim area       MD

                         PSD

0.26

0.32

 * MD- mean deviation, CPSD- corrected pattern standard deviation, CSM-   cup shape measure, RNFLT- retinal nerve fibre layer thickness, CDAR- cup disc area ratio, PSD- pattern standard devation

 

(± 3.99) dB. Only three eyes (2.5%) had MD worse than - 12dB. In previous studies mean MD ranged from - 6 to - 10 Db [18, 19, 22]   with a higher percentage of glaucoma patients with MD worse than - 20 dB (11.5%, [18] 7.5% [22].

In our study we had 65 eyes with mild glaucomatous damage (MD > -6 dB), which means maximum patients had early glaucomatous changes and we found statistically significant correlations between HRT II and visual fields (strongest correlation between PSD and rim area). This suggests that this parameter (rim area) may be studied in detail to diagnose preperimetric glaucoma.

From the scatter plots, we noticed that advanced glaucoma patients had a large influence on the relation between topographic parameters and visual field indices. Since our study had only three advanced glaucoma patients, there were not many high associations. Teesalu et al. [21, 24] had supported this finding. Their scatter plots also showed that the advanced glaucoma patients played an important role in establishing an association.

Few factors can affect the correlation between structural damage and functional damage. Statistically speaking, the sample size and the characteristics of subjects affect the final result. Larger sample sizes and a larger range of values contribute to a better correlation. Thus different studies may get different results because of different sample sizes and different inclusion criteria.

There are few factors which cause a large individual variation in the relations between topographic and visual field indices. First reason being a large variation of these parameters within the normal population. The other is individual difference in the amount of RNFL damage necessary for visual field loss to occur. Bartz- Schmidt et al [31] showed that the amount of rim area loss for certain degree of visual field loss differs for every individual. Therefore, it would be better to evaluate the correlation between optic disc changes and visual field changes by longitudinal study, instead of a cross sectional

data. Thus longitudinal studies that establish the value of HRT in detecting progression, and the expected stronger correlation between the structural changes and functional changes may establish the role of HRT in early diagnosis of glaucoma.

The HRT gives quantitative, rapid and objective measurements of ONH that may be useful for diagnosis and follow up. [30, 32-34] The examination procedure is objective, fast and simple. There are fewer restrictions with respect to pupil size, co-operation of patient and cataract status.

HRT does not provide some important information about the optic disc like, rim pallor, disc haemorrhage, alterations of lamina cribrosa and vessels. Thus clinical evaluation considering all these factors is essential. Nevertheless, the quantitative measurements from HRT assist the clinical judgement for early diagnosis and detection of progression.

The visual field test is more time consuming and depends upon the patient’s responses. Thus it would be better if we could predict the visual function from the optic disc morphology. For evaluation and follow-up of glaucoma patients, a close comparison between optic nerve head parameters and visual field defects is required. The problem of individual variation has to be dealt with by using appropriate parameters.

In our study we found correlation between global visual field indices and HRT II parameters in Indian eyes in established glaucoma as well as glaucoma suspects. This can be studied in further detail by evaluating preperimetric glaucoma patients in a longitudinal study to establish progression structurally and functionally. With this study we conclude that HRTII can be used clinically for diagnosis of glaucoma patients.

 

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ACKNOWLEDGEMENT:

I would like to express my deep gratitude, towards my guide and mentor Dr.Barun Kumar Nayak, who has been a constant source of inspiration, the one who motivated me to give my best all along.

I would also like to thank Dr. Nisheeta Agarwala, Dr. Sunil Moreker, Dr.Ashwin Sainani and Dr. Preetam Samant for their help and support.

 
 

Source of support: Nil.

Conflict of interest: Not Declared

 

 

 


 

 

 





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