Transactions 29th European Strabismological Association Meeting – de Faber (ed) © 2005 European Strabismological Association, ISBN 04 1537 211 9

Thickness of the retinal nerve fiber layer and macular thickness and volume in patients with strabismic amblyopia

Özgül Altıntas, Nursen Yüksel, Berna Özkan & Yusuf Çag˘ lar.

Department of Ophthalmology of Kocaeli University School of Medicine, Turkey

ABSTRACT:

Objective: To evaluate and compare retinal nerve fiber layer thickness (RNFL), macular volume and thickness of amblyopic eye with that of the normal eye in patients with unilateral strabismic amblyopia using optical coherence tomography Model 3000(OCT-3) unit.

Materials and methods: OCT-3 was performed on 14 patients with unilateral strabismic amblyopia who had an absence of neurologic disease. 9 male and 5 female patients, age range 5 to 18 years were enrolled in the study. The RNFL thickness avarage analysis program was used to evaluate superior, inferior, temporal, nasal mean RNFL thickness. The data in all clock quadrants (12 values avaraged) were identified as NFL overall. The retinal thickness/volume analysis program was used to evaluate macular scans Data were compared with Mann Whitney U test.

Results: The mean age was 10,43 4,09 SD years. There were 6 right and 8 left amblyopic eyes, with the amblyopic group having mean visual acuity 0,3 5,70 SE. OCT parameters: RNFL measurements in all quadrants, RNFL overall, macular thickness and macular volume showed no significant differences between two groups.(p 0,05).

Conclusion: Assessment of RNFL and macular thickness, and macular volume by means of OCT-3 revealed no difference between the two eyes in patients with unilateral strabismic amblyopia.

1INTRODUCTION

Amblyopia is defined as a decrease of visual acuity in one eye when caused by abnormal binocular interaction or occuring in one or both eyes as a result of pattern vision deprivation during visual immaturity, for which no cause can be detected during the physical examination of the eye(s) and which in appropiate cases is reversible by therapeutic measures(Von Noorden, 1977).

Previous studies assessing the retinal nerve fiber thickness by means of scanning laser polarimetry did not find any significant difference between the two eyes in patients with unilateral amblyopia (Bozkurt, 2003; Baddini-Caramelli, 2001; Colen, 1985). In the current study we aimed to evaluate and compare the retinal nerve fiber layer thickness, macular thickness and volume of the amblyopic eye and the sound eye in patients with strabismic amblyopia by means of optical coherence tomography Model 3000(OCT-3), which is latest model of commercially available OCT.

2MATERIALS AND METHODS

14 patients with unilateral strabismic amblyopia who had an absence of neurologic disease, 9 male and 5 female patients, age range 5 to 18 years were enrolled in the study. The ophthalmological testing and examination protocol included visual acuity measurements, ocular motility and alignment evaluation, cycloplegic retinoscopy and autorefraction, and examination of external eye, anterior segment, and fundus.

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Figure 1. Macular analysis of a 9 years old male patient with strabismic amblyopia whose visual acuities were 1,0 in the right eye and 0,1 on the left eye.

Each subject eye underwent fast RNFL and macula scan protocols. The RNFL thickness avarage analysis program was used to evaluate superior, inferior, temporal, nasal mean RNFL thickness. The data in all clock quadrants (12 values avaraged) were identified as RNFL overall. The retinal thickness/volume tubular analysis program was used to evaluate macular scans Statistical analysis were performed using a computer program system SPSS 10.0. Mann Whitney U test was used to determine whether differences between data of the amblyopic eyes and the normal fellow eyes were significant. The significance level was set as p 0.05.

3RESULTS

This series of 14 unilateral strabismic amblyopic patients was consisted of 9 male and 5 females. 6 of the patients had exotropia and 8 had esotropia. The age of the children ranged between 5 years and 18 years.The mean age was 10,43 4,09 standart deviation (SD) years. The refraction of the amblyopic eyes ranged between 0,50 and 3 D and the refraction of the normal fellow eyes ranged between 0,50 and 2.0 D. There were 6 right and 8 left amblyopic eyes, with the amblyopic group having mean visual acuity 0,3 5,70 SE. There were no statistically significant differences in the mean values of all OCT parameters between the amblyopic and normal fellow eyes (p 0,05) (Figure 1). The means and SD for the OCT parameters of both groups are shown in Table 1 and Table 2. Mean RNFL overall thickness were 106,85 20,22 microns in ambylopic eyes and 104,35 17,84 microns in normal fellow eyes. Mean macular volume was 7,01 0,41 mm3 in ambylopic eyes and 6,97 0,47 mm3 in normal fellow eyes.

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Table 1. Comparison of the mean retinal nerve fiber layer thickness of the amblyopic and normal eyes of unilateral strabismic amblyopia.

Table 2. Comparison of the mean macular thickness and macular volume of the amblyopic and normal eyes of unilateral strabismic amblyopia.

4DISCUSSION

Binocularly driven cells are found to disappear from the visual cortex and only monoocular neurons are identified in animal studies where binocular vision is prevented by inducing unilateral strabismus or deprivation (Wiesel & Hubel,1965; Crawford & Von Noorden, 1979). Changes in the distribution of cortical neurons and a decrease in size of the layers of the lateral geniculate nucleus (LGN) are found to be common in all forms of unilateral amblyopia(Von Noorden & Middleditch, 1975; Von Noorden & Crawford,1992) . Two types of retinal ganglion cells have been demonstrated: Y and X cells. X cells , which are mainly found in fovea and are thought to have role in providing high visual acuity were shown to be reduced in animals with induced amblyopia (Ikeda & Tremain,1979). Previously done assesments of RNFL thickness by means of scanning laser polarimetry revealed also no difference in RNFL thickness between the two eyes in patients with unilateral anisometropic and strabismic amblyopia (Bozkurt, 2003; Baddini-Caramelli, 2001; Colen, 1985).

The macula is defined anatomically as that region of the retina where the ganglion cell layer is more than one cell thick. The ganglion cells and RNFL contribute 30% to 35% of the retina thickness

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in the macula, where the ganglion cells are known to be most concentrated (Zeimer, 1998). Depending upon the results of the animal studies where ganglion cell lost were shown in amblyopic eyes, we hypothesized it would be logical to expect that macular volume and thickness would be reduced. However, mean foveal thickness and volume measurements of amblyopic eyes were slightly higher than measurements of normal eyes in our study but these differences were not statistically significant. To our knowledge, so far no attempt has been made to measure RNFL thickness and macular thickness and volume in amblyopic eyes by means of OCT. In our study, OCT-3 was performed in only unilateral strabicmic amblyopia patients under between age 5–18 years. This present study also revealed no difference in RNFL thickness and macular thickness and volume between the two eyes in patients with unilateral amblyopia by means of OCT-3.

REFERENCES

1.Baddini-Caramelli C, Hatanaka M, Polati M, Umino AT & Susanna R Jr. 2001. Thickness of the retinal nerve fiber layer in amblyopic and normal eyes: a scanning laser polarimetry study. J AAPOS. Apr;5(2):82–4.

2.Bozkurt B, Irkeç M, Orhan M & Karaag˘aog˘lu E. 2003.Thickness of the retinal nerve fiber layer in patients with anisometropic and strabismic amblyopia. Strabismus 11(1):1–7.

3.Crawford MLJ & Von Noorden GK. 1979.Concomitant strabismus and cortical eye dominance in young rhesus monkeys. Trans Ophthalmol Soc UK. 99:369–374.

4.Colen TP, de Faber JT& Lemij HG. 2000.Retinal nerve fiber layer thickness in human strabismic amblyopia. Binocul Vis Strabismus Q. Summer;15(2):141–6.2.

5.Ikeda H & Tremain KE. 1979.Amblyopia occurs in retinal ganglion cells in cats reared with convergent squint without alternating fixation. Experimental Brain Research 35:559–582.

6.Wiesel TN & Hubel DH. 1965.Comparison of the effects of unilateral and bilateral eye closure on cortical unit responses in kittens. Journal of Neurophysiology. 28:1029–1040.

7.Von Noorden GK & Middleditch PR. 1975. Histology of the monkey lateral geniculate nucleus after unilateral lid closure and experimental strabismus. Further observations. Invest Ophthalmol Vis Sci 14:674–683.

8.Von Noorden GK. 1977. Mechanism of amblyopia. Doc Ophthalmol.;34:93.

9.Von Noorden GK & Crawford MLJ. 1992. The lateral geniculate nucleus in human strabismic amblyopia.

Invest Ophthalmol Vis Sci. 33:2729–2732

10.Zeimer R. 1998. Application of the retinal thickness analyzer to the diagnosis and management of ocular diseases. Ophthalmol Clin North Am. 11:359–379.

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