Dr. Shashank Somani, Dr. Sarang Lambat, Dr. Prabhat Nangia, Dr. Vinay Nangia
Suraj Eye Institute, 559, New Colony, Sadar, Nagpur- 440001.
A male, 23 years of age, presented with blurring of vision in both eyes since 1 month. On examination he is best corrected visual acuity (BCVA) was 6/9, N6 in right eye (RE) with correction of -5.25DS/+2.25DCx 85 and 6/6, N6 in the left eye (LE) with correction of -2.50DS/+1.50DCx 90. On slit lamp examination, BE anterior segment examination was normal. His intraocular pressure (IOP) was 12 mmHg in RE and 10 mmHg in LE. The axial length in RE was 24.14 mm and LE 23.39 mm.
Our patient is a myope. Myopes are highly predisposed to the possibility of developing glaucoma. In myopes the assessment of the optic disc is always difficult. This is specially so because changes in axial length are associated with a change in the shape of the optic disc, the origin of blood vessels, and the development of parapapillary atrophy making it difficult to identify glaucomatous changes, specially early damage. In addition there may be a tilted optic disc, when the contours of the neuroretinal rim do not match those of the normal optic disc.
In our patient, one may see the presence of a parapapillary zone on the temporal side of the optic disc in both eyes This is associated in the RE with the vascular arcade being drawn together towards each other. i.e. the distance between the superior temporal and inferior temporal vessels is much less than one would expect. We have drawn and measured the distance between the veins at the 3.4 mm circumference of the RNFL scan. The RE shows a smaller distance compared to the LE.
Measurement of the RNFL using the 3.4 mm scan centered on the optic disc showed in the RE significant higher RNFL values of 236 microns, whereas the normal values would be about 77 microns. We believe the reason for this is the coming together of the blood vessels which are seen to arise from the optic disc with a relatively more acute angle than in normal eyes. There is also an apparent thinning of the superotemporal and inferotemporal RNFL values. The blue arrow in Fig 3 b shows the temporal area of RNFL which is thickened. The left eye also shows thickened macular RNFL.
In patients with a gamma zone one often finds a somewhat thickened macular RNFL. This may have something to do with the alteration of the temporal area of the optic disc neuroretinal rim and parapapillary area. However this association is seen more often in patients with a significantly increased axial length. The axial length in our patient was RE 24.14 mm. and LE 23.39 mm. In our patient, we also looked at the RNFL scans taken with the Bruch’s Membrane opening reference, however the results were no different. Although clinically there is an appearance that a gamma zone would be present, the OCT did not confirm this.
Fig 5 shows a deviation map of the RNFL and the thickening of the RNFL (black arrows) along with thinning of the superotemporal and inferortemporal areas, was evident.
On enhanced depth imaging of the optic nerve we found that in both eyes. Fig 6 b and figure 7 b. there was axonal thickening ( green arrows) which represents the macular RNFL.
The point to consider is that one may not diagnose these eyes to be glaucomatous, because of the superotemporal and inferotemporal thinning of the RNFL in the RE. It is questionable whether the presence of apparent thickening of the macular segment in the RE and the macular and inferotemporal segment of the LE would be protective from glaucoma, specially with regard to preserving the central field of vision
In the assessment of RNFL in myopia it is important to consider the position of the blood vessels, the size of the optic disc, the presence of parapapillary changes and the tilt of the optic disc. These influence the RNFL thickness and may present with RNFL thinning in different segments depending on the optic disc related changes, even though there is no glaucoma. Therefore using RNFL in assessing for glaucoma must be associated with follow up scans and observation of the changes in the optic disc. The ganglion cell layer would also be useful in these situations, when the axial length is not so much as to prevent a good quality GCL scan. The clinical appearance of the optic disc and the intraocular pressure are important in assessing and in the follow up of glaucoma in myopia. All other diagnostic procedures may be used as indicated and the implications of the test results much be understood.
- Chen JJ, Kardon RH. Avoiding Clinical Misinterpretation and Artifacts of Optical Coherence Tomography Analysis of the Optic Nerve, Retinal Nerve Fiber Layer, and Ganglion Cell Layer. J Neuroophthalmol. 2016 Dec;36(4):417-438. doi: 10.1097/WNO.0000000000000422.
- Hwang YH, Yoo C, Kim YY. Myopic optic disc tilt and the characteristics of peripapillary retinal nerve fiber layer thickness measured by spectral-domain optical coherence tomography. J Glaucoma. 2012 Apr-May;21(4):260-5. doi: 10.1097/IJG.0b013e31820719e1.
Dr Vinay Nangia
MS, FRCS, FRCOphth
Suraj Eye Institute
Email – firstname.lastname@example.org
Q. 1 – Which of the following about the optic disc in myopia is correct.
- All optic discs in myopes are larger than normal
- The optic disc in myopia is always different from a normal optic disc.
- Gamma one is frequently found in myopia
- Beta zone is found more commonly in myopia
Q. 2 – Which of the following is false for the retinal nerve fiber layer in myopia.
- The RNFL may show macular thickening.
- The RNFL may show nasal thinning.
- The RNFL thinning is always associated with glaucoma in myopia
- When the RNFL circle passes over the area of gamma zone the RNFL measurement may not be accurate