FP1172 : The Retinal Nerve Fiber Layer (RNFL) in Myopia. Clinical Implications for Glaucoma.

Dr. Hemangi Rathi, H19777, Dr. Nangia Vinay Kumar B, Dr. Roshani Ramteke, Dr. Priyanka Pardhi

Introduction:   The retinal nerve fiber layer (RNFL) is an important anatomic tissue in the diagnosis and management of glaucoma.  Along with the optic nerve, retinal nerve fiber layer assessment is now a routine part of glaucoma assessment.  The RNFL follows   a similar pattern of  thickness to the neuroretinal rim in non highly myopic eyes, with the inferior RNFl being thicker than the superior RNFL and the superior RNFL being thicker than the nasal. The Temporal RNFL is the thinnest of all.  Changes in the axial length that occur in myopia, are known to result in thinning of the RNFL.  This makes is difficult to assess for glaucoma.  In addition the optic disc shape and size are also altered.  Data on changes of the RNFL in myopia are not easily available for India and for Central India in particular.  It was the purpose to study eyes with myopia and to determine the variation in the RNFL   in the study patients compared to the normative data.

Methods:  The study included 492 eyes of myopes (females 252).   Subjects with Tilted discs, retinoschisis, glaucoma and all subjects less than 10 years were excluded from the analysis.  376 eyes were included ( 196 females).  All subjects underwent an ophthalmic evaluation, including, vision, refraction, slit lamp biomicroscopy, applanation tonometer, pupillary dilatation, indirect ophthalmoscopy, optic disc evaluation, fundus photography and spectral domain OCT.  RNFL images, foveal scans and enhanced depth imaging of the optic disc was done. For high myopes, the high axial length settings were used in the spectralis.  RNFL readings were recorded globally and for  the different segments of  naso superior, nasal, naso inferior, temporo inferior, temporal and temporo superior.

Results:   The mean spherical equivalent was -5.42+-3.85 D.  The BCVA  DU was 0.83+-0.26.  The mean axial length was 25.03+-1.65mm, IOP was 14.84+-2.86.   Peripapillary atrophy was present in 186/190 ( 49.5%) eyes.  The maximum width of PPA was 486.02+-373 microns and the total extent in clock hours was 5.63+-2.94.   Diameter of disc horizontal was 1514.26+-254.47 and vertical was 1610.98+-257.33 microns.  The mean maximum gamma zone values were 385.70+-315.64 u.   The RNFL thickness values were for patients, naso superior 103.84+-24.77 u,  normative values were 102.011+-0.165 u.  Patients nasal values were 72.06+-15.74 u, and normative values were 72.08+-1.57u. Patients naso inferior values were 105.52+-26.15u and normative values were 107.06+-0.94 u.  Patients temporo inferior values were  134.86+-19.13 u and normative  values were 144.99+-8.02 u. Patients temporal values wee 65.55+-12.88u and normative were 76.78+-1.55 u. Temporo superior patient values were  126.90+-19.67u and normative values were 136.91+-1.75u.   Global values for patients were 93.74+-11.46u and normative values were 98.16+-1.1 u.  The foveal optic disc axis  was at an angle of  -6.95+-4.33 in the subjects.

Paired T test was done for the global and segment values for the patients and normative data.

The mean difference, SD and  P values, between  patient and normative RNFL,  were:  Naso superior 1.8+24.7 u ( P = 0.16), Nasal -0.02+15.8 ( P=0.98), naso inferior  -1.5+26.1u ( P =0.265), temporo inferior -10.1+20.8u ( P<0.001), temporal -11.2+12.9u ( P<0.001), temporo superior  -10.00+19.5u ( P<0.001)  and Global -4.4+11.4u ( P<0.001). Significant thinning in the patient values was seen for the temporo inferior, temporal, temporo superior and global regions.

Discussion:    Myopia offers significant challenges in the assessment of retinal nerve fiber layer clinically because of the anatomic changes in the shape, size and the parapapillary area, specially the development of parapapillary atrophy.  In addition due to an increase in axial length, and choroidal thinning, the RNFL is not clinically visualised adequately.  OCT therefore offers an opportunity to assess the RNFL.  The RNFL thickness is expected to decrease with increase in axial length. Axial length has been found to influence the RNFL thickness showing an inverse relationship ( 1). However the authors suggested that after ocular magnification adjustment, this relationship was no longer evident. They did find that with an increase in axial length, the thickness of the temporal RNFL increased than that of the naso superior, nasal and naso inferior RNFL decreased.  The normative database, (privided by Heidelberg Engineering for their Spectralis SDOCT) which includes subjects with a mean age of 48.2+-14.5 years with a refractive error ranging from +5 D to -7.00 D.  Therefore when comparing patient data with the normative database it must be remembered that they are not ideal for comparison. Nevertheless that is the best possible, since there is no specific myopia RNFL database available in the spectralis SDOCT. An attempt to compare   for comparing healthy myopic eyes with eyes with myopia and glaucoma has been made (3) and the authors found that the myopic database  had a higher specificity for detecting  RNFL abnormalities in eyes with myopia and glaucoma than the normative database.  The RNFL  measurements in myopia  have also been evaluated (4) by comparision between measurement in high myopia (<-6.0 D) vs (-6.0D and -0.5D)  The RNFL measurements were significantly  lower in high myopia group compared to  low to moderate  myopia group  at 12,1, and 7 O Clock  positions.

Our study found that there were significant differences, between the myopia group and the normative data for temporo superior, temporal, temporo inferior and for the global regions, with  these regions showing significantly reduced RNFL thickness compared to the normative values. However several variables further need to be taken into consideration.  These include the differences in age, refractive error and  axial length.  It appears, that a more refined normative database for myopia, may be needed for a better assessment of the RNFL.  While even this may have limitations, because of the intraeye anatomic variability, but it may still be an improvement.

It is important to assess the optic disc and the retinal nerve fiber layer in myopia taking into account all the aspects that have been discussed.  The RNFL variability tells us that it should not be the only parameter on OCT for either diagnosing or excluding the diagnosis of glaucoma in myopia.

References:

1. Kang SH, Hong, SW, Im SK, et. al. Effect of Myopia on the thickness of rthe Retinal Nerve Fiber Layer Measured by Cirrus HD  Optical Coherence Tomography. IOVS, August 2010, Vol. 51,No. 8.  4075-4083.
2. Spectralis OCT user manual. V. 6.0. Heidelberg Engineering. August 2014.
3. Biswas S, Lin C, Leung CK.Evaluation of a Myopic Normative Database for Analysis of Retinal Nerve Fiber Layer Thickness.JAMA Ophthalmol. 2016 Sep 1;134(9):1032-9.
4. Leung CK, Mohamed S, Leung KS, et. al. Retinal Nerve Fiber Layer Measurments in Myopia: Optical Coherence tomography Study.  (Invest Ophthalmol Vis Sci. 2006;47: 5171–5176)