Comparison of myopic and glaucomatous eyes using Optical Coherence Tomography Angiography.

INTRODUCTION:

Glaucomais one of the leading causes of blindness in the world.1,2 It ischaracterized by progressive degeneration of the optic nerve associated withcupping of the optic nerve head resulting in gradual, progressive,non-reversible loss of vision. The gold standard for diagnosing Glaucoma is thepresence of characteristic glaucomatous visual field defects demonstrated byAutomated Perimetry. Early identification and prompt initiation of treatmentcan delay the progression of the disease and associated morbidity.   Thoughelevated intraocular pressure is thought to be the most common risk factor forglaucoma development, few studies have shown that vascular insufficiency in theoptic nerve head also plays an important role.3

Myopiais a refractive error wherein parallel rays of light from infinity come to afocus in front of the retina. It has certain specific fundus characteristics.With increase in axial length, myopic eyes show various changes like thinningof the choroid and retina, reduction in retinal nerve fibre layer thickness,increasing area of peripapillary atrophy.4 Vascular changesincluding morphological changes and changes in ocular microcirculation has beenreported in myopic eyes5-7, and these are hypothesized to be theprecursors for the degenerative fundus changes seen in myopia.8,9

Thevisual loss associated with myopia for most part is reversible and itstreatment includes visual rehabilitation using spectacles/ contact lenses / variousrefractive surgeries.

Myopicoptic discs and glaucomatous optic discs may show similar morphology. Sinceglaucoma is a blinding disease whose treatment is life-long, whereas myopiarequires only refractive correction, it is important to differentiate betweenthe two. Automated perimetry in myopic eyes may fallaciously show picturesimulating glaucomatous field defects owing to the retinal changes that may beassociated with myopia and hence may pose a diagnostic dilemma.

Opticalcoherence tomography (OCT) angiography is a novel non- invasive technique usedto evaluate retinal vasculature and blood flow.10,11 It provideshigh- resolution, three dimensional images of retinal vasculature.12Both morphological information about the retinal vessels as well asquantitative measurement of vascular density can be obtained by this method.13,14

Recently,several studies have been done using OCT angiography to assess the vasculardensity in the disc and peripapillary area in glaucomatous subjects, most ofwhich points towards reducing capillary density in these areas in glaucomatous.

Inthis study, we are attempting to use OCT angiography to compare vasculardensity in disc, peripapillary area and macula in myopic and glaucomatous eyesto find the presence of any characteristic vascular density changes so as todistinguish between myopia and glaucoma.

REVIEW OF LITERATURE:

·        Scripsemaet al15 attempted to compare the perfused peripapillary capillarydensity in  primary open angle glaucoma(POAG), normal tension glaucoma (NTG) and normal patients using OCT angiography and found that peripapillarycapillary density in POAG and NTG patients was significantly decreased comparedto normal patients

·        Yarmohammadiet al16 compared retinal nerve fiber layer thickness and OCTangiography retinal vascular measurements in healthy, glaucoma suspect andglaucoma eyes. 261 eyes of 164 healthy, glaucoma suspect and open angleglaucoma patients were enrolled. The study concluded that OCT angiographyvessel density is lower in glaucoma patients as compared to the controls andglaucoma suspects and has a similar diagnostic accuracy as RNFL thickness, fordifferentiating between the three groups.

·        Across sectional observational study by Leveque et al17  tried to detect changes in optic nerve head(ONH) vascularization in glaucoma using spectral domain OCT angiography andreported a significant difference in ONH vascular density between the test andcontrol (normal) group in the total disc and temporal vascular areas.

·        Raoet al18 evaluated the diagnostic ability of peripapillary vesseldensity measurements on OCT angiography in open angle and angle closureglaucoma in a cross- sectional study and concluded that the diagnostic abilityof peripapillary vessel density parameters of OCT angiography, especially theinfero-temporal sector measurement was found to be good in both groups and was comparableto RNFL measurements.

·        OCTangiography images from 12 glaucomatous and 12 age matched normal eyes werecompared by Liu et al19 and found significantly lower peripapillaryflow index and vessel density in glaucomatous eyes.

·        Leeet al20 investigated the topographic relationship between decreased parapapillary microvasculature and RNFLin POAG patients. Vascular impairment was found within the microvascularnetwork in the retina of all POAG eyes and found to correlate with the RNFLdefect.

·        FanH et al21 investigated the change of vascular density in myopic eyesusing OCT angiography and found that with the increase of myopia, the vasculardensity decreased in macular region but not in the optic disc region.

·        Al-SheikhM et al22 studied the retinal capillary microvasculature andchoriocapillaries in myopic eyes using quantitative OCT angiography andobserved that there is reduction in density of the retinal capillarymicrovasculature and increased area of flow deficit in the choriocapillarieswith higher grade of myopia

·        YangY et al23 compared the OCT angiography images from superficial ,deep and whole retinal vascular plexuses at the macular region in highly myopic& mildly myopic / emmetropic eyes and concluded that the microvascular network alterations in highlymyopic eyes correlated with axial length elongation.

AIMS & OBJECTIVES:

Toevaluate the vascular density in the optic disc region and macula using opticalcoherence tomography angiography in order to determine if the two diseasescould be differentiated on the basis of vascular density

MATERIALS & METHODS :

·        StudyArea                     : Giridhar Eye Institute, Kochi

·        StudyPopulation          :

a) Patients between 40-60yrsdiagnosed as high myopia

b) Patientsbetween 40-60yrs diagnosed as Primary Open Angle Glaucoma

c) Agematched controls

·        StudyDesign                 : Prospective, cross-sectionalstudy

·        StudyDuration              : 1 year, from Nov 2017 to Nov2018

·        InclusionCriteria          :

All patients between the ages 40-60years who attend the OPD of Giridhar Eye Institute during the study periodwith-

Ø High Myopia:

Definition: spherical equivalence ≥ -6.00D

 Ã˜ Primary open angle glaucoma:

Definition :

·        Open angles on gonioscopy

·        Optic nerve head changes- thinning/ notching ofNeuro- Retinal Rim or nerve fibre layer defect visible on slit lamp biomicroscopy

·        consistent glaucomatous pattern on Humphrey SITA24-2 visual fields with Pattern standard deviation outside normal limits withP< 0.05 or Glaucoma hemi field test outside normal limits

 Ã˜ Controls:

Definition:

·        Agematched patients

·        refractiveerror between  -1DS to +1DS

·        IOP< 21mmHg

·        noevidence of glaucomatous optic disc changes

·        whovisit the OPD during the study period

 Â·        ExclusionCriteria            :

   Ã˜ H/ointraocular surgery or ocular trauma

 Ã˜ H/oposterior segment laser or scleral buckling surgery

 Ã˜ RetinalDetachment

 Ã˜ Conditionspreventing clear visualization of the fundus such as keratitis, cataract orother ocular diseases

 Ã˜ Presenceof diabetic or hypertensive retinopathy

 Ã˜ Retinaldystrophies

 Â·        SampleSize                    : 25 high myopes, 25 Primaryopen angle glaucoma ,

25controls

Samplesize was calculated using the following formula:

Thesample size was calculated using nMaster 2.0 software.

Z1-α/2*(1.96)is the value of area under the normal curve at a considered level ofsignificance.*

Z1-β    (0.842)is the value corresponding to the power of the study.

METHODOLOGY:

·        Allpatients who satisfies the required inclusion criteria are considered  for the study. Written consent shall be obtained.

·

·        Allpatients will undergo detailed ophthalmologic evaluation including assessmentof best corrected visual acuity ( BCVA), Applanation tonometry, Axial lengthmeasurement, Slit-lamp bio microscopy, dilated funduscopic examination, HumphreyField analysis, OCT glaucoma overview.

 Â·        Thepatients will be categorized as High  Myopes, Primary Open Angle  Glaucoma and Controls as per the abovementioned definitions

 Â·        Allthe patients will be subjected to OCT angiography [Spectralis- OCT angiographymodule, Heidelberg engineering GmbH, Germany]

 Â·        Imageis captured from a) 3x3 mm peripapillary zone centered at the optic disc and b)3x3 mm zone at the macula centred at the fovea.

 Â·        Theimages obtained are extracted and superficial capillary density in each zone iscalculated using Image J software, a public domain Java based image processingsystem. (National Institutes of Health, Bethesda, Maryland, USA )

 Â·        Thecalculated capillary density in each study group is analysed

 STATISTICALMETHODS:

·        Thedata obtained from the patients are recorded and analysed using Microsoft excel.

 Â·        Descriptiveanalysis of the population’s characteristics will be carried out. Thedifferences between quantitative variables will be analysed using One Way ANOVAand non-parametric test Kruskal Wallis test.

 Â·        Resultswill be considered statistically significant if P<0.05