INTRODUCTION:

Glaucoma is one of the leading causes of blindness in the world.^1,2 It is characterized by progressive degeneration of the optic nerve associated with cupping of the optic nerve head resulting in gradual, progressive, non-reversible loss of vision. The gold standard for diagnosing Glaucoma is the presence of characteristic glaucomatous visual field defects demonstrated by Automated Perimetry. Early identification and prompt initiation of treatment can delay the progression of the disease and associated morbidity.   Though elevated intraocular pressure is thought to be the most common risk factor for glaucoma development, few studies have shown that vascular insufficiency in the optic nerve head also plays an important role.³

Myopia is a refractive error wherein parallel rays of light from infinity come to a focus in front of the retina. It has certain specific fundus characteristics. With increase in axial length, myopic eyes show various changes like thinning of the choroid and retina, reduction in retinal nerve fibre layer thickness, increasing area of peripapillary atrophy.⁴ Vascular changes including morphological changes and changes in ocular microcirculation has been reported in myopic eyes^5-7, and these are hypothesized to be the precursors for the degenerative fundus changes seen in myopia.^8,9

The visual loss associated with myopia for most part is reversible and its treatment includes visual rehabilitation using spectacles/ contact lenses / various refractive surgeries.

Myopic optic discs and glaucomatous optic discs may show similar morphology. Since glaucoma is a blinding disease whose treatment is life-long, whereas myopia requires only refractive correction, it is important to differentiate between the two. Automated perimetry in myopic eyes may fallaciously show picture simulating glaucomatous field defects owing to the retinal changes that may be associated with myopia and hence may pose a diagnostic dilemma.

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

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

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

REVIEW OF LITERATURE:

·         Scripsema et al¹⁵ attempted to compare the perfused peripapillary capillary density in  primary open angle glaucoma (POAG), normal tension glaucoma (NTG)  and normal patients using OCT angiography and found that peripapillary capillary density in POAG and NTG patients was significantly decreased compared to normal patients

·         Yarmohammadi et al¹⁶ compared retinal nerve fiber layer thickness and OCT angiography retinal vascular measurements in healthy, glaucoma suspect and glaucoma eyes. 261 eyes of 164 healthy, glaucoma suspect and open angle glaucoma patients were enrolled. The study concluded that OCT angiography vessel density is lower in glaucoma patients as compared to the controls and glaucoma suspects and has a similar diagnostic accuracy as RNFL thickness, for differentiating between the three groups.

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

·         Rao et al¹⁸ evaluated the diagnostic ability of peripapillary vessel density measurements on OCT angiography in open angle and angle closure glaucoma in a cross- sectional study and concluded that the diagnostic ability of peripapillary vessel density parameters of OCT angiography, especially the infero-temporal sector measurement was found to be good in both groups and was comparable to RNFL measurements.

·         OCT angiography images from 12 glaucomatous and 12 age matched normal eyes were compared by Liu et al¹⁹ and found significantly lower peripapillary flow index and vessel density in glaucomatous eyes.

·         Lee et al²⁰ investigated the topographic  relationship between decreased parapapillary microvasculature and RNFL in POAG patients. Vascular impairment was found within the microvascular network in the retina of all POAG eyes and found to correlate with the RNFL defect.

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

·         Al-Sheikh M et al²² studied the retinal capillary microvasculature and choriocapillaries in myopic eyes using quantitative OCT angiography and observed that there is reduction in density of the retinal capillary microvasculature and increased area of flow deficit in the choriocapillaries with higher grade of myopia

·         Yang Y 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 highly myopic eyes correlated with axial length elongation.

AIMS & OBJECTIVES:

To evaluate the vascular density in the optic disc region and macula using optical coherence tomography angiography in order to determine if the two diseases could be differentiated on the basis of vascular density

MATERIALS & METHODS :

·         Study Area                     : Giridhar Eye Institute, Kochi

·         Study Population          :

a)      Patients between 40-60yrs diagnosed as high myopia

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

c)      Age matched controls

·         Study Design                 : Prospective, cross-sectional study

·         Study Duration              : 1 year, from Nov 2017 to Nov 2018

·         Inclusion Criteria          :

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

Ø  High Myopia:

Definition: spherical equivalence ≥ -6.00D

Ø  Primary open angle glaucoma:

Definition :  

·         Open angles on gonioscopy

·         Optic nerve head changes- thinning/ notching of Neuro- Retinal Rim or nerve fibre layer defect visible on slit lamp bio microscopy

·         consistent glaucomatous pattern on Humphrey SITA 24-2 visual fields with Pattern standard deviation outside normal limits with P< 0.05 or Glaucoma hemi field test outside normal limits

Ø  Controls :

Definition:

·         Age matched patients

·         refractive error between  -1DS to +1DS

·         IOP < 21mmHg

·         no evidence of glaucomatous optic disc changes

·         who visit the OPD during the study period

·        Exclusion Criteria            :

Ø  H/o intraocular surgery or ocular trauma

Ø  H/o posterior segment laser or scleral buckling surgery

Ø  Retinal Detachment

Ø  Conditions preventing clear visualization of the fundus such as keratitis, cataract or other ocular diseases

Ø  Presence of diabetic or hypertensive retinopathy

Ø  Retinal dystrophies

·         Sample Size                    : 25 high myopes, 25 Primary open angle glaucoma ,

                                           25 controls

Sample size was calculated using the following formula:

The sample 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 of significance.

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

METHODOLOGY:

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

·  

·         All patients will undergo detailed ophthalmologic evaluation including assessment of best corrected visual acuity ( BCVA), Applanation tonometry, Axial length measurement, Slit-lamp bio microscopy, dilated funduscopic examination, Humphrey Field analysis, OCT glaucoma overview.

·         The patients will be categorized as High  Myopes, Primary Open Angle  Glaucoma and Controls as per the above mentioned definitions

·         All the patients will be subjected to OCT angiography [Spectralis- OCT angiography module, Heidelberg engineering GmbH, Germany]

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

·         The images obtained are extracted and superficial capillary density in each zone is calculated using Image J software, a public domain Java based image processing system. (National Institutes of Health, Bethesda, Maryland, USA )

·         The calculated capillary density in each study group is analysed

STATISTICAL METHODS:

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

·         Descriptive analysis of the population’s characteristics will be carried out. The differences between quantitative variables will be analysed using One Way ANOVA and non-parametric test Kruskal Wallis test.

·         Results will be considered statistically significant if P<0.05