Identifying the real state of primary open angle glaucoma by correlating visual field defects and thickness of retinal nervefiber layer in emmetropic and highly axial myopic eyes

Abstract

Primary open-angle glaucoma (POAG) is one of the leading causes of irreversible blindness worldwide. Glaucoma is an optic neuropathy characterized by specific and progressive injury to the optic nerve and retinal nerve fiber layer (RNFL) .An objective and accurate method of quantifying this damage would enhance our understanding and treatment of this disease. The early diagnosis of RNFL changes associated with glaucoma is crucial for all patients with glaucoma. Once a visual field defect is detectable, the disease has already caused irreversible RNFL loss. Localized RNFL abnormalities have been shown to precede visual field defects earlier than other signs detected during conventional ophthalmologic examinations . Myopia is one of the most common ocular abnormalities reported worldwide, and its association with glaucoma is well recognized. The risk of developing POAG is two to three times higher in myopic individuals than in non-myopic individuals, and this risk is independent of other risk factors for POAG, with an increased prevalence of myopia in patients with ocular hypertension, primary open-angle glaucoma (POAG), and normal-tension glaucoma. Although the mechanisms linking glaucoma and myopia are poorly understood, some population-based studies have shown an increasing risk of glaucoma with increasing severity of myopia,asthe optic nerve head may be structurally more susceptible to glaucomatous damage because of weakness in the connective tissue structure and arrangement , the elongation of the globe in myopic eyes leads to mechanical stretching and thinning of the retina. This elongation of the eyeball is also associated with pathologic changes in the fundus. Structural changes associated with myopia, such as a longer axial length, a larger and/or tilted optic disc, and peripapillary atrophy may make the maculopapillary bundle more susceptible to glaucomatous injury. The reduction in RNFL thickness with increasing axial length is a potential source of vulnerability in highly myopic eyes. Some studies have suggested that optic nerve damage is more pronounced in highly myopic eyes, with a higher susceptibility to glaucomatous optic neuropathy for any given IOP Because of these features, glaucomatous changes cannot be easily interpreted in myopic discs, possibly leading to a misdiagnosis of glaucoma. Currently, glaucoma is diagnosed by considering the appearance of the optic disc , retinal nerve fiber layer (RNFL) and field changes by standard achromatic perimetry. In early glaucoma, structural change is known to precede functional damage. The RNFL is a sensitive indicator for predicting early glaucomatous changes, and the extent of RNFL damage correlates with the severity of functional deficit in the visual field. Thus, RNFL assessment may be more valuable than optic disc assessment in the case of myopic subjects. The optical coherence tomography (OCT) is a modern imaging device designed to measure the RNFL in a noncontact and noninvasive manner. Newer versions of OCT based on spectral domain technology have been developed. These novel techniques offer higher axial resolution and scanning speed than do conventional time-domain techniques.Moreover the newly introduced Macular ganglion complex which shows higher sensitivity in glaucoma detection in high myopes being not affected by longer axial length. The present study was conducted to evaluate the correlation between the peripapillary RNFL thickness and field changes in both glaucomatous and non glaucomatous state in emmetropes and high axial myopic eyes. The study included 47eyes Patients were further categorized according to their axial length:  Group A: included 26 eyes with axial length (22-24mm) named the emmetropic group. further divided into: Control emmetropic group: 8 eyes with no POAG. Glaucomatous emmetropic group: 18 eyes with POAG.  Group B: included 21 eyes with high axial myopia (axial length more than 25 mm); further divided into: Control high axial myopic group: 5 eyes with high axial myopia without POAG. Glaucomatous high axial myopic group: 16 eyes with high axial myopia and POAG.All cases underwent full ophthalmic examination, axial length measurement , RNFLT measurement by spectral domain OCT (SD/OCT) and Humphery field analyzer for assessment of field abnormalities.All obtained data for each eye in the former groups were presented in tables for further statistical analysis and dataintercorrelation.