The morphologic and functional evaluation of optic nerve head in primary open angle glaucoma and non arteritic ischemic optic neuropathy

The aim of this study was to determine the morphologic and functional changes in optic nerve head in subjects with primary open angle glaucoma (POAG) and non arteritic ischemic optic neuropathy (NAION) and to compare the results with age matched normal subjects.Twenty-four patients with POAG being controlled in glaucoma department, 17 NAION patients being controlled in neuroophthalmology department at Afyon Kocatepe University Ophthalmology Clinic were enrolled in the study. Twenty-three normal subjects referred to the same clinic for presbiopia having no other ophthalmologic or systemic disease served as control group. The morphologic evaluation of optic nerve head was performed by Heidelberg retina tomography (HRT) and disc area, rim area, rim volume, C/D ratio, retinal nerve fiber layer (RNFL) and RNFL cross sectional area were recorded. The functional analysis of ganglion cell layer was performed by pattern ERG.We could not find any statistically significant in age among subjects with POAG, NAİON and control group. In POAG, disc area and C/D ratio was greater compared to control group whereas no difference could be detected for rim area and rim volume. No significant difference could be found in disc area between subjects with NAION and control group but C/D ratio was significantly lower in NAION group. Rim area and rim volume was greater in NAION subjects compared to control group. We could not find any difference between groups in terms of RNFL and RNFLCS. Latancies of P50 and N95 in PERG were not different among groups. In subjects with glaucoma, amplitude of P50 was not different compared to control group but amplitude of N95 was significantly lower. In subjects with NAION, both amplitude of P50 and amplitude of N95 were lower compared to control group and subjects with glaucoma. The N95/P50 ratio was significantly lower in both POAG and NAION compared to control group whereas there was no significant difference between study groups.Larger optic discs are more prone to glaucomatous changes but the cupping area may be altered in larger optic discs and so, it should not be forgotton that no topographic change in optic nerve head may be seen before significant glaucomatous damage. Small optic disc and large rim area are risc factors for NAION. In both POAG and NAION, PERG parameters are affected before optic nerve head RNFL. Especially N95 amplitude is affected in POAG whereas both P50 amplitude and N95 amplitude are affected in NAION.As changes in PERG can be seen before changes in RNFL in glaucoma, the functional evaluation of optic nerve head is important in the early diagnosis of glaucoma. As both in POAG and in NAION, ganglion cell functions can be altered without changes in RNFL, electroretinographic evaluation is important in these subjects besides topographic evaluation.