Understanding structure and function in glaucoma

ABSTRACT

OBJECTIVE: Glaucomatous optic neuropathy (GON), defined as definitive damage to the optic-nerve head (ONH) and retinal-nerve-fiber layer (RNFL), involves structural changes in the ONH and RNFL and functional losses in the central visual field. Due to the unique anatomic distribution of the nerve fibers as they enter the ONH, there are specific changes in the ONH correlated with specific findings in the visual-field characteristic of GON. The evaluation of these changes is discussed. METHODS: There are qualitative and quantitative methods in the structural examination of the ONH and RNFL, and quantitative techniques in the functional assessment of the central visual field. They are correlated to one another their strengths and limitations are discussed. RESULTS: Clinical evaluation of the ONH and RNFL consists of five basic rules: (1) identify the limits of the optic disc and determine its size, (2) identify the s of the neuroretinal rim, (3) examine the RNFL, (4) examine the region for parapapillary atrophy, and (5) look for retinal and optic-disc hemorrhages. These steps are simple to use and comprise a portion of the comprehensive eye evaluation. Without proper documentation of the changes in glaucoma progression may be missed. Computer-based digital-imaging technology exemplified by the HRT II, GDx, OCT, provides fast, reproducible, objective measurements of the ONH and RNFL, allowing for more precise diagnosis and monitoring of glaucoma. These changes have good correlation to functional assessments exemplified by the standard automated perimetry (SAP) and the selective perimetry (FDT, SWAP). Typical glaucomatous visu field defects include nasal step, paracentral scotoma, and arcuate defects which follow the RNFL pattern. Progression of these defects is monitored over time and needs to be differentiated from long-term fluctuation. CONCLUSION: The clinician should perform both structural and functional assessments to diagnose and monitor glaucoma. Both examinations provide complimentary information and each has its own place in the clinical care of glaucoma patients. The newer imaging devices and selective perimetry augment the diagnostic armamentarium of the clinician and should, therefore, be used appropriately.