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Evaluation of the optic nerve and nerve fiber layer
Last reviewed: 04.07.2025
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Glaucoma is a common cause of blindness worldwide and can develop in any age group, but is especially common after age 40. Increased intraocular pressure is the most important causative risk factor for glaucoma, but high intraocular pressure is not necessary for the development of glaucomatous damage. The physical effect of glaucomatous optic neuropathy is the irreversible loss of retinal ganglion cells, which is clinically manifested by an increase in the optic disc excavation and the appearance of focal or diffuse defects in the retinal nerve fiber layer. Since glaucomatous damage is irreversible but largely preventable, early and accurate diagnosis is essential.
Functional tests
Evaluation of the optic nerve and nerve fiber layer involves examinations that analyze their structure and function. Glaucomatous retinal ganglion cell loss results in structural changes in the nerve fiber layer and optic nerve, and functional changes in the visual fields, which are assessed by automated perimetry and electrophysiological studies. Glaucomatous visual field defects include focal paracentral scotomas, arcuate defects, nasal steps, and less commonly temporal defects. The most common visual field defects in glaucoma are in the area commonly called Bjerrum's zone, which extends in an arc from the blind spot to the medial raphe.
Automatic perimetry
Automated perimeters test the visual field by presenting static stimuli. These stimuli, of uniform size and varying light intensities, are presented at specific locations for brief periods of time while the patient's responses to each light stimulus are recorded. The Humphrey Field Analyzer (HFA) standard achromatic full threshold test (Humphrey Systems, Dublin, CA) uses white stimuli with a white background illumination; similar programs are available on other automated perimeters. Standard achromatic automated perimetry with clinical examination is the "gold standard" for glaucoma patient care. However, the original automated testing strategy is time consuming, often resulting in patient fatigue and testing errors. Recent improvements in automated perimetry have been developed to reduce testing time and provide a strategy for earlier detection of visual impairment in glaucoma. Half-field testing in glaucoma is a strategy that compares specific portions of the visual fields above and below the horizontal midline. This test is available in the software of most automatic perimeters.
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Swedish interactive threshold algorithms
SITA (Humphrey Systems, Dublin, CA) is a family of testing algorithms designed to significantly reduce test execution time without compromising the quality of the data obtained.
How Swedish Interactive Threshold Algorithms Work
SITA uses the information collected by the program to determine a threshold strategy for adjacent points, measures the response time of each patient, and uses this information to set the test speed. SITA strategies are fairly fast, performing the same or better quality test as the full threshold program. On average, the test time is approximately 5 to 7 minutes per eye with SITA Standard. There is also SITA Fast strategy, which requires approximately 50% less time than SITA Standard, but the sensitivity of the method changes significantly due to the reduced test time.
When Swedish interactive threshold algorithms are used
SITA is becoming the "gold standard" for clinical management of patients with glaucoma.