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The earliest defects in glaucoma are probably small focal paracentral scotomas or depressions in sensitivity close to fixation. These are frequently seen initially in the superior nasal area and may mirror changes observed with the ophthalmoscope, such as erosion of the inferior temporal neuroretinal rim of the optic disc and retinal nerve fibre layer defects. Some authorities suggest that a generalised diffuse loss is also a potential early change. It has to be remembered, however, that these patients are often elderly with lenticular opacities and smaller pupils, changes that can lead to a similar loss of sensitivity. The least common defect is the so-called temporal wedge corresponding to damage at the nasal neuroretinal rim.
With the progression of glaucoma, the paracentral defects may coalesce to form arcuate defects that may become continuous with the blind spot. Typically, asymmetric levels of sensitivity across the horizontal midline produce the characteristic nasal step, the superior hemifield being more affected. Late in the disease, superior and inferior arcuate defects may encircle and advance towards fixation.
The tunnel vision thus created may present the patient with considerable difficulties even though visual acuity may still be normal. Eventually, the patient is usually left with a small temporal island in the visual field.