![]() The optimum amount of dark adaptation (2 to 3 minutes) produces brilliant powder blue arcs, but these can fade to grey and ultimately white with longer periods of dark adaptation. The arcs are always varying shades of blue, depending on the degree of dark adaptation. ![]() The mechanism for how the blue arc entoptic phenomenon is generated remains unknown. The response is transient, lasting only approximately 0.5 seconds, and occurs immediately upon stimulus presentation. 7 The arcs are best seen in the dark after a 2- to 3-minute period of exposure to room light. While stimuli of various shapes and sizes are capable of producing the blue arc entoptic phenomenon, vertical rectangular stimuli are best for eliciting the response. Various investigators reported on the optimum conditions for demonstrating the blue arc entoptic phenomenon. 7 Subsequent investigation 10 indicated that the shape of the perceived blue arc images is strongly influenced by the anatomic topology of the NFL. As Moreland describes it, Purkinje noticed the blue arcs while viewing the embers of a fire in the dark but did not completely understand their signifcance. 9 Purkinje is credited with the first description of the blue arc entoptic phenomenon in the early 19th century. ![]() 8 The most well-known entoptic phenomenon is Moore’s lightning streaks, which are seen when vitreous liquefaction produces retinal traction, resulting in photopsias often referred to as flashes of light. 7 Since glaucoma is a disease that affects the NFL, we postulate that this entoptic phenomenon is diminished in patients with this condition.Įntoptic images refer to positive visual phenomena generated from physiologic or pathologic processes within the eye. Under appropriate psychophysical conditions in normal subjects, when a vertical stripe of light is presented in the macular region such that it stimulates the arcuate retinal ganglion axon bundles immediately above and below the fovea, an entoptic response with the shape of the nerve fiber layer (NFL) topology is generated. We postulate that the macular changes that occur in glaucoma can be leveraged into a fixation-independent psychophysical test. 3 It is now evident that structural and electrophysiologic changes in the macula do occur in glaucoma patients, 4 – 6 although the psychophysical correlates of these changes are not known. 2 Furthermore, the prevalence of fixation loss >20% is relatively high, prompting a recommendation to relax the acceptable fixation loss rate to 33%. The inability to maintain steady fixation or to reliably indicate that visual stimuli are present could profoundly impact study outcome. ![]() 1 Accurate assessment of retinal sensitivity requires an attentive patient to provide reliable subjective responses while maintaining steady central fixation as visual stimuli are projected to various retinal eccentricities. Standard automated perimetry is an accepted reference standard to assist in glaucoma detection. ![]()
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