Dim Flicker: An Endogenous Visual Percept and Its Disease Associations

<b>Background/Purpose:</b> Four patients independently reported episodes of seeing a dimly flickering overlay on an otherwise intact part of their binocular visual field. The aim of the study was to describe the clinical characteristics of this episodic phenomenon, which we call dim flicker. <b>Methods:</b> Retrospective chart review and patient evaluation of an animated reference simulation. <b>Results:</b> The patients described repeated episodes of a seeing a patch of rhythmically oscillating dim flicker overlaid on a circumscribed patch of their otherwise normal binocular visual field. The flicker was typically seen at low ambient light levels and disappeared in bright light or when one or both eyes were covered. Episodes lasted seconds to minutes. Some flicker patches crossed the vertical midline. The flicker was subjectively experienced as coming from one specific eye. Compared to a 7 Hz flicker simulation, patients reported differences in location, prominence, and frequency, with the latter ranging from 3 to 10 Hz. In three patients, the flicker was sometimes experienced during aerobic exercise and in two patients sometimes when they rose at night in the dark. In one patient, the flicker corresponded to an area of ischemic macular edema secondary to central retinal vein occlusion. There was no headache during or after the flicker. Associated maladies included retinal venous congestion, central serous chorioretinopathy, arterial hypertension, atrial fibrillation, and migraine with visual aura distinctly different from the dim flicker. <b>Conclusions:</b> Episodes of seeing an endogenous, rhythmically oscillating transparent overlay within a confined, non-expanding part of an otherwise intact binocular visual field appears to be a distinct nosological entity that can be associated with ocular and systemic vascular disease.