They usually represent embolization from the carotid arteries. There may be more than one. They usually lodge at vessel bifurcations. Although they seem larger than the vessel they occupy, they often do not block blood flow, and the blood column will be visible distal to the plaque. When the ophthalmologist applies pressure to the globe during ophthalmoscopy, the cholesterol shard may rock with the pulse or even dislodge and move on.
Calcific emboli appear as grey-white blobs at or near the optic disc.
They originate from cardiac valvular lesions. They most often result in complete obstruction of the involved vessel, leading to retinal infarction and permanent, rather than transient, visual loss. Such emboli may remain in place for years, and occasionally, a collateral vessel may eventually bypass the obstruction. One of my patients, who had a history of rheumatic heart disease, suffered calcific emboli in each eye and developed collateral shunting bilaterally. These represent platelets and associated thrombotic material combining at sites of intravascular irregularity, especially in patients with hypercoagulable tendencies.