It is caused by many ocular conditions like retinal detachment, glaucoma, macular degeneration, and giant cell arteritis, etc.

Retinal detachment should be considered if there were preceding flashes or floaters, or if there is a new visual field defect in one eye.

[2] Angle-closure glaucoma should be considered if there is painful loss of vision with a red eye, nausea or vomiting.

[4] Wet macular degeneration should be considered in older people with new distortion of their vision with bleeding in the macula.

[6] Giant cell arteritis should be considered in an older person with jaw claudication, temporal pain, and tiredness.