Hello and thank you for your question.
If this is happening in both eyes at the same time, then this may be related to a number of different things, including a transient decrease in blood flow to the brain, an intermittent decrease in blood pressure, or rarer problems affecting the blood flow to the eyes or head. These are all uncommon reasons, but because of their importance in causing serious problems should be ruled out.
A more common cause for this, especially if it is not always both eyes at the same time or only one eye consistently is a dysfunctional tear film over the eyes, or dry eyes. The layer of tears that coats the front of the eye, in addition to contributing to the health of the front of the eyes, also contributes a great deal to the vision. If the tear film is not healthy, then the vision can cloud over as you've described and last for several minutes; however, it usually improves the further away from the hours of sleep you are (both morning and evening) and also usually improves intermittently with blinking. It may be accompanied by mattering of the eyelashes in the morning upon wakening, burning/stinging, itching, foreign-body sensation, and/or tearing/watering of the eyes.
If it is just one eye that is consistently doing this intermittently and does not sound like what I described in the previous paragraph, then it is possible that this may represent a lack of sufficient blood flow to that eye from blood vessel problems in the neck or head.
Regardless, given her age and the possibility that these symptoms could represent a serious problem, she should be examined by her ophthalmologist to determine the exact cause, and to start the necessary diagnostic tests and treatment.
Does that help address your question?
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My opinion is solely informative and does not constitute a formal medical opinion or recommendation. For a formal medical opinion and/or recommendation you must be examined by your doctor.