The most common cause of a loss of vision in a single eye in a person under age 50 is optic neuritis, an inflammation of the optic nerve.
An inflammed optic nerve usually causes a quite dramatic drop in vision in the affected eye, which can come on fairly quickly- over a few hours, and can be accompanied with pain, particularly on eye movements, as the nerve gets tugged around as the eye moves. In contrast, a blood vessel blockage causes a much quicker, near instantaneous drop in vision, and is painless.
The inflammation can last a few weeks to months, and there is often a gradual recovery to near normal standards of vision, but often the affected side will have a colour vision defect- usually a red green colour vision problem, with reds appearing pinker (desaturated) to the affected eye than its fellow. Steroids can be used to help settle the inflammation faster, should the doctors feel it would be beneficial, though the recovery will be just as good without treatment, just taking longer to resolve.
Optic Neuritis can be caused by demyelination- loss of the insulating support cells surrounding the neurones which form the optic nerve. It may also be Post Viral- the most common cause in children, secondary to flu like symptoms, in children the condition can often be bilateral (affects both eyes). It may also be associated with specific acute infections such as measles, mumps, and chicken pox. The optic nerve may also become inflammed if there is an adjacent infection in the meniges (the tissues covering of the brain and central nervous system), orbit (the eye socket) or sinuses- there is an air filled sinus above, between and below the orbit. Granulomatous inflammation, caused by systemic diseases such as Tuberculosis, Sarcoidosis and Syphilis, can cause it also, bur these are rare. Finally Optic Neuritis can occur idiopathically- in many patients who have a single episode, the cause is never established.
In the past, a single episode of optic neuritis would not in itself necessarily have been as thoroughly investigated as it would now- the most common cause of optic neuritis is demyelinating disease- Multiple Sclerosis, and doctors were hesitant to give such a serious diagnosis on a one off episode. Optic Neuritis is the most common initial presentation of Multiple Sclerosis. Should the neuritis recur, or should the patient have other MS symptoms- such as double vision, limb numbness or weakness, tingling, poor balance or unexplained pain, then more investigations would have been ordered.
Because there are now some very effective medicines (Interferons) to deal with MS, doctors would now likely be more willing to discuss this as a possible diagnosis, and an MRI scan is commonly done to look for other areas where nerves may have been affected, which can give a positive diagnosis, but cannot rule out the possibility that no other damage has occured, yet.
If Optic Neuritis recurrs, then there is again a sudden drop off in vision, with a gradual recovery to near normal vision, but each recurrence means that the vision is poorer after the attack than before, and over repeated episodes, vision can be permanently reduced.