Visual symptoms can be functional or dissociative. The following types of symptom are quite common:
Typically people complain that their vision intermittently goes "blurry" and they have to scre up their eyes to make things come back to normal. Sometimes the vision may even be double. This symptom is often due to "convergence spasm". This is when one of the eyes is too actively 'turning in' towards the nose. Convergence is a normal eye movement but sometimes it becomes hyperactive in patients with functional symptoms
Light Sensitivity (Photophobia)
This is common in people with chronic fatigue syndrome. It may also occur in people with functional weakness and sensory symptoms. Often people are more light sensitive on the same side as the weakness. Strong light may even make the weakness worse.
A natural response to this symptom is often to wear dark glasses. The trouble with this is that this tends to make the eyes even more sensitive to light. The best approach to light sensitivity is to gradually expose the eyes to more and more light over time. This may need to be done gradually.
More rarely people may experience significant loss of vision in one or both eyes. Occasionally people cannot see anything at all. In this situation the diagnosis of functional blindness will be made by tests that show the pupils are reacting normally, the eyes are reacting to a moving stimulus and the parts of the brain to do with vision are all intact.
People with functional visual symptoms may have clinical findings such as a 'tubular field defect' or spiral fields when they undergo visual field testing (See pictures oppositie)
"Visual Snow" and Visual Persistence Phenomena
Sometimes people become aware of a snowlike effect in their vision. This could be due to an eye or brain disorder but it can also be a normal phenomenon. If anyone looks at a bright background they may become aware of a fuzzy, 'static' with a sense that things are moving around. In some people this persists probably because of attention paid to it. Normally the brain blocks out this additional visual information. It often helps to understand what it is and that it will improve the less you pay attention to it.
People also can become aware of persistence of visual images, especially after they have been looking at something for a while or if they are trying to follow a rapidly moving object. Like visual snow its really important to exclude a neurological or ophthlamological cause for this but it can occur, like visual snow, as a symptom arising without neurological disease. Its due to the same problem of the normal 'filtering' mechanism in the brain which blocks out the excess visual information going wrong.