Photophobia is an extreme sensitivity to any form of light. It is often marked by pain and discomfort, leading to avoidance of sunlight and well lit places.
Photophobia is the symptom that occurs due to too much light entering the eye. Normally, the amount of light that is able to enter the eye is controlled by the reflex response (meaning it is automatic) of the iris. The iris is controlled by the occulomotor nerve and may constrict and relax in response to different levels of light. When there is a high level of ambient light, the iris constricts, preventing any damage to the eye. The opposite of iris constriction is dilation, which occurs in dark places. This is an important mechanism to allow us to see objects in low light, by letting as much light as possible into the eye.
Causes of Photophobia
The retina is part of the eye that is responsible for detecting light. It is located at the back of the eye and has multiple photoreceptors, which are responsible for sensing light. There are 2 types; rods and cones. The rods are responsible for detecting light, whereas the cones are concerned with detecting colours. When they become activated by rays of light hitting them, they send impulses via the optic nerve to the area of the brain responsible for vision, the occipital cortex. This area is at the back of the brain. It is this pathway that allows us to perceive the environment around us as images.
In some cases this pathway can become damaged, leading to problems with vision. In photophobia, the reflex mechanism to prevent too much light entering the eye is faulty. This can be caused by the iris being constantly dilated, allowing more light into the eye than is necessary. When the retina is exposed to too much light, an excessive number of impulses are sent to the brain (occiptal cortex). This over excitation leads to the perception of pain in the eye, causing the affected individual to shy away from light. This is also known as light aversion. Many people who experience this symptom will often stay indoors in dark rooms, or wear very dark sunglasses when outside.