SYMPTOMS: Sore Eyes, headaches, blurry vision even with glasses, double vision, text looks distorted, can only read for a few minutes, can't concentrate
Binocular vision co-ordinates the eyes resulting in a clear single image, and is essential for comfortable reading.
In reality the eyes are rarely straight (orthophoria) and a certain amount of ocular drift occurs between the eyes. This drifting is controlled by binocular vision, in particular by fusion. If the eyes tend to drift outwards this is known as exophoria and if they drift inwards this is know as esophoria.
The majority of adults have some degree of exophoria which may vary in size from 1-16 dioptres. Esophorias are slightly less common and may exist up to around 10 dioptres before they give rise to binocular vision problems.
There is however a great deal of variation in the amounts of phoria that different individuals manage to control without effort or strain.
In order to focus on objects which are closer than 6m (6m being equivalent to optical infinity) it is necessary to rotate the eyes inwards or converge the eyes.
Converging the eys requires some effort
It is also necessary to alter accommodation so that a clear focus can be achieved. The combination of convergence and accommodation is known as the near reflex. Normal convergence is present if someone can pull their eyes in to view a target placed about 6-10 cms from the end of the nose. Accommodation is an age-related function and deteriorates with age.
left picture shows: eye relaxed right picture shows: eye focussing or accommodating
More Advanced Information
The near reflex must be controlled effectively in order to facilitate comfortable reading. Other aspects of binocular vision such as heterophoria may affect the near reflex. If someone has an exophoria (or divergence) of the eyes they will have to exert more convergence in order to fixate an object close at hand than someone with orthophoria or straight eyes. They do this by using a mixture of proximal convergence and fusional convergence.
Fusion is the ability to combine two images, one from each eye and fuse them in the brain into a single perception. Most people can demonstrate a range of fusion, which is commonly around 40 prism dioptres base out or convergence and 16 prism dioptres base in or divergence for near fixation.
Relax your eyes and try and focus behind the computer screen. If you wait a few seconds you should see some 3D smarties in the picture
The fusion range is used to control drifting of the eyes and maintains single vision. Any difficulties with fusion, convergence or accommodation will affect reading ability and may result in fatigue or ocular discomfort for reading. It has been documented by Stein et al (1989), Evans et al (1996) and Scott et al (2002) that binocular vision anomalies, in particular poor convergence, poor fusion and poorly controlled accommodation may contribute to some of the difficulties that dyslexic readers experience and that these problems affect fixation. If there are problems with binocular vision then the reader may experience blurring of the text, diplopia, movement of the text and ocular fatigue and discomfort and it is important to treat these aspects of visual difficulty.