Prisms can be useful additions to prescription lenses. They can be used to reduce or alleviate some forms of diplopia (double vision), reduce symptoms due to small eye misalignments and also be used to improve cosmesis when a person has an obvious eye turn. Here, I focus on the use of prisms to alleviate symptoms due to small eye misalignments.
By small eye misalignments, I mean conditions such as exophoria and esophoria at distance and/or near. In my experience people typically present with symptoms due to exophoria at near. Prisms can also be used to treat symptoms arising from esophoria, hyperphoria and hypophoria. Here, I focus on using prisms to alleviate symptoms due to decompensating exophoria at near and convergence insufficiency.
A definition of heterophoria I like is: ‘A tendency for the eyes to move out of alignment when one eye is covered or when they view dissimilar objects (fusional vergence is suspended). Usually detected with the cover test or some other type of dissociation test such as Maddox rod for distance or Maddox wing for near.’
In general, symptoms resulting from decompensated heterophoria are associated with a particular use of the eyes for prolonged periods (e.g. computer work). Symptoms are reduced or alleviated by resting the eyes and are less in the morning and increase during the day. Symptoms can be treated by removing the cause of the decompensation (e.g. improving lighting) or modifying the refractive correction or eye exercises. Prisms can be used if these other forms of treatment are not appropriate or have not worked.
An eye which has a tendency to turn outwards such as in decompensating exophoria or convergence insufficiency is in battle with the visual system which wants each fovea to be aligned with the object of regard. This battle between the eyes wanting to turn out and the visual system expecting them to align well causes symptoms such as:
- Blurred vision
- Focusing difficulties
- 3D problems
- Monocular comfort
- Aching eyes
- Sore eyes
- General irritation
Symptoms due to decompensating exophoria or convergence insufficiency can be reduced by incorporating base-in prism (the base in the lens is pointing towards the patient’s nose) into the refractive correction. A prism bends light towards its base. Therefore, a base-in prism will bend light towards the nasal part of the retina. In exophoria, the eyes have a tendency to turn out which means that the foveas have a tendency to be displaced inwards towards the nose. The correct amount of base-in prism will place the image of the viewed object onto the inwardly displaced foveas. This means that the battle between the eyes wanting to turn out and visual system expecting alignment (decompensating exophoria and convergence insufficiency) with the object ceases and symptoms are resolved as long as the spectacles with the prism are worn.
Prism values are, in my experience best determined using a Mallett Unit. More on that in another Question of the Day.
Prism can be incorporated in prescription lenses by decentring the optical centre of the lens or by grinding prism into the lens. Advice on this can be obtained from the people who make lenses.