When an eye turns out such as in exophoria or exotropia, one way to manage the problem is to incorporate base-in prism into spectacles so there is a prism in front of one or both eyes. This can help reduce or prevent symptoms such as diplopia, headaches and eye aches. Similarly, for esophoria and esotropia where base-out prism can be used, hyperphoria and hypertropia using base-down prism and hypophoria and hypotropia where a base-up prism is used.
What possible benefit could come from incorporating prism in the opposite direction to that outlined above? When a person has a misalignment of an eye, it doesn’t look good. The cosmesis is poor would be another way of saying this. And this may have negative psychological effects on the person with the misaligned eyes. If the visual acuity is poor and extraocular muscle surgery (to move the eye to a position where cosmesis is improved) then one way to improve the cosmesis. Sometimes a person has a disfigured face to due trauma or some other reason where one eye looks higher or lower than the eye with a central position.
In these cases, a prism with the base in the ‘wrong’ direction can be helpful. Let me use an example where the poor cosmesis is because one eye socket is higher than the other. A base-up prism incorporated into spectacles in front of the higher eye could help. The light coming from the eye out through the prism will enter the eye of the observer. The base-up prism will displace the image of whatever is behind it downwards. This means that the image of the eye will be shifted downwards and if the correct prism magnitude is used the eye that is too high can look to be more central. Note, this technique is purely to improve the appearance and is not of any use where a functional improvement is required, that is, reducing or negating diplopia or alleviating symptoms.
The amount of prism to use will have to be determined using trial and error with hand-held prisms and a trial frame although it will be useful to remember that 1 prism dioptre = 1 centimetre of displacement when viewed from a distance of 1 metre. So, for an eye that is 3 cm misaligned upwards, a 3 prism dioptre base-up will cause the image of that eye to move downwards by 3 cm when viewed from 1 metre. Cosmesis will be improved when the person wears the prism glasses.
For an eye misaligned downwards use a base-down prism, for an eye misaligned inwards use base-in prism and for an eye misaligned outwards use base-out prism. It’s counterintuitive to the base directions used for the treatment of symptoms but remember this is about appearance and not symptoms.