Prismatic glasses can be used to compensate for a particular form of refractive error called hidden or latent strabismus. “Hidden” is the name given to it because the visual deficit is not visible to other people. Health experts believe that about 80% of the population is affected by this limitation. However, it only causes problems for about 20% of them. The visual ability of patients with latent strabismus is limited. Because of them, children sometimes have learning difficulties at school and – if not corrected – may have problems later in their professional life.
What are prism glasses?
If the ophthalmologist determines the angle defective vision, the patient is prescribed to wear prism glasses. Prism glasses are a special visual aid. They have a wedge-shaped cut and are used specifically to correct angle refractive error (associated heterophoria). At least one of their lenses is processed so that in it the optical center is not congruent with the main focal point. Prismatic glasses are not made with a prism diopter number of 30 or more, which corresponds to an angle of more than 15 degrees. Such thick lenses are not only difficult to manufacture, but also cause aesthetic problems for those affected. In addition, thicker lenses are heavier and make wearing the special glasses less comfortable. However, patients diagnosed with angle refractive error cannot have their prism glasses made by just any optician: There are only a few specialists who are able to perform a measurement in this regard. Also, not all ophthalmologists are specialized in this field. Refractioning is usually not part of their training.
Shapes, types and types
There are two ways to correct angle refractive error: decentering with the help of prismatic glasses or placing a prismatic film on the lens in question. However, the standard medical case is prismatic lenses. Since these are custom-made, they cost accordingly more than conventional lenses. Prismatic lenses should generally only be used for a transitional period of no more than 4 weeks until, for example, the planned eye muscle surgery takes place. They offer 30 to 70 percent poorer imaging quality than prism glasses – which is problematic because the blurring leads to even greater difficulties in the interaction of both eyes. Prismatic goggles are used, for example, by rock climbers as safety goggles. In certain psychological experiments, they can be used as inversion goggles.
Structure and mode of operation
Normally, the six muscles responsible for eye movement are in balance. However, in latent strabismus, this is not the case: the visual axes, which usually meet in the object fixed by the eyes, do not do so in the anomaly. Vertical or horizontal deviations occur. At first, the body tries to correct the imbalance motorically – but after some time this is too strenuous for it. The wedge-shaped cut of prismatic glasses helps to eliminate the need for the affected eye to make the laborious adjustment. The strength previously spent on this can now be used for more important activities. Prismatic glasses not only improve eye mobility. Subsequent movements are also performed with greater precision and more quickly. The maximum deviation that can be corrected with prism glasses is 4 cm per meter. If the deviation exceeds 12 cm/m, thicker prismatic lenses are recommended or – if the affected person finds them mentally stressful – eye surgery. Depending on the medical indication, prismatic lenses can also be ground spherically or cylindrically. They are also available with a sliding-vision effect. Prism glasses are inadequate for patients who also suffer from focusing problems. In such a case, the combination of prism glasses with optometrically guided visual training is recommended.
Medical and health benefits
Prism glasses are used to correct angle refractive error – a visual defect in which both eyes are not aligned 100 percent parallel. Patients with latent strabismus can focus their eyes on the same object, but the affected eye must use a lot of muscle power to do so.This is already the case with only a few degrees of deviation. If the compensation does not succeed or succeeds only inadequately, symptoms occur. The excessive effort leads to rapid fatigue of the eye muscles, visual disturbances despite glasses, increased sensitivity to light, eye pain and headaches. School children with this deformity often have problems with writing and arithmetic as a result. When drawing, the edges of the figures are painted over and groups of letters are sometimes read incorrectly. The immediate consequences are poorer academic performance and unwillingness to solve the tasks set for them. Some of the child patients diagnosed with dyslexia also suffer from angle defective vision. Strabismus with a smaller angle causes greater difficulties for those affected than strabismus with larger deviations. If the latter is present, prism glasses are usually worn for a longer period of time and then – in case of deviations of at least 20 cm/m – surgery is performed. Since the malposition is irreversible, the patient must wear the special glasses for the rest of his or her life. In most cases, there is an improvement after a short period of acclimatization. In some cases, the visual acuity needs to be readjusted later.
