Prismatic film finds application in an area of ophthalmology. What is a prismatic film? What types are there? What is their mode of operation and what are their benefits? That’s what we’re going to talk about here.
What is a prismatic film?
Prismatic films find application in a field of ophthalmology. A prismatic film is a highly transparent film made of flexible plastic that is glued onto an existing eyeglass lens. It is used for flexible therapies for strabismus. The prism deflects the beam of light falling straight ahead so that it passes through the visual axis in the affected eye and strikes the point of sharpest vision, the fovea centralis, in the retina. The function of the prism sheet is to compensate for the measured squint angle and provide binocular single vision for straight-ahead viewing.
Shapes, types, and styles
Because the prism film must be cut to fit the individual’s pre-existing lens, it is offered in a one-size-fits-all, comparable to that of an uncut ophthalmic lens. In order to compensate for the squint angle, it must be adapted to the individual malposition. Therefore, the prismatic film is available in different strengths. These range continuously from deflection of the light beam by 1 cm at a distance of 1 m to 10 cm at a distance of 1 m, which corresponds to the units 1 prism diopter (pdpt) to 10. For stronger squint angles, the film is available for 12, 15, 20, 25, 30, 35 and 40 pdpt.
Structure and mode of operation
In strabismus, the light rays from an object reach non-corresponding – that is, different – retinal areas of both eyes. In the squinting eye, there is an impairment in the control of the external eye muscles, so that the correct alignment of the eye cannot succeed even by motor means. The images of the right and left eye cannot be merged in the visual center to a common visual impression. This results in double images. If a visually weaker eye is not integrated into the visual process through therapy in early childhood, the brain can exclude this eye from the visual process. It then only plays a marginal role in the visual process. Since the visual impressions arrive on different retinal areas of both eyes, spatial vision is then not possible properly. A prismatic film compensates for this circumstance. The parallel light rays emitted by the objects are deflected by the prism at an angle so that they strike the fovea centralis, the visual pit of the eye. This compensates for the squint angle, but does not change the misalignment of the eye itself. Care must be taken when positioning the film to determine whether the strabismus is inward or outward. Depending on the optical system (What is the visual performance of each eye with optimal correction? Is one eye visually weak? Is there a larger dioptric difference in refractive error in both eyes, say more than 4 diopters? Is the visual system already mature? How good was the quality of spatial vision before? ), different quality levels in binocular vision can be achieved with the prismatic film. The first level as a basic requirement is that objects can be perceived simultaneously, i.e. that the right and the left eye are equal. The second stage is reached when the visual center is able to fuse both images into one, called fusion. The highest level of binocular vision is the ability to perceive in three dimensions (stereopsis).
Medical and health benefits
Prismatic films are a transitional solution for fitting before or after eye surgery, or to approximate an optimal solution when strabismus angles should change. They are cheaper to fit than making corrective lenses with prisms. However, visual acuity may be reduced by 10 percent compared to compensating prism with lenses. The prismatic film brings about a noticeable improvement in the quality of vision, as both eyes are once again integrated into the visual process as equally as possible. The images in the right and left eyes again reach corresponding retinal areas in the center and periphery, so that the quality of spatial vision is increased. Spatial vision may become possible in an area where the visual fields of both eyes overlap. Double vision and headaches should no longer occur.There are several things to consider when calculating the squint angle correction:
- The age of the patient
- Are the eyes able to accommodate for near and distance? Physiologically, the closer an object is fixated at near, the more the eyes converge nasally, so the squint angle may be different at near than at distance.
- Are refractive errors properly compensated?
Strabismus may be due to birth. For example, premature birth or lack of oxygen to the brain at birth may be associated with possible infantile cerebral palsy. Infants may exhibit infantile internal strabismus due to a defect in the sensory areas of the brain in the cerebral cortex. These areas perform sensory fusion of the images of both eyes. Further, unilateral high or unequal uncorrected refractive errors may be present, unilateral cataract (cataract) or rarely tumors. Strabismus can also occur after injuries. In adults, circulatory disorders may play a role, for example, as a result of diabetes. Bleeding, inflammation or tumors in the area of the brain stem or multiple sclerosis could be responsible for strabismus. In latent strabismus, the fusion of both images temporarily fails as a result of overtiredness of the eye muscles.
