Prism therapy is a treatment that involves attaching prisms of a power matching the strabismus angle to glasses, optically changing the direction of light rays and stimulating binocular vision in the foveal region. The introduction of Fresnel membrane prisms attached to glasses has expanded its indications.
A prism is an optical device that displaces an image in one direction, and its unit is expressed in prism diopters (Δ). 1Δ means a power that displaces light rays by 1 cm at a distance of 1 m. While lenses focus light rays to correct refractive errors, prisms differ essentially in that they displace the entire light beam in one direction to align the visual axes of both eyes.
In adult strabismus, prism correction is a major non-surgical treatment option and is positioned as an intervention alongside surgery and botulinum toxin1).
A lens focuses light rays to a point to correct refractive errors, while a prism deviates the entire light beam in one direction. By shifting the image position, a prism aligns the visual axes of both eyes, eliminating double vision and maintaining binocular vision.
There are three main types of prisms, used according to the purpose.
Fresnel Membrane Prism
Structure: A thin film type attached to the spectacle lens surface. It has a structure with fine prism rows (grooves) engraved on a vinyl chloride film.
Features: Supports high prescriptions (up to about 40Δ), lightweight and thin. Suitable for trial use and temporary treatment.
Disadvantages: Decreased visual acuity due to diffraction fringes (noticeable above 12Δ), reduced contrast sensitivity, surface contamination, and aesthetic issues.
Ground-in Prism Lens
Structure: A type ground and incorporated into a glass or plastic lens.
Features: Suitable for permanent use. High optical quality with minimal vision loss.
Disadvantages: Increased weight and thickness with high prescriptions. Not easy to change power.
Prism segment glasses
Structure: Design that provides prism effect only in the lower visual field.
Features: Addresses diplopia only for near vision. Functions as normal lenses for distance vision.
The purpose of using prisms is broadly divided into diagnostic and therapeutic.
The base direction of the prism is selected according to the type of strabismus. The correspondence between the type of strabismus and the prism base direction is shown below.
| Type of strabismus | Prism base direction | Notes |
|---|---|---|
| Exotropia | Base-in (BI) | May be only for near vision |
| Esotropia | Base-out (BO) | Refractive correction is prioritized for accommodative esotropia |
| Hypertropia | Base-down (BD) | Attached to the eye with upward deviation |
| Hypotropia | Base-up (BU) | Placed on the eye with downward deviation |
Oblique prisms combining horizontal and vertical components can also be prescribed.
The main indications for prism glasses are listed below.
The prevalence of adult strabismus is reported to be 2.7% based on ophthalmology clinic data1). The annual incidence of convergence insufficiency is 8.4 per 100,000 people, accounting for 15.7% of new adult strabismus cases. The median age of onset is 69 years1).
Divergence insufficiency esotropia (including sagging eye syndrome) is a very common cause of acquired binocular diplopia in individuals aged 60–80 years, and is more frequent in patients with myopia1).
In strabismus after orbital trauma, a series of 54 cases reported that 86% had diplopia preoperatively and 37% had residual diplopia postoperatively. Conservative treatments (occlusion, filters, Fresnel prisms, botulinum toxin, prism glasses) are considered useful for temporary or permanent reduction of diplopia1).
Eye patches and Fresnel prisms are not covered by medical expense benefits.
The prism adaptation test is used to determine surgical indications and the amount of surgery. Under refractive correction glasses, a Fresnel membrane prism is added to detect the maximum strabismic angle that is neutralized. At the same time, the presence or absence of diplopia and fusion is checked to determine whether binocular vision is latent.
If the strabismic angle increases with prism adaptation, increasing the amount of surgery may lead to a good postoperative eye position. In adults with a history of congenital esotropia, a positive response to prism adaptation may indicate the potential to achieve fusion postoperatively 1).
In strabismus with onset in early childhood, abnormal retinal correspondence often coexists, and the response to prisms can be misleading. Although patients may initially complain of diplopia when the strabismus is corrected with prisms, if the same correction is performed surgically, adaptation occurs and persistent postoperative diplopia remains below 1% 1).
Caution is required because membrane prisms exceeding 12Δ cause decreased visual acuity. With high-power Fresnel membrane prisms, diffraction fringes become prominent and contrast sensitivity also decreases.
The near point of convergence (NPC) is the closest distance at which a target can be brought toward the eyes while maintaining fusion. Normal value is 5–8 cm; convergence insufficiency is indicated when it is 10 cm or more.
Abnormal head posture to maintain binocular vision is characterized by improvement when each eye is occluded with an eye patch or corrected with a Fresnel membrane prism.
As a preoperative test for sagging eye syndrome, it is also useful to simulate the expected postoperative eye position with a prism and confirm the range of overcorrection or undercorrection that can be comfortably tolerated 1).
A prism of a power that neutralizes the strabismic angle is worn under fully corrective glasses. Caution is required because membrane prisms exceeding 12Δ can cause decreased visual acuity.
Generally, the optimal prism power is first confirmed using a Fresnel membrane prism, and then transitioned to a ground-in prism. In some cases, treatment can be achieved solely with prism weaning therapy, which gradually reduces the prism power while maintaining binocular vision. However, for large-angle strabismus, it should be attempted with surgery in mind. Prism therapy is also useful for residual strabismus after surgery.
Convergence insufficiency: Base-in prism reading glasses are one of the non-surgical treatment options 1). Convergence training (office-based training) improves positive fusional convergence more than home-based exercise in young adults aged 19–30, but there is no difference in improvement of convergence near point or symptoms. In adults, training effects are less consistent than in children 1). Surgery is considered for convergence insufficiency when distance deviation becomes manifest, symptoms persist, or prism reading glasses are insufficient 1).
Divergence insufficiency esotropia: Fresnel or ground-in prisms are a temporary (preoperative) or long-term treatment option. Explain to the patient that the prism amount may increase over time and that the ability to control diplopia without prisms may also decrease 1). Fusional training has not been shown to be effective for divergence insufficiency 1).
Sagging eye syndrome: Fresnel or ground-in prisms are useful for new-onset deviation (intermittent or constant, small angle). The prism amount may need to be increased over time 1).
Sixth cranial nerve palsy: Occlusion (patch, Bangerter filter, satin tape) may be an alternative. Prisms are used temporarily, and once the deviation stabilizes, ground-in base-out prisms are incorporated into the lenses. Botulinum toxin injection into the medial rectus muscle helps prevent secondary contracture and reduce the final deviation angle 1).
Third cranial nerve palsy: After partial recovery, press-on or ground-in prisms can be useful, but due to incomitance, diplopia persists when the gaze shifts slightly from the primary position. Many patients use an eye patch or occlusive contact lens as needed 1).
In accommodative esotropia with a high AC/A ratio, bifocal glasses with +3.00 D added in the lower segment are prescribed. This is not a prism but convergence suppression by addition power, yet it is closely related to optical management of strabismus with glasses.
In strabismus with abnormal head posture to maintain binocular vision, such as congenital superior oblique palsy, Duane syndrome, and A-V pattern strabismus, early surgery is not necessary if the deviation angle is small. Observation is continued while performing refractive correction or prism therapy.
Prisms optically correct image displacement but do not treat the underlying disease itself. In the acute phase of paralytic strabismus, complete correction is difficult because the deviation angle fluctuates. If the deviation is stable at a small angle, prism glasses often allow daily life with almost no double vision. Large-angle strabismus or strongly incomitant deviation requires surgery.
Prisms refract light toward the base, and the image is displaced toward the apex. The patient’s line of sight moves toward the direction of image displacement, so base-in (BI) prisms are placed for exotropia and base-out (BO) for esotropia. For vertical strabismus, base-down (BD) is placed for the higher eye and base-up (BU) for the lower eye.
Prism unit: 1Δ = the power that deviates a light ray by 1 cm at a distance of 1 m.
Prentice’s rule: Prism effect (Δ) = lens power (D) × decentration distance (cm). Even in ordinary spectacle lenses, a prism effect occurs when viewing away from the optical center.
Fresnel membrane prisms have a structure in which fine prism rows (grooves) are engraved on a thin vinyl chloride film, enabling lightweight, thin, and high-power prism effects. However, they have the following optical limitations.
When a prism projects an image corresponding to the fovea, conditions for sensory fusion are met and binocular vision is stimulated. In intermittent strabismus, it releases suppression and promotes fusion. In constant strabismus with anomalous retinal correspondence, returning to normal correspondence with a prism may be difficult.
In prism adaptation-positive cases, increasing the surgical target has been reported to achieve good postoperative ocular alignment. The relationship between prism adaptation and postoperative fusion acquisition in congenital esotropia is attracting attention.
Prisms are theoretically unable to correct diplopia with a cyclotorsional component. For cyclodiplopia, surgical intervention such as Harada-Ito surgery may be necessary.
Development of variable prisms using liquid crystal elements and other technologies is being attempted, which may broaden the range of indications.
In divergence insufficiency and sagging eye syndrome, the amount of prism correction may increase over time, and the ability to suppress diplopia without prisms may also decrease 1). The overall success rate of surgery is about 80% (with one surgery), and with reoperation, it can exceed 95% 1). The incidence of persistent new diplopia after adult surgery is less than 1%. Even if the preoperative prism test suggests diplopia, patients often adapt after surgical correction and do not develop persistent diplopia 1).
After surgery, prism glasses may be needed for residual small deviations (horizontal ≤8Δ, vertical <3Δ). The benefit of surgery is particularly great in distance-dominant divergence insufficiency type, where diplopia without glasses is problematic for distance vision 1).
It depends on the treatment purpose. In acute paralytic strabismus, the angle of deviation fluctuates, so the power should be reassessed every 1 to 3 months until stable. Once stable, consider switching to ground-in prisms (lens-incorporated type). Long-term use of membrane prisms can cause decreased visual acuity, contamination, and cosmetic issues, so it is advisable to transition to a permanent method as soon as possible.
