Many people do not notice a subtle increase in blurry vision or a slow appearance of double vision. Yet binocular vision dysfunction (the inability of both eyes to work together to form a single, clear image) affects millions of individuals, often going undetected for years while causing symptoms ranging from subtle eye strain to double vision.

One of the tools available to address binocular dysfunction is prism correction. Modern digital surfacing technology enables laboratories to deliver prism-incorporated free-form lenses that help realign the visual system and reduce strain. Now ECPs can prescribe more complex prism solutions, ensuring patients receive lenses that address both their refractive error and their binocular balance.

Binocular vision is an aspect of visual processing that, when functioning correctly, allows both eyes to fuse two images into a single, three-dimensional image with accurate depth perception. The accommodative and vergence systems work in constant partnership to maintain this comfortable vision. The system relies on precise alignment, synchronized eye movements, and the brain's ability to fuse images from each eye into a cohesive visual experience.

When issues disrupt binocular vision, it can manifest in two broad categories of misalignment:

With heterophoria, the eyes have a latent misalignment that is only apparent when an action disrupts binocular vision. This can be when the person is relaxed, closing one or both eyes, or undergoing a cover test. When both eyes are open and functioning, the brain works to fix the misalignment, though the eye muscles constantly strain to do so. 

A compensated heterophoria causes no symptoms because the visual system successfully maintains single binocular vision. When the system becomes stressed—due to factors like prolonged near work, fatigue, illness, or uncorrected refractive error—a previously compensated heterophoria may decompensate, leading to symptoms or even intermittent strabismus.

Also known as strabismus or a squint, heterotropia occurs when one eye can maintain focus on a point, but the other eye deviates in a different direction. It typically results from a muscular or neuromuscular imbalance, leading to misalignment of the visual axes. Unlike heterophoria, heterotropia is always present and prevents the brain from fusing the two retinal images into a single clear picture, often resulting in double vision or suppression of one image.

There are different kinds of heterotropia: 

• Esoptropia: An inward deviation of the eyes.
• Exotropia: An outward deviation of the eyes.
• Hypotropia: One eye points downward
• Hypertropia: One eye points upward. 

Patients with binocular vision issues often present with complaints that overlap with other visual issues and may seem unrelated to eye alignment problems, which is how binocular vision gets misdiagnosed so often. Symptoms frequently worsen during periods of visual stress, such as extended computer use, reading, or when fatigued.

Common issues caused by binocular vision dysfunction include: 

Visual symptoms

• Eyestrain
• Headaches (particularly frontal)
• Blurred vision
• Double vision (diplopia) 
• Text appears to move, jump, or float on the page during reading 

Physical symptoms

• Motion sickness
• Dry eyes
• Dizziness 
• Pulling sensations around the eyes
• Burning or tearing eyes
• Heavy eyelids
• General fatigue after visual tasks 

Functional symptoms

• Difficulty reading, skipping lines, or losing place
• Poor reading comprehension
• Reduced attention span
• Slow writing speed

Children with binocular vision dysfunction often avoid homework and near tasks and have academic struggles misattributed to attention deficit disorders. It’s hard to diagnose young children, as they may not realize their visual experience is abnormal and therefore often don't report visual discomfort.

Adults typically present with work-related symptoms due to heavy digital device use and sustained near work.

Identifying and addressing binocular vision problems requires a patient to undergo specific tests. Common detection methods include:

The patient’s eyes are alternately covered and uncovered to reveal phorias or tropias. If the covered eye moves when uncovered, it reveals a phoria. If the eye moves to take up fixation when the “good” eye is covered, then it confirms a tropia.  Proper technique requires using an accommodative target and occluding each eye for at least two seconds to bring about any existing deviation. By the end of testing, the cover test indicates whether there is a phoria and where a tropia may be present. 

Both tests are dissociation techniques that highlight misalignment by breaking down binocular fusion. The Maddox rod measures horizontal and vertical phorias (heterophorias) at a far distance by creating dissociation between the eyes while maintaining some binocular cues. This technique is beneficial for detecting minor vertical deviations that might be missed with cover testing alone.

Maddox wing testing measures horizontal, vertical, and torsional heterophorias at a near distance. The wing test can measure this rotational deviation, which the rod test cannot. 

Thorington card test

The Thorington card test is a lesser-used test, employed only in some countries, such as the U.S., Spain, and South Africa. It measures horizontal and vertical phoria by having a patient look through a Maddox rod at a card with a numbered scale and a light behind it. Where the patient reports the location of the Maddox rod’s line on the numbered scale reveals the direction and magnitude of the eye deviation in prism diopters. Its simplicity and repeatability make it a reliable test for measuring changes over time. 

Prism bar measurements quantify the degree of deviation in prism diopters. The measurement indicates the strength of the prism required to correct the eye's deviation, which guides treatment decisions. Determining the measurement involves an optometrist observing the eye movement and recording the prism power at the neutralization point. 

The NPC test measures the closest point at which the eyes can maintain single binocular vision. A receded NPC beyond 10 cm is considered abnormal and often indicates convergence insufficiency. A practitioner should repeat the test several times, as patients with unstable binocular vision may show deterioration with successive measurements.

Fixation disparity testing is a routine diagnosis consideration that reveals subtle binocular vision stress that may not be apparent through traditional dissociation tests. When present, fixation disparity indicates that the visual system is working harder than usual to maintain fusion, often predicting future symptoms even when the patient does not have big current complaints. The result reveals disparity in a more natural way and helps refine prism prescriptions. 

When ECPs identify a binocular misalignment, they may prescribe a prism to assist with the imbalance. Indicators for prism correction in lenses are when the eyes cannot maintain comfortable alignment on their own. This typically occurs when a patient's eye muscles cannot provide enough compensatory effort to overcome their natural misalignment, known as inadequate fusional reserves. Since binocular vision dysfunction symptoms overlap with other visual problems, ECPs should only prescribe prism after thorough testing. 

Patients benefit from prism correction when: 

• They experience diplopia (double vision) from various causes

• Convergence insufficiency doesn't improve with eye exercises

• Surgery is contraindicated

• Decompensated phorias

• Tropias / Strabismus

Age plays a crucial role in prism prescription decisions. Younger patients often respond well to vision therapy and may not require permanent prism correction. Older adults, particularly those with presbyopia-related binocular vision changes, may benefit more from prism intervention.

While prism is effective, it introduces challenges in both prescribing and manufacturing:

• Prism accuracy: Even minor errors can cause visual discomfort or worsen symptoms. 
• Adaptation issues: Patients can experience adaptation issues, specifically with high prism and vertical prism.

Eye care professionals must set realistic expectations with patients. With today’s free-form technology, prism accuracy is highly reliable, but clear communication helps ensure patients understand how their lenses will perform and feel during adaptation.

Traditional molded lenses with prism are limited in flexibility. Free-form surfacing, by contrast, treats prism as just another parameter in the digital calculation. This provides:

• Simplified and flexible prism incorporation: Free-form surfacing technology integrates prism directly into the digital calculation process; just like sphere, cylinder, and addition powers. This makes it easier and more flexible to produce lenses with prism correction while maintaining consistent optical quality across the lens surface.
• Ability to process complex prescriptions: Free-form design enables the successful combination of significant refractive errors with prism correction, something that was challenging or impossible with conventional lens manufacturing. A patient can have a complex prescription without sacrificing optical performance.
• Customization: An advanced free-form lens lab can incorporate prism into single vision, wraparound, or occupational lenses with ease. When treated as just another parameter in free-form design, prescriptions become fully customizable. 
• Aesthetics: Lens appearance benefits significantly from free-form manufacturing. The ability to control surface contours precisely means reduced lens thickness variations and improved cosmetic appearance, which is significant for patients requiring higher prism powers.

For patients, the result is better binocular balance, less strain, and a lens that feels natural. 

IOT can seamlessly incorporate prism into free-form designs, so practitioners can confidently prescribe complex corrections knowing they'll be manufactured accurately and consistently. Adding IOT’s lens designs and algorithms means providing binocular balance across the visual field.

Manufacturers gain reliable processes for complex jobs, reducing remake risk and expanding their service range. IOT also supports slab-offs, high prism prescriptions, and custom lenticularization to maintain lens aesthetics and performance.

By partnering with IOT, ophthalmic lens manufacturers and labs can offer their ECP customers advanced lens designs that expand their treatment capabilities and enhance patient outcomes for binocular vision dysfunction.

If your lab is interested in integrating prism correction lenses into your portfolio, contact us to learn more about IOT’s technology and support.