Binocular Vision
Binocular vision is the ability to use both eyes together to create a single, unified visual perception. It provides several advantages over monocular vision:
- Stereopsis: Depth perception from the slight difference between the two eyes' views (binocular disparity)
- Wider visual field: Combined field is approximately 180-200 degrees
- Binocular summation: Visual sensitivity improves when using both eyes
- Redundancy: Loss of one eye's function does not eliminate all vision
Requirements for Binocular Vision
Three conditions must be met for normal binocular vision:
- Both eyes must see reasonably clearly (similar visual acuity)
- Both eyes must be aligned (pointing at the same object)
- The brain must be able to fuse the two images (neural integration)
What Are Phorias?
A phoria (heterophoria) is a latent tendency for the eyes to deviate from perfect alignment. It is revealed only when binocular fusion is disrupted (such as by covering one eye). When both eyes are open and viewing together, the fusional vergence system keeps the eyes aligned despite the underlying tendency to deviate.
Types of Phorias
| Type | Direction of Deviation | When Covered Eye Drifts... |
|---|---|---|
| Esophoria | Inward (nasal) | Toward the nose |
| Exophoria | Outward (temporal) | Away from the nose |
| Hyperphoria | Upward | Upward |
| Hypophoria | Downward | Downward |
| Orthophoria | No deviation | No movement |
The Cover Test
The cover test is the gold standard for detecting phorias and tropias:
Cover-Uncover Test (Unilateral)
- Have the patient fixate on a target
- Cover one eye with an occluder
- Watch the uncovered eye for movement
- If the uncovered eye moves to take up fixation, a tropia is present
Alternating Cover Test
- Quickly move the occluder from one eye to the other
- Watch for recovery movement of the just-uncovered eye
- If the eye refixates after being uncovered, a phoria (or tropia) is present
- The direction of recovery indicates the type of phoria
Fusional Vergence
The eyes maintain alignment through fusional vergence, the motor system that adjusts eye position based on visual feedback. There are several types:
- Positive fusional vergence (PFV): Convergence (turning in), overcomes exophoria
- Negative fusional vergence (NFV): Divergence (turning out), overcomes esophoria
- Vertical fusional vergence: Limited to about 2-3Δ (much less than horizontal)
When a phoria exceeds the available fusional vergence reserves, the patient experiences symptoms: diplopia, eye strain, headaches, or difficulty with sustained near work.
Phorias and Lens Prescriptions
For opticians, phorias affect dispensing in several ways:
- Prescribed prism: Must be accurately incorporated into the lens during fabrication
- Decentration for prism: Prentice's Rule can create prescribed prism through lens decentration
- Binocular balance: Lens prescriptions aim to optimize binocular function, not just monocular acuity
- Progressive lens selection: Some patients with high phorias may have difficulty adapting to certain progressive designs
Key Takeaways
- Binocular vision provides stereopsis, wider field, and binocular summation
- Phorias are latent eye alignment tendencies controlled by fusional vergence
- Esophoria = inward drift; exophoria = outward drift; hyperphoria = upward drift
- The cover test is the standard method for detecting phorias and tropias
- Symptomatic phorias can be managed with prism correction in spectacle lenses