What Is Strabismus?
Strabismus is a manifest misalignment of the eyes where one eye fixates on the target while the other deviates. Unlike phorias (latent deviations controlled by fusion), strabismus is visible and indicates that the fusional vergence system has failed to maintain alignment.
Types of Strabismus
| Type | Direction | Description |
|---|---|---|
| Esotropia | Inward (convergent) | The deviating eye turns toward the nose |
| Exotropia | Outward (divergent) | The deviating eye turns away from the nose |
| Hypertropia | Upward | The deviating eye turns upward |
| Hypotropia | Downward | The deviating eye turns downward |
Strabismus can be further classified as:
- Constant vs. intermittent: Present all the time or only sometimes
- Comitant vs. incomitant: Same deviation in all directions (comitant) or varies with gaze position (incomitant, suggesting a nerve or muscle problem)
- Unilateral vs. alternating: Same eye always deviates vs. either eye can deviate
Common Forms of Strabismus
Infantile Esotropia
Presents before 6 months of age with a large, constant inward deviation. Requires surgical correction.
Accommodative Esotropia
Caused by uncorrected hyperopia. The child accommodates to see clearly, and the linked convergence response turns the eyes inward. Often correctable with plus lenses alone.
Intermittent Exotropia
The most common form of exotropia. The eye drifts outward intermittently, especially when tired, daydreaming, or focusing at distance. May be managed with exercises, prism, or surgery.
What Is Amblyopia?
Amblyopia (commonly called "lazy eye") is reduced visual acuity in one or both eyes that cannot be fully corrected with lenses, in the absence of detectable structural abnormality. It results from abnormal visual development during the critical period (birth to approximately age 7-8 years).
Amblyopia is not a problem with the eye itself but with the brain's visual processing. The brain fails to develop normal connections for the affected eye because it received degraded or conflicting visual input during development.
Types of Amblyopia
| Type | Cause | Mechanism |
|---|---|---|
| Strabismic | Eye misalignment | Brain suppresses the deviating eye to avoid diplopia |
| Refractive (anisometropic) | Large refractive difference between eyes | Brain favors the clearer eye |
| Deprivation | Obstruction of visual input (ptosis, cataract) | No clear image reaches the retina during development |
Treatment of Amblyopia
- Correct any refractive error: Full spectacle or contact lens correction
- Penalize the stronger eye: Patching (occlusion therapy) or atropine drops to blur the good eye, forcing the brain to use the amblyopic eye
- Address underlying cause: Surgery for strabismus, cataract removal for deprivation
Treatment is most effective during the critical period but can show improvement up to age 12-17 in some cases.
Key Takeaways
- Strabismus is manifest eye misalignment; phorias are latent tendencies
- Esotropia (inward) and exotropia (outward) are the most common forms
- Accommodative esotropia can be corrected with plus lenses for hyperopia
- Amblyopia is reduced acuity from abnormal visual development, not an eye defect
- Treatment involves correcting refractive error and penalizing the stronger eye