What Is Strabismus?
Strabismus (commonly called a "crossed eye" or "wall eye") is a condition in which the eyes are not aligned when viewing a target. Instead of both eyes pointing at the same object, one eye deviates in a different direction. Strabismus affects approximately 2-4% of the population and can cause amblyopia (lazy eye) if untreated in childhood.
As a CPOA, you will observe and document strabismus findings, assist during cover testing, and help prepare patients for strabismus evaluations and treatments.
Classification of Strabismus
By Direction of Deviation
- Esotropia (ET): The deviating eye turns inward (toward the nose). Commonly called "crossed eyes." May be constant or intermittent. Common in young children.
- Exotropia (XT): The deviating eye turns outward (toward the ear). Commonly called "wall eye." Often intermittent, especially in children who can control the deviation much of the time.
- Hypertropia (HT): The deviating eye turns upward. Designated by the higher-positioned eye (e.g., "right hypertropia" means the right eye is higher).
- Hypotropia: The deviating eye turns downward. Note: what appears as a left hypotropia is the same misalignment as a right hypertropia -- both describe one eye being high relative to the other.
By Constancy
- Constant strabismus: The deviation is always present.
- Intermittent strabismus: The deviation comes and goes -- the eyes may appear straight some of the time and deviate at others (often when the patient is tired, ill, or not paying attention).
By Laterality
- Unilateral (monocular) strabismus: The same eye always deviates. This eye is at higher risk for amblyopia because it is consistently suppressed.
- Alternating strabismus: Either eye may take fixation. The risk of amblyopia is lower because both eyes are used alternately.
Phoria vs. Tropia
This distinction is critical:
- Phoria (heterophoria): A latent deviation that is controlled by binocular vision (fusion). The eyes appear straight when both are open, but deviate when one is covered (disrupting fusion). Phorias are very common and often asymptomatic. Examples: esophoria, exophoria, hyperphoria.
- Tropia (heterotropia): A manifest deviation -- the eye is visibly misaligned even when both eyes are open. This is true strabismus.
💡 Clinical Tip: The cover test distinguishes phoria from tropia and quantifies the deviation. The CPOA may assist by providing the fixation target, occluding the appropriate eye, and documenting the doctor's findings. Understanding the test helps you assist more effectively.
Common Strabismus Syndromes
| Type | Deviation | Key Features |
|---|---|---|
| Infantile esotropia | Large-angle ET | Onset before 6 months; surgical correction typically needed |
| Accommodative esotropia | Variable ET | Caused by uncorrected hyperopia; corrects with glasses |
| Intermittent exotropia | Variable XT | Eyes straight often; worse when tired; may progress to constant |
| CN VI palsy | Esotropia | Acquired; eye unable to abduct; diplopia |
| CN III palsy | Exo/hypotropia + ptosis | Eye deviated down and out; pupil may be dilated |
| CN IV palsy | Hypertropia | Vertical diplopia; head tilt; worse looking down |
Documenting Strabismus Findings
Standard abbreviations used in chart documentation:
- ET, XT, HT: Esotropia, Exotropia, Hypertropia
- EP, XP: Esophoria, Exophoria (latent deviations)
- CT, ALT: Constant, Alternating
- Ortho: Orthophoria (no deviation detected)
- PD: Prism diopters (quantifies the magnitude of deviation)
- Example: "CT ET 30 PD OD preferred fixator" = constant esotropia of 30 prism diopters, right eye prefers to deviate.
⚠️ Common Mistake: Confusing esophoria (latent deviation, controlled) with esotropia (manifest deviation, visible). They are managed differently -- phorias are often managed with prism or vision therapy; tropias may require surgery.
Key Takeaways
- Strabismus = misalignment of the eyes. Named by direction: esotropia (in), exotropia (out), hypertropia (up).
- Constant vs. intermittent: constant is always present; intermittent appears only sometimes.
- Unilateral strabismus risks amblyopia more than alternating strabismus.
- Phoria = latent deviation (controlled by fusion, detected only by cover test). Tropia = manifest deviation (visible when both eyes open).
- Common types include accommodative esotropia (hyperopia-driven, corrected with glasses) and intermittent exotropia (usually managed with monitoring or surgery).
- Document using standard abbreviations and quantify in prism diopters.