What Is Strabismus?
Strabismus is ocular misalignment, meaning the eyes do not point in the same direction simultaneously. It affects approximately 2-4% of the population and can develop at any age. Proper classification of strabismus type is essential for diagnosis, treatment planning, and communication among eye care providers.
Direction of Deviation
Strabismus is classified first by the direction the deviating eye turns:
- Esotropia (ET): The eye turns inward, toward the nose. Remember "eso" sounds like "inside."
- Exotropia (XT): The eye turns outward, away from the nose. Remember "exo" sounds like "exit" or "out."
- Hypertropia (HT): The eye turns upward. By convention, the higher eye is named (e.g., right hypertropia).
- Hypotropia: The eye turns downward. In practice, this is usually described as a hypertropia of the opposite eye.
Horizontal deviations (esotropia and exotropia) are far more common than vertical deviations.
Phoria vs. Tropia
This distinction is one of the most important concepts in strabismus:
Phoria (Latent Deviation)
A phoria is a latent tendency for the eyes to misalign that is normally controlled by the brain's fusion mechanism. The eyes appear aligned when both are open and viewing together. The deviation only becomes apparent when binocular fusion is disrupted, such as during a cover test.
- Esophoria: Latent inward deviation
- Exophoria: Latent outward deviation
- Hyperphoria: Latent upward deviation
Most people have a small phoria that causes no symptoms. Larger phorias can cause eyestrain, headaches, and intermittent diplopia when the fusion system is stressed (fatigue, illness, alcohol).
Tropia (Manifest Deviation)
A tropia is a manifest eye turn that is present even when both eyes are open. The brain's fusion mechanism cannot overcome the deviation. One eye fixates on the target while the other deviates.
Intermittent vs. Constant
Some tropias are present only some of the time:
- Intermittent: The deviation comes and goes. The patient may maintain alignment most of the time but deviates under certain conditions (fatigue, inattention, illness). Written as X(T) for intermittent exotropia or E(T) for intermittent esotropia.
- Constant: The deviation is always present when both eyes are open.
Concomitant vs. Incomitant
Concomitant (comitant) strabismus means the amount of deviation stays the same regardless of which direction the patient looks. This pattern is typical of childhood strabismus and suggests the problem is not caused by a specific muscle weakness.
Incomitant (non-comitant) strabismus means the deviation changes depending on the direction of gaze. It is larger when looking in the direction of action of the weakened muscle. This pattern strongly suggests a specific cranial nerve palsy or restrictive process (e.g., thyroid eye disease, orbital fracture).
Common Strabismus Types
| Type | Direction | Common Age | Key Features |
|---|---|---|---|
| Infantile esotropia | Inward | Before 6 months | Large, constant, concomitant |
| Accommodative esotropia | Inward | 1-4 years | Related to hyperopia and accommodation |
| Intermittent exotropia | Outward | Childhood | Worse with distance fixation, fatigue |
| CN VI palsy | Inward (esotropia) | Any age | Incomitant, worse looking toward affected side |
Key Takeaways
- Strabismus is classified by direction: esotropia (in), exotropia (out), hypertropia (up)
- A phoria is a latent deviation controlled by fusion; a tropia is a manifest deviation that overwhelms fusion
- Concomitant deviations are the same in all gaze directions (typical of childhood strabismus)
- Incomitant deviations change with gaze direction and suggest cranial nerve palsy or restriction
- Intermittent deviations come and go; constant deviations are always present