Exodeviation Overview

Overview

This lecture discusses exodeviation (exotropia), its types, clinical features, terminology, causes, and underlying theories related to its development.

Definition and Types of Exodeviation

• Exodeviation (exotropia) is a type of strabismus where one or both eyes turn outward, away from the nose.
• Exotropia can be constant (always present) or alternating (switches between eyes), sometimes with eye preference.
• Intermittent exotropia appears only at certain times, often when the patient is fatigued, and orthotropia (straight eyes) occurs at other times.
• Exodeviation may be associated with vertical deviations (hypertropia/hypotropia) or specific patterns (A, V, X, Y).

Terminology and Symbols

• Exophoria: latent outward deviation, visible only with tests like the alternate cover test.
• Intermittent exotropia: outward deviation appears at times, controlled by fusion or convergence.
• Constant exotropia (manifest): deviation is always present.
• Symbol notation: exophoria (X’), intermittent exotropia (X(T)), constant exotropia (XT), base in (BI) prism for measurement.

Epidemiology and Clinical Features

• Intermittent exotropia is the most common type in children and adults, especially in Asian/South Asian populations.
• Prevalence in these groups is 7–18.5 times higher than esotropia.
• Infantile exotropia appears within 6 months of birth, has poor binocular vision, and often requires surgery.

Theories and Causes of Exodeviation

• Innervational Theory: Excessive tonic divergence (baseline outward drift due to nerve supply) causes exodeviation.
• Mechanical Theory: Orbital and muscular variations (shape, elasticity, and insertion points) and scarring/fibrosis influence exotropia.
• Defective Fusion Theory: Poor brain ability to merge images (sensory and motor fusion) leads to progression from latent to manifest exotropia.
• Suppression and Sensory Adaptation: Brain suppresses the deviated eye's image to prevent double vision, especially if onset is before age 7.
• Hemiretinal Suppression Theory: Suppression occurs on the temporal retina in the deviating eye to prevent diplopia.
• AC/A Ratio Theory: Low accommodative convergence/accommodation (AC/A) ratio leads to near exotropia; high AC/A ratio results in distance exotropia.
• Refractive Error Theory: Myopia and high hypermetropia reduce accommodative effort and convergence, facilitating exotropia.

Key Terms & Definitions

• Exotropia — outward deviation of the eye(s).
• Exophoria — latent outward deviation, detected by cover test.
• Intermittent Exotropia — outward deviation present only sometimes.
• Constant Exotropia — continuous, manifest outward deviation.
• Fusion — brain’s ability to merge images from both eyes.
• Suppression — brain’s process of ignoring the image from the deviating eye to avoid double vision.
• AC/A Ratio — accommodative convergence to accommodation ratio.
• Prism Diopter (PD) — unit to measure deviation angle.
• Base In (BI) Prism — prism with base toward the nose, used in exotropia measurement.

Action Items / Next Steps

• Review binocular single vision concepts as foundation for understanding strabismus.
• Read/watch recommended resources on fusion and prism cover testing.
• Practice calculation of AC/A ratio using the lens gradient method.