A 7-year-old girl is referred to the eye clinic for evaluation of exotropia. Her mother reports that the child's right eye turns outward, especially when she is tired or looking at distant objects. On examination, the child is able to fixate with either eye and has good visual acuity (6/6 bilaterally). The cover test shows 15 prism diopters of exodeviation at distance and 8 prism diopters at near. When the child is asked to fixate on a near target, the exodeviation reduces. The child is orthophoric at near when concentrating. There is no history of diplopia or head posture abnormality. What is the most likely diagnosis?

Explanation

## Clinical Analysis **Key Point:** Intermittent exotropia is the most common form of exotropia in children, characterized by periods of normal binocular vision alternating with exodeviation, particularly at distance. ### Classification of Intermittent Exotropia | Type | Distance Deviation | Near Deviation | Key Feature | |------|-------------------|-----------------|-------------| | **Divergence Excess** | >10 PD more than near | Minimal | Greater deviation at distance | | **Convergence Insufficiency** | Similar to near | Minimal | Difficulty maintaining convergence at near | | **Basic** | Similar at distance and near | Similar | Approximately equal deviations | ### Analysis of This Case **High-Yield:** The critical finding is: - **Distance exodeviation:** 15 PD - **Near exodeviation:** 8 PD - **Difference:** 7 PD (>10 PD difference at distance) This **>10 prism diopter difference** between distance and near deviation is diagnostic of **divergence excess type intermittent exotropia**. ### Pathophysiology 1. **Divergence excess** occurs when the divergence fusional amplitudes are deficient or the divergence stimulus is excessive 2. At distance, the eyes diverge excessively because convergence demand is minimal 3. At near, increased convergence demand brings the eyes into better alignment 4. The child maintains orthophoria at near because accommodation and convergence are coupled ### Management Algorithm ```mermaid flowchart TD A[Intermittent Exotropia Diagnosed]:::outcome --> B{Subtype?}:::decision B -->|Divergence Excess| C[Minus lens at distance<br/>or Base-in prism]:::action B -->|Convergence Insufficiency| D[Plus lens at near<br/>or Base-out prism at near]:::action B -->|Basic| E[Symmetric correction<br/>or Surgery]:::action C --> F{Response to<br/>non-surgical?}:::decision F -->|Good| G[Continue spectacles]:::action F -->|Poor| H[Consider surgery]:::action ``` **Clinical Pearl:** Divergence excess exotropia often responds well to **minus lenses** (which increase accommodation and convergence at distance) or **base-in prisms** at distance. Surgery is reserved for cases refractory to optical correction or when the deviation is large. **Mnemonic:** **DECO** — Divergence Excess: Distance > Near + Exodeviation + Convergence helps [cite:Parsons Diseases of the Eye 21e Ch 12]