A 28-year-old woman presents to the ophthalmology outpatient department with a 6-month history of intermittent horizontal diplopia that worsens toward the end of the day and with prolonged near work. She reports that her right eye drifts outward when she is tired, but the deviation is not constant. On examination, the visual acuity is 6/6 in both eyes. The cover test shows that when the right eye is covered, the left eye makes a small inward movement to take up fixation (phoria). When the right eye is uncovered, it quickly realigns. Ocular motility is full in all gazes. There is no ptosis or pupillary involvement. What is the most likely diagnosis?

Explanation

## Clinical Diagnosis: Intermittent Exotropia ### Definition and Key Characteristics **Key Point:** Intermittent exotropia (IXT) is a manifest outward deviation of the eye that occurs intermittently — the eye visibly drifts outward during periods of fatigue, inattention, or prolonged near work, but alignment is maintained at other times through fusional control. It is the most common form of exotropia in children and young adults. ### Diagnostic Criteria Met in This Case | Feature | Finding in Case | Significance | |---------|-----------------|---------------| | **Nature of deviation** | Intermittent, not constant | Characteristic of IXT — manifest but not always present | | **Visible drift** | Right eye drifts outward when tired | Manifest deviation (tropia), not merely latent | | **Symptoms** | Diplopia worse at end of day | Fatigue reduces fusional reserves, allowing tropia to manifest | | **Trigger factors** | Prolonged near work | Depletes fusional capacity, unmasking the tropia | | **Cover test finding** | Inward movement of left eye to take up fixation | Consistent with phoric component between manifest episodes | | **Spontaneous realignment** | Eye realigns when uncovered | Intact fusional mechanisms that control the deviation between episodes | | **Ocular motility** | Full in all gazes | Rules out paralytic/restrictive causes | | **Pupillary function** | Normal | Rules out cranial nerve III involvement | ### Understanding Intermittent Exotropia vs. Exophoria **High-Yield (Kanski's Clinical Ophthalmology):** The critical distinction between IXT and exophoria lies in whether the deviation becomes **manifest (visible)** during normal viewing conditions: - **Exophoria:** Purely latent deviation — the eye never visibly drifts outward during normal binocular viewing; deviation is only revealed by the cover test (disrupting fusion). No diplopia during normal activities. - **Intermittent Exotropia:** The eye **visibly drifts outward** during periods of fatigue or inattention, even without the cover test disrupting fusion. The patient in this vignette reports the right eye "drifts outward when she is tired" — this is a manifest deviation, not merely a latent one. ```mermaid flowchart TD A[Exodeviation]:::outcome --> B{Does eye visibly drift<br/>during normal viewing?}:::decision B -->|No - Only on cover test| C[Exophoria - Latent]:::outcome B -->|Yes - Intermittently| D[Intermittent Exotropia - Manifest]:::outcome B -->|Yes - Always| E[Constant Exotropia]:::outcome D --> D1[Diplopia when manifest<br/>Fusional control between episodes] ``` ### Clinical Features of Intermittent Exotropia 1. **Presentation:** - Visible outward drift of one eye, typically triggered by fatigue, illness, bright light, or prolonged near work - Diplopia when the deviation is manifest - Normal alignment and binocular vision between episodes 2. **Symptoms:** - Intermittent horizontal diplopia - Photophobia (characteristic — patients often close one eye in bright light) - Asthenopia (eye strain) - Symptoms worsen toward end of day 3. **Cover test findings:** - Alternating cover test reveals the full magnitude of the deviation - Between manifest episodes, a phoric component may be detected - No restriction of ocular motility ### Management of Intermittent Exotropia **Clinical Pearl (Kanski / AAO):** Management depends on the frequency and control of the deviation. 1. **Observation:** If well-controlled and infrequent 2. **Optical correction:** Minus lens therapy (over-minus) to stimulate accommodative convergence 3. **Orthoptic exercises:** Convergence training, antisuppression exercises 4. **Surgery:** Bilateral lateral rectus recession or unilateral recession-resection for poorly controlled or frequent deviations ### Why This Is NOT Exophoria In exophoria, the deviation is **purely latent** — the eye never visibly drifts outward during normal binocular viewing. The patient in this vignette explicitly reports the right eye **drifting outward when tired**, indicating a manifest (visible) deviation, which defines a tropia, not a phoria. The cover test finding of an inward movement represents the phoric component present between manifest episodes, which is consistent with IXT. ### Why This Is NOT Constant Exotropia Constant exotropia shows a persistent, visible outward deviation at all times. This patient maintains alignment during non-fatigued states, ruling out a constant tropia. ### Why This Is NOT Sixth Nerve Palsy Sixth nerve palsy would present with restricted abduction, esotropia (inward deviation), and typically constant diplopia. This patient has full ocular motility and an outward deviation, ruling out sixth nerve involvement. **Reference:** Kanski's Clinical Ophthalmology, 9th edition; American Academy of Ophthalmology Basic and Clinical Science Course, Section 6 (Pediatric Ophthalmology and Strabismus).