Overview
This lecture covers the classification of exotropia, including key terminology, concepts of convergence, Burian’s classification, and Kushner’s classification, with a focus on mechanisms for measuring and interpreting different types of exodeviation.
Key Terminology in Exotropia
- Primary exotropia is an outward deviation not caused by secondary or adaptive issues.
- Primary exotropia is subdivided into intermittent (appears occasionally, often in children) and constant (present at all times).
- Secondary exotropia results from disruption of binocular vision due to sensory or structural eye problems (e.g., cataract, amblyopia).
- Consecutive exotropia arises after esotropia surgery due to overcorrection.
- Residual exotropia is remaining deviation after under-correction in exotropia surgery.
Types of Convergence
- Tonic convergence maintains the eyes aligned at rest without a visual stimulus.
- Accommodative convergence is linked to focusing on near objects, measured by AC/A ratio.
- Fusional convergence corrects minor misalignments for single binocular vision.
- Proximal convergence is triggered by awareness of a near object, not by visual stimulus.
Vergence After Effect / Adaptation
- Vergence after effect is a temporary persistence of convergence after focusing on near objects, masking the true exodeviation at near.
- Mono-ocular occlusion test (Scoobee-Burian test) removes this masking to reveal the true near deviation.
Burian's Classification of Intermittent Exotropia
- Divergence Excess: Deviation is greater at distance than near by ≥10 prism diopters.
- Basic Intermittent Exotropia: Deviation is equal at distance and near.
- Convergence Insufficiency: Deviation is greater at near than at distance by ≥10 prism diopters.
- Pseudo Divergence Excess: Appears like divergence excess, but after occlusion testing, the near deviation increases to match the distance deviation.
Kushner's Classification of Exotropia
- Uses AC/A ratio and plus 3 diopter test to assess accommodative convergence.
- Basic Exotropia: Equal deviation at distance and near.
- Tenacious Proximal Fusion: Near deviation increases after monocular occlusion, due to persistent fusion.
- High AC/A Ratio: Increase in near deviation with +3D lens; accommodative convergence masks near exodeviation.
- Proximal Convergence: Near deviation remains less despite occlusion and normal AC/A ratio.
- Low AC/A Ratio & Fusional Convergence Insufficiency: Near deviation exceeds distance; low AC/A or poor fusional amplitude.
- Pseudo Convergence Insufficiency: Near deviation reduces after occlusion.
Measurement Techniques
- AC/A ratio measures accommodative convergence per diopter of accommodation (use lens gradient method with +3D lens).
- Fusional convergence amplitude is tested with base-out prisms; normal values: Distance (break 15–20, recovery 10–15 PD), Near (break 30, recovery 10–25 PD).
Key Terms & Definitions
- Exotropia — outward turning of one or both eyes.
- AC/A Ratio — amount of accommodative convergence per diopter of accommodation.
- Monocular Occlusion Test — patching one eye to eliminate fusional convergence and reveal true deviation.
- Prism Diopter (PD) — unit measuring the degree of deviation.
- Tenacious Proximal Fusion — persistent near fusion effect masking true deviation.
- Scoobee Phenomena — masking of near deviation due to retinal disparity, not only fusion.
Action Items / Next Steps
- Review prism cover test and AC/A ratio calculation methods.
- Practice classifying clinical cases using Burian and Kushner schemes.
- Study normal values for fusional convergence amplitude.