Strabismus and Hirschberg's Test

Overview

This lecture covers the basics of strabismus (squint), key definitions, clinical appearances, and a detailed explanation of the Hirschberg's corneal reflex test for detecting and estimating squint.

Strabismus: Concepts and Types

• Strabismus (squint) is when the visual axes of the eyes are not parallel.
• Visual axis refers to a line from fixation point to the fovea.
• Orthotropia is the normal state when both eyes look straight ahead and visual axes are parallel.
• Heterotropia (manifest squint) is when one eye visibly deviates from the fixation point.
• Heterophoria is a latent squint, controlled by fusion, only revealed when fusion is disrupted.
• Types of deviation include: exotropia (outward), esotropia (inward), hypertropia (upward), and hypotropia (downward).

Clinical Detection of Squint

• Inspect for abnormal head posture: chin up/down (vertical muscle issues), face turn (horizontal muscle issues), or head tilt (cyclovertical muscle issues).
• Observe for craniofacial abnormalities, eyelid shape/position, or asymmetric palpebral fissures—can indicate underlying strabismus.
• Diseases like Graves' orbitopathy may cause esotropia and hypertropia due to muscle involvement.

Hirschberg's Corneal Reflex Test

• Shine a torch at 33 cm from the glabella; observe corneal light reflexes with both eyes open.
• Central corneal reflex in both eyes indicates orthotropia; deviations suggest manifest squint.
• In esotropia, the reflex is displaced temporally; in exotropia, nasally; in hypotropia, upwards; in hypertropia, downwards.
• The test is based on angle kappa, the angle between the visual and pupillary axes.

Understanding Angle Kappa

• Positive angle kappa: corneal reflex is nasal to center, common in normals (≈3°).
• Large positive angle kappa can cause pseudo-exotropia (appears as outward deviation but not true squint), often in hypermetropes or ectopic macula.
• Negative angle kappa: corneal reflex is temporal, may cause pseudo-esotropia, seen in myopes.
• Central fixation means image lands on the fovea; eccentric fixation is when it lands elsewhere.

Estimation of Squint Angle

• For every 1 mm displacement of corneal reflex from center = ≈7° of deviation (≈15 prism diopters).
• Von Noorden’s rule: reflex at pupil edge = 15°, between edge and limbus = 30°, at limbus = 45°, beyond limbus = >45°.
• Microtropias are very small angle strabismus (<10 prism diopters) and may be hard to see.

Key Terms & Definitions

• Strabismus (Squint) — Misalignment of the visual axes.
• Orthotropia — Normal eye alignment; axes parallel.
• Heterotropia — Manifest squint; visible deviation.
• Heterophoria — Latent squint; controlled by fusion.
• Hirschberg’s Test — Corneal reflex test to detect and estimate squint.
• Angle Kappa — Angle between visual axis and pupillary axis.
• Pseudo-exotropia/esotropia — False appearance of deviation due to abnormal angle kappa.

Action Items / Next Steps

• Review the concept of angle kappa and its clinical implications.
• Memorize Von Noorden’s rule for squint angle estimation by Hirschberg’s test.
• Prepare for a future session on the cover test for squint detection.