Strabismus — the medical term for any misalignment of the eyes — affects approximately 4% of children and is closely linked to amblyopia development. It ranges from a subtle latent deviation (phoria) that most people never notice, to a constant, visible eye turn that causes significant visual and psychosocial impact. Understanding strabismus is essential for paraoptometrics because accurate documentation of eye alignment, the cover test results, and how the deviation changes with treatment are core clinical measurements.
The terminology of strabismus is precise and tested on both the CPO and CPOA exams. The direction of the deviation (esotropia, exotropia, hypertropia), whether it is manifest or latent (tropia vs phoria), constant or intermittent, and comitant or incomitant each carry distinct clinical implications.
Beyond terminology, the paraoptometric's practical skills include performing the Hirschberg corneal light reflex test, the cover-uncover test, and the alternating cover test — fundamental assessment tools used at every strabismus visit.
Types of Strabismus
Esotropia
Eye turns inward (toward the nose). May be accommodative (hyperopia-driven), non-accommodative, or infantile (onset before 6 months). Most common childhood strabismus. Associated with high hyperopia and amblyopia risk.
Exotropia
Eye turns outward (away from the nose). Often intermittent initially — the eye drifts out during inattention, fatigue, or distance viewing, but can recover alignment. More common at distance than near.
Hypertropia
Eye turns upward. Usually indicates superior oblique palsy (CN IV), inferior rectus restriction (orbital fracture), or thyroid eye disease. Head tilt compensation is common.
Hypotropia
Eye turns downward. Same causes as hypertropia — the fellow eye is hypertropic relative to the hypotropic eye. The designation (hyper vs hypo) depends on which eye is the reference.
Esophoria
Latent tendency for convergence. Controlled by fusion. Only revealed on cover test (covered eye adducts, moves back to primary on uncovering). May cause asthenopia (eye strain) at near if decompensated.
Exophoria
Latent tendency for divergence. Most common phoria in the general population (small exophoria at distance is normal). Large exophoria can cause intermittent exotropia under stress or fatigue.
Clinical Testing: What the Paraoptometric Performs
Hirschberg Corneal Light Reflex
Penlight at 33cm. Symmetric reflexes = aligned. Temporal reflex displacement = esotropia. Nasal displacement = exotropia. 1mm ≈ 15 prism diopters. A quick, child-friendly screening test.
Cover-Uncover Test
Detects manifest tropias. Cover dominant eye → watch fellow eye for fixation movement (indicates tropia in fellow eye). Uncover → watch previously covered eye return. Esotropia: uncovered eye moves outward. Exotropia: moves inward.
Alternating Cover Test
More sensitive. Alternate cover between eyes rapidly. Any movement as the cover shifts = deviation (phoria or tropia). Total deviation magnitude > cover-uncover deviation = phoria component present.
Prism Cover Test
Neutralize deviation with prism bars. Increase prism until cover test shows no movement → that prism power equals the deviation in prism diopters. Measured at distance (6m) and near (33cm).
Monocular Occlusion (Suppression Test)
Worth 4-dot test or Bagolini lenses check for suppression — if one eye is being ignored by the brain. Present in strabismic patients. Important for treatment planning.
Practice strabismus questions for your certification exam
Opterio covers strabismus types, cover test interpretation, and amblyopia screening with AI-powered explanations.
Strabismus and Amblyopia: The Connection
Constant unilateral strabismus in a child under 7-8 years is the classic cause of strabismic amblyopia. The brain suppresses the image from the deviated eye to avoid diplopia, and this constant suppression prevents normal cortical development. Not all strabismus causes amblyopia — intermittent exotropia often spares acuity because the eye is only suppressed intermittently. Alternating strabismus (where either eye can fixate) rarely causes amblyopia because the brain does not consistently suppress the same eye.
Key Principle: Amblyopia Can Persist After Strabismus is Fixed
Correcting the eye alignment (with glasses or surgery) does not automatically reverse amblyopia. The acuity must be actively treated with patching or atropine even after alignment is achieved. Conversely, treating the amblyopia first (before alignment is corrected) with patching alone sometimes improves alignment — particularly in accommodative esotropia corrected with hyperopic glasses.