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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.
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.
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.
Eye turns upward. Usually indicates superior oblique palsy (CN IV), inferior rectus restriction (orbital fracture), or thyroid eye disease. Head tilt compensation is common.
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.
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.
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.
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.
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.
Types, critical period, detection, and treatment of lazy eye.
Muscle actions, innervation, and nine cardinal positions of gaze.
Monocular VA testing, pediatric charts, and recording results.
All CPO and CPOA study topics organized by category.
A phoria is a latent tendency for the eyes to deviate that is controlled by the fusional vergence system — the binocular mechanism that keeps the eyes aligned. Phorias are only revealed when fusion is disrupted, such as by covering one eye during the cover test. The vast majority of people have some degree of phoria (usually a small exophoria at distance) without any symptoms or functional impact. A tropia is a manifest deviation — the eyes are actually misaligned even without any disruption of fusion. Tropias can be constant (always present) or intermittent (the eyes sometimes achieve alignment). The cover-uncover test distinguishes tropias from phorias: a tropia causes the uncovered fellow eye to move to take up fixation when the dominant eye is covered; a phoria only causes movement under the cover (revealed on the alternating cover test).
Accommodative esotropia is the most common type of childhood esotropia, typically appearing between 18 months and 3 years of age. It is caused by excessive hyperopia (farsightedness). Hyperopic children must accommodate (increase lens power) to see clearly at all distances. The accommodative-convergence reflex links accommodation with convergence — when a hyperopic child accommodates maximally, excess convergence drives the eyes inward, producing esotropia. Treatment is glasses with the full hyperopic prescription. This reduces the need for accommodation, reducing the excess convergence. Many children with pure accommodative esotropia achieve straight eyes with glasses alone, and the strabismus resolves or improves significantly. Some children require bifocal addition for near esotropia that persists with glasses, because near vision demands even more accommodation.
The Hirschberg corneal light reflex test is a quick estimate of ocular alignment. A penlight or the slit-lamp illuminator is held approximately 33cm (arm's length) from the patient. The patient fixates the light, and the examiner observes where the corneal reflections (Purkinje images) fall relative to the pupil center in each eye. Normally, the reflex falls slightly nasal to the pupil center bilaterally (due to positive angle kappa, ~5 degrees). If the reflexes are symmetric, alignment is approximately normal. Asymmetry indicates deviation: a reflection displaced temporally (toward the ear) relative to the pupil indicates that eye is deviated inward (esotropia). A reflection displaced nasally indicates exotropia. Each 1mm of displacement from the normal position corresponds to approximately 15 prism diopters of deviation. The Hirschberg is a rapid screening tool but less precise than prism cover testing.
Comitant (or concomitant) strabismus is an ocular deviation that remains the same magnitude in all positions of gaze. The angle of deviation is constant regardless of which eye is fixating or which direction the patient looks. This type usually results from cortical causes (refractive, binocular development) rather than neuromuscular pathology. Childhood-onset strabismus is typically comitant. Incomitant (or non-comitant) strabismus has a deviation that varies with gaze direction — it is larger in some positions and smaller in others. This pattern strongly suggests a specific muscle or nerve problem: a muscle palsy, restrictive myopathy (thyroid eye disease, orbital floor fracture with entrapment), or neuromuscular disorder. New-onset incomitant strabismus in an adult always requires further investigation to rule out a serious underlying cause such as a cranial nerve palsy, myasthenia gravis, or orbital mass.
Strabismus surgery involves adjusting the extraocular muscles to reposition the eyes. The surgeon weakens or strengthens muscles by recession (moving the muscle attachment point further back on the globe, weakening its pull) or resection (shortening the muscle, strengthening its pull). For example, an esotropia (eye turns in) might be treated by recessing the medial rectus (weakening adduction) and/or resecting the lateral rectus (strengthening abduction). The surgery is performed under general anesthesia in children. Adjustable suture techniques (performed in cooperative adults) allow fine-tuning of alignment in the recovery room. Postoperatively, the paraoptometric may assist with: documenting preoperative and postoperative cover test measurements, monitoring for postoperative complications (infection, wound dehiscence, diplopia), administering postoperative antibiotic drops, and supporting patient/parent education about recovery expectations and activity restrictions.