What is the difference between a tropia and a phoria?

A tropia is a manifest strabismus, meaning the misalignment is present and visible even with both eyes open under normal viewing conditions. A phoria is a latent deviation that is controlled by the fusional mechanisms of the visual system and only becomes apparent when fusion is disrupted, such as when one eye is occluded during the alternating cover test. Most people have some degree of phoria; tropias are pathological.

What is accommodative esotropia?

Accommodative esotropia is an inward eye turn in children caused by excessive convergence driven by accommodation for uncorrected hyperopia. When a hyperopic child accommodates to see clearly, the associated convergence reflex overcomes fusional divergence and the eye turns in. Correcting the hyperopia with glasses reduces the accommodative demand and, in pure accommodative esotropia, eliminates the deviation.

What is the difference between comitant and incomitant strabismus?

Comitant strabismus has the same angle of deviation in all directions of gaze, typically from a childhood onset primary motor disorder. Incomitant strabismus has a deviation that varies with gaze direction, indicating a paretic cause (cranial nerve palsy) or restrictive cause (thyroid eye disease, orbital fracture). New incomitant strabismus in an adult warrants investigation for an underlying neurological or systemic cause.

Strabismus Types: CPO Exam Study Guide

What Is Strabismus?

Strabismus is a condition in which the two eyes are not aligned in the same direction simultaneously. One eye fixates on the target while the other deviates. Strabismus affects approximately 4% of children and can cause amblyopia if not treated early. As a CPO, you need to recognize strabismus, understand the terminology, and assist with documentation.

Key Terminology: Tropias vs. Phorias

Tropia (manifest deviation): a constant or intermittent misalignment visible without any special testing. The deviated eye is actively suppressed by the brain to avoid diplopia.
Phoria (latent deviation): a tendency for the eyes to deviate that is controlled by fusion. The deviation only becomes apparent when fusion is disrupted (e.g., with an occluder during the cover test).

The cover test distinguishes tropias from phorias:

Cover-uncover test: detects manifest strabismus (tropia) — the uncovered eye moves to take up fixation when the other is covered
Alternating cover test: disrupts fusion and reveals latent deviations (phorias)

Types of Strabismus

Type	Direction of Deviation	Common Forms
Esotropia (ET)	Eye turns inward (nasal)	Infantile ET, accommodative ET, sixth nerve palsy
Exotropia (XT)	Eye turns outward (temporal)	Intermittent XT, sensory XT, third nerve palsy
Hypertropia (HT)	Eye turns upward	Superior oblique palsy, skew deviation
Hypotropia	Eye turns downward	Inferior rectus restriction (thyroid), blow-out fracture

Comitant vs. Incomitant Strabismus

Comitant strabismus: the angle of deviation is the same in all directions of gaze. Usually a primary motor defect from childhood.
Incomitant strabismus: the angle of deviation varies with the direction of gaze. Usually indicates a paretic or restrictive cause (cranial nerve palsy, thyroid eye disease, orbital fracture).

Incomitant strabismus in an adult with diplopia is more worrisome than comitant strabismus because it suggests an acquired cause such as a cranial nerve palsy (CN III, IV, or VI) that may have a serious neurological or systemic etiology requiring investigation.

Accommodative Esotropia

Accommodative esotropia is the most common form of childhood esotropia. It results from excessive accommodative convergence driven by uncorrected hyperopia. As the child accommodates to clear their blurred distance vision, the associated convergence overcomes fusional divergence and the eye turns in. Full correction of the hyperopia with glasses often dramatically reduces or eliminates the esotropia.

Always perform a cycloplegic refraction (with cycloplegic drops to relax accommodation) when evaluating a child with esotropia. Without cycloplegia, latent hyperopia may be masked by the child's accommodation, and the full hyperopic correction needed to treat the accommodative component of the esotropia will be missed.

Amblyopia Risk

In infantile and unilateral strabismus, the brain may suppress the deviated eye's image to avoid confusion or diplopia. Prolonged suppression of a young eye can lead to amblyopia (reduced vision that does not correct with glasses). This is why early detection and treatment of strabismus in children is so important.

Assuming strabismus is always obvious. Intermittent exotropia, for example, may only appear when the child is tired or daydreaming, and parents may not notice it during a quick office visit. Careful cover testing in multiple gaze positions is needed to detect subtle or intermittent deviations.

Key Takeaways

Tropia = manifest deviation; phoria = latent deviation revealed only when fusion is disrupted
Esotropia = inward turn; exotropia = outward turn; hyper/hypotropia = vertical deviations
Comitant = same angle in all positions; incomitant = varies with gaze direction
Accommodative esotropia in children often resolves or improves with full hyperopic correction
Early treatment of strabismus prevents amblyopia in children