Suppression, correspondence and microtropia

Sensory Adaptations to Strabismus

When ocular misalignment occurs during the critical period of visual development (before age 7 to 8), the immature visual cortex undergoes neuroplastic adaptations to eliminate two intolerable visual disturbances:

1. Visual Confusion: The perception of two entirely different objects superimposed in the same subjective visual direction, caused by dissimilar images falling on corresponding foveae.
2. Pathological Diplopia: The perception of a single object situated in two different spatial locations, caused by the image falling on the fovea of the fixating eye and an extrafoveal retinal locus in the deviating eye.

1. Suppression Scotomas

Suppression is the active cortical inhibition of visual input from the deviating eye under binocular conditions. In strabismic suppression, the cortex generates two distinct scotomas:

• A foveal suppression scotoma in the deviating eye, which eliminates visual confusion.
• A peripheral fixation point scotoma over the extrafoveal retinal area in the deviating eye that receives the fixation image, which eliminates diplopia.
• Facultative Suppression: Present only under binocular conditions; visual acuity returns when the fellow eye is occluded.
• Obligatory Suppression: Persists monocularly, developing into deep strabismic amblyopia.

2. Abnormal Retinal Correspondence (ARC)

Abnormal retinal correspondence is a state of binocular neuroplastic reorganization in which the fovea of the fixating eye acquires a shared subjective visual direction with an extrafoveal retinal point (a) in the deviating eye (termed the pseudofovea):

$$\text{Objective Angle } (H) = \text{Angle of physical ocular deviation measured by cover testing}$$ $$\text{Subjective Angle } (S) = \text{Angle of separation perceived by the patient during testing}$$ $$\text{Angle of Anomaly } (A) = H - S$$

• Normal Retinal Correspondence (NRC): The foveae share the same visual direction. $A = 0$, meaning $H = S$.
• Harmonious Abnormal Retinal Correspondence (HARC): The angle of anomaly equals the objective deviation ($A = H$). Therefore, the subjective angle is zero ($S = 0$). The pseudofovea has fully adopted the visual direction of the fellow fovea, allowing anomalous binocular cooperation without diplopia under everyday conditions.
• Unharmonious Abnormal Retinal Correspondence (UARC): The adaptation is incomplete ($0 < A < H$, and $0 < S < H$). The subjective angle is less than the objective angle, but greater than zero; diplopia persists during testing.

3. Horror Fusionis

An intractable sensory defect characterized by the complete inability of the visual cortex to achieve binocular sensory fusion, despite precise motor alignment with prisms or surgery. On the synoptophore, as targets are brought toward the objective angle, the images actively jump over or slip past one another, provoking persistent, distressing diplopia.

---

Microtropia (Monofixation Syndrome)

Microtropia (described comprehensively by Parks as monofixation syndrome) is a subtle, small-angle strabismus of $\le 5$ prism dioptres (rarely up to 8 prism dioptres) that frequently escapes routine detection:

Clinical Triad of Microtropia

1. Small-angle deviation ($\le 5$ prism dioptres), most commonly esotropia.
2. Harmonious abnormal retinal correspondence with preserved peripheral fusion and gross stereopsis ($> 60 - 3000$ arcseconds), but complete absence of bifoveal high-grade stereopsis.
3. A small, absolute central foveal suppression scotoma ($1 - 2^\circ$) in the microtropic eye.

The 4-Prism Dioptre Base-Out Test

A small base-out prism displaces the target image. In normal bifoveal viewing, both eyes first make a version movement toward the prism apex, followed by convergence of the eye without the prism to restore fusion. Test each eye and watch the complete sequence.

In monofixation, placing the prism before the non-fixating eye may produce no response because the displacement remains within its suppression scotoma. Placing it before the fixating eye can produce the initial version without the fellow eye's restoring convergence. Interpret this with cover testing and stereoacuity; cooperation and fixation can affect the result.

Case: sensory adaptation versus new diplopia

A patient with longstanding childhood esotropia reports no diplopia, whereas an adult with a recently acquired similar angle does. The motor angle alone does not explain the sensory experience. Childhood suppression can remove conflicting input from awareness, and anomalous retinal correspondence can alter the relationship between a fixating fovea and the fellow eye's percept. An adult with previously normal binocular function may lack those adaptations. Use appropriate sensory tests and compare their degree of dissociation rather than assuming all tests interrogate identical everyday conditions. A result obtained with strongly dissociating filters may differ from a less dissociated test. Persistent absence of diplopia does not prove normal stereopsis. Before changing alignment, assess acuity, fixation, suppression and correspondence, explain potential sensory consequences, and distinguish a chronic adaptation from an acute neurological or mechanical cause.