Accommodative esotropia 20 marks
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Accommodative Esotropia
Definition
Accommodative esotropia is a convergent misalignment of the eyes associated with activation of the accommodative reflex. Near vision involves both accommodation (focusing by altering lens curvature) and convergence (bifoveal fixation). The quantitative relationship between these two processes is the AC/A ratio (accommodative convergence/accommodation ratio). Abnormalities of this ratio are central to the pathophysiology.
- Age of onset: 6 months to 7 years (average ~2.5 years)
- Accounts for the majority of acquired childhood esotropias
AC/A Ratio
The AC/A ratio expresses how much convergence (in prism diopters) occurs per dioptre of accommodation. Normal is approximately 3–5 Δ/D.
- Normal AC/A → Refractive accommodative esotropia
- High AC/A → Non-refractive (convergence excess) accommodative esotropia
Classification
1. Refractive Accommodative Esotropia
Mechanism: Normal AC/A ratio. The child has significant hypermetropia; the excessive accommodation required to focus even at distance drives a proportionate but uncontrollable degree of convergence, exceeding the fusional divergence amplitude.
Features:
- Hypermetropia typically +2.00 to +7.00 D (Kanski) / +3.00 to +10.00 D, average +4.75 D (Wills Eye Manual)
- Deviation is equal at distance and near (difference <10 Δ)
- Onset: 18 months to 3 years
- Amblyopia is common at presentation
- AC/A ratio is normal
Subtypes:
- Fully accommodative esotropia — deviation completely eliminated by full optical correction of hypermetropia; binocular single vision (BSV) restored at all distances
- Partially accommodative esotropia — deviation reduced but not eliminated by full hyperopic correction; residual angle is the non-accommodative component. Associated with bilateral congenital superior oblique weakness. Suppression common; ARC may occur (but of lower grade than in microtropia)
2. Non-Refractive Accommodative Esotropia (High AC/A Ratio)
Mechanism: A unit increase in accommodation triggers a disproportionately large increase in convergence. This occurs independently of refractive error, although hypermetropia frequently coexists.
Feature: Deviation greater at near than at distance
a) Convergence Excess
- High AC/A ratio due to increased accommodative convergence (accommodation is normal, convergence is excessive)
- Normal near point of accommodation
- Straight eyes with BSV for distance
- Esotropia for near, usually with suppression
- Straight eyes through bifocals
b) Hypoaccommodative Convergence Excess
- High AC/A ratio due to decreased accommodation (weak accommodation requires increased accommodative effort → over-convergence)
- Remote near point of accommodation
- Straight eyes with BSV for distance
- Esotropia for near, usually with suppression
Clinical Features (Summary)
| Feature | Refractive | Non-Refractive (Convergence Excess) |
|---|---|---|
| AC/A ratio | Normal | High |
| Refractive error | High hypermetropia | Mild–moderate hypermetropia or normal |
| Distance vs. near deviation | Equal | Greater at near |
| Effect of glasses | Fully/partially corrects deviation | Corrects distance, not near |
| Bifocals | Not needed | Required for near control |
Investigations
- Cycloplegic refraction (retinoscopy) — mandatory in all children with esotropia, irrespective of age
- Cover test — assess deviation at distance and near, with and without glasses
- Prism and cover test — quantify angle of deviation
- AC/A ratio measurement — gradient method or heterophoria method
- Sensory tests — assess suppression, ARC, BSV (Bagolini glasses, Worth 4-dot test)
- Assessment for amblyopia — visual acuity in each eye
Treatment
Step 1 — Optical Correction of Refractive Error
The first and most important treatment step:
- Under 6 years: Full cycloplegic hypermetropic refraction is prescribed (deduction only for the working distance of the retinoscope)
- Over 8 years: Refraction without cycloplegia; prescribe the maximum "plus" tolerated (manifest hypermetropia)
In fully accommodative refractive esotropia, full optical correction controls the deviation for both near and distance.
Step 2 — Bifocals
- Prescribed for convergence excess esotropia (high AC/A) to relieve accommodation for near, reducing accommodative convergence
- Executive bifocals are preferred so the child is forced to look through the lower segment at near
- Miotics (e.g., echothiophate iodide) — act as peripheral cholinergic agents, stimulate ciliary muscle directly, reducing the accommodative effort needed → decrease accommodative convergence; used as an adjunct or when glasses are poorly tolerated
Step 3 — Amblyopia Treatment
- Occlusion (patching) of the dominant eye when amblyopia is present
- Must be addressed before surgical correction
Step 4 — Surgery
Indicated for:
- Partially accommodative esotropia — glasses reduce but don't eliminate the deviation; surgery addresses the residual non-accommodative component
- Convergence excess not controlled by bifocals
- Bilateral medial rectus recession is the standard procedure
- Surgery is performed after the angle is stable and optical treatment has been maximised
Step 5 — Ongoing Review
- Regular refraction is essential; hypermetropia may increase or decrease with age
- An accommodative element should be suspected if eyes appear straight post-surgery then become convergent again
Prognosis
- Fully accommodative refractive esotropia: Excellent prognosis for BSV if treated early; hypermetropia often decreases in adolescence, and glasses may eventually be discontinued
- Partially accommodative: Requires surgery; prognosis for BSV is guarded
- Convergence excess: Good if controlled by bifocals or miotics; surgery when needed
Key Clinical Images
Sources: Kanski's Clinical Ophthalmology: A Systematic Approach, 10th ed., pp. 755–756 | The Wills Eye Manual, pp. 499–500
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