6.6 Intermittent Exotropia

Definition

  • Exophoria (XP) that breaks down to exotropia (XT) when it’s no longer controlled by convergence

History

  1. Onset < 5 years (usually ~ 2yrs)
  2. Parents notice eye is “turned out” under certain conditions (when the child is daydreaming, tired- end of the day, sick)
  3. Patient closes the affected eye in bright light
  • The natural history is debatable. The traditional teaching that intermittent XT progresses towards a constant XT has not been replicated in a retrospective review.[iii]

Romanchuk et. al. Natural History of surgically untreated intermittent exotropia. JAAPOS 2006; 10:225-31.

Classification

1. Divergence Excess (XT Distance (D) > Near (N) by >10-15Δ, High AC / A)

True

XT still D>N after 1-hour monocular patch or with +3D lens

Pseudo / Simulated

XT D=N after:
1 hour monocular patch (“fusional”)
+ 3D lens (“accommodative”)

Exo deviations for near are controlled by fusional convergence. This control can be abolished by dissociating the eyes with monocular occlusion or removing accommodative drive with a +3D lens. In pseudo-divergence excess, this causes the XT for near to increase to the same degree as that for distance (i.e. it breaks down to a basic XT). The significance of this is debated.

2. Basic (D=N)
3. Convergence Insufficiency (D<N, low AC / A)
  • Very rare in young children
  • Worse outcomes
Figure 6.6.1 Classification of Intermittent Exotropia

Figure 6.6.1
Classification of Intermittent Exotropia

Examination Structure

1. Visual Acuity (Best Corrected)

  • Amblyopia is uncommon

2. Spectacles

3. Inspection

  1. Abnormal head position?
  2. Hirschberg (pupil margin=30Δ, limbus=90Δ) - ± Exotropia (at time of examination)
  3. (Lids normal)
  4. (Pupils normal)
Figure 6.6.2 Intermittent Exotropia

Figure 6.6.2
Intermittent Exotropia

Left exotropia when looking in the far distance

4. Cover-Testing

± Comitant XT

  1. Without glasses: CTD, CTN
  2. ± With glasses: CTD, CTN
A. Assess Type

Divergence Excess

XT Distance (D) > Near (N)

Basic

D = N

Convergence Insufficiency

D < N

If divergence excess, test for divergence excess or pseudo-divergence excess by remeasuring after 1 hour of monocular occlusion. If divergence excess is confirmed, test with +3D lenses to differentiate between true and simulated divergence excess, see above)

B. Assess Control

Excellent

Only evident with cover test and controls rapidly

Good

Controls after blink

Fair

Controls after 5 - 10 seconds

Poor

Does not regain control / breaks spontaneously

C. Far Distance

The XT is often worst when asking the patient to look in the far distance (e.g. out a window). However, this may be difficult to examine in young children

5. Ocular Rotations

  • Full ductions

6. Stereopsis

  • Usually excellent because straight most of the time

7. Dilated Fundoscopy

  • Exclude sensory XT

8. Cycloplegic Refraction

Summary

  • Intermittent exotropia… divergence excess / basic / convergence insufficiency…
  • Excellent / good / fair / poor control

Management

Consider

1. Age
  • Risk of amblyopia with early surgery
2. Type / Size
  • Cosmesis
3. Control (Estimates Binocularity)
  • There are scoring systems for this (e.g. Newcastle, Mayo Clinic)
  • The size of the tropia does not predict control
  1. Percentage of day in XT, progressing?
  2. Vision (amblyopia rare): Binocular VA chart, Worth 4 dot, Synoptophore
  3. Speed of recovery of tropia (see above)
  4. Stereopsis for (distance) and near (eyes need to be straight for this)

A. Treat Amblyopia / Refractive error

B. Non-surgical

These methods are rarely definitive, but can be used to delay surgery until the child is older. There is little consensus between clinicians / orthoptists as to best practice

1. Spectacles to Correct Refractive Error
  • Myopia, astigmatism, high hypermetropia (consider if >+4D)
2. Consider Additional Minus power -2D to Stimulate Accommodative Convergence
  • Best if high AC / A (Pseudo- divergence excess with +3D lens)
  • Ensure patient still have VA = 6 / 6 for distance with spectacles after giving the additional minus
3. Part-time Occlusion (2-6hrs / day)
  • Alternate patching or dominant (non-deviating) eye
4. Active Orthoptics

Requirements:

  • Motivated co-operative child >5yrs,
  • Constant XT <20Δ for distance
  • No vertical component

1. Fusional Convergence Training

  • e.g. “Pencil push-ups”
  • Especially for convergence insufficiency (must be straight at some distance)

2. Diplopia Awareness

  • Red filter over the fixing eye (make child aware of diplopia / when eye is out, then teach them to fuse the images)

3. (Base in Prisms Maximum 8Δ / Eye)

  • This may reduce the fusional vergence amplitude and is rarely used

C. Surgical

1. Indications
  1. There is potential to preserve motor fusion
  2. There is a risk of losing near stereopsis
  3. The patient or their family are concerned by cosmesis
2. Timing

The child must be old enough to check vision for amblyopia

  • “Early” (< 4 years)
    • Advantage: Greater success of “cure” (no tropia), preserve stereopsis (although this is usually good anyway!)
    • Disadvantage: Higher rate of amblyopia / microtropia ( 10 - 15%).
  • Traditional (> 4 years, before school)
    • Advantage: cosmetically reasonable time before school, more reliable measurements
3. Amount
  • Operate for the largest angle found (far distance after monocular occlusion).
  • In the case of true divergence excess, operating on the (larger) distance angle has a risk of post-operative esotropia (with diplopia) for near. This may be avoided by operating on slightly less than the distance angle and is often manageable by prescribing post-operative bifocals for near.
4. Operation

  1. Bilateral LR recessions for divergence excess (or basic) e.g. 6mm and 6mm if 30Δ
  2. Ipsilateral LR recession + MR resection (for basic)
    • Kushner [iv] demonstrated improved outcomes with unilateral surgery for
    • intermittent XT of basic type

5. Target
  • Initially aim for ~10Δ ET- less risk of recurrence but greater risk of amblyopia (monitor carefully). Others prefer small under correction to avoid amblyopia that can occur with esotropia.

Managing Over-correction (persistent overcorrection in 10%)

  • Check ductions to exclude a slipped muscle
  • Wait at least 4 weeks post-operation:
    • Spectacles
      • Correct hyperopia. Bifocals for high AC / A ratio
    • BO prism (Fresnel)
    • Re-operate if persists after 6 months
      • Bilateral MR recession or
      • Contralateral MR recession + LR resection

Managing Under-correction (40%)

  • Similar indications to original management
  • Consider bilateral MR resections if LR surgery has already been performed

          

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