Intermittent XT Flashcards
What are the aetiologies of intermittent XT?
Innervation Factors -
Imbalance of convergence & divergence for classification
Hereditary factors are also shown to influence the prevalence. However, in the majority of these genetic studies exotropia and esotropia have been included so it is unclear whether different subtypes have different inheritance models. Sub-analysis of this group would appear that there are possible differences.
In the presence of neurological abnormalities and the deviation of an IXT at near exceeds distance by 10PD 66% have associated neurological abnormalities.
What did Von Noorden find out about the natural progression of Intermittent XT? & how did Burian define this?
Von Noorden undertook a study to observe the natural progression of IXT in 51 subjects. He noted that in the majority (75%) progression of the deviation occur, however, there were a minority who didn’t change (9%) and in 16% there was actually improvement.
Burian explained this difference by segregating groups into the type. By doing so believed that those with distance exotropia who were true type – remained stable, whereas those with simulated exotropia, near exotropia or non-specific/basic types progressed.
What did Burian find out about the stability for Distance XT’s?
- Distance exotropia of true type remain stable,
- Distance exotropia of simulated type progress to decompensate for near
- Near exotropia rapid progression to symptomatic and constant deviation
- Non-specific/ basic type progress to larger size
What is distance XT also known as?
Divergence Excess XT
How is a Distance XT defined?
Fusion at Nr (often latent and can be large depending on ability to control) and distance XT (>10PD difference between Nr & Dist)
When do Distance XT’s occur?
Distance exotropia has onset by approximately 12 month/1 year of age although has been reported to occur later.
What are symptoms of a Distance XT?
Symptoms:
- Typically none
- Diplopia – rare
- Panoramic vision
- Photophobia
- Asthenopia (eye strain/fatigue)
- Diplophotophobia: May close one eye in bright sunlight
How do we investigate a Distance XT?
Fundus and media
Refraction
History
Visual acuity
Cover test
Ocular movements
Convergence
Assess control of deviation
Measurement of deviation
Diagnose true / simulated
What ocular movements do we need to be aware of in a distance XT?
- Need to carefully check adduction
- A or V patterns more likely (V is often an over-action of the IO)
- Lateral incomitance
How do we test for control in an intermittent Nr XT deviation?
- Cover test
- Near point of convergence (good for Nr deviations)
- Motor fusion
- CBA (Read down chart to see when they become manifest; doesn’t necessarily have to be at the point their VA is)
How do we assess control using CBA?
1) Accommodation
Accommodative Convergence (AC/A)
2) Fusional Convergence
Convergence Accommodation
The consequences of this are blur, asthenopic symptoms, problems with school work/avoiding detailed work
Need to improve positive relative vergence
What is control like in a distance XT?
Exo deviation control varies over a day, and over an examination in most children with intermittent XT.
How do we test stereoacuity in an intermittent distance XT?
1) FD2
2) Distance Randot Stereotest
Holmes et al 2007 - FD2 stimulates fusion in intermittent XT even when control poor. New distance Randot test very sensitive to disturbances of binocularity
If someone has smaller XT on the sides (laterally) during PODT then what is this called?
Lateral incomitance
What in a distance XT causes a risk of over-correction?
Lateral incomitance have a higher risk of overcorrection (to a consecutive ET) and homonymous diplopia on lateroversions
BUT
Variability as to whether this truly exists. Induce some artifacts of the prisms if not held in the correct position. If higher on side-gaze = higher risk of over-correction during surgery
What are the types of Intermittent XT?
Distance XT (True or Simulated)
Near XT
Non-Specific XT (Basic XT)
What will a True Distance XT look like?
- XT on distance
- Fixation with BSV on Nr
- Fixation under all testing conditions
What does Simulated Distance XT look like?
The Nr angle of deviation INCREASES with either:
1) Prolonged disruption of fusion (i.e. patching) - 30-60mins of disruption without letting them fuse again
2) Elimination of accommodation (high AC/A ratio = simulated due to accom; assess near angle with +3.00DS)
What did Kushner find out about simulated XT?
340 patients with exotropia. 154 were found to have a high AC:A ratio prior to occlusion. After occlusion only 22 patients had a high AC:A ratio and the remainder had pseudo high AC:A. The presence of high AC:A is infrequent in patient with XT but is highly predictive of a post op ET at near.
BUT some think this isn’t real as other studies have shown no difference
What does True Distance XT look like in terms of AC/A and Occlusion?
AC/A - Normal
Occlusion - No Change
What does Simulated Fusion Distance XT look like in terms of AC/A and Occlusion?
AC/A - Normal
Occlusion - Increases Nr Angle
What does Simulated Accommodative Distance XT look like in terms of AC/A and Occlusion?
AC/A - High
Simulated Accommodation - No Change
What did Buck et al (2012) find out about XT management?
The findings of the present study highlight that:
a) Many children with X(T) do not experience adverse outcomes from observation or non-surgical treatment;
b) The risk of conversion from intermittent to constant exotropia is minimal;
c) A significant proportion of children who undergo surgery for X(T) experience an overcorrection, occasionally with loss of near stereoacuity.
What’s the Newcastle control score?
In 2004, Haggerty et al devised a scoring chart that incorporated control observed at home and control found in clinic to determine whether further intervention would be required. In 2007, Buck et al, obtained data on 272 children with IXT and followed up 157 of them. They noted that a high (poor) NCS (≥4) predicted surgery would be required.