Lecture 4 Flashcards
How can strabismus occur?
Ocular alignment of the eyes depend on normally functioning sensory and motor fusion system.
- Sensory fusion is the ability of the eyes to perceive the information from both eyes as a single image
- Motor fusion is the ability to sustain that image through a range of eye movements
When there is a failure of either the sensory or motor component of the fusion system through interruption or failed development, a strabismus can occur.
What is strabismus?
AND
How is it described by?
Strabismus (heterotropia) is when the ocular alignment of the eyes is not ‘straight’. The two eyes are not working together as a pair to observe the one object.
Strabismus is described by:
- Affected eye
- Direction - exotropia, eso…
- Frequency - constant, intermittent, latent
- Size - tiny, small, moderate, large
How else may strabismus be classified?
Clinicians need to consider the following when classifying strabismus:
- Onset - congenital or acquired
- Secondary factors causing the strabismus
- Residual/consecutive after surgical correction
- Accommodative/convergence relationship
- Decompensating
How do we test for manifest deviation (heterotropia)?
Cover and uncover test
- Corneal reflections are observed for symmetry
- If the corneal reflection of one eye is off centre, it is suspected to be deviated
- An occluder is used to cover the fixing eye to see if the deviated eye moved to take up fixation
- Once the occluder is taken away, we observe to see if the deviated eye maintains fixation or it deviates again
- The patient has small right esotropia
How to record/document the cover/uncover test
Remember to cover the good eye (unturned eye) to test the turned eye
CR: Left reflection displaced temporally on the pupil margin
CTN s gls mod L ET mf briefly s dip c detailed target
CTN - cover test near
s gls - without glasses
mod L ET - moderate left esotropia
mf briefly - maintained fixation briefly
s dip - without diplopia
c detailed target - with detailed target
And detailed target is just pretty much whether the target they used induced convergence/accomodation - e.g. back of the occluda or toy with fine details
Prevalence of strabismus globally
Prevalence of childhood strabismus is - 0.8 - 5.8%
Variations in the prevalence reported occur due to
- clinical populations will overestimate prevalence due to using a sample of patients with concerns, then a study looking at everyone
- who did the strabismus examination?
Prevalence of strabismus in the Sydney Children’s Eye Disease study
n = 6531 children
Types of strabismus by age
Risk factors for strabismus
Although there was higher prevalence of strabismus with age, further analysis of this sample showed the risk factor included:
- Hyperopia for strabismus, esotropia (highest) and exotropia
- Myopiam strabismus and exotropia
- Anisometropia for strabismus, esotropia and exotropia
Analysis of children aged 6 months to 6 years showed that:
- Admission to NICU as an infant put a child at 2,34 x greater risk for strabismus
- Prematurity was a risk factor for esotropia and low birth weight for exotropia
Normal ocular posture
When BSV is present (sensory and motor) the eyes will appear straight
This is because, when there is normally functioning BSV, fusion will ensure that there is bifoveal fixation on the object of interest
However, this does not mean that the eyes are naturally straight
The eyes are straight because the brain is ensuring they are, through controlling the ocular posture with fusion. This is to maintain a single perceived image.
Anatomical or absolute position of rest
Due to the position of our ocular orbits, the anatomical or absolute position of rest for our eyes is deviated outwards.
This position when measured doe not correspond with the alignment of the orbit because of other mechanical factors, but does tend to be outwards and slightly elevated
Physiological position of rest
The anatomical position of rest is never seen in a living person, because the EOM always have some tone to hold the eyes in a straighter position
The position of the eyes when dependent on only the normal tension of the EOMs is called the physiological position of rest or the fusion-free position.
Active position (position of rest)
The physiological position of rest is not the same as the ocular posture in primary position because primary position is an active position where motor fusion is occuring.
Thus, the eyes will assume the physiological position of rest only when fusion is disrupted by occluding an eye.
Perfect ocular alignment and orthophoria
- Ideally, the physiological position of rest would allow the visual axes to be exactly parallel for distance fixation
- AND visual axes to be crossed to acheive the appropriate amount of convergence and accommodation for near fixation
- In this case, the physiological and active position of rest where BSV is achieved are identical
- If fusion was suspended through covering an eye there would be no change in ocular position
- This is called orthophoria, meaning that there is no underlying or latent deviation (eye perfectly straight)
Latent deviation
- For most people, there will be differences in the position of rest for near or distance fixation or both
- When fusion is present the deviation will be controlled by fusion and eyes will remain straight
- The difference between the positions will be come apparent when the eyes are dissociated and fusion is suspended
- When a difference exists it is described as a latent deviation or latent strabismus or heterophoria
How to detect latent deviation?
To detect a latent deviation we perform an alternate cover test
- The occluder is alternated between the eyes
- The eye under the cover moves into the physiological position of rest (deviates) because fusion is suspended
- The refixation movement as the occluder is alternated is observed to determine the direction
- Once the occluder is taken away there is a recovery of BSV
Do latent deviations vary between individuals?
Heterophoria varies between individuals depending on where their physiolgocal position of rest is.
The deviation may vary in size and direction (in the same way as manifest deviation).
- Esophoria
- Exophoria
- Hyperphoria/ hypophoria
Esophoria
Esophoria is rare at distance fixation (1%) and has a low prevalence at near fixation (9-10%) in children.
It is more common in European Caucasian populations
Types include:
- Convergence excess - the size of the esophoria is bigger at near than at distance. N > D
- Divergence weakness - the size of esophoria is bigger for the distance than at near. D > N
- Mixed or non specific - the deviation measures the same for near and distance. N =D
Exophoria
Most commonly seen at near fixation 53-58% of children. More common in East Asian children compared to Caucasian children.
Types include:
- Convergence weakness - the size of the deviation is bigger at near than at distance. N > D
- Divergence excess - the size of the deviation is bigger for distance than at near. D > N
- Mixed or non-specific - the deviation measures the same for near and distance. N = D
Distance heterophoria
- The deviation is also likely to vary between near and distance fixation
- Distance: the fixation is based solely on anatomical factors and is called the basic deviation
- The basic deviation is fixed and determined by your genetics and the anatomical position of your orbits
- A wide interpupillary distance (IPD) is associated with exophoria
- A narrow IPD is associated with esophoria
- The normal IPD is approximately 60 mm
Normal range of heterophoria at distance
Near heterophoria
Near: the deviation depends on the basic deviation + the impact of accommodation and convergence
- The visual system isn’t perfect and the amount of convergence performed to fixate at near for a given amount of accommodation is often is a little less or a little more than needed
- The fusional system makes up the difference to control the deviation and maintain BSV
Motor fusion and maintaining BSV
Motor fusion:
Positive & Negative relative fusion
