Lecture 5 Flashcards
What’s the significance of normal sensory fusion system and ocular alignment?
Ocular alignment of the eyes depends on a normally functioning sesnory fusion system.
Sensory fusion is the ability of the eyes to perceive the information from both eyes as a single image.
What significance does motor fusion play in ocular alignment?
Ocular alignment of the eyes depends on a normally functioning motor fusion system.
Motor fusion is the ability to sustain that single image through a range of eye movements.
How does pathological diplopia occur?
When the eyes are not aligned (a manifest strabismus is present) pathological diplopia will occur. This is because non-corresponding retinal points are being stimulated.
In the case of a constant or intermittent strabismus, diplopia will occur and will either be:
- consciously appreciated by the patient (when there is normal BV) or,
- not appreciated
Can pathological diplopia occur in people with normal BV?
In those with normal BV and normally straight eyes:
- Pathological diplopia may occur if the eyes become misaligned (e.g. when fusion is not possible) - vergence failure
What is contralateral image point (CIP)?
The location of the diplopic image will depend on the area of the retina stimulated.
The non-corresponding retinal points stimulated in the deviating eye is called the contralateral image point (CIP).
Pathological diplopia is esotropia and exotropia?
Esotropia would result in uncrossed diplopia as the image falling on the temporal retina gets projected nasally.
Exotropia would result in crossed diplopia as the image falling on the nasal retina gets projected temporally.
The star would fall on top of the heart
What happens when different images are presented to each fovea?
When different images are presented to each fovea, they are superimposed. This is because both fovea have principle visual direction and are projected straight forward ahead (cylopean eye).
When this occurs it is called confusion.
This only occurs when there is NRc e.g. both foveas are cortically linked as corresponding retinal points.
Why would you not have diplopia?
Having diplopia and or confusion can be very uncomfortable. The cortical response to diplopia that cannot be fused is to inhibit or suppress the diplopic image.
In the circumstance where there is a misalignment of the eyes present and the patient does not appreciate diplopia, suppression is occurring.
Define suppression
Suppression is defined as:
The subconscious cognitive inhibition of visual stimuli, either partially or totally, in a binocular situation (e.g. both eyes must be open and seeing).
Suppression can be:
Physiological, pathological
Constant or intermittent
Unilateral or Bilateral or alternating
Physiological suppression occurs in response to?
Physiological suppression occurs in response to:
Physiological diplopia:
You aren’t aware of the diplopia images in front of and behind fixation.
Retinal rivalry:
When dissimilar images are presented to both fovea (corresponding retinal points) images are superimposed but not fused. The individual will see both images, but will alternate between them (alternating the other image)
Pathological suppression occurs in response to?
Pathological suppression occurs in response to:
Diplopia: images stimulating non-corresponding retinal points
Confusion: different images falling on each fovea
Incompatible images: as in anisometropia where the images may be different in size, shape and clarity preventing central fusion (peripheral fusion still occurs)
Pathological suppression in manifest strabismus
In the case of a manifest strabismus that is present from childhood, the brain chooses to ignore one image (suppress) when the patient is viewing with both eyes open.
Suppresion is a BINOCULAR ANOMALY, i.e. you need to have both eyes open for it to occur. It is not a blind eye.
When you cover the non-suppressing eye (the fixing eye) the suppressing eye will see the fixation target.
Pathological suppression in adults and children
Pathological suppression develops very rapidly in response to diplopia in young children. This corresponds to the period of neural plasticity. Slower suppression development occurs in older children.
Adults may however learn to ignore one image over time. Once suppression develops in childhood it will remain throughout life (unless treated)
What is suppression scotoma?
The area and density of the suppression varies depending on the binocular condition. Generally, only part of the retina in the deviating eye is suppressed.
The retinal area which is suppressed is called a ‘suppression scotoma’.
In manifest strabismus, suppression generally includes the fovea and CIP.
Pathological suppression in Esotropia
In ET the suppression tends to be elliptical in shape incorporating the fovea and CIP with the size depending on the size of the deviation.
Suppression at the fovea is very dense, with the density decreasing into the peiphery.
Pathological suppression in exotropia
In XT, the suppression scotoma may be elliptical or incorporate the whole temporal retina.
Intermittent, alternating pathological suppression
The suppression scotoma may be present intermittently, only when required, e.g. intermittent strabismus.
It can also alternate between eyes, depedning on which eye is deviating and which is th efixing eye.
Alternating suppression is especially common in children with large infantile strabismus and cross-fixation behaviour.
Investigation of suppression
Cover test:
Manifest deviations - appreciation of diplopia
Intermittent and decompensating latent deviations: recovery of BSV on CT-slow or sluggish recovery can indicate suppression is impacting the strength of BSV
Worths lights: absence of one set of lights
Bagoline glasses: part or all of the image to one eye is missing
Synoptophore: missing images, parts of images or controls
Appreciation of diplopia when beyond the range of motor fusion
Sbisa Bar
To measure both the density of the suppression and the size of the suppression scotoma, a sbisa bar can be used.
The patient fixates on a torch and the intensity of filter over the fixating eye is gradually increased until the deviating eye moves to take up fixation.
Use of synoptophore in suppression
Synoptophore: Adjusting the rheostat in front of the suppressing eye to increase the brightness of the image and decreasing the brightness to the contralateral eye.
Continue until both images are seen.
Measuring the depth of suppression is only relevant when assessing the progress of treatment
Qualitative and quantitative measurement of suppression
Measurement of the area of suppression is performed either qualitatively or quantitatively
Qualitatively: on the synoptophore, the slide size (using fusion slides) is increased until both of the image controls are visible outside the scotoma.
Quantitatively: by mapping the boundaries of the scotoma by moving the image until it is seen (with prisms or the synoptophore)
Treatment of suppression
Remember that the presence of suppression is usually a good thing. There are treatments that can be conducted at home and in clinic to eliminate supression to improve binocular functionality and control of a deviation. BUT, these should be performed at patients with:
- Latent or intermittent deviations or
- Before surgery for those with the potential for BSV, e.g. late onset constant strabismus
The aim of treatment is to allow for appreciation of diplopia/ teaching the patient to recognise the second image. This is often used as a starting point for treatment. The elimination of suppressio itself can help improve BSV (the presence of suppression can be a barrier to BSV)
