Binocular sensory adaptation

Changes in strabismic eye to avoid diplopia and confusion

• Binocular sensory adaptation: Harmonious anomalous retinal correspondence, Unharmonious anomalous retinal correspondence, Suppression

• Monocular sensory adaptation: Eccentric fixation, amblyopia

Differentiate diplopia and confusion

• Diplopia: eye sees double images of 1 object, object imaged on non-corresponding retinal points

• Confusion: 2 images superimposed, object seen by strabismic eye’s fovea and fellow eye’s fovea are different, both foveas are corresponding points, causing images of OU to be superimposed

Early onset strabismus adaptations to avoid diplopia and confusion

Binocular vision not fully developed, sensory adaptation to avoid symptoms: strabismus, ARC, amblyopia, EF

Recent onset strabismus in adults

Binocular vision system past its plasticity period, cannot adopt sensory adaptation to avoid diplopia and confusion

they will have normal retinal correspondence: both fovea share the same visual direction

will adopt abnormal head posture and cover one eye

Suppression

active inhibition resulting in loss of awareness of visual impression for 1 eye in binocular vision

young enough patient with sensory plasticity will develop HARC

Physiological suppression

• present in normal BSV to prevent awareness of physiological diplopia and retinal rivalry

Pathological suppression

• developed to overcome binocular diplopia, confusion, incompatible images (retinal images of different size and shapes, preventing central fusion)

Anomalous retinal correspondence purpose

For a point of the retina of a good eye to correspond with a new point of the retina of a strabismic eye

Alternative to suppression, only present under binocular viewing conditions

Key requirements of HARC

• Unilateral strabismus

• Small angle strabismus

• Constant strabismus

• Early onset strabismus (plastic visual system)

Objective angle and subjective angle of ARC

Objective angle: manifest deviation of strabismus, measured objectively (cover test)

Subjective angle: angle of strabismus perceived by patient (Maddox rod, von graefe)

Angle of anomaly

Angle which retinal correspondence has shifted from normal

Objective angle-subjective angle

Angle of anomaly: 0 (NRC)

Not equal to 0 (UARC)

Equals of objective angle with allowance of 4 (HARC)

Unharmonious anomalous retinal correspondence

• rare

• objective angle: angle of new strabismus

• subjective angle: difference between angle of old and new strabismus

Presence of suppression tests

• Bagolini striated lens

• Worth 4-dot test

• Mallett modified OXO test

Worth 4-dot tests

• If px has strabismus but sees 4 dots=ARC

• 4 dots without strabismus=Normal

• 5 dots=crossed/uncrossed diplopia

Interpret this:

1. Bifoveal or anomalous BSV

2. Heterotropia with NRC and diplopia

3. Central or paracentral suppression scotoma in one eye

4. Central and peripheral suppression of one line image

Draw out all possible results of near Mallett modified OXO test

Mallett unit suppression test

• Wear polaroid glasses

• test calibrated at 35cm

• RE see red letters

• LE see yellow letters

• OU see green letters

Mallett unit suppression test procedure

1. Get patient to wear polaroid glasses

2. Read down the chart— if patient does not read all letters at the bottom row

3. Occlude better eye and ask them to read the smallest letter they see

4. Compare the acuity in binocular and monocular conditions

5. Difference in readings= foveal suppression in minutes of arc

Depth of suppression tests

ND filter

Sbisa bar: if suppression still present at number 7-9, deep suppression indicated

Bagolini lens with ND filter

• ND filter placed on unsuppressed eye

• ND filter increased by 0.3 log units until suppressed eye sees an image

• Suppression depth—>log unit before reversal

Step 1: If left eye suppressed and strabismic, image only seen by right eye

Step 2: Increase density of ND filter on right eye, until streak seen by left eye appears

Step 3: Further increase ND filter will cause retinal illuminance of suppressed eye to be better, resulting in suppression of right eye. Left eye now assumes primary position and RE becomes strabismic

Sbisa bar procedure

Place bar on unsuppressed eye, only unsuppressed eye can see a red spot of light

Slowly increase density of the sbisa bar until suppressed eye can see a white spot of light (diplopia occurs)

Continue to increase the density of the filter until image of the unsuppressed eye cannot be seen

Good eye is now suppressed and shows strabismus, Suppressed eye shifts to primary position

Log unit recorded before reversal

Management of suppression

• Aim: encourage patient to become aware of suppressed image and integrate it correctly with image from other eye

• Method of treatment should: Ensure simultaneous stimulation of foveal areas/corresponding points of both eyes, angle of squint must be relieved

• Treatment for anti-suppression: relieve deviation so that eye can fuse both images (red-green TV trainer)