3/6/2015
Diplopia in patients with longstanding strabismus
Yvonne Ng MBChB, FRANZCO
Why do we get double vision?
1. Ocular misalignment that develops after visual maturity 2. Surgical overcorrection of patients with long-standing strabismus e.g. Patient with Intermittent exotropia will develop diplopia with surgical overcorrection 3. Adult patients with childhood strabismus may develop diplopia, when they were previously diplopia-free. Why does this happen? What can we do about it?
1. Is the diplopia monocular or binocular?
• The question of whether or not the diplopia could be eliminated by closing one eye may falsely suggest a binocular cause. • Only when asked if the diplopia could be eliminated by closing the other eye that the monocular cause is identified. • Monocular causes include:
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Causes of Monocular Diplopia
Refractive error High errors, irregular astigmatism, edge effect Eyelid Mechanical compression of cornea Tear film Dry eye, fb Cornea Scar, Oedema, Keratoconus Lens Cataract, Lens subluxation Iris and Pupil Iridotomy, Iridodialysis Retina Epiretinal membrane, CSR Cerebral Polyopia Trauma, migraine, MS, encephalitis
2. What changed at the onset of diplopia?
• Change in alignment • Change in refractive management • Change in refractive need
Patients with childhood-onset strabismus • Have sensory adaptations: • Suppression • Monofixation syndrome
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Suppression
= a sensory adaptation in the visually immature that eliminates an unwanted image, caused by e.g. strabismus or anisometropia • occurs in response to differing inputs from each eye to cortex
Suppression
• The whole area of the visual field of the deviating eye that overlaps the fixing eye is suppressed
• When the image of the fixation target crosses the midline of the retina it stimulates a “trigger” mechanism that determines either suppression or diplopia
Suppression – “Hemiretinal trigger” • Patient with childhood ET with no fusion: • Suppresses the area of VF of the deviating eye that overlaps the VF of the fixing eye, so long as the deviation remains in ET. • If deviating eye becomes XT, causing the image to cross the retinal midline by even 1 diopter diplopia is triggered.
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a) Change in Alignment
• Determine the situation that existed before the onset of diplopia • Return the patient to that prior situation • Subjective refraction can be inaccurate in an amblyopic eye – consider cycloplegic refractions in adults with amblyopia and in whom you suspect accommodative convergence may be influencing the angle of the squint
Fixation Switch Diplopia
• Occurs in patients with non-alternating strabismus • The suppression that occurs in the previously deviated eye cannot be transferred to the dominant eye when the patient fixates with the non-dominant eye.
Fixation Switch Diplopia
Can occur if: • The fixating eye become the more myopic • The fixating eye develops media opacity • The patient is optically encouraged to fix with the non- dominant eye when removing their spectacles • Inaccurate refractive correction • Monovision treatment for presbyopia
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Vertically Incomitant Strabismus and bifocal / progressive glasses
• A or V pattern strabismus • Vertical deviation that increased on downgaze May be well-aligned in primary position Develop diplopia when switch to bifocal glasses or from bifocal to progressive glasses As glasses force the patient to look further down beyond the vertical range of BSV
b) Changes in refractive management
• Patients with poor fusion, longstanding strabismus with suppression of deviated eye are not good candidates for monovision • Beware of vertically incomitant strabismus when prescribing progressive glasses
c) Change in refractive need
Presbyopia: • Common cause of decompensation of previously well controlled accommodative ET • Secondary to increased accommodative effort
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Cycloplegic Refraction
• May be necessary in patients with a history of accommodative ET to ensure their glasses remain effective in relaxing their accommodative effort • May be necessary in amblyopic eyes - to obtain an accurate refraction
A systematic approach to diplopia in adults with pre-existing strabismus
1. Is the diplopia monocular or binocular? - if monocular, look for corneal, lenticular or macular problems 2. Did the diplopia begin with a change in the angle of misalignment? - Restore the previous asymptomatic angle (spectacles, prisms, surgery) 3. Has there been a change in the patient’s refractive management? Inaccurate refraction Optical centration Switch to bifocal Induced fixation switch 4. Has there been a change in the patient’s refractive needs? Presbyopia Fixation Switch Undercorrected Hyperopia Overcorrected Myopia Bifocal use in vertical incomitant strabismus
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