Functional Visual Loss

A diagnosis of exclusion May coexist with organic pathology Possible indicators of functional visual loss

• Visual functioning does not correlate with history eg. navigating well, signing name, using phone

• Recent or impending stressful events

• Normal examination eg. no RAPD

• No uncorrected refractive error

• Optokinetic nystagmus demonstrable

• Visual field findings

  • ‘Spiralling’ isopters
  • Crossed isopters

• Inconsistent responses to Ishihara plates

Tests of visual function

• Normal stereoacuity suggests normal VA

• ‘Crossed cylinder technique’

  • Fog the ‘normal’ eye with a +6 and place a +0.25 in front of ‘blind’ eye
  • Rotate a crossed +3D cyl over a -3D cyl so they negate each other and present to the blind eye
  • Patient may be encouraged to read through the superimposed cylinders

• Normal reading vision demonstrates normal visual potential

• Normal Ishihara results imply visual acuity of at least 6/24

Aetiology

• Conversion disorder: psycho-social difficulties
• Malingering: feigned

Investigate if any uncertainty

• EDTs
• Neuroimaging
• Consideration of chronic optic neuropathies eg Leber’s