Lecture 14 a- Visual field Loss Flashcards
Visual field loss is generally divided into:
- peripheral loss, where the patient may have good central visual acuity
- central loss, which will always result in poor visual acuity and is very disabling for the patient.
Common conditions causing peripheral field loss:
• Glaucoma (very gradual onset)
• Retinitis pigmentosa (very gradual over many years, overall constriction (tunnel vision)
• Cerebrovascular accident (hemianopia)
- homonymous hemianopia or quadrant defects
- usually sudden onset due to stroke, seldom gradual onset due to tumour
- other associated problems hemiparesis, dysarthria (impaired speech)
•Nyctalopia
Brain lesion caused by Hemaninopia can also have which defects:
• Hemiplegia
- ‘Handling’
• Loss of “higher visual functions”
- Agnosia: Visual (object) or Prosopagnosia (face)
- Alexia: reading
• Oculomotor problems
• Personality/behaviour/attention change
- Anosoagnosia - “denial of illness”
- Extinction
- Neglect
What are the functional affects of limited field?
• In early stages of VF loss, px may compensate by scanning, but when VF reduced to 10 deg or less, their mobility will be significantly impaired
• This can vary depending on if distance, intermediate or near VA is affected
Describe symptoms of distance, intermediate and near visual field defects
• Distance
- bumping into obstacles - poor mobility
- not being aware of layout of environment
• Intermediate
- difficult to get whole task into view simultaneously (TV, computer)
- can’t find objects put down on cluttered table/desk
• Near
- Page navigation - doesn’t realise reached end of line; difficulty in finding start of line without missing one
Management strategies for VF loss:
Depending on the cause of the VF loss, management strategies include:
1. Illumination & glare control
2. Optical methods:
- Visual field (VF) enhancement for sighting where a minified image is produced, so that more information can be squeezed into the patient’s remaining field of vision
- VF enhancement for mobility where objects from the missing part of the field are projected into unaffected parts of the field.
(Unfortunately, optical VF enhancement can be quite confusing for the patient, which is why its success rate in practice is relatively low.)
Field enhancement: For sighting
• For Sighting:
- Reverse telescope -‘field expanders’
- Amorphic lens
- Concave lens
- Convex mirror
Field enhancement: For mobility
• For Mobility:
- Mirror system (hemianopia mirrors)
- Prism system (Fresnel, Inwave lens)
- Scanning
- Electronic systems
Reverse telescopes:
• For tunnel vision
- Not suitable for hemanopia
• Magnification <1.0 (minification)
- Increased FoV equal to power
- VA is reduced in proportion
• Therefore, unlikely to be able to wear permanently
- Hand held for intermittent spotting tasks
- Bioptic mounting, clip on
• Door peep-hole viewer (substantial VF expansion > 100Deg)
Amorphic lens:
• Amorphic lens:
- minifies only in the horizontal plane-expands the field horizontally (where it will be most useful),
- preserves VA - no size change vertically
- image distortion
• Low success rate (due to distortion, cosmetic factors)
• Very rare in UK, as illegal to drive in UK with VF defects
Hand held minus lens:
• Holding negative lens 30-50cm from eye produces minified image + expanded VF
• Can be thought of as “ reverse Galilean” telescope, with Px accom or add as positive eyepiece
• Increasing lens power increases VF, but reduces acuity
• Calculation for optimal size : d = D/Tan (a) or D = d tan(a)
Convex mirrors:
Convex mirrors give a wider field of view than plane mirrors and can be used for field enhancement. Placed in kitchens, hallways, public places etc. they can help detect people, open doors and other obstacles. Note the minification effect.
Mirror system : (Hemianopia mirror)
- mirror could be permanently fixed, or (preferably) clip-on with variable angle
- convex mirror to expand field
- tinted mirror to help distinguish direct and reflected images
- semi-reflecting mirror so doesn’t obscure view completely
- rarely used
Prism systems:
• Px with Bitempotal hemianopia and px with < 20 deg field, acuity > 6/30.
• As much prism as possible (15-30^) with base towards field defect
Disadvantages:
- Causes jump at apex, (1° for each 1.75^)
- reduction if VA (Fresnel)
- Prism scotoma
- Needs to train eye movements
- Min 6/30 VA required
Prism systems:
• Where should edge of prism be placed?
• Monocular or binocular?
• Can align with anatomical features (pupil/limbus) or edge of measured field
• Conventional is binocular; disadvantages is the jump, scotoma. But also one viewing distance only - not diverging or converging
- To avoid monocular prism placement suggested
