L17,18 - Low Vision Flashcards
Registration of visual impairment
Sight impaired (SI) Partially sighted Severely sight impaired (SSI) blind
Sight impaired
VA 3/60 to 6/60, full visual fields
VA up to 6/24, moderate VF constriction, medial opacities or aphakia
VA 6/18 or better with gross field defect
Severely sight impaired
VA less than 3/60 or less with full VF
VA ranging from 3/60 to less than 6/60 with significantly contracted VF
VA of 6/60 or better with gross VF constriction, particularly lower field.
Main causes of visual impairment
Unaddressed refractive error
Near vision impairment caused by unaddressed presbyopia
Cataract
Glaucoma
A person with low vision (UK) …
Has an impairment in visual function which cannot be corrected by spectacles, CLs or medical intervention that causes restriction in that persons everyday life.
Visual assessment for low vision
Visions - LogMAR distance, Baile- lovie N for near Visual fields Optical aids Non optical aids Pupils, look for pathology Advice and management
Distance vision and VA for low vision
Use LogMAR chart at 4m (or 2m/1m/50cm) Binocular and monocular Record preferred eye Give time to respond Allow eccentric viewing Place chart close enough to read one or two lines
Near vision and NVA low vision
Bailey-lovie often used
Record speed
Record near WDs
Record if they use reading specs
Contrast sensitivity and visual fields for low vision
Contrast sensitivity - use Pelli robson
Visual fields - adapt test to suit px (eg amsler chart)
Near/ reading performance reference standard
X1 mag for object at 25cm required +4.00 add (or adequate accommodation)
Magnification equation
M = F/4
F is dioptric power
Divide by 3 rule
Gives an estimate of px near acuity based on snellen acuity
Divide the denominator by 3
(Doesn’t apply to central Scotomas)
Two ways to determine starting magnification
Practical approach - increase mag by using lenses and shortening focal length
Theoretical approach - using formula
What px can see / what px wants to see
4 types of magnification
- Relative size
- Relative distance
- Transverse
- Angular
Relative size
Increasing the actual size of the object being viewed
Relative distance
Reducing the distance between the object and the eye
M = old object distance / new object distance
Transverse / linear magnification
Ratio of image to object length
Angular magnification
Increasing angular subtense of the image being viewed (telescope magnifier)
Hand magnifiers
Range from +4 to +50DS
Biconvex - offers larger lens diameters, more for low powered.
Aspheric - reduces weight and thickness and helps reduce aberrations for high powered
Stand magnifiers
Fixed lens to object working distance
Thick lens formula
Fe = Fm + Fa - (z x FmFa) Fe - equivalent power of the system Fm - equivalent power of the magnifier Fa - equivalent power of reading add Z - distance between eye and hand magnifier distance
Field of view equation
w = y/(FmZ)
y = lens diameter Fm = equivalent power of magnifier z = distance from eye to magnifier
Telescopes - types
Allow image even if far distance to be seen as larger by using Angular Mag.
There’s 2 types:
-Galilean
-Keplarian
Telescopes - the objective Lens, the eye piece
Objective lens - lens furthest from the eye, always a converging lens (positive), must have longer focal length compared to the eye piece.
Eye piece - lens closest to the eye, can be positive (Keplerian) or negative (Galilean), must be positioned so image is formed by objective lens at primary focal point of the eye piece so must have shorter focal length compared to objective lens.
