MIDTERMS Flashcards
Classification of Functional Vision Deficits
- Cloudy media
- Central Field Deficit
- Peripheral Field Deficit
no field deficit (overall blur)
Cloudy media
Any condition that alters the clarity of several clear structures may result in overall blurred
or hazy vision, impaired detail vision or significant glare without a field deficit
Cloudy Media
Functional Implications of Symptoms:
o Generalized hazy or blurred vision
o Lost of ability to perceive sharp details
o Printed materials appeared faded, as do colors
o Focal opacities sometimes cause monocular diplopia
o Sunlight and direct lighting tend to cause glare
o Contrast is usually reduced causing poor border detection of both small and large
objects
directly related to the extent of disorder, often 20/200 or worse
Visual Acuity
blurred
Amsler Grid
strong positive
Glare Test
loss at all frequencies
Contrast Sensitivity
Functional Tests Results
Visual Acuity –
Amsler Grid
Glare Test –
Contrast Sensitivity
Management of Cloudy Media
Careful refraction which may involve automated refraction and keratometry. Visual
acuity with pinhole and/or refraction over a bandage contact lens can neutralize tear
film abnormalities, focal, opacities or irregular corneal surface.
- Moderate increase light can help patients with reduced contrast to perform better,
especially with proper positioning to avoid glare
use of the following is advisable:
o Sun visor or hat
o Typoscope
o CCTV
o Sunglasses
o UV filters
o Polarizing filters
o Anti-reflective coating
General measures to improve contrast are helpful like:
o Yellow filters or amber glasses
o CCTV with variable contrast
o Large print books
o Felt-tipped markers
Improving contrast almost always helps. Measures include enhancing print contrast
through large print and with dark, felt-tipped markers, along with yellow filters indoors and
plum (light or medium) outdoors. CCTV provides a range of magnification along with
improved contrast
Central Field Deficit
Certain diseases preferentially affect the peripheral retina or nerve pathways that carry
information from the peripheral visual field. The functional implications indicate mobility
and night vision affections.
- Peripheral field defects causing functional deficits generally fall into two categories: the
general constriction and the sector or hemianopic field defect.
Peripheral Field Deficit
Functional Implication of Symptoms Peripheral Field Deficit
o Performance of the peripheral retina worsens at night or under poor illumination
o Patients may be unable to perceive people or objects in the surroundings unless the
object appears in their remaining visual field
o Difficulty ambulating in unfamiliar environment and anxiety about bumping into
peripheral objects.
o Difficulty in locating objects
– usually good if there is no macular disease or cataract, may be 20/20
Visual Acuity
normal if there is no macular disease, periphery is missing if field
constriction is advanced
Amsler Grid
negative unless media pathology co-exists
Glare Test
usually normal (except glaucoma) but may be difficult to test if
the central field is less than 5 degrees.
Contrast Sensitivity
reduced
Visual Field
allows you to predict the add from the best corrected distance acuity
Kestenbaum’s Rule
allows you to read the predicted add directly from the near visual acuity chart.
Lighthouse Method
predicts an add based on both the patient’s best-corrected distance
acuity level and the actual desired near acuity level.
Reciprocal Vision
Consider the primary goal of the patient particularly the tasks and history
Deciding on Style
adapt by looking slightly off center so the healthy part of the
retina is pointing straight ahead to receive the image.
Eccentric viewing
defined as the optical power needed to focus parallel rays of light at 1 meter. As the
distance is shortened, the power needed to focus an image increase on inverse
proportion.
Diopters
the distance at which a lens focuses parallel rays of light. Focal length is determined by
dividing 1m by the dioptric power of the lens.
Focal Length
An object of a given size “projects” an image on the retina of the viewing eye. This retinal
image is also of a given size, usually measure in seconds, minutes, or degrees of an arc
X notation
The purpose of low vision aids is to magnify the retinal image of an object. By enlarging the size
of the image that is projected onto the retinal surface, it is more likely that the image will be seen
by the remaining healthy tissue surrounding the compromised areas
Magnification
- Also known as Linear Magnification
- The object is actually made larger while the working distance remains the same. There is
a direct relationship between increased object in size and increased retinal image size. - By just doubling the size of the actual object, the retinal image size is also double and
easier to see.
Relative Size Magnification
Also known as Approach Magnification
- As objects are brought closer to the eye, the retinal image size is again enlarged
proportionally. A closer object takes up a larger portion of the visual field and the image
will be larger on the retina. It is still necessary to focus image by means of accommodation
or lenses.
Relative Distance Magnification
Optical Magnification
- It is a complicated type of magnification that occurs from a system of lenses, such as
telescopes and binoculars, wherein it increases the angular subtense of the object.
- Divergent light rays leaving the system cause images to appear to be coming from a closer
distance than the actual location of the object. The image seen is virtual. Our brain is
aware that objects appear smaller as they get further away, so when distant object appear
larger, our brain interprets them as being closer.
- It uses 2 or more lenses to create magnification.
Angular Magnification
- An optical instrument designed to make objects appear nearer, containing an arrangement
of lenses by which rays of light are collected and focused and the resulting image
magnified. - It can help people with low vision to improve distance vision
Telescope
The two specifications that differentiate telescopes from one another are the:
o Magnification power
o Field of view
- A Galilean telescope consists of a convex objective lens separated from a concave
ocular lens by the sum of their focal lengths. - Magnification (M) is derived by dividing the dioptric value of the ocular lens (Fe) by
the dioptric value of objective lens (Fo).
Galilean Telescope (Terrestrial)
consists of convex objective lens separated from a convex
ocular lens by the sum of their focal lengths.
- Magnification (M) is derived by dividing the dioptric value of the ocular lens (Fe) by
the dioptric value of objective lens (Fo)
Keplerian Telescope System (Astronomical)
Print or images may be enlarged by projection.
- In Low Vision, closed circuit televisions (CCTVs) and other electronic magnifying
devices use this type of magnification very successfully.
Projection Magnification
M = F/4
For the purposes of classification and ordering, magnifiers tend to be calibrated in terms
of magnification. This relies on some formulation allowing a magnification value to be derived
from knowing the dioptric value of the lens in the aid. However, this does not really represent how
an aid might be used.
Nominal Magnification Formula
