Refractive management Flashcards
year 3 exam
schematic eye
a basic or diagrammatic reprenstation of a real eye
Reduced eye
simplified versions of real eyes or previous models that will help us with calculation
optical axis
a line through the Centres at all refractive Surfaces - best Centration is given by this
visual Axis
a line from the Centre of the Macula through pupll Centre
the angle between the Visual + optical axes approx 5 Degrees horizontally
( Positive Vergence if the Visual axis in Space lies nasal to opic disc)
Axial length
the distance between the anterior corneal surface + the foveola - mean value 24mm ( adult)
Principal point
Position of a single equivalent lens which can replace a lens System
Nodal Point
point on a lens system to enable graphical construction light entering a System heading towards the first nodal Point exists Via the Second nodal Point
standard reduced emmetropic eye
visual + optical axis coincide pupil is at reduced Surface The Macula (M' ) is at the second focal point ( fe') of the eye the far point ( MR) is at Infinity the far point and macula are conjugate P= Principal point C= Centre of curvature N = Nodel point
light travels from left to right
Snells law applies n Sini = n’ Sin i ( light into denser medium is refracted towards the normal so light passing from air into the eye will be refracted towards the normal
Parallel light will Converge towards or appear to diverge from the Second Principle focus of a Surface
F= n / - f f = n’- n / r. L ‘ = L + F
in reduced eye. k’ = K + Fe
k = Vergence required at the eye to focus at M ‘
k’ = Vergence required leaving reduced Surface to focus on M ‘
k = for Point distance at ♾
k = Ocular refraction
k = n/ k = 1 / ♾ = + 0.00D
k’ = Axial length of eye
k’ = n’ / K’ = 4 / 3 / + 60 .OODx 1000 = + 22.22 mm
k’ = Dioptric length of eye
k’ = n’ / k ‘ = 4/3 / + 60.00 X 1000 =+ 22.22 mm
the Cornea
Transparent Medium - most of refraction Total Power = + 42.00D
the Aqueous
fills anterior and Posterior chamber n= 1. 336
The Iris
Contains pupil variable aperture 2-8mm
crystalline lens
Biconvex changes shape to become more Convex = Positive Power Increase = accomodation supplies the remaining balance of eyes refractive Power
Vitreous
largely homogenous gel like fluid - fills Space between lens + retina
Schematic and reduced eyes are used in optics to allow us to calculate
retinal image size
axial length
refractive Power
magnification
the Retina
.light sensitive neural layer containing rods and cones
Lies in contact with the vitreous and Vascular choroid
Rods -130 million Scotopic Achromatic ill defined
cones - 7 million Photopic full detailed Colour Vision
contains Macula and foveola - concentration of cones - highest Visual Acuity
Ametropia
An uncorrected ,unaccomodated ametropic eye will not focus distant light on the macula.
the eyes far point is not at infinity and there is Some degree of refractive error
The eyes Second Principal focus does not coincide with the macula
we can describe ametropia as either Myopia or hyperopia or value of refractive error
in the reduced eye
k’ = k + Fe
k = Vergence required at the eye to focus at M’
k ‘ = Vergence required leaving reduced surface to focus on M’
what is meant by the term emmetropia
when light from a distant object is focused on the macula of an uncorrected unaccommodated eye k = ♾ K = 0. K'= Fe k' = fe'
vertex distance
the distance from the back Plane of the Spectacle lens to the Corneal apex
BS 2738 Any Rx over + / - 5.OOD must include a tested vertex distance
True far point
MR = a point conjugate with the macula in the uncorrected unaccommodated eye
Ocular refraction
K = Vergence required at the eye to focus rays of light from a distant object on the macula
Spectacle Correction fsp
Fsp = the placement of a thin Spectacle lens in front of the eye So that the lens’ Second Principle focus coincides with the eyes far point
( for thick lens the back Vertex focal length must coincide with the eyes far point
FSp=
fsp’ =
d=
k=
FSP = Power of spectacle lens
fsp ‘ = focal length of Spectacle lens
d= vertex distance
k = far point distance
fsp’ = k + d
FSP =K / 1 + dk
k = fsp / 1- d Fsp
explain the difference between subjective refraction and objective refraction
Subjective = requires a response or input from patient objective = does not require a response or Input from Patient
retinoscopy a subjective or objective method
objective
In subject refraction what is meant by the term best vision Sphere
the Sphere Power that achieves the best line on the snellan chart
BVS = Sph + 1 / 2 Cyl (Push plus onto Patient
if a cx has Rx -4.00 / -3.00 x45 What is best Vision Sphere would you expect for the subject
-5.50DS
Define spectacle magnification and state a formula which may be used to calculate it
SM =
Size of corrected retinal Image.
- - - - - - - - - - - - - - - - - - - - - - - - - -
Size of uncorrected retinal image.
h’ k
- - - - - -
hu’ fsp
Define relative Spectacle magnification and state a formula which may be used to calculate it
RSM =
Size of corrected retinal image.
- - - - - - - - - - - - - - - - - - - - - - - - -
Size of retinal Image in Standard.
reduced eye
k. x ke’
fSp k ‘
Heterophoria and Heterotropia
eyes associated
the eyes are both Viewing in the primary Position and appear to have taken up the primary Position of gaze
Heterophoria and Heterotropia
eyes disassociated
one eye occluded therefore the fusion reflex is prevented
Heterophoria and Heterotropia
Active and passive Positions
when both eyes are uncovered they are associated and are in the Active Position
If only one eye fixates and the other is occluded then the occluded eye is in the passive Position - No fusion reflex for binocular Vision
Orthophoria
Ortho = straight phoria = state
Ideal state for distance and near objects
the visual axes always remain directed towards the fixation object- even when the eyes are dissociated
typical values are less than 1Prism horizontally and less than 0.25
Prism vertically
Some patients can be orthophoric for distance but not for near vision
Heterophoria
Heter - other
phoria - State
or latent strabismus (dormant Squint)
the visual axis are directed towards the fixation object when the eyes are associated but deviate when dissociated
Heterophoria classification
classified according to the movement of the eye under Cover from it’s active to its Passive position by observing the recovery Movement
classification Orthophoria Exophoria Esophoria Hyperphoria Hypophoria in- cyclophoria EX-cyclophoria
NO Movement Temporalwards abduction Nasalwards - adduction Turns up Turns Down upper pole - nasal upper Pole temporal
Direction of prism in Heterophoria
exo - Base in
eso - Base out
R. Hyper- base down
L. Hyper. Base down
Maddox Rod
a Series of Parallel glass rods/ cyls that creates an image of light Source into a streak of light Perpendicular to its axis
Compensated Heterophoria
Dissociated phoria which usually not corrected no Symptoms or binocular discomfort
Subjective amount depending on the patients fusional reserves
not usually Prescribed
uncompensated Heterophoria
De compensated
A phoria becomes uncompensated when BV is under Stress
Cx who attempt to maintain binocular vision by straining will have symptoms which include headaches,blurred vision diplopia and vertigo
caused by eye fatigue age,bad light
Prisms prescribed to decompensated
uncompensated phoria checked using a fixation technique : Mallet unit
Mallet unit
O X O target viewed through polarising filter
2 bars are plane polarised So each eye See’s one of the lightbars
light increased
Any Misalignment of nonios lines is Compared to the ‘ X ‘ fixation letter
Prism added to realign lines
Detects weakest prism needed
causes of heterophoria
refractive under corrected Hypermetropia for distance- need to accommodate under corrected myope- need to Converge Dissociation with eyes - Pro longed use abnormality of the orbits exophthalmos Iarge PD- prone exo Endothalmos Narrow PD - prone eso
Concomitant Squint -
The angle between the visual axis is the same for all distances + directions of gaze
InConmitant Squint
the angle between the visual axis varis with direction of case
Amblyopia
A unilateral or bilateral decrease in vision for which there is no obvious cause found on physical examination of the eye
Retinoscopy
key technique it allows an objective measure of the patients prescription where barriers such as language,mental Capacity and compliance do not allow fora subjective examination need a visible light reflex Spot /Streak collar up: Convergent bean Collar down -Divergent beam
Reflex
Fsp = Frev - WD
fsp= final Rx
frev =the lens used to get reversal
WD= working Distance
Your final RX is +4.00 / -3.50 x180
WD= 1 / 2 a metre
What lens did you use to find reversal
fsp = frev - WD
100/ 50 =+ 2.00 D
frev = fsp + WD = +4.00 + +2.00 = +6.00
final = +6.00 / -3.50 x180
Mohandria Technique
Alternative to cycloplegia instead of drops the room is made dark child expected to look at light of the retinoscope
accomodation assumed to be relaxed
Working distance reduced to 50cm (lens is 2.00D)
-1:25 adjustment may be made to compensate
Accommodation + Convergence
Binocular movements
versions
vergences
versions - conjugate Movements. eyes move together binocularly and the angles between the Visual axes is constant
Vergences - Disjunctive Movements .eyes move In opposite Directions
Binocular Movements version and movement Dextroversion Laevoversion Superversion infraversion
looking Right
looking Left
looking up
looking down
Binocular Vision vergence and Movement convergence Divergence Right Supra vergence Right in fra vergence In- cyclo Vergence Ex. cyclo vergence
Eye turns Inwards Divergence eye turn out RE turns up + left RE turns down I left upper poles of eyes + Inwards upper poles of eyes rotate out
what 3 ocular movements occur when viewing a near object
Convergence- Aligns Visual Axes with object, 2 Images will fall on corresponding retinal points Accomodation -the Power of the crystalline lens Increases to create a Sharp Image of the near object on the retina Pupil miosis (constriction) -reduces blur + increases Depth of field (ocular motor nerve CNIII )
Binocular vision Problems can therefore be the result of an excess of deficiency of
- vergence or control problem (Prism dioptres)
2. Power or focusing (dioptres)
Examples of everyday tasks that could lead to binocular vision problems
Reading
VDu use
Driving ( distance to dash board+ back)
TV
accommodative Convergence
the amount of convergence Stimulated by the act of accommodation
AC /A ratio
the amount of accommodative Convergence per 1.00D of accommodation
Accommodative convergence
- - - - - - - - - - - - - - - - - - - - - - -
Accomodation
an average 1.00D of Accommodation = 4.00Prism of Convergence
accomodation
Hyperopes
uncorrected hyperopes need to accomodate to see a distant object but most not converge
myopes
uncorrected myopes must converge to see a near object but will not need to accommodate
optometers
Opthalmic devices used to measure ametropia
Subjective
Objective
subjective - refractive error - needs response from patient( Snellen)
objective -no response ( Auto refractor)
Scheiner Disc
an opaque disc 38mm in diameter can be used to distinguish Spherical ametropia
2 pin holes around 0.75 mm - 1mm in diameter and 2-3mm apart
Badal optometer
optameter comprises of a linear Scale,Produces an Image size that is nearly constant
consists of Single lens( + 16.00D )
lens positioned so 2nd focal Point coincides with Spec lens plane = Spec Rx
one eye occluded -target Moved closer to cx until clear
xx’ = ff’
Telescope optometer page 248-252
Galilean telescope used to determine the Spectacle refraction.when retinalImage is clear the Position of the objective lens Indicates the refractive error
young Porterfield optometer page 248-252
consists of a black Rule with a white line down the centre.
A + 10D lens and 2 Parallel Slits are mounted at the end.Each SIit form an Image of a white line Cursor Can be moved by CX to indicate the Intersection of the white line
grades of binocular vision
1 simultaneous
2 fusion
3 Stereopsis
Simultaneous
lowest grade
the abilly to perceive two Seperate Images and then to Superimpose them
fusion
The ability to fuse two images into a single Image (percept)
Sensory - neurological ability to percieve 2 Similar Images
motor extraocular muscles work during vergence to maintain Single image
why is fusion Important
who doesn’t have fusion?
What happens if lost
if 2 eyes cannot produce 2 Similar Images which can be fused diplopia will result
Someone with Squint
anisometropic Rx
Someone with one eye
poor depth perception
Reduced Fov
clumsy
Supression
Squints when young
Suppression rarely Possible if diplopia occurs as adult ( brain can’t cope)
can occur in adults If VA Poor In one eye or one eye occluded due by cataract
3 types of Diplopia
Pathological - ( muscles affected)
Physiological - ( most common ) crossed + uncrossed
Prism induced
Exam Question
A patient Views a Spotlight at a distance of 6m through a 3 prism base down in front of the left eye . Explain what the Patient will see
which eye has Image on macula - Right eye
Image of the spotlight will be seen on RE Macula but the Image of Spotlight is displaced down due to prism base in LE
Image is seen double
Px will see the image in the LE as being above the image Seen by RE
exam Questions Monocular cues
overlapping
objects in the foreground are percievedto be closer to us compared to those behind that are obsecured by objects in foreground
Exam Q
Geometric perspective
Straight lines - narrowing into the distance giving them perception of distance away from you
Exam Q
Relative Size
your understanding of the size of an object when smaller than normal you Percieve it to be further away
Exam Q
Light and shading
the change of light or shading of a object is different when nearer to us Compared to when further away we can use that detail like shadow to See as 3 D
Exam Q
Aerial Perspective
the lower the contrast of an object denotes how far it may be away in the distance
Exam Q Monocular cues
Parallax
is the ability to judge distance based On the percieved Speed of the object if further away will Seem like going slower
Explain how astigmatism may be classified according to the position relative to the retina of the focal lines formed by a distant point object
Simple myopia . 1 focal line in front other on retina
Compound =both focal lines in front- myopia
Simple Hypermetropia 1 line on retina are behind
Compound Hyperemetrope both behind retina
Mixed lines either side of retina
the cross cyl method is often used to refine the Astigmatic Component during Subjective refraction with aid of diagram describe the Cross cylinder lens
2 plano cyls Perpendicular to each other
most common + / - 0.25 and + / - 0.50
an astigmatic Subject re quires a negative Cylinder with its axis at 90 if cross cyl used what would the orientation of the cross cyl handle
45 degrees to axis Or @ 135
Handle will be at 45/135 axis will be alinged with 90/180 medians
Keratometry
Keratometry measures the radius of Curvature of the Central anterior surface of the cornea over a 3-4mm diameter ( optic cap)
the practioner can then select the most appropriate back optic Zone Radius ( BOZR) when fitting CL
4 Purkinje Images
formed
Image 1 - Corneal reflex
Image 2 - formed by refraction and reflection( Posterior corneal Surface)
Image 3 - formed by refraction and reflection (anterior lens)
Image 4 - formed by refraction and reflection ( Posterior lens)
Purkinje Image 1
Corneal reflex reflection at the anterior Corneal Surface Virtual upright falls inside crystalline lens Brightest of all used in beratemetry and to measure PD
Purkinje Image 2
formed by refraction and reflection at the Posterior corneal Surface
virtual and upright
falls inside the crystalline lens just behind Image 1
1 / 100th the brightness of Image 1
Purkinje Image 3
formed by refraction and reflection at the anterior Surface of the crystalone lens
Virtual and upright
falls inside the vitreous
1 / 100th of brightres of image 1
’ Purkinje Image 4
formed by refraction and reflection at the posterior Surface of the Crystalline lens
Real and inverted
falls inside crystanline lens
1 /100th the brightness of Image 1
Keratometry
uses Purkinje Image 1 and by treating the cornea as a curved mirror makes use of the relationship between focal length and radius of Curvature
Keratometry Exam Q
However the eye is constantly making Small tremor,Microsaccadic and drift Movements while trying to hold fixation
Because of these constant Movements the Image size cannot be measured against a fixed Scale
Image doubling ( formula 2)
fixed doubling System power and Position of Prism= fixed
size of corneal Image calculated by
h’ = Pd / 100
Power of Prism x distance
use this then r= 2 h’ / h xd
Keratometry formula 3
using variable doubling System there is only one fixed object
no calculate the sizeof the corneal Image
h’ = - P S / 100 Mo
Mo -magnification observation System
P. Power of Prism
s = Distance of object to Image
one position instrument
has single mire and uses 2 doubling Prisms@ 90degrees opposed to one another This allows it to readboth principal meridians of the Cornea
Two position
can only take reading from are Corneal meridian at a time
instrument must be rotated through 90 to a Second Position to measure Second principal meridian
Why is doubling necessary
Indirect method of Measuring the image size of the Cornea that’s not affected by these constant Movement
Exam Q
Heterophoria
in normal binocular vision with both eyes fixating there is no deviation.If one eye occluded or binocular Vision prevented then one eye will deviate
Exam Q
Heterotropia
one eye will always deviate. the 2 visual axis are never directed in the same place
What is Keratoscopy
overall shape of the cornea
what is the main use of Keratoscopes and Keratometers in practice
fitting of CL
what is entoptic ehenomena
happens within the eye itself visual disturbances
Give are example of an entopic phenomenon Caused by ocular media
floaters casting Shadows on the retina
Exam Q
what lightening should be used to Illuminate the Ishihara test ? (5)
Exam Q
Ishihara colour Vision test what is recommended working distance
75mm
what is most common type of congenital Colour vision defect
Exam Q explain meaning - Colour vision for Deuteranomaly Protanopia Rod monochromacy
exam Q
state 2 causes acquired Colour vision defects
Exam Q
state 2 ways in which acquired Colour vision de fects may differ from congenital Defects
