Transcript 10_Instruments for research and correction of the human eye

Instruments for investigation and
correction of the human eye
disorders
Department of medical physics of diagnostic and treatment
equipment
I. Ya. Horbachevsky Ternopil State Medical University
By Oksana Bagrij-Zayats and Oleksandr Tokarskyy
Anatomy
Posterior Pole of fundus
Diagram of Fundus Layers
Rods and Cones
• Because of their different functions, rods and cones are
present in varying densities in the retina. The blind spot
is due to the connection of the optic nerve
Light Sensitivity
• Remember we talked about rods and cones
• Cones:
– Sensitive to bright light, photopic conditions
– Densely packed in the fovea
– Only a few cones per nerve fiber
• Rods:
– Sensitive to low light, scotopic conditions
– Widely distributed across the retina
– Up to 1000s of rods per nerve fiber (think of this as
many many drops falling into the same pipe, one drop
can’t be detected, but many drops generate some
water flow that can be measured)
Dark Adaptation
Object must be
very bright to be
seen
Dim objects can
be seen
Near and Far Points
• The eye lens has two extreme points, fully relaxed
and fully “bulged”, called fully accommodated.
When the lens is fully accommodated, which object
is in focus on the retina?
A. A distant star
B. A tree outside in your yard
C. Your cell phone screen when texting
Far Points
• When the lens is fully relaxed, a normal eye cornea
and lens will focus distant objects (at infinity) on the
retina
• This is known as the “far point” of the eye
Near Points
• When the lens is fully accommodated (bulged), the
eye will focus an object at about 25 cm (10 inches)
away onto the retina
• This is known as the “near point” of the eye
25 cm (10 inches)
Basic Physical Exam
• General physical examination should include :
–
–
–
–
–
Visual acuity
Pupillary reaction
Extraocular movement
Direct ophthalmoscope
Dilated exam (in case of visual loss or retinal pathology)
Visual Acuity
• Distance or Near
• Distance visual acuity at age 3
– early detection of amblyopia
• Terminology
–
–
–
–
VA - Visual acuity
OD - ocular dexter
OS - ocular sinister
OU - oculus uterque
Imperfect Vision
• Let’s consider light coming into the eye from a distant
object, approximated as parallel rays. In a normal
eye, these rays focus on the retina when the eyelens
is fully relaxed
• If the cornea is not properly shaped, these rays will
not focus on the retina
parallel rays focus past the retina
parallel rays focus in front of the retina
Myopia (Nearsightedness)
Myopia
• Myopia occurs when the cornea is too powerful.
• When the eyelens is fully relaxed, the far point is
not at infinity, but closer
• This results in distant objects appearing blurry
far point is less than infinity
Hyperopia (Farsightedness)
• Hyperopia is the opposite problem, when the
cornea is not powerful enough, and parallel rays
are not focused by the time they reach the retina.
• The eyelens can partially accommodate to
increase the power of the cornea-lens system,
and focus these rays on the retina
eyelens partially accommodated to increase lens power
Hyperopia
• Because the eyelens has to partially accommodate to
focus rays from distant objects, its range will not be
sufficient to focus near objects on the retina
25 cm
more than 25 cm
This results in a near point that is more distant than the standard 25 cm
Power of a Lens
• It’s going to be easier to think about corrective lenses
using lens power rather than focal length, so let’s
review what this means
• Remember:
• The more a lens bulges, the shorter its focal length,
and the larger its ray-bending power
Power of a Lens
When the eyelens is fully relaxed, the power of the
cornea plus the eyelens is 60 diopters in a normal
eye.
If the eyelens then fully accommodates, does the
power of the cornea plus eyelens
A. increase
B. stay the same
C. decrease
Power of a Lens
• When fully accommodated, the power of the
cornea plus eyelens increases by about 4
diopters.
• Your eyeglass or contact lens prescription is given
in diopters, the power of the lens needed to
correct the imperfect curvature of your cornea
• Converging lenses have a positive power (positive
focal length) and diverging lenses have a negative
power (negative focal length)
Corrective Lenses
• Myopic (nearsighted) eyes have a cornea plus
lens that is too powerful
– They will require a negative (diverging) lens to
compensate
• Hyperopic (farsighted) eyes have a cornea plus
lens that is not powerful enough
– They will require a positive (converging) lens to
compensate
Lens Power
A myopic eye is too powerful, say it has a power of
63 diopters. What power of lens should we put next
to it to get a combined power of 60 diopters
(normal eye)
A.
B.
C.
D.
-2 diopters
-3 diopters
2 diopters
3 diopters
Lens Power
If we have a hyperopic eye of power 58 diopters
wearing corrective lenses of power 2 diopters, what
is the focal length of the combined set of lenses?
A. 1.5 cm (0.015 m)
B. 1.7 cm (0.017 m)
C. 2 cm (0.02 m)
Myopia - Definition
• When parallel rays of light enter the eye (with
accommodation relaxed) and come to a single
point focus in front of the retina
Etiology
• Axial length
– The axial length of the eye is longer than normal due to
imperfect emmetropization
– The most common cause of myopia for high myopes
Etiology
• Refractive power
– The refractive power of the eye is too strong
• Curvature myopia
– Cornea or lens has a steep curvature (e.g., keratoconus)
• Increased index of refraction (e.g., cornea, lens)
• Anterior movement of the lens (e.g., nuclear sclerosis)
Corrective Lenses: Myopia
To correct myopia (nearsightedness), a diverging lens
creates an intermediate image of a distant star at your
far point so that your eye can see it even though the
star is beyond your far point.
Corrective Lenses: Myopia
To correct myopia (nearsightedness), a diverging lens
creates an intermediate image of a distant star at your
far point so that your eye can see it even though the
star is beyond your far point.
far point
image of distant object
Corrective Lenses: Hyperopia
To correct farsightedness your contact lens creates an
(intermediate) image of a book 25 cm away at your near
point so that your farsighted eye can see it even though
the book is closer than your near point
near point
25 cm
Corrective Lenses: Hyperopia
To correct farsightedness your contact lens creates an
(intermediate) image of a book 25 cm away at your near
point so that your farsighted eye can see it even though
the book is closer than your near point
near point
25 cm
focal point of corrective lens
Determining Prescription
Determining Prescription
Determining Prescription
You are near sighted and your far point is 1 meter
away. What is your prescription?
A.
B.
C.
D.
E.
+1 diopter
-1 diopter
+2 diopters
-2 diopters
+3 diopters
Determining Prescription
You are far sighted and your near point is 1 meter
away instead of 25 cm. What is your prescription?
A.
B.
C.
D.
E.
+1 diopter
-1 diopter
+2 diopters
-2 diopters
+3 diopters
Presbyopia: Bifocals
• It is possible to have both a near point that is
more distant than 25 cm and a far point that is
closer than infinity.
• In this case, you need bifocals, which have two
lenses in them, one to correct each imperfection
The top part of the lens (the
picture shows a pair of bifocals
upside down) corrects the far
point
The bottom part of the lens
corrects the near point
Contact Lenses
Contact lenses are just a thinner and smaller version
of glasses that rest directly on the cornea, with a
thin layer of fluid in between.
Image Size on the Retina
• The size of an object on your
retina is related to the angle
between the axis and the ray
passing through the center of
the lens
• A large angle means a large
image on the retina.
• Here we see an example that
you all know intuitively: that
objects look smaller when
they are farther away
this angle is large so the object is large
this angle is small so the object is small
What can we use Slit Lamps for?
On their own
• Routine examination of
anterior segment
– Adnexa through to anterior
vitreous
• Problem-based examination
of anterior segment
With accessories
• Gonioscopy
• Fundoscopy
• Ocular photography
• Contact tonometry (Goldmann)
• Pachymetry
• Contact lens examination
• Corneal sensitivity
measurements (aesthiometry)
• Assessment of anterior
chamber depth and angle
• Laser photocoagulation
Basic Design of a Slit Lamp
• Viewing arm
– Biomicroscope
– Adjustable focus eyepieces
– Magnification dial
• Illumination arm
– The “slit lamp”
– Slit size, shape and filter controls
– Variable size, shape, colour and brightness
• Biomicroscope and illumination are mechanically coupled
around central pivot point (copivotal)
– Both focus at the same point (parfocal)
– Both arms can swing independently 180º along horizontal –
there is a scale in degrees
– Both always central regardless of angle (isocentric)
• Moveable base plate and joystick control
Slit Lamp Magnification
• Slit lamps provide variable magnification
• Lower magnifications are used for general
assessment and orientation
• Higher magnifications are used for detailed
inspections of areas of interest
• There are several ways to do this
– Common methods: Littmann-Galilean telescope and
zoom systems
– Less common methods: Change the eyepieces and/or
change the objective lens
Littmann-Galilean telescope method
• A separate optical system is placed in between the
eyepiece and the objective
• It consists of a rotating drum that house 2 Galilean
telescopes plus a pair of empty slots
– Optics refresher: Galilean telescopes consist of a positive and
negative lens that provide magnification based on the lens
powers and their separation
• It is easy to identify whether the slit lamp you are
using has this inside
– The magnification dial will click into place as you turn it, and
there will be numbers on the dial that correspond to the
magnification in each position
 Two telescopes produce two
magnifications
 Mag highest when the
convex lens is near
objective
 Reversal of these two
telescopes produces two
further minifications
 No telescope provides 5th
option
Change eyepieces or objective
Eyepieces
• Often two sets provided with
slit lamp
– Typical values 10x, 12.5x, 15x or 20x
• Inconvenient so rarely used
• Generally unnecessary on
modern slit lamps
Objective
• Flip arrangement for rapid
change
• Usually only two options due
to space confinements
• Typical values are 1x and 2x
Lever
Slit width
• Continuously variable (0 to
12-14mm)
• May be graduated to allow
measurement
• Narrow slits are used to
“slice” through the cornea to
determine depth or
thickness
• Wide slits are used to inspect
surfaces
Definition:
Gonioscopy is a clinical technique used to
examine structures in the anterior chamber
angle.
Trantas, using limbal indentation in an eye with keratoglobus
in 1907, first visualized the anterior chamber angle in a
living eye and coined the term gonioscopy.
The normal angle of the eye is not visible to us due to total internal
reflection of light emanating from the angle.
DIRECT Gonioscopy:
The anterior curve of the goniolens is such that the critical angle is not
reached, and light rays are refracted at the contact lens- air interface
EG: Koeppe, Shaffer, Layden, Barkan, Thorpe, Swan Jacob
. INDIRECT Gonioscopy:
The light rays are reflected by a mirror/ prism in the contact lens and leave
the lens at nearly a right angle to the contact lens- air interface.
Eg: Goldmann single, and three mirror lenses, Ziess four mirror lenses,
posner and susmann four mirror lenses, Thorpe four mirror, Ritch
trabeculoplasty lens
Various Diagnostic Gonio Lenses and
Specifications
Direct Goniolenses:
-Koeppe- Prototype
-Shaffer. – small Koeppe
lens(infants)
-Barkan- prototype surgical
goniolens
-Thorpe- surgical and
diagnostic lens.
-Swan Jacob- surgical
goniolens for children
Indirect goniolenses:
Goldmann single mirrormirror inclined at 62
degree for gonioscopy.
Central well- dia of 12 mm,
post radius of curvature of
7.38 mm
Goldmann three mirror- 59
degrees
Zeiss four mirror- all four
mirrors inclined at 64
degree.
Ritch trabeculoplasty lens.
CORNEAL WEDGE
Identification of Schwalbe’s line
How to do Gonioscopy?
• Anesthetize the cornea.
•Insert the lens with or without
coupling device.
• Short beam of light, avoid
illuminating the pupil
• To manipulate - ask patient to
look in the direction of the
mirror
•Indent the cornea with a four
mirror lens ( appearance of
Descemet’s folds)
Direct Ophthalmoscopy
• Tropicamide or phenylephrine for dilation
– unless shallow anterior chamber
– unless under neurological evaluation
• Use own OD to examine OD
– Same for OS
Direct Ophthalmoscope
Examiner right eye, hand, right
patient eye
Structures of the retina
nasal
temporal
• Have patient sit in a comfortable position
• Tell them to look at something straight ahead and level
over your shoulder
• Dim light in the room, so patients pupils dilate a little. You
can also use mydriatic eyedrops to dilate the pupil
• Hold ophthalmoscope in same hand as eye you are
looking at, and looking through (e.g. left hand for
examining patients left eye, using your left eye)
• Hold head steady with thumb above eyebrow, or hold
shoulder
• At about 30cm distance with light on eye, locate red reflex (seen as
an orange glow in the pupil)
• Follow red reflex into the eye as 15 degrees lateral to the patients line
of vision, this will get you directly into the optic disc
• If you cannot find the disc, trace any blood vessels back to it
• Examine vessels in all 4 quadrants of eye (upper and lower nasal and
temporal quadrants)
• Identify macula – slightly darker pigmented area, 2 optic disc widths
lateral away from the optic disc
• You can tell the patient to look at the light – this will put the macula
in your focus, however don’t look at it too long as it can be irritating
•
•
•
•
•
•
1 The size, shape and borders of the optic disc
2 The disc to cup ratio
3 The relative size of the arteries and veins
4 The texture of the retina
5 The color of the retina
6 Trace the vascular structure to the equator of
the retina.
• 7 Find the macula and note its color and size
Glaucoma
• Identify disc-to-cup ratio
• The pink rim of disc
contains nerve fibers. The
white cup is a pit with no
nerve fibers. As glaucoma
advances, the cup
enlarges until it occupies
most of the disc area.
Opthalmoscopy
• Turning the dial to positive (or green) numbers increases
the refractive index – short focal length lenses – for
examining cornea, iris, or opacities in vitreous or lens. e.g.
start at +20 and use the slit light
• Turning the dial to negative (or red) numbers decreases –
infinite focal length lens that fits your refractive power
(individual) – for examining retina, start at +10 as you
move in and dim the scope light about halfway
• Rule of thumb: You will focus on the retina with same
number as your refractive error, then correct for your
patients refractive error
Glaucoma
A disease of progressive optic neuropathy with
loss of retinal neurons and their axons (nerve
fiber layer) resulting in blindness if left
untreated.
“High IOP (intraocular pressure) is the strongest
known risk factor for glaucoma but it is neither
necessary nor sufficient to induce the
neuropathy.”
• Classification:
– Open-angle glaucoma
GLAUCOMA
Types of glaucoma
I. Primary:
A. Congenital
B. Hereditary
C. Adult (common types)
1. Narrow angle
2. Open angle
(Normal tension glaucoma)
II. Secondary
A. Inflammatory
B. Traumatic
C. Rubeotic
D. Phacolytic
etc.
GLAUCOMA
How do we diagnose it?

IOP is not helpful diagnostically until it reaches
approximately 40 mm Hg at which level the
likelihood of damage is significant.

Visual fields are also not helpful in the early stages
of diagnosis because a considerable number of neurons must be lost before VF
changes can be
detected.

Optic nerve damage in the early stages is difficult
or impossible to recognize.

50% of people with glaucoma do not know it!
Glaucoma Evaluation
• Complete history
• Complete examination
–
–
–
–
IOP
Gonioscopy
Optic disc
Visual Fields
GLAUCOMA
How do we measure IOP?
Applanation
Tonopen
Schiotz
Air
Non-contact
GLAUCOMA
Tonometry
Applanation
Schiotz
Intraocular Pressure Measurement
• Range: 10 - 22
GLAUCOMA
Goldmann applanation
tonometer
GLAUCOMA
Tonopen
GLAUCOMA
Goldmann perimeter
Glaucoma visual fields
THE VISUAL FIELD
Humphrey automated perimetry
GLAUCOMA
Cup-to-disk ratio
GLAUCOMA
DISK CUPPING
Normal
Glaucoma
GLAUCOMA
Surgical treatment of glaucoma
Argon laser
trabeculoplasty
Filtration
procedures
Cataract
•
•
•
•
Opacification of the lens
Congenital vs. acquired
Often age-related
Different forms
– Nuclear, cortical, PSCC
• Very successful surgery
Keratometry: Main Points so Far
• Keratometry uses the anterior cornea as a mirror
• Distant object: h  rAC
2 b h
• Keratometer Equation:
rAC  
h
• Virtual corneal image (h) inaccessible, small and unstable (eye
movements), so:
– Use objective lens to focus reflected rays as a real image
– magnify the real image with an eyepiece lens (~ 5 mag)
– split the real image inside the keratometer into two using a half-field prism;
adjust prism to “double” images
Moving prism toward image plane decreases image displacement (x)
Previously doubled images are no longer doubled (now overlap)
What new corneal radius would this prism position “suit”?
MIRE
PRISM (P)
IMAGE
PLANE
CORNEA
½h
h
h
C F
P
½h
x
< h
OBJECTIVE
What happens if we move the prism?
Fig 13.17, Page 13.18
Schematic View of the B & L Optical System
ILLUMINATED
MIRE
HORIZONTAL &
VERTICAL PRISMS
OBJECTIVE
LENS
EYEPIECE
OBSERVER
PV
CORNEAL
MIRE
IMAGE
APERTURE
PLATE
PH
GRATICULE
PLANE
OBJ
Fig 13.22, Page 13.27
Two prisms means two deviated images
Topcon Keratometer
What the Clinician Sees
V 90 / H 180
B & L: Oriented to Measure r90 and r180
Corneal vertex
h'90
OBJ
Question: If most corneas are
aspheric, what is one drawback
with a keratometer?
Answer: only measuring radius at
one location (annulus) on cornea;
h'180
and
it is NOT central radius
B & L: Oriented to Measure r60 and r150
Corneal vertex
120
135
150
105 90
75
60
45
30
165
15
180
(0) 180
Q2: What does this appearance indicate?
Ag
a
W
it h
-t h
e-
Ru
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As
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in
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m
-t h
at
ism
eRu
le
As
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. ..
at
is m
m
As
tig
at
ism
m
iq
ue
Ob
l
rA
st
ig
su
rfa
gu
la
ea
l
Irr
e
or
n
lc
ic a
Spherical corneal surface
Irregular Astigmatism
Oblique Astigmatism
With-the-Rule Astigmatism
Against-the-Rule Astigmatism
Sp
he
r
1.
2.
3.
4.
5.
ce
20% 20% 20% 20% 20%
Basis of Corneal Power Estimate - Exact Eye
Fe (cornea)
+43.05 D
r2 = +6.8 mm
F1 = +48.83 D
F2 = 5.88 D
naqueous
1.336
nair
1.000
ncornea
1.376
r1 = +7.7 mm
Page 13.23
Electrooculography - DEFINITION
• The clinical electro-oculogram is an
electrophysiological test of function of the outer
retina and retinal pigment epithelium in which the
change in the electrical potential between the
cornea and the fundus is recorded during
successive periods of dark and light adaptation.
• The eye has a standing electrical potential between front and back,
sometimes called the corneo-fundal potential. The potential is mainly
derived from the retinal pigment epithelium (RPE), and it changes in
response to retinal illumination
• The potential decreases for 8–10 min in darkness. Subsequent retinal
illumination causes an initial fall in the standing potential over 60–75
s (the fast oscillation (FO)), followed by a slow rise for 7–14 min (the
light response). These phenomena arise from ion permeability
changes across the basal RPE membrane.
Measurement of the clinical EOG
•
The calibration of the signal may be achieved by having
the patient look consecutively at two different fixation
points located a known angle apart and recording the
concomitant EOGs .
• By attaching skin electrodes on both sides of an eye the
potential can be measured by having the subject move his
or her eyes horizontally a set distance .
• Typical signal magnitudes range from 5-20 µV/°.
• A ground electrode is attached usually to either
the forehead or earlobe.
• Either inside a Ganzfeld, or on a screen in front of
the patient, small red fixation lights are place 30
degrees apart .
• The distance the lights are separated is not critical
for routine testing.
The standard method
•
Typically the voltage becomes a little smaller in the dark
reaching its lowest potential after about 8-12 minutes, the socalled “dark trough”.
•
When the lights are turned on the potential rises, the light rise,
reaching its peak in about 10 minutes.
•
When the size of the "light peak" is compared to the "dark
trough" the relative size should be about 2:1 or greater .
•
A light/dark ratio of less than about 1.7 is considered
abnormal.
Electroretinogram: An electrical diagnostic
test of retinal function in situ
• Electro -part
– Currents, wires, voltage, resistance
• Retino - part
– Cell types, membrane potential, radial currents.
• Gramo - part
– Diagnostic test of patient retinal health
– Research test retinal circuitry, cell function, disease
states, drug efficacy
The Eye generates a lot of electrical signal,
some fast . . some slow. . .
Methods
•
•
•
•
Dark adapt 20-45 min
Anesthetize subjects cornea (paracaine)
Dilate iris (tropicamide; phenylephrine)
Attach electrodes: Burian-Alled, Or
– Forehead (neg)
– Corneal (pos) (DTL microfiber)
– Behind Ear (reference)
Burian-Allen Electrode for Human Use
Electroretinogram (ERG)
600
r (µV)
400
200
B wave
0
-200
A wave
0.0
0.1
0.2
time (s)
0.3
Basic Clinical ERG tests
• Dark adapted, dim (blue) flash response
– Isolated rod-driven response
• Dark adapted, bright (white) flash response
– Generates Max a-wave, b-wave, also generates OPs :
• Light adapted, bright flash
– Isolated cone-driven response
• 30 Hz Flicker
– Another method of isolating cone responses.
Different conditions yield different
responses
Rod
Rod & Cone
Cone