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ophthalmoscopy
‘seeing the eye’
ophthalmos - the eye
skopeos – to see
from greek:
structure
●principles of fundus examination
–direct ophthalmoscopy
–indirect ophthalmoscopy
●magnification in ophthalmoscopy
●field of view in ophthalmoscopy
classic article
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principles of ophthalmoscopy
● in the emmetropic
patients, light rays
emanating from a point in
the fundus emerge as a
parallel beam
● if this beam enters the
pupil of the observer, the
rays are focussed on his
retina and an image is
formed
principles of ophthalmoscopy
● so why do we
generally not see the
fundus when looking
directly into a patient’s
pupil?
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principles of ophthalmoscopy
● problem:
● fundus of patient not
illuminated – light
source required
● pupillary axis of
observer must be
aligned with incoming
light rays from light
source to allow
observation of the
fundus
● limiting factor
● patient’s pupil size
principles of ophthalmoscopy
● in large animals or
patients with
extremely dilated
pupils, it may be
possible to observe
the fundus by simply
aligning a bright light
source with your
visual axis
principles of ophthalmoscopy
● direct ophthalmoscopy
● ‘key-hole’ view
●indirect
ophthalmoscopy
●‘aerial’ image
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direct ophthalmoscope
● first developed by
Charles Babbage
1847
● failed to prove its
function when showing
to eminent ophthalmic
surgeon and
abandoned project
http://www.college-optometrists.org/
direct ophthalmoscope
● clinical use introduced by
Hermann von Helmholtz
1851
● initially called ‘Augenspiegel’
(eye mirror)
● within 10 years physicians
using it call themselves
‘ophthalmoscopists’
● first fundus photograph
1864
http://www.college-optometrists.org/
direct ophthalmoscope
● Helmholtz’ principles of
direct ophthalmoscopy
● a source of illumination
● a method of reflecting the
light into the eye
● an optical means of
correcting an unsharp
image of the fundus
● greatest improvements
made were re. source
of illumination
● candle
● oil burning wick
● development of
incandescent bulbs
● halogen
● ….LED light source
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direct ophthalmoscopy
direct ophthalmoscopy
advantages
● ‘real’, upright image
● high magnification
● does not require full dilation
● cheap and mobile
disadvantages
● closeness to patient required
● small field of view
● no stereoscopy
● poor penetration of cloudy media
direct ophthalmoscope head
observer’s view
hole
fast diopter switch
diopter indicator
diopter dial
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direct ophthalmoscope head
shutter
light reflecting
mirror
various sizes
round beam
observers’
viewing
aperture
slit beam setting
red free light
direct ophthalmoscopy – light filters
● green filter (red free
light)
● blood (vessels) appear
black
● grid pattern
● facilitates lesion
documentation
● slit beam
● evaluate surface
topography of retina
and optic nerve
Direct ophthalmoscope head
Shutter
Light
reflecting
mirror
Various
sizes round
beam
Observers’
viewing
aperture
Slit beam setting
Red free light
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Direct ophthalmoscope on a slit
Direct ophthalmoscope on a slit
direct ophthalmoscope
● if patient and observer are
emmetropic, no lenses
required to focus on retina
● why are lenses used?
● lenses can be used to
correct known refractive
error of observer
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direct ophthalmoscope
● if observer and patient
are emmetropic,
lenses can be used to
● move the point of
focus onto structures
posterior to the retina
or structures more
anterior within the eye
direct ophthalmoscope
● negative lenses can
be used to focus onto
● structures posterior to
the retina
●colobomata, areas of
scleral ectasia, cupped
disc
direct ophthalmoscope
● positive lenses can
be used to focus onto
● structures more
anterior within the eye
● lens, iris, cornea,
adnexa
courtesy J Mould
0 8 12 20
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Mirror
Mirror
12 dioptre lens
Mirror
15 dioptre lens
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Mirror
20 dioptre lens
direct ophthalmoscope
●diopter equivalent
●distance (in mm) the focal moves anterior/posterior within the eye per diopter change
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focus on ONH on + 6 D0 D = focus on retina
Diopter equivalent - clinical
Diopter equivalent - clinical
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direct ophthalmoscope
● positive lenses can
be used to focus onto
● structures more
anterior within the eye
● lens, iris, cornea,
adnexa
courtesy J Mould
0 8 12 20
direct ophthalmoscopy
● how to do it
● align ophthalmoscope with your visual axis
● dim light beam to minimal intensity
● gain control over patient’s head with your free hand
● pick up fundus reflex at arms length
● (compare fundus reflex of both eyes)
● following the fundus reflex, come close to the eye until
the fundus becomes visible
● search fundus in reproducible manner
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Tora
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Fig. 1.12a-c
● Milk bottles
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direct ophthalmoscopy
● apply a replicable,
systematic approach to
the assessment of the
fundus
● keep ‘landmarks’ in mind
● consider that varying
species may require a
varying routine!
indirect ophthalmoscopy
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indirect ophthalmoscopy
indirect ophthalmoscopy
http://medical-
dictionary.thefreedictionary.com/_/viewer.aspx?path=ElMill&name=F0O-01-
S2958.jpg
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light options
‘all pupil’ setting
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indirect ophthalmoscopy
advantages
● larger field of view
● stereoscopic (with head
mounted)
● penetrates opaque
media better
disadvantages
● limited magnification
● inverted & upside down
image
condensing lenses
● acrylic or glass
● glass superior optics
● biconvex
● coated to reduce
reflections
● variety of strenghts
● most commonly used
●2.2 pan-retinal
●20 D
●30D
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high power condensing lenses
image – direct vs indirect
direct: 2.5 mm 20D lens: 13 mm
magnification and field of view
●magnification
● process of enlarging something only in appearance
but not in physical size
● field of view● (angular or linear or areal) extent of the observable
world that is seen at any given moment
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magnification and field of view
of the fundic image depend on two main factors
● optical properties of the eye to be examined
● small eyes = high dioptric power
● large eyes = less dioptric power
● form of ophthalmoscopic technique employed
● direct ophthalmoscopy = high magnification, small field of
view
● indirect ophthalmoscopy = low magnification, larger field of
view
magnification
● optical properties of the eye to be examined
● small eyes = high dioptric power
●high magnification
● large eyes = less dioptric power
● low magnification
optical properties
of the eye to be
examined
●small eyes = high
dioptric power
●high
magnification
●large eyes = less
dioptric power
●low
magnification
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ophthalmoscopy - magnification
●lateral magnification
●magnification of an area
●axial magnification
●magnification in depth
lateral magnification
● magnification across an
axis perpendicular
towards viewing
● e.g. left-right, dorsal-
ventral with regards to
fundus
● exemplified by action of
slide projector throwing a
much magnified image
onto a screen
lateral magnification – direct ophthalmoscopy
● the magnification M of
a direct
ophthalmoscope is
equal to
● M = Fe/4
● where Fe is the total
refractive power of the
eye.
● e.g.
● horse 8
● dog 17
● cat 20
● Rat 77
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lat magnification – indirect ophthalmoscopy
● lat magnification of a
lens =
● total refractive power
of eye/power of lens
dependent on lens
power and eye to
be examined
lateral magnification - indirect ophthalmoscopy
● power of lens inversely related to magnification
● high power = low magnification
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axial magnification
● describes magnification in depth
● precisely, the ratio of the distance along the optical axis
between two points in image space (A’B’) to the
distance along the optical axis between the
corresponding two points in object space (AB)
● this magnification is useful when considering an image
in its three dimensions
axial magnification
● magnification along
the axis of viewing
● e.g. elevation of
optic nerve head,
colobomata
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axial magnification
● axial magnification is approximately the lateral
magnification squared
● e.g. ● lateral magnification = 2 means axial magnification = 4
(axial) = (lateral)2
indirect ophthalmoscopy
● in any given species,
the dioptric power of
lens used determines
● magnification
● field of view
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axial magnification in diff. species
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axial magnification in diff species
field of view - direct ophthalmoscopy
● field of view in direct
ophthalmoscopy is
determined by
● pupil size of
●examiner
●patient
● size of light beam
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indirect ophthalmoscopy
● in any given species,
the dioptric power of
lens used determines
● magnification
● field of view
indirect ophthalmoscopy
● high diopter
● large field of view,
● (low magnification)
● low diopter
● small field of view
● (high magnification)
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hand held ‘PanOptic’
● cross between direct and
indirect ophthalmoscope
● increased field of view
compared to direct
● 25 degree vs. 5 degree
● increased magnification
● (by 26% compared to
indirect ophthalmoscopy
with 14 D lens)
● virtual upright image
● greater working distance
● monocular view
hand held ‘PanOptic’
http://www.welchallyn.com
Optics of Direct Ophthalmoscopy
Species Field of View
(using large
aperture)
Lateral
Magnification
Axial
Magnification
Dioptric
Equivalent
(mm/D change)
Horse 5.5mm 8x 84x 1.33mm
(0.75 D = 1mm)
Dog 2.5mm 17x 405x 0.28mm
(3.5 D = 1mm)
Cat 2.2mm 19x 508x 0.22mm
(4.5 D = 1mm)
Rat 0.6mm 77x 7965x 0.014mm
(71 D = 1mm!)
1. Field of view varies directly with axial length
2. Dioptric equivalent varies directly with axial length
3. Lateral and axial mag vary inversely with axial length
4. Axial mag ~= lateral mag2
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Optics of Direct Ophthalmoscopy
Species Field of View
(using large
aperture)
Lateral
Magnification
Axial
Magnification
Dioptric
Equivalent
(mm/D change)
Horse 5.5mm 8x 84x 1.33mm
(0.75 D = 1mm)
Dog 2.5mm 17x 405x 0.28mm
(3.5 D = 1mm)
Cat 2.2mm 19x 508x 0.22mm
(4.5 D = 1mm)
Rat 0.6mm 77x 7965x 0.014mm
(71 D = 1mm!)
1. Field of view varies directly with axial length
2. Dioptric equivalent varies directly with axial length
3. Lateral and axial mag vary inversely with axial length
4. Axial mag ~= lateral mag2
Magnification-Indirect Ophthalmoscopy
Species Lateral Magnification Axial Magnification
14D
Lens
20D
Lens
30D
Lens
14D
Lens
20D
Lens
30D
Lens
Horse 1.2x 0.8x 0.6x 1.9x 0.8x 0.4x
Dog 2.6x 1.8x 1.1x 9.0x 4.0x 1.7x
Cat 2.9x 2.0x 1.3x 11.3x 5.1x 2.1x
Rat 11.5x 7.7x 5.0x 177x 89x 18
1. Lateral and axial mag vary directly with axial length
2. Lateral and axial mag vary inversely with dioptric power of indirect lens
3. Axial mag ~= lateral mag2
*Pan Retinal lens has similar magnification to 20D lens, and similar field of view to 28D
lens
Field of View - Indirect Ophthalmoscopy
Species Field Size (mm)14D
Lens
20D
Lens
30D
Lens
Horse 17.3mm 27.6mm 38.3mm
Dog 7.9mm 12.5mm 17.4mm
Cat 7.0mm 11.2mm 15.5mm
Rat 1.8mm 2.8mm 3.9mm
1. Field of view varies directly with axial length
2. Field of view varies directly with dioptric power of indirect lens
*Pan Retinal lens has similar magnification to 20D lens, and similar field of view to 28D lens
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Optics of Direct vs. Indirect Ophthalmoscopy
Species Direct Ophthalmoscopy Indirect Ophthalmoscopy (20D
Lens)
Field
Size
Lateral
Mag
Axial
Mag
Field
Size
Lateral
Mag
Axial
Mag
Horse 5.5mm 8x 84x 27.6mm 0.8x 0.8x
Dog 2.5mm 17x 405x 12.5mm 1.8x 4.0x
Cat 2.2mm 20x 508x 11.2mm 2.0x 5.0x
Rat 0.6mm 77x 7965x 2.8mm 7.7x 70x
1. Field of view varies directly with axial length
2. Dioptric equivalent varies directly with axial length
3. Lateral and axial mag vary inversely with axial length
4. Axial mag ~= lateral mag2
*Direct = small field size, large mag; Indirect = large field size, small mag
Direct PanOptic Indirect (20D)Ophthalmoscope Ophthalmoscope Ophthalmoscope
FOV 9o (2.5 mm) 29o (7 mm) 41 (12.5mm)
LM 17.24x 3.2x 1.74x
AM 405x 7.43x 4.04x
PanOptic field size and magnification falls between direct and indirect
www.volk.com