Brachycephalic Obstructive Airway Syndrome (BOAS)

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ophthalmoscopy ‘seeing the eye’ from greek: ophthalmos - the eye skopeos – to see

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: ● limiting factor ● fundus of patient not ● patient’s pupil size illuminated – light source required ● pupillary axis of observer must be aligned with incoming light rays from light source to allow observation of the fundus

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 ●indirect ophthalmoscopy ophthalmoscopy

● ‘key-hole’ view ●‘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 ● greatest improvements direct ophthalmoscopy made were re. source ● a source of illumination of illumination ● a method of reflecting the ● candle light into the eye ● oil burning wick ● an optical means of ● development of correcting an unsharp incandescent bulbs image of the fundus ● halogen ● ….LED light source

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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 dial

diopter indicator

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direct ophthalmoscope head

shutter observers’ viewing aperture

light reflecting mirror

slit beam setting various sizes round beam

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 Observers’ viewing aperture Light reflecting mirror Slit beam setting Various sizes round beam Red free light

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Direct ophthalmoscope on a slit

direct ophthalmoscope

● if patient and observer are ● why are lenses used? emmetropic, no lenses ● lenses can be used to required to focus on retina 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 0 8 12 20

courtesy J Mould

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Mirror

12 dioptre lens

Mirror

15 dioptre lens

Mirror

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20 dioptre lens

Mirror

direct ophthalmoscope ●diopter equivalent ●distance (in mm) the focal moves anterior/posterior within the eye per diopter change

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0 D = focus on retina focus on ONH on + 6 D

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 0 8 12 20

courtesy J Mould

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

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 ● e.g. a direct ophthalmoscope is ● horse 8 equal to ● dog 17

● M = Fe/4 ● cat 20 ● Rat 77 ● where Fe is the total refractive power of the eye.

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lat magnification – indirect ophthalmoscopy dependent on lens ● lat magnification of a power and eye to lens = be examined ● total refractive power of eye/power of lens

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

(axial) = (lateral)2

● e.g. ● lateral magnification = 2 means axial magnification = 4

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

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 Lateral Axial Dioptric (using large Magnification Magnification Equivalent aperture) (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 20D 30D 14D 20D 30D Lens Lens Lens Lens Lens 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 20D 30D Lens Lens 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 Lateral Axial Field Lateral Axial Size Mag Mag Size Mag 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

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