IDO mechanism

1. INDIRECT OPHTHALMOSCOPY
Dr. Anupama Manoharan
Junior resident , Dept of ophthmology
UGMCH ooty (TN)

2. INTRODUCTION:
 IDO was introduced by Nagel in 1864
 Examination of the posterior segment
 Consists of headset with binocular
viewing system and adjustable lighting
system
 Portable – rechargeable power packs/
spectacle mounted
 Uses condensing lens(+14 to +30 D)

3. OPTICAL PRINCIPLE:
 To make the eye highly myopic by placing a strong
convex lens in front of patient’s eye
 Emergent rays from an area of fundus is brought to
focus as a Real, inverted image

4. OPTICAL SYSTEM
 Binocularity is achieved by reducing the observer’s
interpupillary distance from about 60 mm to approx
15 mm by prisms/ mirrors.

5. FIELD OF ILLUMINATION:
 More in Myopia
 Less in Hypermetropia

6. IMAGE FORMATION IN EMMETROPIA
 Emergent rays are parallel
 Focus at its principal focus
 Inverted image is formed between condensing lens
& observer
 Size of the image remain same

7. IMAGE FORMATION IN HYPERMETROPIA:
 Emergent rays are divergent
 Condensing lens uses imaginary enlarged upright
image behind the eye
 Final image is in front of principal focus

8. SIZE OF THE IMAGE:
 Equal to an emmetropic eye if lens is held that
principal focus correspond to ant focus of eye
 Larger than emmetropic eye if cond lens is held
at distance that principal focus is nearer than ant
focus of the eye
 Smaller than emmetropic eye when cond lens is
farther away than ant focus of the eye

9. IMAGE FORMATION IN MYOPIA:
 Emergent rays are convergent
 Inverted image in front of the eye
 Final image is within its own focal length

10. SIZE OF THE IMAGE:
 Equal to an emmetropic eye, if cond lens –
principal focus corresponds to ant focus of the eye
 Smaller than emmetropic eye, if cond lens -
principal focus is nearer than the ant focus of the
eye
 Larger than emmetropic eye, principal focus of
cond lens is farther away from ant focus of the eye

11. IMAGE CHARACTERISTICS:
 Image- Real, Inverted and magnified

12. IMAGE MAGNIFICATION:
 Function of power of condensing lens
 Power of patient’s eye ( 60 D)
 Mag= power of eye
power of cond lens
 Ex) 20 D lens- 60/20 = 3x lateral mag
30 D lens- 60/30= 2x lateral mag

13.  Magnification:
 - Diopteric power of the convex lens
 - Position of the lens in relation to eyeball
 - Refractive state of the eyeball

14. Condensing lens Magnification Field of view
15 D 4x 40°
20 D 3x 45°
25D 2.5x 50°
30D 2x 60°
40D 1.5x 65°

15. FIELD OF ILLUMINATION
 Field of observation is larger than the field of
illumination
 Field of observation is func of magnification and
condensing lens diameter

16. TECHNIQUE:
 Procedure explained to pt
 Light thrown into pt’s eye at arm’s distance
 Keeping eye on that reflex, interpose condensing
lens in path of beam of light – slowly move the lens
away from the eye until image clearly seen
 Examiner moves around the head of the patient
 Examiner has to stand opposite the clock hour
position to be examined

19. SCLERAL DEPRESSION:
 Depressor placed on pt’s lid
 Examiner move depressor in a direction opposite to
that he/she wishes the depression to appear
 Depressor rolled gently and tangentially over eye
surface
 Most sensitive- superonasal quadrant

22. FUNDUS DRAWING:
 Image – vertically inverted and laterally reversed
 Top of retina chart placed towards foot end of the pt.
 Made on special Amsler ‘s chart that has 12 clock hrs
marked and 3 concentric circles made on it.
Innermost circle- Equator
Middle circle-Ora serrata
Outermost circle-midpoint of pars plana

24. NORMAL ANATOMICAL LANDMARKS:
 Vortex veins ampulla- along the equator
 Long ciliary vein- 3 o’ clock and 9 o’ clock position
 Branching vessels marked to draw the pathology seen

26. COLOUR CODE FOR FUNDUS DRAWING
COLOUR REPRESENTATION
Red Retinal arterioles
Attached retina
Retinal hemorrhage
Microaneursyms
Neovascularisation
Retinal break or hole
Blue Retinal venules
Detached retina
Outline of retinal break or hole
Yellow Exudate
Edema
Green Vitreous opacity
Brown Pigmentation
Detached choroid
Black Hyperpigmentation
Ora serrata

27. ADVANTAGES:
 Larger field of retina is visible
 Lesser distortion of image
 Useful in hazy media because of bright light and
optical property
 Can be used during retinal surgery as it preserve
sterile operative field
 Laser energy can be directed thro IDO

28. DISADVANTAGE:
 Less magnification( 5 times)
 Impossible in very small pupil
 Patient uncomfortable because of intense light of
IDO & scleral intentation

29. FEATURE DIRECT
OPHTHALMOSCOPY
INDIRECT
OPHTHALMOSCOPY
Condensing lens Not recqired Recquired
Examination distance As close to patient’s eye
as possible
At an arm’s distance
Image Virtual, Erect Real, Inverted
Magnification 15 times 4-5 times
Illumination Not so bright Bright

30. FEATURE DIRECT
OPHTHALMOSCOPY
INDIRECT
OPHTHALMOSCOPY
Examination thro hazy
media
Not possible Possible
Area of field in focus About 2 disc diopter About 8 disc diopter
Stereopsis Absent present
Accessible fundus
view
Beyond equator Upto ora serrata
Teaching mirror None Can be attached

31. THANK YOU