Day 7(4): Visualizing the Ocular Fundus Flashcards
What is direct ophthalmoscopy?
Visualizing the fundus using a direct ophthalmoscope
Indications for direct ophthalmoscopy.
- Need for a high power study of the optic disc, macula and SMALL lesions in the posterior pole
- Measure elevations and depressions:
- slit beam curved back: depression
- slit beam curved forward: elevation - Measure dioptric power of the eye (myopia, hyperopia or emmetropia)
Advantages of direct ophthalmoscopy.
- High magnifications: 15X
- Erect image
- Ability to take measurements
- Easy to learn
Disadvantages of direct ophthalmoscopy.
- Limited field of view: 10 - 12 degrees
- can only view upto the equator
- due to small aperture - Poor illumination: affected by media opacities
- Monocular: loss of binocularity and stereopsis
- Distortion in the periphery
What is the near point?
- Closest point at which an object can be placed and still form a focused image ON the retina within the eye’s accommodative range
- 8.0 inches or 20 cm: after adjusting for AL
Steps in doing direct ophthalmoscopy.
Set-up:
- Pt sitting upright
- Focused at an object at a distance
- Dim lights
- Pupils pharmacologically dilated
Examiner position:
- Scope resting against the cheek
- Standing or sitting obliquely to the pt’s side
- Do NOT obstruct pt’s fixation point
- Pt’s R eye = Examiner’s R eye = Ophthalmoscope in R hand
- Pt’s L eye = Examiner’s L eye = Ophthalmoscope in L hand
Steps:
1. Shine light from a short distance and slowly move closer.
2. Adjust light aperture:
- too much light: uncomfortable for pt
- too little light: poor illumination of internal eye
3. Look for Red-Orange Reflex: move towards that direction as close as possible, almost touching the eye with fingers touching the pt’s cheek
4. Look for the optic disc then adjust focus until clear.
5. Follow vessels emanating from the disc and examine each quadrant
6. Move temporally to examine macula/fovea last.
- will cause intense glare and discomfort as this is the most sensitive area to light
What is indirect ophthalmoscopy?
Visualizing the fundus using an indirect ophthalmoscope
Indications for indirect ophthalmoscopy.
- (+) media opacities: due to stronger illumination
- High refractive errors: less distortion
- Children
- Total fundus examination: upto periphery and pars plana
- Examination of LARGE lesions: due to wider field of view
Advantages of indirect ophthalmoscopy.
- Wider field of view: 30 - 35 degrees
- vs 10 - 12 degrees in DO
- peripheral retina (ora serrata) to pars plana can be examined - Stronger illumination: can penetrate media opacities
- Stereopsis: due to binocularity
Disadvantages of indirect ophthalmoscopy.
- Low magnification: 2 - 5X
- vs 15X in DO - Inverted image
- vs erect in DO - Harder to master
Prerequisites for proper indirect ophthalmoscopy.
- Maximal mydriasis: wider field of view
- Good control of the eye: pt able to fixate, understand and cooperate; NO nystagmus
What is stereopsis?
- Perception of depth and three-dimensionality
- Requirement: binocularity or a pair of optimally functioning eyes
- Result of retinal disparity: images formed on the left and right retina are different causing the visual cortex to integrate both images and perceive difference as depth
- Objects should be at a DISTANCE: accommodation is relaxed
- Problems with NEAR objects:
1. If object is closer than the near point (8 inches), image will form in front of the examiner’s retina thus losing focus
2. Fusional Vergence Amplitude: limited inward rotation of the eyes to maintain focus
3. Amplitude of accommodation: limited accommodative power of the examiner’s lens
How does indirect ophthalmoscopy work to view the fundus stereoscopically?
To use binocular vision and stereopsis:
- need to move object FARTHER away to neutralize the near point, vergence and accommodation
- problems:
1. loss of focus: object may not fall at the line of sight
2. as you move farther away, the visualized area of the retina gets SMALLER: limited by pupillary aperture
3. difficulty perceiving minute details and pathologies
Solutions:
1. Reflecting mirrors: bend or focus the line of sight
2. Strong illumination
3. Condensing lens: to overcome the limited accommodation of examiner’s lens at close distances
Image: forms BETWEEN examiner and condensing lens
1. Real
2. Inverted
Steps in doing indirect ophthalmoscopy.
Set-up:
- Pt lying down
- Fixated at a distance
- Dim lights
- Pupils pharmacologically dilated
Examiner position:
- Standing above the pt’s head or at the side
- Arms stretched
- Headpiece, lens and examiner’s arms moving as a unit
- Thumb or middle finger to keep eye open
- Lens positioned one-finger length from the pt’s eye
- Lens held by both index fingers and thumbs with the other fingers resting on the periorbital area
Steps:
1. Adjust head piece to desired fit.
2. Adjust pupillary distance of eyepieces until SINGLE image is seen with good focus and depth
3. Adjust light source to a CENTRAL position.
4. Systematically examine the internal eye as in DO:
- optic disc
- retinal vessels: follow each in all 4 quadrants to the periphery
- peripheral retina and ora serrata
- ciliary body (pars plana)
- macula/fovea: examine last to avoid glare and discomfort as this is the most sensitive area to light
What are the common lenses used for IO?
20D Large
- Double aspheric lens
- Volk Optical
- M: 3.13X
- F: 46 degrees (static), 60 degrees (dynamic)
28D Large
- Volk Optical
- M: 2.27X
- F: 53 degrees (static), 68 degrees (dynamic)
Note: HIGHER dioptric power = WIDER field of view = SMALLER magnification
- D and F are DIRECTLY proportional: the higher the power, the wider the field illuminated
- F and M are INVERSELY proportional: the wider the field illuminated and examined, the smaller the image
Optional contraptions:
1. Adapter Set
- transforms standard 20D lens into 16D, 24D or 28D
2. Yellow Filter
- decreases light scatter and glare to increase comfort
- enhances image contrast
