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Duke Elder: Optics (Alice) > Chapter 14: Instruments A > Flashcards

Chapter 14: Instruments A Flashcards

1
Q

Broadly speaking how does a direct ophthalmoscope function?

A
  • system of lenses focus light from an electric bulb onto a mirror where a real image of the bulb filament is formed
  • mirror reflects the emitted light in a diverging beam which illuminates the patient’s eye
  • image of the bulb is formed just below the hole so corneal reflection doesn’t lie in visual axis of observer
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2
Q

How does a myopic eye influence the field of view in direct ophthalmoscopy?

A

field of view smaller in a myopic eye

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3
Q

How does a hypermetropic eye influence the field of view in direct ophthalmoscopy?

A

field of view is larger in a hypermetropic eye

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4
Q

What is the influence on the field of view when the pupil is dilated in direct ophthalmoscopy?

A

the field of view is enlarged

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5
Q

What are 3 factors that increase the size of the field of view in direct ophthalmoscopy?

A
  1. hypermetropic eye
  2. pupil dilation
  3. smaller istance between patient and observer
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6
Q

Why is there often a dark shadow during examination of peripheral parts of the retina in direct ophthalmoscopy?

A

due to total internal reflection of light at the periphery of the crystalline lens

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7
Q

In direct ophthalmoscopy, where is the image of the patient’s retina formed, and how does this reach the observer’s retina?

A
  • image xy of illumianted retina XY is formed at the patient’s far point
  • a ray from the top of this image passes through the observer’s nodal point and locates the position of the top of the image X’Y’ on the observer’s retina
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8
Q

What are the properties of the final image formed by the direct ophthalmoscope?

A

it is inverted in observer’s eye, so is therefore seen as erect, + virtual

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9
Q

How is the image size in direction ophthalmoscopy influenced by refractive state of the patient?

A

image is smaller in hypermetropia

image is larger in myopia

(remember field of view greater in hypermetropia, smaller in myopia - field of view and image size are opposites)

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10
Q

How will the observer perceive the image from a hypermetropic patient’s retina in direct ophthalmoscopy?

A

diverging beam leaves patient’s eye - need to accommodate or use a correcting convex lens

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11
Q

How will the observer perceive the image from a myopic patient’s retina in direct ophthalmoscopy?

A

converging beam of light enters observer’s eye - observer needs to use concave lens to view

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12
Q

What can influence the discrepency in size of the image on the observer’s retina from direct ophthalmoscopy due to refractive error?

A

the use of a correcting lens can reduce the discrepancy

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13
Q

In which scenario is the observer’s image size the same regardless of patient’s refractive error in direct ophthalmoscopy?

A

when the patient’s and observer’s anterior focal points coincide and the correcting lens is placed at that point (rarely fulfilled in practice)

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14
Q

How can direct ophthalmoscopy inform the observer of the patient’s refractive state?

A

characteristic view of the fundus -

in high hypermetropia - small image of wide field of view (easy to scan whole fundus quickly)

high myopia - large image with small field of view (difficult to examine fundus)

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15
Q

Why is it impossible to secure a perfect view of the fundus of an astigmatic eye in direct ophthalmoscopy?

A

the only corecting lenses in the ophthalmoscope are spherical - only possile to correct one meridian at a time

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16
Q

What shape will the fundus appear in high degrees of astigmatism in direct ophthalmoscopy?

A

oval - due to distortion of the image due to disparity of dioptric power of eye in two principal meridians

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17
Q

What is the best way to view a highly myopic fundus with a direct ophthalmoscope?

A

patient keeps glasses on

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18
Q

What is the magnification of the direct ophthalmoscope for a dioptric power of +60 D in an emmetropic eye?

A

M = F/4

M=60/4 =x15

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19
Q

What is acting as a loupe in the process of direct ophthalmoscopy?

A

observer is using the dioptric power of the patient’s eye as a loupe - approx +60 D dioptric power in an emmetropic patient

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20
Q

What can make detection of micro-aneurysms easier with a direct ophthalmoscope?

A

red-free filter - resulting green light causes microaneuryms to show up as lack dots against a green background, detection is easier

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21
Q

How does the field of view compare with direct ophthalmoscopy?

A

large - 25o (vs small, 6o for direct ophthalmoscopy)

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22
Q

How does magnification compare for indirect vs direct ophthalmoscopy?

A

smaller magnification for indirect: x3 (+20D power) or x5 (+13D) rather than x15 for direct.

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23
Q

What binocularity is available for direct vs indirect ophthalmoscopy?

A

not available for direct, stereoscopic view available for indirect

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24
Q

What is the relative influence of the patient’s refractive error in indirect ophthalmoscopy?

A

small (whereas it is large in direct)

25
Q

What teaching facility is available for indirect ophthalmoscopy (unlike direct)?

A

teaching mirror

26
Q

How does indirect ophthalmoscopy work?

A
  • powerful convex lens (condensing lens) held in front of patient’s eye (powers +20D and +13D)
  • illuminating bulb shines from observer headset → condensing lens converges onto pt retina → reflected by retina and refracted (converged) by consending lens
    • real image between condensing lens and observer formed
27
Q

What are the properties of the image formed by the indirect ophthalmoscope?

A
  • real
  • inverted vertically and horizontally
  • situated beween condensing lens and observer
28
Q

What is the effect of the condensing lens on light in indirect ophthalmoscopy?

A

converges it - convergent beam enters pt eye, brought to focus within vitreous by eye’s refractive system then diverges onto retina

29
Q

Why is illumination of the retina bright and even in indirect ophthalmoscopy?

A

comes from the real image of the light source within the patient’s eye

30
Q

How does refractive error influence the field of illumination in indirect ophtalmoscopy?

A
  • myopia: large field of illumination
  • hypermetropia: small field of illumination
31
Q

What factor limits the field of illumination in all refractive states in indirect ophthalmoscopy?

A

size of subject’s pupil

32
Q

What does it mean that the patient’s pupil and observer’s pupil are conjugate foci in indirect ophthalmoscopy?

A

light arising from a point in the subject’s pupillary plane is brought to a focus by the condensing lens in the observer’s pupillary plane and vice versa

33
Q

What is formed in the subject’s pupillary plane in indirect ophthalmoscopy?

A

reduced image of observer’s pupil (4mm pupil is 0.7mm)

34
Q

Why is the size of the image of the observer’s pupil in the subject’s pupillary plane important?

A

only rays of light which leave the patient’s eye via the area of the image of the observer’spupil can, after refraction by the condensing lens, enter the observer’s pupil and be seen by him

35
Q

What are the 2 important factors that influence the field of view in indirect ophthalmoscopy?

A
  • observer’s pupil size
    • aperture or size of condensing lens
36
Q

Why is it usual to dilate patient’s pupil widely before indirect ophthlamoscopy?

A

to widen field of illumination

37
Q

Why is the largest possible aperture of condensing lens chosen for indirect ophthalmoscopy?

A

to give widest field of view

38
Q

How can aberrations from the condensing lens in IO be minimised?

A

use lens of aspheric form

39
Q

Where is the image situated in IO?

A

between condensing lens and observer, at or near to second principal focus of the condensing lens (approx 8cm in front of a +13D lens)

40
Q

At what distance does the observer view the image in IO?

A

40-50cm as holds condensing lens at arms length

41
Q

What lenses are incorporated into binocular indirect ophtalmoscopes eyepieces and why?

A

+2.0 D lenses so observer doesn’t need to accommodate

42
Q

How can the linear magnification be calculated from an indirect ophtalmoscope image?

A

linear magnification = ab/A (ab is image on retina, AB is size of image on subject’s eye)

also linear magnification = focal length of lens (in mm) / 15

43
Q

What is the linear magnification of a +13D lens?

A

f = 1/13 = 76mm

76/15 = approx. 5x

44
Q

Where is the image of the retina of an emmetropic eye always located in indirect ophthalmoscopy (regardless of position of lens relative to the eye)?

A

at the second principal focus of the condensing lens (because all rays emerging frmo an emmetropic eye are parallel)

45
Q

Where will rays from a hypermetropic patient vs myopic patient be in relation to the second principal focus of the condensing lens?

A

a) hypermetropic: outside second principal focus (closer to examiner, further from lens)
b) myopic: within second focal length of the lens (i.e. closer to lens, further from examiner)

46
Q

Where is the principal focus of the condensing lens, F1, in relation to the anterior focus of the patient eye, Fa, in emmetropia?

A

F1 is closer to the patient than Fa; the ray through F1 is parallel to the ray of light through Fa

47
Q

What happens to image size in myopia as the condensing lens is moved away from the patient’s eye?

A

the image size increases

48
Q

hat happens to image size in hypermetropia as the condensing lens is moved away from the patient’s eye?

A

the image size becomes smaller

49
Q

What is the portability of the indirect vs direct ophthlamoscope?

A

direct ophthalmoscope smaller and lighter, easier to transport

50
Q

Which type of ophthalmoscopy is more useful for patients with opacities in the ocular media and why?

A

indirect ophthalmoscopes - more scope for more power light source

51
Q

What is the instrument of choice for examining patients with retinal detachments and why?

A

indirect ophthalmoscope - combination of good illumination and wide field of view

52
Q

What may be missed in the case of extensive subretinal fluid with direct ophthalmoscopy?

A

uderlying malignancy

53
Q

What is the instrument of choice for retinal detachment surgery and why?

A

indirect ophthalmoscope - used at a distance to maintain sterile operative field

54
Q

Which can be used for retinal photocoagulation and how?

A

indirect - laser energy can be delivered through the direct ophthalmoscope

55
Q

How can annoying reflections be removed from the line of view in indirect ophthalmoscopy?

A

subtle tilting of the angle of the condensing lens

56
Q

What are 4 (3 positive and 1 negative) effects of examiner moving head towards the condensing lens in IO?

A
  • P: enhanced brightness
  • P: enhanced image size and detail
  • P: enhanced clarify
  • N: requires larger pupil
57
Q

How can the examiner overcome the patient’s small pupil in IO and why?

A

moving their (examiner’s) head away from the condensing lens - because smaller area of patient’s pupil requires illumination, leaving more available for image transmission

58
Q

What are the properties of the image from indirect ophthalmoscopy?

A

real, vertically and horizontally inverted

Duke Elder: Optics (Alice) (19 decks)

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