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

Chapter 14: Instruments C Flashcards

1
Q

What type of slit lamp illumination is the most generally useful for illuminating the eye?

A

direct focal illumination - slit beam accurately focused upon that part of the eye under inspection

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2
Q

What is diffuse illumination with the slit lamp?

A

beam of light thrown slightly out of focus across the structure being examined so that a large area is diffusely illuminated

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3
Q

What part of the eye is diffuse illumination with the slit lamp useful for?

A

anterior capsule of the lens

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4
Q

What does lateral illumination with the slit lamp involve?

A

illuminating the structure by light which is refleced from tissue just to one side of it e.g. directed at margin of pupil, illuminates outer rim of sphincter muscle

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5
Q

What is retroillumination with the slit lamp?

A

method of examining part of the eye by light reflected from a structure behind it - structure behind is used as a mirror

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6
Q

What is a key example of a use of retroillumination with the slit lamp?

A

areas of iris atrophy identified by light which is reflected from the choroid

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7
Q

Where should the illuminating column of the slit lamp be moved to to perform retroillumination?

A

should be brought to lie between the objective lenses of the microscope so the illuminating and viewing systems are co-axial

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8
Q

What is specular reflection with a slit lamp?

A

examining rays of light reflected from a mirror-like surface (corneal surfaces and anterior lens capsule)

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9
Q

Where is the patient’s gaze directed in specular reflection with the slit lamp?

A

to bisect the angle between axis of illumination and that of the microscope

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10
Q

What 2 structures can be examined with specular reflection?

A
  1. corneal surfaces including corneal endothelium
  2. anterior lens capsule
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11
Q

What is sclerotic scatter?

A

the slit beam and the binoculars are dissociated so that the limbal sclera is illuminated, while the binoculars are centered on the cornea. the whole limbus glows and the light from the beam is reflected back and forth between the two internal limiting surfaces of the cornea and is scattered centrifugally all around the cornea

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12
Q

If the slit beam is directed to the limbus at 9 o clock to achieve sclerotic scatter, where will the maximum glow be?

A

3 o’clock

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13
Q

What are 2 types of slit lamp filters which can be used?

A
  1. blue cobalt filter
  2. blue and green (red-free) filters
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14
Q

What is the blue cobalt filter used for with the slit lamp?

A

applanation tonometry

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15
Q

What are th eblue and green (red free) filters useful for with the slit lamp? 2 reasons

A
  1. examination of the vitreous
  2. to create dark fundus background to easier detect structures in the vitreous e.g. vitreous cortex in PVD
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16
Q

Why are blue and green (red-free) filters useful for examining the vitreous?

A

scattering of light is greatest when the incident light is of short (blue) wavelength - visibility of vitreous depends on light being scattered

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17
Q

What is the furthest back in the eye that can be seen with the slit lamp on its own?

A

anterior third of the vitreous

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18
Q

Why is it impossible to see further than the anterior third of the vitreous with the slit lamp alone?

A

refractive power or cornea and lens renders light emerging from deeper points of the eye parallel so no image formed within focal range of hte slit lamp microscope

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19
Q

How can the fundus be viewed with a slit lamp?

A

using additional lens to overcome the refractive power of the eye; improved if light reflected from cornea doesn’t enter slit lamp (no overlap at the cornea of illuminating and viewing systems)

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20
Q

What can be done in order to avoid reflection of light from cornea entering slit lamp microscope when performing fundal examination?

A

if there is no overlap at cornea of illuminating and viewing systems - tilt illumination column so axis is below that of viewing system

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21
Q

What are 4 devices which can be used to visualise the fundus with the slit lamp microscope?

A
  1. Hruby lens
  2. Fundus-viewing contact lens
  3. 90D and 78D lenses
  4. Panfundoscope contact lens
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22
Q

What is the Hruby lens used for?

A

examination of fundus and posterior vitreous with slit lamp

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23
Q

What is the Hruby lens used for?

A

examination of fundus and posterior vitreous with slit lamp

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24
Q

What is the Hruby lens and how does it work?

A
  • powerful plano-concave lens, -58.6 D
  • placed with concave surface towards eye
  • held immediately in front of eye, forms virtual, erect and diminished image of illuminated retina
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25
Q

What 2 qualities are met to give the best image of the fundus and posterior vitreous with the Hruby lens?

A
  1. lens held near eye
  2. retinal image found in pupillary plane
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26
Q

What can be examined with the fundus viewing contact lens?

A

posterior vitreous and posterior pole of fundus

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27
Q

What type of lens is the fundus viewing contact lens?

A

plano-concave with higher refractive index than eye

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28
Q

What 2 things may be used as a fundus-viewing contact lens?

A
  1. central zone of gonioscopy lens
  2. central zone of three-mirror contact lens
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29
Q

How do 90D and 78D lenses work to view the fundus with the slit lamp?

A

work like indirect ophthalmoscope - high power condensing lenses which shorten light path and bring retinal image within focal range of slit lamp

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30
Q

What compensates for the loss of image size with high power condensing lenses (90D and 78D) used with the slit lamp?

A

magnification of slit lamp miroscope

31
Q

What is a benefit of the panfundoscope contact lens used with slit lamp?

A

wider field of view to view fundus

32
Q

How does the panfundoscope contact lens work to view the fundus with the slit lamp?

A

high convex power contact lens, acts as condensing lens and forms image within spherical glass element within panfundoscope; this flattens the image and redirects light diverging from it towards the observer.

33
Q

What are the properties of the image formed by the panfundoscope?

A

real, inverted

34
Q

What type of lens may the panfundoscope lens be based on the principles of?

A

Rodenstock lens - condensing lens applied to eye as contact lens and transmitted light collected and redirected towards observer

35
Q

How does the applanation tonometer work?

A

applied to cornea with sufficient force to produce standard area of contact; force required is directly proportional to IOP when area of contact is approx 3mm in diameter
when area is 3.06mm in diameter, rigidity and surface tension cancel out and force of application is is proportional to intraocular pressure

36
Q

Is there a change in IOP when area of contact with the applanation tonometer is 3.06mm?

A

no - ocular volume hange is very small, doesn’t significantly alter IOP

37
Q

How is the standard area of corneal contact achieved in applanation tonometry?

A

two prisms are mounted with their bases in opposite directions; when operator sees two half circles edges just overlap each other, area of contact is approx 3.06mm diameter

38
Q

What can cause error, and how much, with applanation tonometry?

A

high degree of corneal astigmatism (area of contact elliptical, not circular); error of 1mmHg per 4 dioptres of astigmatism

39
Q

How can the correct area of contact be achieved with applanation tonometry in corneal astigmatism?

A

if measurement is made at 43 degrees to the meridian of lower corneal power (43 degrees to axis of minus cylinder) (marked with red line on some tonometers)

40
Q

How does the non-contact tonometer work?

A
  • cornea flattened with puff of air
  • paralell beam of light directed on central cornea at angle of 30 degrees, reflected light measured by photodetector at angle of reflection of 30 degrees - strongest reflected beam when cornea is flat
  • force of air required to flatten cornea measured
41
Q

What are 2 fundamental ways in which pachymetry can work?

A
  • optical
  • ultrasound
42
Q

What principles do optical pachymeters use?

A

Purkinje-Sanson images formed by anterior and posterior surfaces of the cornea (images I and II) to measure thickness, and those at the posterior cornea and anterior lens (II and III) to measure depth of anterior chamber

43
Q

What instrument are optical pachymeters used in conjunction with and what is done to the observer’s image?

A

slit lamp - field of view doubuled

44
Q

How is image doubling achieved in optical pacyhmetry?

A

either splitting incident beam of light or splitting observer’s view

45
Q

What are 3 types of pachymeters which are used?

A
  1. Jager pachymeter
  2. Maurice and Giardine pachymeter
  3. Ultrasound pachymetry
46
Q

How does the Jaeger pachymeter work?

A
  • axis of illumination perpendicular to cornea, axis of observation 40 degrees to this
  • image split horizontally through 2 glass plates. upper tilted to displace upper half of image.
  • observer aligns anterior surface of cornea in one image with posterior surface in second image
  • degree of tilt required is proportion to corneal thickness
47
Q

How can Jaeger pachymetry be used to measure the depth of the anterior chamber?

A

posterior surface of cornea of one image is aligned with anterior surface of lens of second image

48
Q

What are the 2 different jaeger pachymeters available?

A
  1. No 1: measures depths up to 1.2mm
  2. No 2: depths up to 6mm
49
Q

How does the Maurice and Giardine pachymeter work?

A
  • perspex plate covered by coloured celluloid with cut out area placed in slit lamp beam
  • some light passes through cut out zone, some laterally displaced by passage via perspex plate
  • images formed by 2 beams seen via slit lamp, plate rotated until superimposed
50
Q

How does ultrasound pachymetry work?

A

US probe applanates cornea gives only a reading when perpendicular to posterior surface

51
Q

What is the advantage of ultrasound pachymetry?

A

much more precise

52
Q

When is ultrasound pachymetry used?

A

invaluable for planning depth of corneal incisions in graft andrefractive surgery

53
Q

What is specular reflection?

A

reflection of light at different angles by structures with different refractive indices

54
Q

Why is specular microscopy viewed with only 1 eye?

A

angle between axes of illumination and observation is critical

55
Q

What can specular microscopy demonstrate?

A
  • any irregularity of a smooth reflecting surface occurring at the **boundary of structures **that have different **refractive indices **
  • e.g. when cell body and intercellular material of corneal endothelium are seen by specular microscopy, one appears dark and the other light
56
Q

How is specular microscopy performed?

A

reflection from the surface of the cornea is reduced by direct contact between the instrument and the cornea; effect is seen by focusing on the area of interest then carefully changing the angle of illumination

57
Q

What is a common practical application of specular microscopy?

A

assess health of a donor cornea

58
Q

How does ocular coherence tomography work?

A
  • Light from infrared source (843 nm) is split into a **reference beam **which is reflected off a mirror and a sample beam which is reflected off the retina.
  • Temporal differences between the two reflections result in an interference signal which is processed to produce a digital image
59
Q

What is OCT analogous to?

A

B scan ultrasound

60
Q

What are 3 basic principles used in automated refraction?

A
  1. Scheier principle
  2. Optometer principle
  3. Meridional refrctometry
61
Q

What is the Scheiner principle?

A
  • point at which eye is focused can be determined with double pinhole apertures
  • form single focus on retina if eye emmetropic but two if any refractive error
  • by adjusting the position of an object until one focus of light is seen by the patient, the far point of patient’s eye and refractive error can be determined
62
Q

What type of method of automated refraction does the Scheiner principle apply to?

A

zonal focus (refractive conditino determined by examining through small zones of optical aperture)

63
Q

What is the optometer principle?

A
  • convex lens placed in front of eye so focus is in spectacle plane
  • if movable target lies at first principal focus of the lens, light from it will be parallel at the specetacle plane and focused on retina in emmetropic eye
  • when target within focal length of lens, light will be divergent in spectacle plane, light outside focal length will be convergent in spectacle plane
  • vergence of light in plane of second principal focus of lens linearly related to distance of target from first principal focus of the lens
64
Q

What is meridional refractometry?

A
  • in astigmatism, axes of principal meridians must be found and refraction in both measured
  • if spherical refraction measured in at least 3 arbitrary meridians, position of principal axes and their refractive power can be found by mathematical calculation
  • more accurate with >3
65
Q

What is instrument accommodation?

A

inappropriate accommodation which often occurs when a target is viewed which is konwn to be within an instrument and therefore near the eye (major problem in optometer design)

66
Q

What are 3 principles which modern infrared optometers may work on?

A
  1. Scheiner principle
  2. simulating retinoscopy
  3. use of the otpometer principle
67
Q

How does an infrared optometer work?

A
  • patient’s eye refracted using invisibel infrared light to overcome instrument accommodation
  • separate fixation target provided, designed to encourage relaxation of accommodation
  • instruments calibrated due to difference in refraction of infrared vs visible light
68
Q

What are 2 reasons why infrared in infrared optometers has different refraction in eye from visible light?

A
  1. chromatic aberration
  2. IR light not reflected by same layers of retina as visible light
69
Q

What order of difference in refraction of IR and visible light may be present?

A

0.75 D to 1.50D more hypermetropic to IR

70
Q

What are 3 situations when infrared optometers do not perform well?

A
  1. small pupil
  2. distorted pupil e.g. broad iridectomy
  3. ocular media not clear
71
Q

What is photoscreening?

A
  • ancillary staff screen preverbal children to detect factors which may cause amblyopia
  • polaroid photograph taken -flash from polaroid camera and lens offset
  • child photographed from set distance in horizontal and vertical meridians
  • alteration in red reflex captured on film
  • image examined by trained personnel
72
Q

What is the Scheiner principle?

A
  • point at which eye is focused can be determined with double pinhole apertures
  • form single focus on retina if eye emmetropic but two if any refractive error
  • by adjusting the position of an object until one focus of light is seen by the patient, the far point of patient’s eye and refractive error can be determined
73
Q

What are 6 methods of illumination with the slit lamp?

A
  1. Direct focal illumination
  2. Diffuse illumination
  3. Lateral illumination
  4. Retroillumination
  5. Specular reflection
  6. Sclerotic scatter

Duke Elder: Optics (Alice) (19 decks)

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