optics Flashcards
What is the distance measured using a A scan US
Axial length is traditionally measured using A-scan ultrasound
Measures distance from anterior cornea to internal limiting membrane
Types of A scan ?
- Applanation biometry
- Immersion A scan
How does he optical biometry measure axial length
Uses delay and intensity of infrared light reflected back from a diode to determine axial length
Measures distance from anterior cornea to retinal pigment epithelium
Advantage of optical biometry compared to US
Measures to fovea versus ultrasound measuring longest axis to posterior pole
In highly myopic/ staphyloma eyes, ultrasound overestimates axial length, underestimation of IOL power
More accurate in pseudophakic and silicone oil-filled eye
Disadvantages of optical biometry
Need clear path for infrared laser to travel to the fovea
Opacities can interfere with measurement
Cornea pathology
Dense/ hypermature, posterior subcapsular cataracts
Vitreous opacity
Maculopathy and retinal detachments
What biometry to use for axial length <22
Haigis, Hoffer Q
What biometry to use for Axial length between 22-26
SRK/T or Barret Universal II
What biometry to use for previous refractive surgery ?
Haigis L
What biometry to use for Axial length >26mm
Haigis, SRK/T
when should contact lens be removed prior to biometry
Soft lenses 1 week
Rigid gas permeable contact lens 2-4 weeks
Perimetric errors 4
Miotic pupil-pupil size is documented on HVF printout
Cloverleaf defect-pattern of visual field does not correlate to any anatomical defect and is a sign of poor visual attention/ malingering
Lens rim artifact-thick rim cause blockage of peripheral stimuli
Refractive errors
Pachymetry false positive:
The patient signals when no target is displayed
Pachymetry false negative
The patient fails to signal when a target brighter than the previously detected stimulus is displayed in the same spot
What does SITA stand for and it benefits
SITA: Swedish Interactive Threshold Algorithm
Shortens the time needed to complete the test
50% less time
SITA standard typically takes 7 minutes per eye.
Advantages over kinetic perimetry such as Goldman Visual Fields include
Standardized testing conditions
Less user dependent
Numerical data as output for statistical analysis
The HVF 24-2 evaluates the – with how many points?
The HVF 24-2 evaluates the central 24 degrees with 54 points.
The HVF 10-2 evaluates the – with how many points?
The central 10 degrees with 68 points
The HVF 30-2 evaluates the – with how many points?
evaluates the central 30 degrees with 76 points
What is the measure of raw light intensity used in automated perimetry?
Apostib
Fluroscein is excited and emited - at what colors and wavelengths?
Fluorescein is a water-soluble dye which helps visualize the choroidal and retinal vasculature. It is stimulated by blue light (490nm), and subsequently emits green light ( 530nm
what are the 6 phases of fluroscein ?
- Arm to retina
- Choroidal
- Arterial
- Arteriovenous
- Venous
- Tissues
How many after injection does the tissue phase occur?
5-10 after injection
How long des the arm to retina phase take ?
Fluorescein reaches retinal vessels from the arm to retina in 10-12 seconds.
when is a bscan useful?
Particularly useful to visualize the retina if there is an opacity anterior to the retina: Corneal scarring, cataracts, vitreous haemorrhage. Can be visualized through the eyelid.
Quadrants and nomenacluture of the eye when using US?
Four quadrants of the eye are typically denominated with the following nomenclature based on clock hours
T12 ( superior quadrant)
T9 ( lateral or nasal quadrant)
T3 ( nasal or lateral quadrant)
T6 ( inferior quadrant)
US and density of tissue ?
Dense - hyperechoic
Less Dense - hypoechoic
Relationship between gain and detecting signals ?
Higher gain allow weak signals - vitreous opacity, small foreign body, PVD
Lower gain allow high strong signal - masses and tumours
Common A scan error with misalignment
Probe not perpendicular to lens or macular, or aligned to optic nerve
Falsely short reading
Cornea compression as discussed above
Falsely long reading
Fluid meniscus between probe and cornea, posterior staphyloma
Incorrect velocity
Important to consider if the eye is phakic, aphakic, pseudophakic, or if there is silicone oil as this can result in changes the sound velocity. A correction factor should be applied.
A scan error when gain is too high
High gain increases sensitivity, but reduces resolution of spikes, causing retina and scleral spikes to merge together
A scan error for falsely short reading
Cornea compression as discussed above
Falsely long reading
Fluid meniscus between probe and cornea, posterior staphyloma
Incorrect velocity
A scan falsely long reading ?
Fluid meniscus between probe and cornea, posterior staphyloma
A scan incorrect velocity
Important to consider if the eye is phakic, aphakic, pseudophakic, or if there is silicone oil as this can result in changes the sound velocity. A correction factor should be applied.
Readout spikes of the A scan biometry
Initial spike ( probe tip and cornea)
Anterior lens capsule
Posterior lens capsule
Retina
Sclera
Orbital fat
Features of keratoconus progression
Kmax change =≥1D
Topographical astigmatism change =≥1D
Corneal thickness change >30um
Remember keratoconus can present in nearly all pattern
Features of early keratoconus
1) Kmax => 47D
2) Kmax difference between both eyes is > 2D
Red flags for ectasia :
Superior and inferior thickness difference greater than 30 um.
A difference in thinnest value between both eyes of more than 30um.
- A difference between the apex thickness and thinnest location of more than 10um.
Posterior elevation map > 15um, or Anterior elevation > 12um
Corneal thickness maps important for 3 things:
1) Important for ectasias i.e. keratoconus and PMD.
2) Useful for Fuch’s endothelial dystrophy
3) Helps determine the need for cross-linking
What is the k max ?
This is the maximal K reading of the anterior corneal surface. Should be measured using the tangential map. Its location tells you the apex of the cone.
What is the supposed K max?
47.2 D
A k max value that is a risk of keratocnus ?
> 50D
What shape of the corneal surface is at increased risk of keratoconus?
Hyper-prolate <-1
Vertical bow tie
Astigmatism with the rule - vertical meridian is steeper
Horizontal bow tie
Astigmatism against the rule - horizontal meridian is steeper
What is the progression index ?
Progression index: assess CHANGE in corneal thickness of the whole cornea.
Useful for assessing ectatic disease
> 1.1 suggests ectatic disease
What is the q val?
Q-val: Tells you about corneal shape (normally based on 6mm zone)
Normal value between -1 and 0
Positive > 0 = Oblate cornea
Negative < -1 = Prolate (keratoconus)
Lipofuscin is primarily derived from what structure?
Outer photoreceptor segment
Confocal microscopy and function
Principle of confocal-single point of tissue illuminated by a point source of light; while simultaneously imaged by a camera in the same plane.
Uses:
Identify organisms causing infectious keratitis such as Acanthamoeba, fungus, microspores, herpetic eye disease
Evaluate cornea nerve morphology
Evaluate cornea endothelial layer
Differentiate corneal dystrophy
Each small square on a Hess Chart subtends to a degree of?
5 degrees at 50cm working distance
IOP is derived from the amount of force require to flatten an area of how many diameter on the cornea?
3.06mm diameter
Instrument used to preform ERG
Ganzfeld bowl which illuminates the whole retina with a full-field luminance stimulus, based on the ISCEV standard.
what inferometry
A light beam is split into two beams. One beam is sent and reflected off the ocular structures. The other is reflected off a reference mirror
Which of the following method is best used to evaluate paracentral defects?
Humphrey 10-2
Mechanism of OCTA
Multiple OCT B-scans are taken at the same point in the retina. Based on en-face OCT technique, reconstruct scans performed multiple times in a vertical plane into a single image
differences between the B scans generate movement related contrast
particularly contrast related to erythrocyte/ red blood cell movement
Loss of which cells from which layer will occur from retinal laser treatment?
Retinal pigment epithelium
Loss of which cells from which layer will occur from retinal laser treatment?
Green
Where is the inner retinal capillary plexus typically found?
NFL and ganglion cell layer
Where is the inner retinal capillary plexus typically found?
Pattern standard deviation
What is a normal field of vision i degrees ?
A normal visual field is an island of vision measuring 90 degrees temporally to central Fixation, 50 degrees superiorly and nasally, and 60 degrees inferiorly
The IS/OS junction represents the junction of the inner and outer segments of the photoreceptors. Why is it more pronounced near the fovea compared to the more periphereal macula?
The cones have a longer outer segment
The 30-2 visual field static perimery how many degrees are the central points away from fixation?
3
the 30-2 visual field static perimery how many degrees are the central points away from fixation?
focal cone problem
What is a significant axial length difference between both eyes, where you should consider repeating biometry?
If axial length difference > 0.3mm between eyes
What is the compression of the cornea in biometry
Compression of the cornea s between t0.2 4mm to 0.27mm
How long do cotton wool spots last ?
4-12 weeks