Corneal Topography Real Flashcards
In what cases would you use corneal topography to measure peripheral cornea?
In complicated conditions for example post refractive surgery.
What is a limitation of keratometry?
It cannot be used to accurately measure corneal shape if the cornea does not have a constant radius or is not radially symmetrical (I.e. keratometry is a crap indication of corneal shape for those with astigmatism)
What does a keratometer do?
It measures the curvature of the cornea along a fixed chord length usually two to three millimetres long (which lies within the optical spherical zone of the cornea - I.e. we are only getting an indication of central corneal curvature)
What is the average corneal diameter?
11-12 millimetres
What is corneal topography also known as?
Keratoscopy
What is the difference between keratometry and keratoscopy?
Keratometry is the measurement of the anterior curvature of the cornea in 2 principle meridians of about 3mm. Keratoscopy is the study of light reflected from the anterior surface of the cornea (360 degrees) it measures about 8-10mm of the diameter of the cornea.
What are some limitations of corneal topography?
The whole cornea is not measured. RGPs will on average be 9.5mm whereas the keratoscopy measures 8-10 mm of the cornea. The average cornea is 11-12 mm in diameter. Further more in keratoscopy not all of the cornea can be measured ( the concept is based on the reflection of light but nasally we shall not see this occur due to the shadow of the nose upon the cornea) - this is usually approximated (and seen by black approximation dots) — furthermore other features such as eyelids/eyelashes may also cause shadows on the cornea.
What is the aim of corneal topography and how does it do this?
To accurately describe the shape of the corneal surface in all meridians. It will do this using a range of concentric circle targets (this is called a placido disk target) - this allows both central and peripheral curves to be calculated.
How has the output of keratoscopy changed?
Historically the machine would take photos of the reflections off the cornea and these were called photo-keratoscopy and measurements were calculated subsequently. Now the output is mapped on software so we can see the shape of the cornea diagrammatically on a computer - this is called Video Keratoscopy.
What have we learn’t from video keratoscopy?
The cornea is aspheric and that it can best be described as a flattening or prolate elongated eclipse who’s rate of flattening is asymmetrical around its centre. This rate if flattening is given the name P value (which may also be described as rate of eccentricity)
In keratoscopy what is a healthy P value?
Between 0.75 and 0.85
How Is a placido disk used as a qualitative assessment of corneal topography and what do possible results mean?
Essentially the disk is positioned in front of the cornea and the reflections are observed. The closer the rings are to eachother, the steeper you know the corneal curvature is. Whereas widely separated rings indicate a flatter cornea. The shape of the rings can indicate the presence of astigmatism or keratoconus.
What’s a disadvantage of using the placido disk as a qualitative assessment of corneal topography?
Only large/gross irregularities in the corneal surface are visible - hard to detect small changes such as minimal astigmatism. Only high astigmatism of more than three dioptres can be detected
How does a quantitative assessment taken place in corneal topography?
What is a disadvantage of the surface slope method?
Law of reflection applies - mire rings are reflected off the cornea and sent back into the camera. This is then analysed and converted into numbers.
The most frequent computational way of assessing the radius of curvature of the cornea is the slope of surface method. Slope is measured directly as a function of distance from a central reference axis - a curvature is derived from these results. (Important to note these distance based instruments are estimating average shape of the cornea since algorithms are based on radially symmetrical surfaces). Since algorithms are based on symmetrical surfaces - for steeper slopes we get an underestimate of corneal curvature and for flatter slopes we get an overestimate of corneal curvature. Algorithms are now based on radii to be slightly more accurate.
What type of astigmatism is being shown via this placeido disk?
With the rule astigmatism
(as then rings are steeper i.e closer together in the horizontal meridian)
How do you interpret a curvature map?
Bluer colours represent slower/flatter rates of change.
Red colours represent steeper/faster rates of change.
[Green is used as a reference colour]
What are the three scaling options used in topography maps?
[Note Auto-scaling can give a dramatic look because it can note steps of 0.25D by a specific colour]
What is the advantage of using axial curvature maps?
Each point’s value is the average curvature along a line from that point to the centre point. Because it’s an average from that value to the centre point these maps are less dramatic so look ‘smoother’ and less ‘distorted’.
i.e. the map is smoothed out so it it easier to spot any astigmatism or keratoconus.
What do meridional curvature maps show?
The show data value at every location (i.e. not the average of that point to the centre) thus they look more dramatic and can be harder to interpret (as with the axial map you have filtered out the ‘noise’).
What do elevation maps show?
These show the difference/variations in a Px’s cornea compared to a reference cornea (set by software).
What is the reference/alignment point in elevation maps?
The centre of the cornea
Describe how to determine an elevation map by colour.
Red areas show steepness i.e. where the cornea is higher than the reference shape (>sag).
Blue/Cooler colours show the declining or decreasing saggital height i.e. where the cornea is lower than the reference shape. (
What does the centre of a normal cornea look like on an elevation map?
Centrally the surfce rises above the reference shape - giving rise to the appearance of a central hill - i.e. a red circle shape in the map.
When do we use topography?
What does a ‘bow-tie’ pattern on a curvature map indicate?
The presence of a toric cornea.
What is being shown in the following curvature map?
Keratoconus
[In early stages it looks like an uneven bow tie - as the disease progresses it becomes more of a circle shape - this circle is called a cone]
[Where Keratoconus is present you will see a thinning of the cornea in that area]
What are Ortho-K lenses?
How do they work?
They are lenses that are slept with on and removed during the day.
They work by flattening the centre of the cornea while you sleep - this is done by redistributing corneal endothelium cells (towards the mid periphery).
The effect/degree to which this happens can be seen using topography.
What is an Oximap?
A map that shows oxygen transmissibility to the cornea through a lens.
Name at least two other measurements modern topographers can now measure?
Why would an eye surface profiler (Eaglet-Eye) be used and why?
For checking the fit of contact lenses bigger than 9mm e.g. scleral lenses (which can be up to 20mm in size).
[An eaglet eye profiller requires installation of fluorescein]
(Basically the chord of a corneal topographer is limited to 9mm - this means we can only check the central 9mm of the cornea which obviously isn’t sufficent for monitoring the impact of bigger lenses and or conditions that affect peripheral outer surface of the cornea).
True or False- You can use OCT for topography purposes.
True - You can map the anterior eye surface using OCT.
