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Corneal topography final
- 1. CORNEAL TOPOGRAPHY PRESENTED BY MAJANJANI KUMAR RESIDENT (OPHTH) MODERATOR LT COL MANEESH JHA CL SPL (OPHTH)
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- 6. SHAPE OF THENORMAL CORNEA Cornea is not a perfect sphere ,assumed to have a conic section. X 2 + Y 2 + (1 + Q)Z 2 – 2RZ = 0 Z axis - axis of revolution of the conic R is the radius at the corneal apex Q -asphericity, used to specify type of conicoid.
- 7. Perfect sphere Q=0 Ellipsoid (Oblate surface), Q> 0 Ellipsoid (Prolate surface), -1<Q<0 X 2 + Y 2 + (1 + Q)Z 2 – 2RZ = 0
- 8. Anterior corneal configurationtends to be prolate, i.e., the cornea progressively flattens out towards periphery by 2–4 diopters . The asphericity of the normal cornea, depending on different studies, ranges from –0.26 to –0.11. This tendency to flatten towards periphery can be detected in the topographic map. Toward the periphery, dioptric power appears to decline, and the nasal area flattens more than the temporal area . The topographic patterns of the two corneas of the same individual often show mirror-image symmetry. SHAPE OF THE NORMAL CORNEA
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- 11. Corneal topography ina normal right eye. There is a flattening towards the periphery, more pronounced at the nasal area
- 12. Corneal profile inprincipal meridians
- 13. Corneal Topographic Patterns: •Depending on corneal curvature • Rabinowitz et al in 1996 described 10 different patterns: • REGULAR PATTERNS : – Round – Oval – Steepening : Superior or Inferior • ASTIGMATIC PATTERNS: – Symmetrical & Orthogonal : (Bow-Tie Effect) • With or without skewed axis – Asymmetrical & Orthogonal: • With superior steepening • With inferior steepening • Bow-tie with skewed radial axis – Irregular : no pattern and non-orthogonal
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- 15. FUNDAMENTALS AND TECHNOLOGICALAPPROACHES TO CORNEAL TOPOGRAPHY PLACIDO DISC SYSTEM- Based on specular reflection technoque
- 17. Placido Disc: theOriginal Corneal Topographer Placido Disc: observer views the pattern of concentric white rings (mires) reflected from the patient’s cornea through a central +2 D lens. Very “qualitative”
- 18. Images formed byPlacido Disc • Based on the overlay of concentric mires on the cornea. – The closer the mires, the steeper the axis. – The wider the rings, the flatter the axis.
- 20. Nidek Sun PhotokeratoscopePKS-1000
- 21. Limitations of PlacidoDisc System • It misses data on the central cornea • It is only able to acquire limited data points • It measures slope not height • It is difficult to focus and align • In most topographers, the patient is exposed to high light
- 22. • Computerized VideoKeratoscopy •Capturing the keratoscopic details onto a video and displaying data analysed with mutiple algorithms • Measures larger area with more points • Produce permanent reproducible records • One of the most important developments in diagnostic instrumentation
- 23. The Real Need– Analysing each & every point over cornea Video -keratoscopy
- 24. Types of ComputerizedTopographers
- 25. Key Points •Avoid alleye drops, particularly local anaesthetics as they decrease TBUT • Explain the patient & make comfortable • Ask patient to blink normally • Other contact procedures on cornea (tonometry, A-scan) should be done after topography
- 26. Self-illuminated bowl ofa topographer with Placido rings on
- 27. READING OF TOPOGRAPHICALDATA • Check the name of the patient, date of exam and examined eye. • Type of measurement (height in microns, curvature in mm, power in D) • Check the scale & step interval • Study the map (type of map, form of abnormalities) • Evaluate statistical information • Compare with topography of the other eye • Compare with the previous maps
- 29. HOW TO INTERPRETA CORNEAL TOPOGRAPHY MAP Photokeratoscope raw image
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- 32. • Most widelyused • Most useful • Quick interpretation possible • User-friendly COLOUR-CODED TOPOGRAPHIC MAPS Louisiana State University Color-Coded Map 1987 by Stephen Klyce
- 33. Interpretation of acolour map: 1. Colour Codes: – Hot colours: red-orange – steep portions – Cool colours: blue-purple – flat portions 1. The Scale used: – Absolute Scale: routine practice / screening • 35-50D : each color = 1.5D interval • <35D or >50D : each color > 5D interval – Normalized Scale: more minute details • 11 equal colours spanning ‘that’ eyes’ dioptric power
- 34. Normalized scale (variablescale) Uses a given colour for different curvatures or elevations on each cornea analyzed, depending on the range for that particular cornea, determined by its flattest and steepest values. Difficult to interpret and can lead to an incorrect diagnosis Magnify subtle changes in corneal surface if the scale is too narrow, or minimize large distortions if the scale is too wide. Color recognition is lost with a variable scale, since it uses different colors for different eyes.
- 35. ii. Absolute scale(fixed scale) uses the same color for the same curvature or elevation no matter which eye is examined. However, there are many different absolute scales since the examiner can choose different variables such as range or step size (intervals in color changes)
- 36. Topographic Displays: CornealMaps Axial Map (Sagittal Map) Most commonly used map, good approximation for the paracentral cornea Measures the radius of curvature for a comparable sphere (with the same tangent as the point in question) with a center of rotation on the axis of the videokeratoscope. Localized changes in curvature and peripheral data are poorly represented
- 37. Displays tangential radiusof curvature or tangential power, which is calculated by referring to the neighboring points and not to the axis of the videokeratoscope . Reflect local changes and peripheral data better than axial maps. Very useful in detecting local irregularities, corneal ectactic diseases, or surgically induced changes. Local Tangential Curvature Map (Instantaneous Map)
- 38. A) Tangential curvature(left); (B) Sagittal curvature (right)
- 39. Refractive Map displays therefractive power of the cornea, which is calculated based on Snell’s law of refraction, assuming optical infinity . Correlates corneal shape to vision, and is useful in understanding the effects of surgery.
- 40. Corneal topography aftermyopic LASIK
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- 42. Distortion of thePlacido rings because of tear film breakup Topographic irregularities and patches of the map without analysis because of a tear film with large instability
- 43. Loss of informationof certain areas of the cornea due to eyelids not opened enough (A), and due to nose (B)
- 44. Tear film abnormalitiescausing pseudolesions in a keratograph
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- 46. CLINICAL USES OFCORNEAL TOPOGRAPHY Keratoconus 1. An area of increased corneal power surrounded by concentric areas of decreasing power. 2. A inferior-superior power asymmetry. 3. A skewing of the steepest radial axes above and below the horizontal meridian.
- 47. PELLUCID MARGINAL DEGENERATION Inferiorcornealthinning between 4 and 8 o’clock positions above a narrow band of clear thinned corneal stroma. The ectasia is extremely peripheral and it presents a crescent-shaped morphology. This pattern has a classical “butterfly” appearance that results in a flattening of the vertical meridian and a marked against-the-rule irregular astigmatism
- 49. Keratoglobus Rare bilateral disorder Entirecornea is thinned out most markedly near the corneal limbus Reliable topographic examinations show an arc of peripheral increase in corneal power (steepening) and a very asymmetrical bow-tie configuration
- 51. Terrien´s marginal degeneration Flatteningover the areas of peripheral thinning. When thinning is restricted to the superior and/or inferior areas of the peripheral cornea, there is a relative steepening of the corneal surface approximately 90 degrees away from the midpoint of the thinned area. High against-the-rule or oblique astigmatism is a common feature, as this disorder involves more frequently the superior and/or inferior peripheral cornea. If the area of thinning is small or if the disorder extends around the entire circumference of the cornea, central cornea may remain relatively spared with a spherical configuration
- 52. Patient with Terrienmarginal degeneration displaying superior stromal thinning with intact epithelium, overlying pannus, and lipid at the leading edge.
- 54. Pterygium Triangular encroachment ofthe conjunctiva onto the cornea usually near the medial canthus When the lesion continues to grow out onto the cornea, it could lead to a high degree of astigmatism. When the growth of pterygium is about 2 mm or more, a flattening of the cornea at the axis of the lesion occurs . This produces a marked with the-rule astigmatism, even of more than 4 D. The evolution of the pathology and the surgical outcome could be monitored by changes in corneal topography.
- 55. Photorefractive keratotomy (PRK) Topographic patternafter a myopic ablation Pattern of decentered myopic ablation after PRK
- 56. Topographic pattern aftera hyperopic ablation Central island after myopic photoablation
- 57. Topographic pattern afterpenetrating keratoplasty Superior corneal steepening caused by a tight suture
- 58. Topographic patterns ofLASIK decenterd ablations after myopic treatment after hyperopic treatment
- 59. Topographic analysis ina post-LASIK cornea with an epithelial in-growth at the inferonasal area: Placido rings image axial map
- 60. CONTACT LENS INDUCEDCORNEAL WARPAGE Characterised by topograhic changes in cornea following contact lens wear as a result of mechanical pressure exerted by lens Usually 4 different form which occur alone or with one another i.Peripheral steepening ii. Central flattening iii. Furrow depression iv. Central moulding
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- 62. Other Uses ofCorneal Topography 1. To guide removal of tight sutures after corneal surgery (keratoplasty, cataract surgery, etc.) that are causing steepening of the cornea. 2. To help in the designing the astigmatic keratotomy. 3. To guide contact lens fitting: election of the probe lens and design of the lens. 4. To calculate the keratometry values for the calculation of the required power of an intraocular lens for implantation. 5. To evaluate the effect and evolution of a keratorefractive procedure.
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