Manual Keratometry PDF

Manual Keratometry Ariette Acevedo, O.D. PPO 2 Keratometry Evaluates the curvature, power and toricity of the cornea Assess the integrity of the cornea and tear surface Helps speed up retinoscopy and subjective refraction Provides axis and power of astigmatism Helps with keratoconus diagnosis by pro...

Manual Keratometry Ariette Acevedo, O.D. PPO 2 Keratometry Evaluates the curvature, power and toricity of the cornea Assess the integrity of the cornea and tear surface Helps speed up retinoscopy and subjective refraction Provides axis and power of astigmatism Helps with keratoconus diagnosis by providing obvious signs Mire distortion, irregular form of mires Provides proper assessment for CL evaluations Astigmatism Anterior Corneal Astigmatism: measured by the keratometer Refractive (total) Astigmatism: measured by retinoscopy and/or subjective refraction Corneal and refractive astigmatism may not coincide due to: Physiologic lenticular astigmatism Usually ATR and varies with age Effectivity changes: ~ 25% increase in astigmatism going from the corneal plane to the spectacle plane Corneal posterior surface curvature Basic Components of a Keratometer Adjustable eyepiece Adjustable chin rest and forehead support Knob to raise and lower the instrument and align the patient's eye Two power wheels to measure corneal power in each principal meridian Axis scale of the 2 meridians Mires (target) that are reflected in the cornea Focus knob or joystick to focus mires on patient’s cornea Keratometer The design of the keratometer provides a measurement of the size of the reflected image and leads to the determination of the radius of curvature of the anterior corneal surface. Keratometer Taking the refractive index of the cornea into account, the radius of curvature is converted to the dioptric power of the cornea, which is the number indicated on the side of the dial. Keratometers can measure corneal curvature between 36.00D and 52.00D Auxiliary lenses have to be added to measure anything above 52.00D or below 36.00D Keratoconus, Cornea plana Keratometry Limitations Only measures a small region of the cornea 3-4mm zone Assumes the cornea is symmetric with a major and minor axis separated by 90 degrees Ignores spherical aberrations Is susceptible to focusing and misalignment errors Mire distortion prevents accurate measurement of irregular corneas Set Up Disinfect the instrument Chin rest and forehead rest Focus eyepiece Reticule in focus Incorrect focusing can produce significant error Adjust the patient Turn on and unlock Align the barrel with the patient’s eye (lateral canthus with mark) until you see the reflection of the mires in the patient's cornea. Procedure Ask the patient to look at their own eye If you cannot find the patient’s eye you can use a penlight to help guide you Procedure Focus the mires and adjust the instrument Focus the crosshair in the center of the lower hand circle Procedure Using the focusing knob, bring the bottom circle into focus as a single image. Procedure Locate the principal meridians Make the “plus” signs parallel by rotating the instrument and then overlap them with the horizontal meridian wheel. Align the “minus” signs with the vertical meridian wheel. Keep one hand on the focusing knob Adjust and re-center as necessary during the procedure Procedure Observe corneal integrity by observing the conditions of the mires Patients with dry eyes will need to blink or use artificial tears before the procedure After evaluating OD proceed with OS If irregular astigmatism is present the 2 principal meridians will not be 90 degrees apart. In this case first do the horizontal meridian, readjust the barrel and then do the vertical https://youtu.be/IFRJw1xeVJI Recording Record each eye separately Flatter K’s/ Steeper K’s 44.12@180/45.25@090 Always record to 2 decimal points Also include the amount of corneal astigmatism in diopters with the axis CC: -1.13x180 Record the condition of the mires: Clear or distorted Can also include if astigmatism is ATR, WTR, OBL or irregular Astigmatism Regular Astigmatism: principal meridians are perpendicular (90 degrees away) With-the-rule (WTR): more power (greatest curvature) in the vertical meridian and horizontal meridian is flatter. Ex: 43.25@180/45.00@090 Against-the-rule (ATR): more power (greatest curvature) in the horizontal meridian and vertical meridian is flatter Ex: 42.50@115/44.87@025 Oblique: principal meridians lie between 30-60 degrees and 120-150 degrees Irregular Astigmatism: principal meridians are not perpendicular to each other. Produce distorted mires Troubleshooting Unable to locate keratometric mires: instrument and/or patient are not aligned properly. Mire clarity is transient: measure quickly after allowing the patient to blink. Mire focus is transient: ensure the patient’s forehead is secure against the headrest. Patient gaze is unsteady: ensure that fellow eye is occluded. H&V mires cannot be measured concurrently: patient may have irregular astigmatism. Only 1 plus sign is visible: patient’s eyelid is drooping, have them open wide. Only 1 minus sign is visible: the occluder is in the way. Extended Ranges If the power in a meridian falls outside the power range of the keratometer, lenses can be used to extend the range. Tape or hold loose lenses over the opening of the keratometer barrel and perform the measurement as described before. +1.25D loose lens: to increase the diopter range -1.00D loose lens: to decrease the diopter range Then multiply the K reading by the corresponding correction factor. Increasing the Range 9 Correction Factor 6 +1.25D will increase range by 9.00D +2.25D -1.00D will decrease range by 6.00D 1.166 1.3123 0.8576 For K’s above 52.00D For K’s above 52.00D For K’s below 36.00D Increasing Range Either you add +9.00D with the +1.25D lens to the K reading or -6.00D with the -1.00D lens to the K reading. OR Multiply the K reading by the correction factor Increasing Range Examples K’s are over 52.00D Add a +1.25D lens in front of the barrel New K drum reading is 49.00D True K value: 49.00 x 1.166 = 57.13D or 49 + 9= 58.00D K’s are less than 36.00D Add a -1.00D lens in front of the barrel New K drum reading is 38.00D True K value: 38.00 x 0.8576= 32.59D or 38 – 6= 32.00D Expected The 2 principal meridians are expected to be 90 degrees apart If not, it is irregular astigmatism Average K readings are between 43.00 to 44.00 Reminders for recording: Eye: reading flat@meridian/steep@meridian; quality of mires; corneal cylinder (cc) with axis Diopters: 0.125 steps (each line in the dial) Axis of the corneal astigmatism is from the flattest meridian Recording Examples Mires can be recorded as: Clear And Regular (MCAR), irregular and distorted, or egg shaped. OD: 42.50@180/43.00@090 MCAR CC -0.50x180 OS: 41.00@170/43.00@010 MCAR CC -2.00x170 OD: 42.50@180/43.50@090 MCAR, 1.00D WTR OS: 41.37@180/47.37@090 distorted mires, 6.00 AR OD: 43.13@060/44.00@150 distorted; -0.87x060 OS: 47.37@180/41.37@090 egg shaped mires; -6.00x180 Javal’s Rule It is an empirical determination of the relationship between corneal astigmatism and refractive astigmatism. Total refractive astigmatism = 1.25 (CC) ± 0.50 axis 090 Add or subtract 0.50 cyl according to the axis If CC axis is ATR astigmatism will be higher = subtract If CC axis is WTR astigmatism will be lower = add Javal’s Examples Total refractive astigmatism = 1.25(CC) ± 0.50 axis 090 Ex 1: CC= -1.00 axis 180 1.25(-1.00x180) + 0.50axis 090 = -0.75x180 Ex 2: CC = -1.00 axis 090 I 1.25 (-1.00 x 090)-0.50 axis 090 = -1.75x090 go 8 Simplified Form of Javal’s Rule By Grosvenor et al Refractive astigmatism = (Keratometric astigmatism) –/+ 0.50x090 Add or subtract 0.50 cyl according to the axis If CC axis is ATR astigmatism will be higher = subtract If CC axis is WTR astigmatism will be lower = add -2.00x090 predicted will be -2.50 x 090 −2.00𝑥090 − 0.50𝑥090 = −2.50𝑥090 -1.00 x 090 predicted will be -1.50x 090 −1.00𝑥090 − 0.50𝑥090 = −1.50𝑥090 0 predicted will be -0.50 x 090 -2.00 x 180 predicted will be -1.50x180 −2.00𝑥180 + 0.50𝑥090 = −1.50𝑥180 -1.00x180 predicted will be -0.50 x 180 −1.00𝑥180 + 0.50𝑥090 = −0.50𝑥180 Practice #1: 43.00@180/46.00@090 What is the total refractive astigmatism using Javal’s rule? What is the total refractive astigmatism using the Simplified form? #2: 47.00@180/49.00@090 What is the total refractive astigmatism using Javal’s rule? What is the total refractive astigmatism using the Simplified form? #3: 47.00@090/49.00@180 What is the total refractive astigmatism using Javal’s rule? What is the total refractive astigmatism using the Simplified form? Practice #1: #4: #2: #5: #3: For all answer: Over 52.00D, placed a +1.25D lens over the keratometer New K’s: 46.00@180/47.00@090 Less than 36.00D, placed a -1.00D lens over the keratometer New K’s: 37.00@175/38.00@085 Over 52.00D, placed a +2.25D lens over the keratomter New K’s: 42.50@090/46.00@180 Over 52.00D, placed a +1.25D lens over the keratomter New K’s: 42.50@090/52.00@180 Less than 36.00D, placed a -1.00D lens over the keratometer New K’s: 38.75@010/40.25@100 What is the keratometry value and CC? What is the expected refractive astigmatism according to Javal’s Rule and Simplified Javal’s?