Corneal shape and refractive power are the basic means to measure astigmatism. Each measurement may be utilized in incisional surgery (ie, cataract, astigmatic keratotomy, limbal relaxing incisions), refractive laser procedures, and postkeratoplasty for suture removal or IOL power calculations. Accurate measurement of the magnitude and orientation of astigmatism may improve visual outcomes and patient satisfaction through more effective treatment. Astigmatism can be measured using several methods described herein.
Corneal topography. This measurement provides qualitative and quantitative evaluations of the corneal curvature. Most topographers evaluate 8,000 to 10,000 specific points over the entire cornea and center the acquisition on the corneal apex. This method of measuring astigmatism identifies multiple steep and flat meridians at 3-, 5-, and 7-mm optical zones. Topographers that incorporate scanning slit photography also measure the power and the astigmatism of the posterior corneal surface, which may improve correlation with the refractive astigmatism.1
Keratometry. In contrast to topography measurements, manual keratometry only has four data points within 3 mm to 4 mm of the central anterior surface of the cornea. The reading does not provide data from the central or peripheral cornea, and therefore conditions such as keratoconus or pellucid marginal degeneration may not be detectable. In cases of irregular astigmatism, manual keratometry measurements may be quite difficult due to the distorted appearance of the mires that do not allow for accurate superimposition. In such cases, the contours of topography are more meaningful than the mild variations shown between the orthogonal steep and flat meridia in keratometry. Naturally occurring lenticular astigmatism, astigmatism of the corneal posterior surface, or tilted IOL is not taken into account. Despite this, keratometers are more readily available in most consulting suites and provide a quick and reliable means of gauging corneal astigmatism with an experienced observer. Automated keratometers can be a useful screening device, although not as sensitive with low magnitudes of astigmatism for accuracy of axis.
Manifest refraction. Subjectively measuring astigmatism that extends from the anterior cornea to the perceptual levels at the visual cortex, manifest refraction considers the total amount of astigmatism the patient accepts or rejects, despite what has been measured objectively on the cornea or by wavefront aberrometry. Refraction can identify only one steep and one flat refractive meridian orthogonal to each other (ie, regular astigmatism).
Wavefront refraction. These measurements are done by centering the mires on the middle of the pupil. It measures the aberrations within the optical system and does not deal with the nonoptical component of refractive astigmatism. The second-order astigmatism magnitude and axis must be within a certain range of that measured by manifest refraction to be acceptable for use as treatment. Advising patients not to read in the waiting area and leaving them in a dark room for approximately 10 minutes before the aberrometry measurement may improve the accuracy of a wavefront refraction. Obtaining at least three captures of each eye is recommended for consistency.
Furthermore, it is a very useful exercise to crosscheck the manifest refraction obtained with the wavefront refraction, particularly for the cylinder magnitude and axis. If they differ, repeat manifest refraction may be in order. Using the same instruments pre- and postoperative and similar lighting conditions, where possible, adds to the accuracy of any postsurgical outcome analyses.
Treating astigmatism would be considerably simpler if refractive and corneal astigmatism always coincided in magnitude and axis. Variance between manifest or wavefront refraction and keratometry or topography is widely prevalent, however, and an inevitable amount of astigmatism remains in the eye after treatment.
Refractive laser surgery relies on manifest refraction. In-corporating corneal and refractive parameters into this treatment plan using vector planning may potentially improve visual outcomes,2,3 by reducing the remaining corneal astigmatism. During cataract surgery, preferential reliance is on keratometry or topography. Subjective refraction information is inaccurate because of the cataract and its subsequent removal. Incisional surgery including astigmatic keratotomy (AK) or limbal relaxing incisions (LRIs) to correct pre- or postoperative astigmatism may involve keratometry, topography, refraction or a combination of corneal and refractive parameters using vector planning. Postkeratoplasty, for suture removal or laser surgery retreatment, can involve keratometry, refraction, and topography parameters.
To quantify the discrepancy between corneal and refractive astigmatism measurements, calculate the vectorial difference between the refractive cylinder—measured by wavefront or manifest refraction—and the corneal astigmatism measured by topography or keratometry. This vectorial difference is known as the ocular residual astigmatism (ORA), and it is expressed in diopters.2,3 The greater its magnitude, the (1) greater the astigmatic difference between the refractive and corneal astigmatism in magnitude and/or axis and (2) more remaining postoperative astigmatism. Eyes with irregular astigmatism (ie, keratoconus, keratoglobus, pellucid marginal degeneration) have, in general, a poorer correlation between corneal and refractive values. Hence, they have a higher ORA than a normal astigmatic eye. Studies have shown that ORA in healthy astigmatic eyes is between 0.73 D and 0.81 D.3,4 In keratoconic eyes, it is calculated at 1.34 D. This was derived from a series of 45 eyes with myopic astigmatism treated with photoastigmatic refractive keratectomy (PARK) in an ongoing study at Newvision Clinics, in Melborne, Australia. The ORA has also been referred to as intraocular, lenticular and noncorneal astigmatism.
Figure 1a displays (1) refractive astigmatism measured by manifest or wavefront refraction and (2) simulated keratometry measured by topography. To calculate the difference between these two parameters, the axes are doubled and the magnitudes remain unchanged (Figure 1b). The ORA, which has a direction from the simulated keratometric reading (SimK) to the refractive astigmatism, is then calculated using basic trigonometric principles, and the ORA vector is transferred to the origin (x=0, y=0) and halved to simulate how it would exist within the eye (Figure 1c).
Topographic disparity (TD) is a precise vectorial measure of corneal irregularity. The greater the topographic disparity, the greater the ORA.4 The TD quantifies both the nonorthogonal and asymmetrical component of corneal irregularity as a single number with an axis. It is a precise and convenient way of assessing the variable of irregularity. The TD is calculated as the dioptric distance between the displays of superior and inferior topographical values on a 720º double-angle vector diagram.
ASTIGMATIC TREATMENTs, CORRECTION
Refractive laser surgery. Treatment of astigmatism using excimer laser surgery should incorporate corneal and refractive parameters to leave the minimum amount of astigmatism in the eye and reduce the amount of astigmatism remaining on the cornea. This has the potential for improving BCVA and reducing higher-order aberrations. With this astigmatism treatment, vector planning is used.
Figure 2 displays a wavefront refraction of -1.25 DS (Spherical Diopter)/-1.70 DC (Diopter of Cylinder) x 165 at the spectacle plane and a simulated keratometry of 42.87/45.62@82 from topography. The ORA has been calculated as 1.25 D x 1. The best that can be done for this patient is to leave 1.25 D entirely on the (1) cornea, (2) spectacle refraction, or (3) apportioned between the two in varying amounts ranging from 1% topography/99% refraction to 99% topography/1% refraction, as indicated by the emphasis bar. It is important to note that the corneal target is 90º away from the ORA axis to neutralize the vectorially calculated amount of astigmatism. In this example, the emphasis to correct the residual astigmatism (ORA) was set at 40% topography/60% refraction. Instead of treating -1.23 DS/-1.61 DC x 165 by using refractive parameters alone, the laser treatment (ie, -1.48 DS/-2.05 DC x 169) was derived from the refractive and corneal parameters.
The maximum correction of astigmatism is achieved with this method of treatment, and the corneal plus refractive astigmatism values are at the minimum, wherever the emphasis lies. The desirable result should achieve this optimized minimum amount of reduced corneal astigmatism.
Cataract surgery and refractive lens exchange. Using an incision to minimize postoperative astigmatism is effective in cataract surgery. Application of the incision at varying points around the limbus will control the amount of remaining astigmatism when the keratometry is accurate. Patients with previous corneal surgery require a more detailed topography measurement to accurately calculate the IOL power. The use of manifest refraction here is limited apart from gauging the BCVA, as it represents a combination of lenticular and corneal astigmatism as well as other components of the optical pathway and nonoptical psychophysical components of subjective astigmatism. If these measurements do not match up, then the first parameter to be removed from consideration is the refraction, followed by the steepest meridian (ie, incision site).
Incisional surgery. AK and LRIs are also effective at reducing preoperative astigmatism. In the postoperative cataract eye, it is important to know what astigmatic effect the major incision paracentesis had on the corneal astigmatism and to plan the corrective incisional surgery on the resultant shifted magnitude and axis. This is particularly important for surgeons who perform LRIs simultaneous to cataract surgery. Accurate postoperative astigmatism measurements will give reliable outcome analysis (ie, the amount of flattening and torque produced by the incisions5 with rotation and reduction of the existing astigmatism). The measurements may also be applied to future procedures for for further patient satisfaction. Taking into account keratometry, topography, and refraction when planning the treatment is invaluable in these cases.
Postkeratoplasty astigmatism. Keratometry, corneal topography, and manifest refraction are used to determine the steep meridian for postoperative suture removal. Agreement in these measurements indicates a good visual prognosis, while disagreement may result in a greater chance of a decrease in visual acuity.6,7 Refraction and keratometry are inaccurate in keratoplasty patients who have irregular astigmatism (ie, nonorthogonal and asymmetrical), since they only evaluate the astigmatism as one steep and one flat meridian. Topography allows for more information about the corneal shape in separate parts of the cornea and can quantify the amount of nonorthogonal asymmetrical irregularity using the vector parameter TD.4 Postoperative suture removal is easily determined by keratometry measurement.
When planning an excimer laser retreatment, it is useful to compare the outcomes of the first treatment by refraction, keratometry and topography—looking for over- or undercorrection of astigmatism and adjusting the amount of the second treatment according to the effect of the first surgery.
Keratometry, topography, and refraction, all provide useful information regarding the astigmatic status of patients. When the astigmatism measured by these tools is not in agreement either in (1) magnitude, (2) axis, or (3) both, then the surgeon needs to decide where to place more emphasis—either on the corneal or refractive astigmatism.
Noel Alpins, FRACS, FRCOphth, FACS, is an associate fellow at the University of Melbourne, in Victoria, Australia. Dr. Alpins states that he has a financial interest in the ASSORT program used for calculation of the treatment parameters and examination of outcomes. He may be reached at alpins@newvisionclinics.com.au; phone: + 61 3 9584 6122; or fax: + 61 3 9585 0995.
George Stamatelatos, BSc(Optom), is a senior optometrist at Newvision Clinics, in Melbourne, Australia. Mr. Stamatelatos states that he has no financial interest in the products or companies mentioned. He may be reached at george@newvisionclinics.com.au or +61 3 9584 6122.
Up Front | Nov 2006
Tricks of the Trade: Measuring Astigmatism
Corneal shape and refractive power are fundamental determinants of astigmatic status.
Noel Alpins, FRACS, FRCOphth, FACS, and George Stamatelatos, BSc(Optom)
