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KERATOMETRY USING THE GOLDMA NN TONOMETER
M. C. CORBETT, G. A. SHUN-SHIN and P. N. AWDRY
Oxford
SUMMARY
Keratometry using the Goldmann tonometer is a reliable
and readily available guide to corneal astigmatism fol
lowing cataract surgery. In regular corneal astigmatism the Goldmann tonometer rings are distorted into skewed ellipses. The axis of the cylinder can be measured by rotating the tonometer head until an undistorted ellipse is obtained. The power is then assessed by comparison with standard ellipses. The difference in the intraocular pres sure readings (mmHg) in the two principal meridians was also a good guide to the presence of astigmatism. Gold mann keratometry was performed by a single masked
observer in 71 patients 8 weeks after routine extracapsu
lar cataract surgery. This was compared with Javal Schiotz keratometry performed by an independent
observer. In 83% of patients the axis was measured to
within 20°. The 95% confidence interval for the power
was ±2.90 DC; and 87% of patients would have sutures
removed appropriately. Significant astigmatism acquired during cataract surgery is often due to over-tightening of one or more sutures. 1 These must be accurately identified then removed if the optimum visual result is to be achieved.2 A technique used to identify tight sutures should be able to detect when 3.00 DC or more of corneal astigmatism is present,I,2 and
to locate the axis to within 20° (the approximate distance
between sutures). The aim of this study was to determine whether the Goldmann tonometer could measure corneal astigmatism sufficiently accurately to be of use in determining which sutures, if any, should be removed following cataract sur gery. Existing techniques include refraction and keratom etry, which may be more time-consuming or more technically demanding. Astigmatism measured by Gold mann tonometry was compared with results of conven tional keratometry. Our technique depends upon the observer's ability to distinguish a circle from an ellipse, and whether an ellipse is skewed or symmetrical. A second study was therefore performed to determine the certainty with which observers could distinguish a circle from ellipses of different dimensions.
Correspondence to: Miss M. C. Corbett, FRCS, The Oxford Eye Hospital, Walton Street, Oxford OX2 6AN, UK.
Eye (1993) 7,43-
PATIENTS AND METHODS
The main study group comprised 71 patients who had
undergone routine extracapsular cataract extraction with posterior chamber lens implantation at The Oxford Eye Hospital. The majority of procedures were performed through corneal sections, and all were closed with inter
rupted 10.0 monofilament nylon sutures.
At the outpatient appointment 8 weeks following sur
gery, keratometry was performed on all patients using a Javal-Schiotz keratometer. The astigmatism was recorded as 'keratometric power' and 'keratometric axis'. Assessment of the power and axis of astigmatism was then made using the Goldmann tonometer, by a single observer who had no prior knowledge of the keratometry measurements. Keratometry using the Goldmann tonom eter is performed as follows. The tension of the tonometer is adjusted as for measuring the pressure, and the shape of the rings is noted. If astigmatism is present the rings
appear either skewed or elliptical (Fig. 1). The axis of the
corneal astigmatism is assessed by repeating the process,
with the tonometer head rotated successively through 30°.
The orientation of the tonometer head producing the most symmetrical ellipse is nearest to the axis of the corneal astigmatism. Further small rotations of the tonometer head can be made until the rings are no longer skewed, and a reading of the orientation in degrees is taken from the scale on the side of the tonometer head ('observed axis'). A series of standard ellipses was computer-generated
Horizontally split Ellipse Vertically split Ellipse SkewedEllipse
Fig. 1. Appearance of the Goldmann rings in a patient with
+3,00 corneal astigmatism at 90°. When the tonometer head is
orientated along the principal meridians (0° and 90°) the ellip
ses are symmetrical and split along their long (horizontal) or short (vertical) axis. When the tonometer head is at any other orientation, the ellipses are skewed.
Cylindrical Horizontally^ split^ Verllcally^ split Power (DC) Ellipse^ Ellipse
0
n ·u
ru �
ru �
4
f"\
(^8) ....J
9
10
� ru
Fig. 2. The series of standard ellipses with which the appear ance of the rings on Goldmann tonometry were matched to determine the 'observed power' of the astigmatism.
showing the configuration of the rings when 0 to 10 DC
astigmatism is present (Fig. 2). When attached to the slit
lamp beside the patient's head the standard 0 DC ellipse
appears the same size as the rings seen on Goldmann tonometry of a spherical cornea. For construction of the other ellipses, the average corneal power was taken to be
40 DS. The difference between the horizontal and vertical
diameters of the constructed ellipses was altered by one fortieth for each dioptre of astigmatism. Ellipses split both along their long axes (,horizontally split ellipses') and their short axes (,vertically split ellipses') were constructed. The power of the corneal astigmatism was assessed by comparing the shape of the rings when orientated along the two principal meridians with the series of standard ellipses ('observed power'). The pressure readings in these two dimensions were also taken ('power by pressure difference'). With experience there is a reduction in the number of manoeuvres required to attain the result. After practice,
the procedure takes about 3 minutes.
A second study was performed to determine with what accuracy an observer could distinguish a split-circle from a split-ellipse. Twenty normal subjects were given a sheet
bearing 40 computer-generated horizontally and vertically
split ellipses representing O-S DC astigmatism. These
M. C. CORBETT ET AL.
were orientated at 0°, 4So and 90° for all powers, with addi
tional ellipses at 30° and 60° for the 2 and 3 DC powers.
They graded each shape as: 0, definitely a circle; 1, prob
ably a circle; 2, probably an ellipse; 3, definitely an ellipse.
The score for each shape was calculated as the total of the
grades given by all subjects, expressed as a percentage of
the maximum possible score. Therefore a score of 0 means
that all subjects were sure that the shape was a circle, and a
score of 100 means that all subjects were certain that the
shape was an ellipse. The scores of the same ellipse at all orientations were amalgamated.
RESULTS
Iaval-Schiotz keratometry and Goldmann keratometry
were performed in single eyes of 7 1 patients 8 weeks fol
lowing cataract extraction. One elderly patient was
uncooperative, and therefore the results of 70 patients are
presented.
Of the 70 patients there were 2 in whom astigmatism
could not be detected by Goldmann tonometry. On Iaval Schiotz keratometry these pateints had astigmatism of
o DC and 1.S DC. There were 2 other patients in whom it
was noted that the axis was very difficult to detect. One
had an axis error (keratometric axis-observed axis) of 30°
with only 1 DC of astigmatism. The other had an axis
error of 3So in the presence of 3.S DC on keratometry, but
it was noticed that the eye was very soft, and the pressure
was measured at 4 and S mmHg in the two principal mer
idians. The technique can be very sensitive. For example
there was 1 patient with only O.S DC on keratometry in
whom the axis was determined to within So.
Scatter diagrams of the cylinder measured by obser
vation (Fig. 3) and by pressure difference (Fig. 4) were
plotted against the cylinder on keratometry. The line of equality is the line on which all points would lie if there was total agreement between the two methods. The corre-
Observed Power (DC)
Keratometric Power (DC)
� Sutures removed inap�Topriately
EI Sutures not removed when necessary
Fig. 3. Scatter diagram of the power of corneal astigmatism determined by comparison with standard ellipses against the power determined by laval-Schiotz keratometry. The line of equality is shown.
Table II. Ability of 20 observers to distinguish ellipses from circles
does not vary consistently with the orientation of the shape
Orientation: 0° 30° 45° 60° 90°
2 DC ellipse Horizontally split 23 38 58 60 73 Vertically split 32 18 25 32 23 3 DC ellipse Horizontally split 68 85 88 75 78 Vertically split 40 45 50 45 32
Scores are expressed as percentages. Score of 0, all subjects were certain the shape was a circle; score of 100, all subjects were certain the shape was an ellipse.
therefore did not need sutures removing anyway. The smaller the astigmatism, the more difficult it is to detect the axis correctly. The results of the second study are shown in Table I, which lists the scores for ellipses split horizontally and
vertically, and the average of the two. A score of over 50
means that subjects tended to think that a shape was an ellipse rather than a circle. This was the case for horizon
tally split ellipses of 1 DC or greater, and vertically split
ellipses of 4 DC or greater. An ellipse is much easier to
detect and quantify when split horizontally rather than ver tically. This was also our experience when performing Goldmann keratometry. It was felt during the main study that the technique was most accurate when the axis was
near either 0° or 90°, but the second study showed no con
sistent difference in the ability to identify a given shape at different orientations (Table II).
DISCUSSION
The results of extracapsular capsular cataract extraction can be compromised by induced surgical astigmatism. The most significant cause of acquired post-operative astigmatism is altered corneal contour due to inappro priate suture tension or, more rarely, wound misalign ment.1,4 The intraocular lens implant contributes very little to post-operative astigmatism. This is because it is of uni form refractive index; and to produce significant astig
matism it has to be tilted by more than 20° or markedly
displaced, both of which can be detected on slit lamp examination. Following cataract surgery,6 astigmatism is minimised by adjustment of sutures under topical anaesthesia at the slit lamp, once the wound has healed. Interrupted sutures may be removed from the steepest axis of the cornea, or a continuous suture can be eased round towards that meridian. Refraction is the only means of assessing the astig matism of the whole ey e, but Misson8 has shown that results are not significantly different from the anterior cor neal astigmatism measured by keratometry. The correla tion for axis was stronger than that for power. Both the Javal-Schiotz keratometer and the Goldmann tonometer take readings from a central area of the cornea approxi
mately 3 mm in diameter, and therefore assume that the
astigmatism is sy mmetrical.
M. C. CORBETT ET AL.
The technique using the Goldmann tonometer depends upon the observer's ability to distinguish a circle from an ellipse, and whether an ellipse is skewed or sy mmetrical. A normal subect can distinguish a rectangle with a sides
ratio of 1.05 with about 95% certainty, 1.1 with 99% cer
tainty, and 1.2 with 100% certainty.
In our second study recognition rates were not quite as high, probably because the distinction is slightly more dif ficult to make for a circle than a square, especially when split. A horizontally split ellipse with a diameter-ratio of
1.05 (2 DC) was detected with 53% certainty ; and a dia
meter-ratio of 1.1 (4 DC), with 89% certainty.
CONCLUSIONS
Assessment of astigmatism by Goldmann tonometry is an easily learned technique. It reduces the need for keratom etry and refraction in the early post-operative period fol lowing cataract surgery. The technique is sufficiently accurate to enable sutures to be removed appropriately. Best results are achieved if the indication for suture
removal is: 'either the observed power is 3 DC and
greater, or the pressure difference between the two major
meridians 3 mmHg and greater'. The observed power is
most accurately assessed using the horizontally split
ellipse. If the ey e is very soft (e.g. less than 6 mmHg)
Goldmann readings underestimate the astigmatism, and in those cases conventional keratometry should be performed.
Key words: Astigmatism, Cataract surgery, Goldmann tonometry, Keratometry.
REFERENCES
- Swinger CA. Post-operative astigmatism. Surv Ophthalmol 1987;31:219-48.
- Roper-Hall MJ. Control of astigmatism after surgery and trauma. Br J Ophthalmol 1982;66:556-9.
- Bland JM, Altman DG. Statistical methods for assessing agreement between two methods of clinical measurement. Lancet 1986; 1 :307-10.
- van Rij G, Waring GO. Changes in corneal curvature induced by sutures and incisions. Am J OphthalmoI1984;98:773-83.
- Lakshminarayanan V, Enoch JM, Raasch T, Crawford B, Nygaard RW. Refractive changes induced by intraocular lens tilt and longitudinal displacement. Arch Ophthalmol 1986; 104:90-2.
- Kronish J W, Foster RK. Control of corneal astigmatism fol lowing cataract extraction by selective suture cutting. Arch OphthalmoI1987;105:1650-5.
- Atkins AD, Roper-Hall MJ. Control of postoperative astig matism. Br J OphthalmoI1985;69:348-51.
- Misson GP. Keratometry and post-operative astigmatism. Eye 1992;6:63-5.
- Efron R. What is perception? Boston studies in the philo sophy of science. Humanities Press Inc; 1968:137-73.
