goldmann applanation tonometry in patients with regular corneal astigmatism
TRANSCRIPT
GOLDMANN APPLANATION TONOMETRY IN PATIENTS W I T H REGULAR CORNEAL ASTIGMATISM
JACK T. HOLLADAY, M.D. , MARK E. A L L ISO N , M.D. , AND THOMAS C. PRAGER, P H . D .
Houston, Texas
By averaging vertical and horizontal applanation readings from the Goldmann tonometer, we developed a simple technique that accurately measures intraocular pressure in patients with regular corneal astigmatism. This procedure eliminates the need for keratometry and oblique alignment of the applanator.
When the applanator is oriented normally, with the mires displaced horizontally, intraocular pressure is underestimated for with-the-rule corneal astigmatism and overestimated for against-the-rule corneal astigmatism. The error is approximately 1 mm Hg for every 4 diopters of astigmatism. For oblique axis corneal astigmatism, the error is smaller and approaches zero when the axis is either 45 or 135 degrees.
The Goldmann applanation tonometer has served as the standard for intraocular pressure measurements since 1955.x A circular applanation area of 7.36 mm2, which Goldmann determined was optimal to neutralize the corneal structural resistance, creates capillary attraction during applanation, by surface tension of the corneal tear film. This results in an accurate measurement of intraocular pressure independent of corneal rigidity and capillary attraction.2,3
When regular corneal astigmatism is present, the applanated area is elliptical, not circular. To eliminate error resulting from the elliptical applanation area, the applanator must be oriented at approximately 43 degrees from the flattest corneal meridian.4
We have developed a second method
Accepted for publication April 11, 1983. From the University of Texas Medical School,
Hermann Eye Center, Department of Ophthalmology, Houston, Texas.
Reprint requests to Jack T. Holladay, M.D., Hermann Eye Center, P.O. Box 20420, Houston, TX 77030.
that does not require determination of the flattest corneal meridian, is as accurate as Goldmann's procedure, and is technically easier to perform. Our technique requires measuring the intraocular pressure with the applanator oriented horizontally, then vertically, and determining the average value.
SUBJECTS AND M E T H O D S
We selected a consecutive series of 11 patients (12 eyes) with regular corneal astigmatism of more than 3 diopters. Each patient underwent keratometric measurements once and intraocular pressure measurements four times. The first measurement was done in the standard manner with the applanation mires displaced horizontally (180 degrees). We then measured the intraocular pressure three more times: first, after rotating the applanation mires vertically (90 degrees); second, after rotating the applanation mires to 43 degrees from the flattest corneal meridian (Goldmann's procedure); and, finally, as a control, repeating the initial measurement with the applanator oriented horizontally to assure
90 © AMERICAN JOURNAL OF OPHTHALMOLOGY 96:90-93, 1983
VOL. 96, NO. 1 APPLANATION TONOMETRY 91
that none of the measurements were artificially low as a result of repeated appla-nation.5 If the first and last intraocular pressure measurements were not identical, we repeated all four measurements until this criterion was satisfied. Measurements were done independently by two observers and the results were statistically evaluated by analysis of variance and Pearson product-moment correlation.
R E S U L T S
A one-way analysis of variance performed after collapsing data across both sets of observed data showed no significant difference (P>.05) between Gold-mann's method of measuring intraocular pressure and our averaging technique (Goldmann: X = 13.11 mm Hg, S.D. = 2.36; averaging technique: X = 13.08 mm Hg, S.D. = 2.59). A Pearson product-moment correlation between the two methods was almost linear, r = +0.9936 (Table).
Our data showed that an error of 1 mm Hg for every 4 diopters of with- or against-the-rule astigmatism was present when we used only the normal, horizontal measurement. For oblique axis corne-al astigmatism, the difference was less than a ratio of 1:4 and approached zero as the axis neared 45 or 135 degrees.
D I S C U S S I O N
Goldmann and Schmidt2,3 were aware of the applanation error induced by regular astigmatism when the mires were not aligned 43 degrees from the flattest cor-neal meridian. To our knowledge, the specific amount of the applanation error has never been reported, because for astigmatism of more than 3 diopters Goldmann's method is used.
Goldmann's determination of 43 degrees as the proper applanator alignment is an average value of an angle that varies with the amount of astigmatism.4 Gold
mann's intraocular pressure error is roughly 0.3% for 5 diopters, 1.8% for 10 diopters, and 2 .1% for 20 diopters of regular corneal astigmatism. In comparison, our averaging technique theoretically underestimates the intraocular pressure by 0.0%, 0.5%, and 0.7%, respectively. This underestimation is a result of the difference between the true geometric mean and our arithmetic mean. Although these differences are of theoretic interest, they are not significant clinically.
The horizontal-vertical averaging technique offers three advantages that Goldmann's original method does not: (1) Ker-atometric measurements are not necessary. (2) Goldmann's method of aligning the applanator at 43 degrees from the flattest corneal meridian usually requires oblique orientation of the mires. It is difficult to perform this technique, which requires simultaneous vertical and horizontal movement of the slit lamp. Using the averaging technique obviates the need for oblique alignment. (3) The vertical and horizontal measurements determine the precise applanation intraocular pressure for that patient's cornea, and thus determine the error of the routine horizontal applanation.
The averaging technique is clinically useful after cataract surgery, because a significant incidence of with-the-rule astigmatism results from the use of 9-0 and 10-0 nonabsorbable sutures that compress the corneoscleral wound.6,7 Compressing sutures, combined with wound swelling, often produce 3 to 10 diopters of corneal astigmatism during the early postoperative period.6 Measuring intraocular pressure with applanation mires in the normal, horizontal orientation leads to an underestimation of approximately 1 mm Hg for every 4 diopters of corneal astigmatism. For example, with +12 diopters of with-the-rule astigmatism, intraocular pressure is underestimated by 3
CD
to
TAB
LE
APP
LA
NA
TIO
N I
NT
RA
OC
UL
AR
PR
ESS
UR
ES
IN P
AT
IEN
TS
WIT
H R
EG
UL
AR
CO
RN
EA
L A
STIG
MA
TIS
M
Eye
No.
1 2 3 4 5 6 7 8 9 10
11
12
Ker
atom
etric
Rea
ding
s (D
iopt
ers)
Flat
test
40.0
0 at
170
° 41
.62
at
2°
41.7
5 at
170
° 41
.12
at
4°
42.5
0 at
30
° 41
.12
at
4°
41.5
0 at
17
° 41
.12
at 1
80°
40.6
2 at
8°
41
.00
at
1°
41.2
5 at
2°
42
.00
at 1
80°
Stee
pest
50.2
5 at
80
° 48
.12
at
90°
47.7
5 at
80
° 48
.50
at
92°
47.2
5 at
118
° 46
.00
at
95°
49.3
7 at
107
° 46
.25
at
85°
47.5
0 at
98
° 47
.50
at
90°
49.0
0 at
92
° 47
.00
at
90°
Asti
gmat
ism
10.2
5 6.
50
5.00
6.
38
4.75
4.
88
7.88
5.
13
6.88
7.
50
8.75
5.
00
Hor
izon
tal
15.0
11
.0
12.0
9.
5 12
.0
11.5
14
.5
15.0
8.
0 12
.0
12.5
12
.5
Obs
erve
r 1
(mm
Hg)
Ver
tical
20.0
13
.0
15.0
11
.5
13.0
14
.0
17.0
19
.0
10.5
15
.0
15.5
14
.5
Ave
rage
17.5
12
.0
13.5
10
.5
12.5
12
.8
15.8
17
.0
9.8
13.5
14
.0
13.5
43
Deg
rees
*
17.0
12
.0
13.0
10
.5
12.5
13
.0
15.5
17
.0
9.0
14.0
14
.5
13.5
Diff
eren
ce
+0.5
0
+0.5
0 0
-0.2
+0
.3
0 +0
.8
-0.5
-0
.5
0
Hor
izon
tal
12.0
7.
5 9.
0 9.
0 11
.0
10.5
12
.0
13.0
8.
0 11
.0
15.0
15
.5
Obs
erve
r 2
(mm
Hg)
Ver
tical
18.0
10
.0
11.0
11
.0
13.0
13
.5
16.0
18
.0
11.0
14
.0
19.0
18
.0
Ave
rage
15.0
8.
8 10
.0
10.0
12
.0
12.0
14
.0
15.0
9.
5 12
.5
17.0
16
.8
43
Deg
rees
*
15.0
9.
0 9.
5 10
.0
12.0
11
.5
14.0
15
.5
10.0
12
.5
17.0
16
.5
Diff
eren
ce
0 -0
.2
+0.5
0 0
+0.5
0
-0.5
-0
.5
0 0 +0
.3
*Gol
dman
n's
met
hod
of ta
king
the
mea
sure
men
t at
43
degr
ees
to t
he f
latte
st c
orne
al m
erid
ian.
D
iffe
renc
es i
n in
trao
cula
r pr
essu
re b
etw
een
the
two
tech
niqu
es.
VOL. 96, NO. 1 APPLANATION TONOMETRY 93
mm Hg. Thus, using the averaging technique after surgery prevents underestimates of the intraocular pressure without keratometry and oblique alignment of the applanator.
The averaging technique may be clinically important in measuring the intraocular pressure of an older patient. Older individuals have an increased incidence of against-the-rule astigmatism,6 resulting in an overestimation of the intraocular pressure if the. applanation mires are orientated horizontally. Unnecessary treatment may be an unfortunate consequence of this error.
We have found that using this simple averaging technique prevents significant applanation intraocular pressure errors in patients with regular corneal astigmatism. We suggest that whenever the applanation mires do not appear to be circu
lar they should be rotated to the vertical orientation and a second reading taken.
REFERENCES 1. Goldmann, H.: Applanation tonometry. In
Newell, F. W. (ed.): Glaucoma. Transactions of the Second Conference, December 3, 4, and 5, 1956. New York, Josiah Macy Foundation, 1957, pp. 167-220.
2. Goldmann, H., and Schmidt, T.: Über Applanationstonometrie. Ophthalmologica 134:221, 1957.
3. : Weiterer beitrag zur Applanationstonometrie. Ophthalmologica 141:441, 1961.
4. Schmidt, T.: Zur Applanationstonometrie an der Spaltlampe. Ophthalmologica 133:337, 1957.
5. Moses, R. A.: Repeated applanation tonometry. Ophthalmologica 142:663, 1961.
6. Jaffe, N. S., and Clayman, H. M.: The patho-physiology of corneal astigmatism after cataract extraction. Trans. Am. Acad. Ophthalmol. Otolaryngol. 79:615, 1975.
7. Emery, J. M., and Paton, D. (eds.): Current Concepts in Cataract Surgery. Selected Proceedings of the Fourth Biennial Cataract Surgical Congress, 1975. St. Louis, C. V. Mosby, 1976, pp. 179-193.