A C T A O P H T H A L M O L O G I C A VOL. 5 3 1 9 7 5
The University Eye Clinic, Rigshospitalet, Copenhagen (Heads: E. Gregersen, V . Dreyer, J . Edmund, S . Kessing, and H . H . Seedorff)
THE NONCONTACT TONOMETER Clinical Evaluation on Normal and Diseased Eyes BY
PER NELLEMANN S0RENSEN
The measurement of intraocular pressure with a noncontact tonometer was correlated to a Goldmann tonometer in 20 normal persons, 20 glaucomatous patients, and 8 patients suffering from corneal diseases. In normal persons the error in measurement for the noncontact tonometer was related to their skill in fixation, and in eye patients to height of pressure and corneal state. Acceptable correlation was found between noncontact tonometry and Goldmann applanation tonometry when the cornea was normal and the pressure below 35 mmHg (Goldmann), otherwise noncontact tonometry was only a guide, and in the presence of corneal disease, unreliable. Good fixation reduced the method error. The standard deviation was 1.09 mmHg at poor fixation and 0.60 mmHg at good fixation. Repeated measurements on the same eye with noncontact tonometry did not alter the intraocular tension.
Key words: applanation tonometry - fixation - glaucoma tonometry - intraocular pressure - noncontact tonometry.
-
Goldmann
In 1971 Grollman took out a patent for a noncontact tonometer, and in 1972 he published the principle of this means of measuring the intraocular pressure. The clinical use of this instrument was first described in 1974 by Forbes, Pic0 & Grollman. There is, however, disagreement about the measuring accuracy of the instrument on normotensive subjects (Stepanik 1974) as compared with the Goldmann applanation tonometer to which the noncontact tonometer is calibrated. The noncontact tonometer (NCT) was investigated Received December 4, 1974.
513 Acta ophthal. 53, 1
34
Per Nellemann Ssrensen
on normotensives with special reference to fixation skill, since this is a n important factor and has never been investigated. Furthermore, the measuring accuracy of the instrument was studied o n hypertensive (>24 mmHg) and normotensive eyes with pathological corneae because there a r e reasons to suppose that i n using the NCT the measuring error will affect these eyes particularly. The hypertensive eyes investigated included both eyes with clear, oedematous o r scarred corneae. The construction and function of the instrument is described by Grollman (1972) and Forbes et al. (1974) and is summarized in the following: With linear increasing power, an air current applanates a central area of 3.6 mm in diameter. A pencil of rays is directed towards the central area oblique to the air current. On the other side of the air current a photoelectric cell records the reflected light. With increasing air current power the corneal surface changes its form from a steeper to a flatter convex mirror, thus directing more light to the photoelectric cell. The time for reaching maximum light intensity indicates the time for applanation. This interval is thus a measure for the intraocular pressure, and the time is measured by an oscillator clock released at the same time as the air current. The time interval is transformed from the clock to a recorder (digital display) to millimetre mercury, the tonometer being calibrated to the Goldmann tonometer. The time interval for normotensive eyes averages 2 msec. The instrument is adjusted by a system similar to that of the radioscope; a red dot target must be focused and placed in the centre of a white circle. A mirror system ensures that the examinee also looks towards the red dot target which in relation to the examinee is imaged in an optically infinite distance. Another system (Secundary Automatic Alignment Verification System) ensures that the air current and the pencil of rays hit the optical centre of the cornea. The system consists of infrared light which is sent out in the same direction as the air current, is perpendicularly reflected, and received in a detector. If the red dot target falls outside the white circle, the AAVS ensures that the tonometer button cannot be released.
Materials and Methods
In experiment I the intraocular pressure (IOP) was measured on 20 normal eyes, only one eye i n each person. W i t h the noncontact tonometer, 10 measurements on the same eye were m a d e successively. The noncontact applanation was made with American Optical's instrument HS 1258. At the first measurement, the time from the discovery of the red focusing dot target to t h e fixation of the dot target within the white circle is read on a stop watch; this time interval being taken as expression of the fixation skill. The IOP was measured every '/z min. The subjects were seated comfortably. A few minutes after the NCT recording, one single Goldmann applanation measurement was made. 514
Evaluation of Noncontact Tonometer
The Goldmann applanation tonometry was made with Haag Streit’s model mounted on Haag Streit slit lamp model 500 on which the reading was covered by paper so that the adjustment could not be read till after the procedure. NovesinB (oxibuprocainum NFN) local anaesthetic with diluted fluorescein (0.125 O/O) was given and the adjustment was made exactly between the cardiac cycles. In experiment I1 the measurements were made on hypertensive eyes with clear, cloudy or scarred corneae, and normotensive eyes with pathological corneae, one eye in each person. The IOP was measured with the NCT and Goldmann applanation tonometry as in experiment I, omitting the timing. Measurements wercjmade on: a) 14 eyes which presumably had increased IOP due to simplex glaucoma and with clear cornea judged by slit lamp examination. Visual acuity was better than 6/9. b) Six patients with acute glaucoma and oedematous cornea. c) Eight patients with normal pressure and irregular cornea due to keratitis, keratoconus or leucoma. The automatic verification system was disconnected during b) and c).
Table I . One single Goldmann applanation compared with noncontact applanation in 20 normal eyes (one eye / subject). 10 succesive measurements are performed every half minute. The subjects are divided into two groups according to ability of fixation, e.g. short and long adjustment time
I
applanation
Mean reading (max./min.)
1
Noncontact applanation
Mean of first reading
(maximin.)
Mean of mean Mean of lowest readings in each reading subject f (maximin.) standard deviation
515 34
Per Nellemann Serensen Table II. One single Goldmann applanation compared with noncontact applanation in patients with elevated IOP and pathological cornea. The patients with elevated IOP are divided into classes according to tension and present corneal oedema. 10 succesive noncontact measurements are performed every half minute on one eye in each
1 Elevated IOP 35>X>24 n = 8 Clear cornea Elevated IOP
x > 35
n=6
applanation
I
Noncontact applanation
Mean of lowest Mean of mean reading readings in each (max./mi,) patient f s. d.
Mean reading (max./min.)
Mean of first reading (max./min.)
30.88 mmHg (26 135)
32.50 mmHg (48 155)
32.00 mmHg (24 138)
4 1.OO (36 / 47)
51.00 (48 / 55)
48.83 (44 150)
53.00 (42 166)
72.00 (61 190
66.00 (61 174)
Clear cornea Elevated IOP n=6 Corneal oedema "Normal" IOP n=8 Pathological cornea
33.08 mmHg f 1.06 f=72 51.43
k 2.05 f = 54 72.90
f 5.44 f=54
19.00 (17 125)
21.38 (18 / 28)
16.50 (13 120)
20.66
f 3.53 f-72
Results Normal eyes
Table I demonstrates that 10 repeated noncontact applanation measurements with half-minute intervals do not indicate major deviation between the measurements. The adjustment time of the instrument was taken as a measure of the fixation skill, and the normal eyes were arbitrarily subgrouped into two groups according to whether the adjustment was relatively quick (< 3 sec) or time-consuming (> 3 sec). However, reservations must be made regarding the division of the population into good or bad fixators. It is worth noticing that if the adjustment of the instrument is more timeconsuming (> 3 sec), e. g. if the fixation is poor, standard deviation will in516
Evaluation of Noncontact Tonometer
crease. Bartlett’s test showed variance homogeneity and a common standard deviation could be calculated. The standard deviation is thus 0.60 mmHg a t good fixation (< 3 sec) and 1.09 mmHg at poor fixation (> 3 sec). All error is attributed to the NCT measurement. If the corresponding estimations of the variances are F-tested, disparity is found (P = 0.05, two-tailed), and consequently it may be assumed that the two groups (good or poor fixation) cannot be regarded as belonging to the same population. Table I11 demonstrates good correlation between Goldmann tonometry and the NCT in the first, the lowest, and the mean of 10 readings. However, it is more relevant that the corresponding measurements with Goldmann
Table 111. Correlation between Goldmann and noncontact (NCT) applanation
N C T reading
NCT reading
of 10 readings
r
r
r
b
s z
b
s~~
b
s z
Normal IOP. n= 9 Adjustment time < 3 sec
0.98 1.01 0.001 0.001
1.06 0.98 0.90 0.001 0.001
Normal IOP. n = 11 Adjustment time < 3 sec
0.77 0.56 0.001 0.01
2.03 0.88 0.62 1.54 0.91 0.001 0.001 0.001
0.63 0.001
1.32
ElevatedIOP. n= Clear cornea 35 2 X > 24 mmHg
8
0.92
1.96 0.96
1.37
1.91
Elevated IOP. Clear cornea X > 35 mmHg
n=
6
Elevated IOP. Corneal oedema
n=
“Normal” IOP. n = Pathologic cornea r
=
1.29
0.001 0.005
6
8
0.53
0.20
-
-
0.53
0.58
-
0.37 -
-
0.94 -
1.40
0.001 0.001 2.45 0.56
9.98 0.49
-
4.12 0.60
-
0.42
0.28
-
0.70
-
0.80 0.98 0.96 0.001 0.001
1.42 0.94
0.89
0.001 0.001 3.13 0.53
5.35 0.76
-
2.48 0.67
-
0.35
2.81
0.58
5.38
0.05
0.56
-
2.55
coefficient of correlation. b represents byIx = regression coefficient.
s a = standard deviation of mean NCT. The significance limits for difference from zero for r and b are shown. (-) denotes that r or b are not different from zero for P = 0.05.
517
Per Nellemann Serensen
GoldmandFirst NCT reading
GoldmandLowest NCT reading
GoldmandMean of 10 readings
tonometry and the NCT do not show great difference between the two types of measurements regarding the first, the smallest, and the average of 10 readings (Table IV). Thus the standard deviation of the difference was 1.01 mmHg at good fixation and 2.09 at poor fixation. Also with the Wilcoxon test for paired differences, the H, hypothesis (hypothesis of no differences) is accepted for P > 0.10, (two-tailed). Hypertensive eyes; clear or oedematous corneae and normotensive eyes with pathological corneae
Table I1 demonstrates that the standard deviation is small (1.06 mmHg) if the cornea is clear and the tension below 35 mmHg. If, on the other hand, the cornea is oedematous and the tension more than 35 mmHg, the standard deviation will be considerable and completely unacceptable in oedematous 518
Evaluation of Noncontact Tonometer
and pathological cornea. (Bartlett’s test showed variance homogeneity for the 10 readings in the subgroups, permitting the calculation of a common standard deviation.) Table I11 demonstrates that if the cornea is clear and the tension less than 35 mmHg, an acceptable correlation between Goldmann and noncontact tonometry is found. Furthermore, the standard deviation of the differences between pairs of Goldmann and NCT measurements was low, 3.10 mmHg at the first reading. With the Wilcoxon test there is no significant difference between the two types of measurements. The H, hypothesis will be accepted for P > 0.10 (twotailed) whether the first, lowest or average of the 10 readings is used. If the tension is higher than 35 mmHg, or the cornea is irregular, no correlation between Goldmann and noncontact applanation is found, whether the first, lowest or the average of the 10 readings is used. The standard deviation of the paired differences supports this statement (Table IV). The Wilcoxon test also shows difference in the two types of measuring (P > 0.01, two-tailed]. Repeated noncontact applanation
Table V indicates that 10 NCT measurements every half minute leave the IOP unchanged if IOP is below 35 mmHg and if the cornea is normal. (Wilcoxon test; H, accepted for P > 0.10, two-tailed.)
Table V . Repeated noncontact applanation every half minute for 5 min in normal subjects and glaucoma patients with intraocular pressure below 35 mmHg (Goldmann). The mean of the first and tenth measurement is shown. s. d. is the standard deviation.
Normal eyes Adjustment time
> 3 sec
n = 11 Adjustment time > 3 sec n= 9 Elevated IOP 35 > X > 24 Clear cornea n= 8 (one eyehubject)
1st. measured tension
10th. measured tension
f s.d.
f s.d.
15.73 f 2.13 mmHg
15.83 f 1.95 mmHg
15.67 f 2.90
15.54 f 1.98
32.50 f 4.78
33.12 f 5.98
519
Per Nellemann Ssrensen
Discussion The investigation has shown that the standard deviation, sD, of differences between pairs of NCT and Goldmann measurements was lower than 3.10 mmHg provided the intraocular tension is below 35 mmHg and the cornea normal. Also, but less relevant, a high correlation coefficient (r > 0.9) was found between the paired NCT and Goldmann measurements under the same circumstances. Forbes et al. (1974) found a similar correlation coefficient (r = 0.9) and a similar standard deviation between pairs of NCT and Goldmann measurements (sD = 2.8G mmHg) among 570 eyes, and their results were supported by Grollman (1972) who, among 168 normal eyes, found r = 0.83 and SD = 2.0 mmHg. The simple comparative study of Ducrey et al. (1975) involving 90 eyes with mixed tensions but no stratification also shows satisfactory agreement between the NCT and Goldmann measurements. Forbes et al., who measured on hypertensive and normal eyes, made no stratification but a considerable discrepancy seemed to exist between the Goldmann and the NCT applanation with tensions above 40 mmHg. In comparative studies on normal eyes the average of several NCT measurements might be more accurate than the first measurement (Stepanik 1974). Furthermore, Forbes et al. (1974) stress that the lowest reading is the most accurate when fixation is poor or impossible. These statements may be true, but they have not been definitely substantiated in this study. The present investigation proves that the method error is reduced by good adjustment and good fixation. In amblyopic patients who cannot fixate exactly, the method error may be intensified as was assumed, but not investigated, by Forbes et al. (1974). The method error in normal eyes was found to be considerably less (s.d. = 0.60 - 1.09 mmHg) than suggested by Stepanik (1974). Stepanik measured the intraocular pressure in both eyes of 10 normal persons by 10 fast NCT measurements followed by 10 Goldmann applanations. He found the standard deviation to be 2.37 mmHg by the NCT, and 0.60 mmHg by Goldmann applanation. With repeated applanation, the IOP decreases 3-4 mmHg in average during 5 min and the drop in tension is described as fairly exponential (Moses 1961. Krakau & Wilke 1974). This decrease exhibits a considerable individual variation. With series of Goldmann applanations, a systematic error will be found which exceeds the method error found by Stepanik. The NCT instrument does not provoke a decrease of tension by measuring every half min for 5 min as stated by Forbes et al., and verified in the present work. (The vibration tonometer has the same characteristic (Krakau 1970).) 520
Evaluation of Noncontact Tonometer
In practice, the NCT measurement has proved to be favourable in certain respects. This applies particularly to children and eye-squeezers where the instillation of eye drops and Goldmann applanation tonometry or Schiotz tonometry is unreliable or entirely impossible. If the patient has a corneal disease, the present investigation confirms the suspicion that the NCT measurement is not reliable. However, Goldmann applanation tonometry would also be unsatisfactory in these circumstances. With an IOP exceeding 35 mmHg the NCT instrument could still be used as a guide to indicate whether or not the eye is normotensive.
References Ducrey, N., Geinoz, J. & Faggioni, R. (1975) Le tonomktre A air. Ophthalmologica (Basel) 170, 446-449. Forbes, M., Pico, G. & Grollman, B. (1974) A noncontact applanation tonometer. Arch. Ophthal. 91, 134-140. Grollman, B. (1972) A new tonometer system. Amer. /. Optom., 49, 646-660. Krakau, C. E. T. (1970) A vibration tonometer. Ophthal. Res. I, 129-139. Krakau, C. E. T. & Wilke, K. (1971) On repeated tonometry. Acta ophthal. (Kbh.) 49, 611-614.
Moses, R. A. (1961) Repeated applanation tonometry. Ophthalrnologica 142, 663-668. Stepanik, J. (1974) Das AO-non contact-Tonometer. I. Klinische Messungen in normotensiven Augen. Albrecht v. Graefes Arch. klin. exp. Ophthal., 190, 47-49. Wilke, K. (1974) On Repeated Tonometry and the Episcleral Venous Pressure in Human Eyes. Thesis. University of Lund.
Author’s address: Per Nellemann Ssrensen, Sslleredvej 48, 2840 Holte, Denmark.
52 1
The University Eye Clinic, Rigshospitalet, Copenhagen (Heads: E. Gregersen, V . Dreyer, J . Edmund, S . Kessing, and H . H . Seedorff)
THE NONCONTACT TONOMETER Clinical Evaluation on Normal and Diseased Eyes BY
PER NELLEMANN S0RENSEN
The measurement of intraocular pressure with a noncontact tonometer was correlated to a Goldmann tonometer in 20 normal persons, 20 glaucomatous patients, and 8 patients suffering from corneal diseases. In normal persons the error in measurement for the noncontact tonometer was related to their skill in fixation, and in eye patients to height of pressure and corneal state. Acceptable correlation was found between noncontact tonometry and Goldmann applanation tonometry when the cornea was normal and the pressure below 35 mmHg (Goldmann), otherwise noncontact tonometry was only a guide, and in the presence of corneal disease, unreliable. Good fixation reduced the method error. The standard deviation was 1.09 mmHg at poor fixation and 0.60 mmHg at good fixation. Repeated measurements on the same eye with noncontact tonometry did not alter the intraocular tension.
Key words: applanation tonometry - fixation - glaucoma tonometry - intraocular pressure - noncontact tonometry.
-
Goldmann
In 1971 Grollman took out a patent for a noncontact tonometer, and in 1972 he published the principle of this means of measuring the intraocular pressure. The clinical use of this instrument was first described in 1974 by Forbes, Pic0 & Grollman. There is, however, disagreement about the measuring accuracy of the instrument on normotensive subjects (Stepanik 1974) as compared with the Goldmann applanation tonometer to which the noncontact tonometer is calibrated. The noncontact tonometer (NCT) was investigated Received December 4, 1974.
513 Acta ophthal. 53, 1
34
Per Nellemann Ssrensen
on normotensives with special reference to fixation skill, since this is a n important factor and has never been investigated. Furthermore, the measuring accuracy of the instrument was studied o n hypertensive (>24 mmHg) and normotensive eyes with pathological corneae because there a r e reasons to suppose that i n using the NCT the measuring error will affect these eyes particularly. The hypertensive eyes investigated included both eyes with clear, oedematous o r scarred corneae. The construction and function of the instrument is described by Grollman (1972) and Forbes et al. (1974) and is summarized in the following: With linear increasing power, an air current applanates a central area of 3.6 mm in diameter. A pencil of rays is directed towards the central area oblique to the air current. On the other side of the air current a photoelectric cell records the reflected light. With increasing air current power the corneal surface changes its form from a steeper to a flatter convex mirror, thus directing more light to the photoelectric cell. The time for reaching maximum light intensity indicates the time for applanation. This interval is thus a measure for the intraocular pressure, and the time is measured by an oscillator clock released at the same time as the air current. The time interval is transformed from the clock to a recorder (digital display) to millimetre mercury, the tonometer being calibrated to the Goldmann tonometer. The time interval for normotensive eyes averages 2 msec. The instrument is adjusted by a system similar to that of the radioscope; a red dot target must be focused and placed in the centre of a white circle. A mirror system ensures that the examinee also looks towards the red dot target which in relation to the examinee is imaged in an optically infinite distance. Another system (Secundary Automatic Alignment Verification System) ensures that the air current and the pencil of rays hit the optical centre of the cornea. The system consists of infrared light which is sent out in the same direction as the air current, is perpendicularly reflected, and received in a detector. If the red dot target falls outside the white circle, the AAVS ensures that the tonometer button cannot be released.
Materials and Methods
In experiment I the intraocular pressure (IOP) was measured on 20 normal eyes, only one eye i n each person. W i t h the noncontact tonometer, 10 measurements on the same eye were m a d e successively. The noncontact applanation was made with American Optical's instrument HS 1258. At the first measurement, the time from the discovery of the red focusing dot target to t h e fixation of the dot target within the white circle is read on a stop watch; this time interval being taken as expression of the fixation skill. The IOP was measured every '/z min. The subjects were seated comfortably. A few minutes after the NCT recording, one single Goldmann applanation measurement was made. 514
Evaluation of Noncontact Tonometer
The Goldmann applanation tonometry was made with Haag Streit’s model mounted on Haag Streit slit lamp model 500 on which the reading was covered by paper so that the adjustment could not be read till after the procedure. NovesinB (oxibuprocainum NFN) local anaesthetic with diluted fluorescein (0.125 O/O) was given and the adjustment was made exactly between the cardiac cycles. In experiment I1 the measurements were made on hypertensive eyes with clear, cloudy or scarred corneae, and normotensive eyes with pathological corneae, one eye in each person. The IOP was measured with the NCT and Goldmann applanation tonometry as in experiment I, omitting the timing. Measurements wercjmade on: a) 14 eyes which presumably had increased IOP due to simplex glaucoma and with clear cornea judged by slit lamp examination. Visual acuity was better than 6/9. b) Six patients with acute glaucoma and oedematous cornea. c) Eight patients with normal pressure and irregular cornea due to keratitis, keratoconus or leucoma. The automatic verification system was disconnected during b) and c).
Table I . One single Goldmann applanation compared with noncontact applanation in 20 normal eyes (one eye / subject). 10 succesive measurements are performed every half minute. The subjects are divided into two groups according to ability of fixation, e.g. short and long adjustment time
I
applanation
Mean reading (max./min.)
1
Noncontact applanation
Mean of first reading
(maximin.)
Mean of mean Mean of lowest readings in each reading subject f (maximin.) standard deviation
515 34
Per Nellemann Serensen Table II. One single Goldmann applanation compared with noncontact applanation in patients with elevated IOP and pathological cornea. The patients with elevated IOP are divided into classes according to tension and present corneal oedema. 10 succesive noncontact measurements are performed every half minute on one eye in each
1 Elevated IOP 35>X>24 n = 8 Clear cornea Elevated IOP
x > 35
n=6
applanation
I
Noncontact applanation
Mean of lowest Mean of mean reading readings in each (max./mi,) patient f s. d.
Mean reading (max./min.)
Mean of first reading (max./min.)
30.88 mmHg (26 135)
32.50 mmHg (48 155)
32.00 mmHg (24 138)
4 1.OO (36 / 47)
51.00 (48 / 55)
48.83 (44 150)
53.00 (42 166)
72.00 (61 190
66.00 (61 174)
Clear cornea Elevated IOP n=6 Corneal oedema "Normal" IOP n=8 Pathological cornea
33.08 mmHg f 1.06 f=72 51.43
k 2.05 f = 54 72.90
f 5.44 f=54
19.00 (17 125)
21.38 (18 / 28)
16.50 (13 120)
20.66
f 3.53 f-72
Results Normal eyes
Table I demonstrates that 10 repeated noncontact applanation measurements with half-minute intervals do not indicate major deviation between the measurements. The adjustment time of the instrument was taken as a measure of the fixation skill, and the normal eyes were arbitrarily subgrouped into two groups according to whether the adjustment was relatively quick (< 3 sec) or time-consuming (> 3 sec). However, reservations must be made regarding the division of the population into good or bad fixators. It is worth noticing that if the adjustment of the instrument is more timeconsuming (> 3 sec), e. g. if the fixation is poor, standard deviation will in516
Evaluation of Noncontact Tonometer
crease. Bartlett’s test showed variance homogeneity and a common standard deviation could be calculated. The standard deviation is thus 0.60 mmHg a t good fixation (< 3 sec) and 1.09 mmHg at poor fixation (> 3 sec). All error is attributed to the NCT measurement. If the corresponding estimations of the variances are F-tested, disparity is found (P = 0.05, two-tailed), and consequently it may be assumed that the two groups (good or poor fixation) cannot be regarded as belonging to the same population. Table I11 demonstrates good correlation between Goldmann tonometry and the NCT in the first, the lowest, and the mean of 10 readings. However, it is more relevant that the corresponding measurements with Goldmann
Table 111. Correlation between Goldmann and noncontact (NCT) applanation
N C T reading
NCT reading
of 10 readings
r
r
r
b
s z
b
s~~
b
s z
Normal IOP. n= 9 Adjustment time < 3 sec
0.98 1.01 0.001 0.001
1.06 0.98 0.90 0.001 0.001
Normal IOP. n = 11 Adjustment time < 3 sec
0.77 0.56 0.001 0.01
2.03 0.88 0.62 1.54 0.91 0.001 0.001 0.001
0.63 0.001
1.32
ElevatedIOP. n= Clear cornea 35 2 X > 24 mmHg
8
0.92
1.96 0.96
1.37
1.91
Elevated IOP. Clear cornea X > 35 mmHg
n=
6
Elevated IOP. Corneal oedema
n=
“Normal” IOP. n = Pathologic cornea r
=
1.29
0.001 0.005
6
8
0.53
0.20
-
-
0.53
0.58
-
0.37 -
-
0.94 -
1.40
0.001 0.001 2.45 0.56
9.98 0.49
-
4.12 0.60
-
0.42
0.28
-
0.70
-
0.80 0.98 0.96 0.001 0.001
1.42 0.94
0.89
0.001 0.001 3.13 0.53
5.35 0.76
-
2.48 0.67
-
0.35
2.81
0.58
5.38
0.05
0.56
-
2.55
coefficient of correlation. b represents byIx = regression coefficient.
s a = standard deviation of mean NCT. The significance limits for difference from zero for r and b are shown. (-) denotes that r or b are not different from zero for P = 0.05.
517
Per Nellemann Serensen
GoldmandFirst NCT reading
GoldmandLowest NCT reading
GoldmandMean of 10 readings
tonometry and the NCT do not show great difference between the two types of measurements regarding the first, the smallest, and the average of 10 readings (Table IV). Thus the standard deviation of the difference was 1.01 mmHg at good fixation and 2.09 at poor fixation. Also with the Wilcoxon test for paired differences, the H, hypothesis (hypothesis of no differences) is accepted for P > 0.10, (two-tailed). Hypertensive eyes; clear or oedematous corneae and normotensive eyes with pathological corneae
Table I1 demonstrates that the standard deviation is small (1.06 mmHg) if the cornea is clear and the tension below 35 mmHg. If, on the other hand, the cornea is oedematous and the tension more than 35 mmHg, the standard deviation will be considerable and completely unacceptable in oedematous 518
Evaluation of Noncontact Tonometer
and pathological cornea. (Bartlett’s test showed variance homogeneity for the 10 readings in the subgroups, permitting the calculation of a common standard deviation.) Table I11 demonstrates that if the cornea is clear and the tension less than 35 mmHg, an acceptable correlation between Goldmann and noncontact tonometry is found. Furthermore, the standard deviation of the differences between pairs of Goldmann and NCT measurements was low, 3.10 mmHg at the first reading. With the Wilcoxon test there is no significant difference between the two types of measurements. The H, hypothesis will be accepted for P > 0.10 (twotailed) whether the first, lowest or average of the 10 readings is used. If the tension is higher than 35 mmHg, or the cornea is irregular, no correlation between Goldmann and noncontact applanation is found, whether the first, lowest or the average of the 10 readings is used. The standard deviation of the paired differences supports this statement (Table IV). The Wilcoxon test also shows difference in the two types of measuring (P > 0.01, two-tailed]. Repeated noncontact applanation
Table V indicates that 10 NCT measurements every half minute leave the IOP unchanged if IOP is below 35 mmHg and if the cornea is normal. (Wilcoxon test; H, accepted for P > 0.10, two-tailed.)
Table V . Repeated noncontact applanation every half minute for 5 min in normal subjects and glaucoma patients with intraocular pressure below 35 mmHg (Goldmann). The mean of the first and tenth measurement is shown. s. d. is the standard deviation.
Normal eyes Adjustment time
> 3 sec
n = 11 Adjustment time > 3 sec n= 9 Elevated IOP 35 > X > 24 Clear cornea n= 8 (one eyehubject)
1st. measured tension
10th. measured tension
f s.d.
f s.d.
15.73 f 2.13 mmHg
15.83 f 1.95 mmHg
15.67 f 2.90
15.54 f 1.98
32.50 f 4.78
33.12 f 5.98
519
Per Nellemann Ssrensen
Discussion The investigation has shown that the standard deviation, sD, of differences between pairs of NCT and Goldmann measurements was lower than 3.10 mmHg provided the intraocular tension is below 35 mmHg and the cornea normal. Also, but less relevant, a high correlation coefficient (r > 0.9) was found between the paired NCT and Goldmann measurements under the same circumstances. Forbes et al. (1974) found a similar correlation coefficient (r = 0.9) and a similar standard deviation between pairs of NCT and Goldmann measurements (sD = 2.8G mmHg) among 570 eyes, and their results were supported by Grollman (1972) who, among 168 normal eyes, found r = 0.83 and SD = 2.0 mmHg. The simple comparative study of Ducrey et al. (1975) involving 90 eyes with mixed tensions but no stratification also shows satisfactory agreement between the NCT and Goldmann measurements. Forbes et al., who measured on hypertensive and normal eyes, made no stratification but a considerable discrepancy seemed to exist between the Goldmann and the NCT applanation with tensions above 40 mmHg. In comparative studies on normal eyes the average of several NCT measurements might be more accurate than the first measurement (Stepanik 1974). Furthermore, Forbes et al. (1974) stress that the lowest reading is the most accurate when fixation is poor or impossible. These statements may be true, but they have not been definitely substantiated in this study. The present investigation proves that the method error is reduced by good adjustment and good fixation. In amblyopic patients who cannot fixate exactly, the method error may be intensified as was assumed, but not investigated, by Forbes et al. (1974). The method error in normal eyes was found to be considerably less (s.d. = 0.60 - 1.09 mmHg) than suggested by Stepanik (1974). Stepanik measured the intraocular pressure in both eyes of 10 normal persons by 10 fast NCT measurements followed by 10 Goldmann applanations. He found the standard deviation to be 2.37 mmHg by the NCT, and 0.60 mmHg by Goldmann applanation. With repeated applanation, the IOP decreases 3-4 mmHg in average during 5 min and the drop in tension is described as fairly exponential (Moses 1961. Krakau & Wilke 1974). This decrease exhibits a considerable individual variation. With series of Goldmann applanations, a systematic error will be found which exceeds the method error found by Stepanik. The NCT instrument does not provoke a decrease of tension by measuring every half min for 5 min as stated by Forbes et al., and verified in the present work. (The vibration tonometer has the same characteristic (Krakau 1970).) 520
Evaluation of Noncontact Tonometer
In practice, the NCT measurement has proved to be favourable in certain respects. This applies particularly to children and eye-squeezers where the instillation of eye drops and Goldmann applanation tonometry or Schiotz tonometry is unreliable or entirely impossible. If the patient has a corneal disease, the present investigation confirms the suspicion that the NCT measurement is not reliable. However, Goldmann applanation tonometry would also be unsatisfactory in these circumstances. With an IOP exceeding 35 mmHg the NCT instrument could still be used as a guide to indicate whether or not the eye is normotensive.
References Ducrey, N., Geinoz, J. & Faggioni, R. (1975) Le tonomktre A air. Ophthalmologica (Basel) 170, 446-449. Forbes, M., Pico, G. & Grollman, B. (1974) A noncontact applanation tonometer. Arch. Ophthal. 91, 134-140. Grollman, B. (1972) A new tonometer system. Amer. /. Optom., 49, 646-660. Krakau, C. E. T. (1970) A vibration tonometer. Ophthal. Res. I, 129-139. Krakau, C. E. T. & Wilke, K. (1971) On repeated tonometry. Acta ophthal. (Kbh.) 49, 611-614.
Moses, R. A. (1961) Repeated applanation tonometry. Ophthalrnologica 142, 663-668. Stepanik, J. (1974) Das AO-non contact-Tonometer. I. Klinische Messungen in normotensiven Augen. Albrecht v. Graefes Arch. klin. exp. Ophthal., 190, 47-49. Wilke, K. (1974) On Repeated Tonometry and the Episcleral Venous Pressure in Human Eyes. Thesis. University of Lund.
Author’s address: Per Nellemann Ssrensen, Sslleredvej 48, 2840 Holte, Denmark.
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