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
Department of Ophthalmic Optics (Head: Professor M . Millodot) University of Wales Institute of Science and Technology, U.K.
EFFECT OF HARD CONTACT LENSES ON CORNEAL SENSITIVITY AND THICKNESS BY
MICHEL MILLODOT
Corneal sensitivity and central corneal thickness were measured on 11 subjects before and after 8 hours of hard contact lens wear. Both factors were monitored for 30 min after removal. It was found that hard contact lenses affect both corneal sensitivity and thickness, significantly. After 8 hours the reduction of sensitivity was found to be, on average, 94 "lo in the centre and 116O/o in the periphery, and the average increase in thickness was 6.9 o/o. Thirty minutes after removal corneal sensitivity had recovered 82Vo and 100°/o in the centre and periphery, respectively, and corneal thickness had decreased by 6.3 O/o. A high correlation was found between central and peripheral corneal sensitivity. This investigation shows that corneal sensitivity measurements offer an alternative method of assessinp corneal integrity which is simple and precise.
Key words: hard contact lenses oedema.
-
cornea - sensitivity
-
pachometry -
The wear of hard corneal contact lenses affects corneal metabolism to some extent even if the lenses are well fitted and the patient perfectly adapted. Some oedema is noted either by sclerotic scatter illumination using a slit lamp or by Received April 10, 1975.
576
Effect
of
Hard Contact Lenses
measuring corneal thickness with a pachometer (e.g. Smelser 1952, Miller SExford 1967, Mandell & Polse 1969, Farris, Kubota & Mishima 1971, El Hage et al. 1974). Concomitant with the oedema induced by the wear of contact lenses is a reduction of corneal sensitivity. This diminution of corneal sensitivity after the wear of hard contact lenses has been noted by several authors (Boberg-Ans 1955, Bronner, Gerhard &. Coeurdevey 1958, Byron & Weseley 1961, KO & Tomiyama 1963, Dixon 1964, Knoll & Williams 1970, Larke & Sabell 1971). That corneal sensitivity reflects the changes that occur in the cornea is indisputable. Contact lenses affect the oxygen uptake which is causing oedema and this is accompanied by a loss of corneal senstivity. T h e purpose of this study is to contribute more systematic measurements of this diminution of corneal sensitivity with the wear of hard contact lenses, particularly how corneal sensitivity recovers after removal of the contact lenses. And also to examine the relationship between the oedema (as evinced by corneal thickness variations) and corneal sensitivity, as the latter may well serve as a method of assessing corneal integrity.
Materials and Method The Cochet-Bonnet Aesthesiometer (1960) based on the instrument devised by Boberg-Ans (1955, 1956) was used to stimulate the cornea. Two models of the instrument with nylon monofilament of 0.12 and 0.08 mm diameter were used to produce pressures from 1 1 to 200 mg/O.O113 mm? and from 2 to 90 mg/0.005 mm* respectively. The aesthesiometer was mounted in a holder allowing movement in x, y and z axes by means of three knobs. This set-up makes it possible to obtain reliability in stimulation of a corneal point, and stead speed of application of the monofilament at right angles to the cornea. All measurements were taken when the humidity in the room did not exceed 55 O/O because the nylon monofilament is affected by humidity (Millodot & Larson 1967). Central corneal thickness was measured by using a pachometer mounted on a slit lamp (HaagStreit 900). Five readings were taken and the mean calculated for each determnation of corneal thickness. Eleven subjects (six females, five males) between 21 and 29 years of age participated in this experiment. All were successful hard corneal contact lens wearers frce of any complaints or any discernible eye pathology. Their refraction varied between + 1.00 and - 7.50 with astigmatism not exceeding 2.75D. They had been wearing their lenses for more than 3 months and the median time was 14 months. 57 7 Acta ophthal. 53, 4
3s
Michel Millodot
Each subject reported in the morning prior to inserting his lenses and a measurement of corneal touch threshold was made subjectively (Millodot 1973) in the centre and in a peripheral point (6 o'clock) of the cornea. The measurements began with stimulation of the cornea at the lowest pressure and continued in an ascending fashion. A t each predetermined length (in increments of one half a cm) of the monofilaments, four to six contacts were made and the slightest bend of the nylon wire visible through a X 4.3 magnifier was defined as corneal contact. The subject was fixating an object on the opposite wall and he indicated when he felt the probe by pressing a bell. From these readings the touch threshold was defined as the length of the monofilament for which the subject responded for 50 O/O of the number of stimulations. This length was converted into pressure using a previously established calibration curve between length and pressure. Corneal thickness was also recorded with the pachometer following the instructions provided with the instrument. The subjects were then asked to report to the laboratory after having worn their lenses continuously for 8 hours. A measurement of corneal touch threshold was carried out within 1 min of removal. It was followed by other readings 4, 7, 10, 15, 20, 25 and 30 min after removal. The measurements of central and peripheral corneal sensitivity and of central corneal thickness were each carried out on a different day after the subject had kept his or her lenses for 8 hours.
Results The effect of continuous wear of hard contact lenses for 8 hours on corneal sensitivity (threshold-') is illustrated in Fig. 1. The upper curve (continuous line) represents the peripheral measurements, and the lower curve (broken line) the central measurements. Each data point is the mean of 1 1 subjects. The mean corneal touch threshold before inserting the lenses in the morning was 13.86 mg/ sq.mm. (s.d. 4.42) and 19.64 mg/sq.mm (s.d. 4.90) in the centre and periphery, respectively. The well-known difference between centre and periphery is again confirmed (Boberg-Ans 1955, Cochet & Bonnet 1960, Millodot & Larson 1969). After 8 hours wear the corneal touch threshold was found to be 25.91 mg/ sq.mm (s.d. 7.88) and 45.64 mg/sq.mm (s.d. 17.5) in the centre and periphery, respectively. Both these values are significantly higher (P < 0.001) than they were prior to insertion. The threshold in the corneal periphery increased on average by 1160/0 (range from 50 to 2000/0) and by 940/0 (range from 34 to 2000/0) in the centre. This decrease in sensitivity after wearing hard lenses is in qualitative accord with other studies. 578
Effect of Hard Contact Lenses
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COYIACI L I W S WEAR
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LENS
. 1
I
mrrOlE C O N I A C I
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1
1
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,
1
1
11
cormci
1
1
1s LENS
1
1
20
1
1
1
2.
1
2s
nmovhL mnn.
Fig. 1. Relationship Re1 ationship between central and peripheral corneal touch threshold (sensitivity-'), and subjiects. the wearing of hard contact lenses for 8 hours. Each data point is the mean of 1 1 subjects.
The loss of corneal sensitivity as found in this study is less in the centre than in the periphery. This fact may be accounted for by the following consideration. When testing the sensitivity in the centre of the cornea, subjects are usually apprehensive at first and therefore the threshold obtained is lower. After a few minutes of testing they relax and their threshold is less dependent on apprehension, a phenomenon which has already been demonstrated (Bonnet & Millodot 1965). This factor would contribute to the reduction of the height of the peak of the broken curve in Fig. 1. After removal of the contact lenses there is a progressive decline in corneal touch threshold (i.e. an increase in sensitivity) which is greater within the first 20 minutes and diminishes more slowly afterwards. Thirty minutes after removal the corneal touch threshold was found to be 15.82 mg/sq.mm (s.d. 4.59) and 22.73 mg/sq.mm (s.d. 5.46) in the centre and periphery, respectively. These values are significantly lower (P < 0.001) than those found immediately after removal. Although these values are close to the thresholds obtained in the morn579 38'
Miclzel Millodot
ing, when the subjects have not worn their lenses overnight for some 10 to 12 hours, the differences are nonetheless significant (P < 0.01) which means that corneal sensitivity has not fully recovered within 30 min. Yet it has regained 82 and lOOO/o (for the centre and periphery, respectively) of what it had lost during the 8 hours of wear. The effect of continuous wear of hard contact lenses for 8 hours on central corneal thickness is illustrated in Fig. 2. Each data point is the mean of 1 1 subjects. The mean corneal thickness before inserting the lenses in the morning was 0.520 mm (s.d. 0.033). This value is in good agreement with the result of other authors (see Hansen 1971). After 8 hours, corneal thickness increased to 0.556 mm (s.d. 0.041). This result is significantly higher (P< 0.001) than before inserting the lenses. Thus the corneal thickness increased on average by 6.9 O/o (range from 1 to 120/0). This finding is in good qualitative accord with the results of the investigations cited earlier. In only one subject was there only a very slight increase (1 U/O) in corneal thickness but there was nevertheless a decrease in central (34 o/o) and peripheral (50 O/O) corneal sensitivity. On the other hand, the subject displaying the greatest increase in corneal thickness had also the largest reduction of corneal sensitivity.
OW CONlACl LENS WEAR 0HOURS
-
AFIER
CONIACI LENS WEAR
0.56
t
I
BEFORE C O N l A C l LENS W E A R
0 4 0 12 16 20 24 20 TIME FOLLOWING C O N l A C l LENS R E M O V A L mtn.
I I
I 1
1
,
,
,
,
,
,
1
,
,
,
1
,
,
,
Fig. 2. Relationship between central corneal thickness and the wearing of hard contact lenses for 8 hours. Each data point is the mean of 1 1 subjects. 580
E f f e c t of Hard Contact Lenses
Thirty minutes after removal, corneal thickness decreased to 0.524 mm (s.d. 0.037) which is significantly (P 0.001) lower than immediately after removal. The decline of corneal thickness after 30 min was on average 6.3 O/O. Although the mean corneal thickness 30 rnin after removal is very close to the mean corneal thickness in the morning, it is significantly different (P 0.05),which means that corneal thickness has not fully recovered within that time. A comparison between central and peripheral measurements of corneal sensitivity obtained in the morning prior to inserting the lenses yielded a correlation coefficient of 0.78 which is significant (P 0.01). Thus it appears to be valid to evaluate the effect of contact lenses on corneal sensitivity by measuring either area of the cornea.
Department of Ophthalmic Optics (Head: Professor M . Millodot) University of Wales Institute of Science and Technology, U.K.
EFFECT OF HARD CONTACT LENSES ON CORNEAL SENSITIVITY AND THICKNESS BY
MICHEL MILLODOT
Corneal sensitivity and central corneal thickness were measured on 11 subjects before and after 8 hours of hard contact lens wear. Both factors were monitored for 30 min after removal. It was found that hard contact lenses affect both corneal sensitivity and thickness, significantly. After 8 hours the reduction of sensitivity was found to be, on average, 94 "lo in the centre and 116O/o in the periphery, and the average increase in thickness was 6.9 o/o. Thirty minutes after removal corneal sensitivity had recovered 82Vo and 100°/o in the centre and periphery, respectively, and corneal thickness had decreased by 6.3 O/o. A high correlation was found between central and peripheral corneal sensitivity. This investigation shows that corneal sensitivity measurements offer an alternative method of assessinp corneal integrity which is simple and precise.
Key words: hard contact lenses oedema.
-
cornea - sensitivity
-
pachometry -
The wear of hard corneal contact lenses affects corneal metabolism to some extent even if the lenses are well fitted and the patient perfectly adapted. Some oedema is noted either by sclerotic scatter illumination using a slit lamp or by Received April 10, 1975.
576
Effect
of
Hard Contact Lenses
measuring corneal thickness with a pachometer (e.g. Smelser 1952, Miller SExford 1967, Mandell & Polse 1969, Farris, Kubota & Mishima 1971, El Hage et al. 1974). Concomitant with the oedema induced by the wear of contact lenses is a reduction of corneal sensitivity. This diminution of corneal sensitivity after the wear of hard contact lenses has been noted by several authors (Boberg-Ans 1955, Bronner, Gerhard &. Coeurdevey 1958, Byron & Weseley 1961, KO & Tomiyama 1963, Dixon 1964, Knoll & Williams 1970, Larke & Sabell 1971). That corneal sensitivity reflects the changes that occur in the cornea is indisputable. Contact lenses affect the oxygen uptake which is causing oedema and this is accompanied by a loss of corneal senstivity. T h e purpose of this study is to contribute more systematic measurements of this diminution of corneal sensitivity with the wear of hard contact lenses, particularly how corneal sensitivity recovers after removal of the contact lenses. And also to examine the relationship between the oedema (as evinced by corneal thickness variations) and corneal sensitivity, as the latter may well serve as a method of assessing corneal integrity.
Materials and Method The Cochet-Bonnet Aesthesiometer (1960) based on the instrument devised by Boberg-Ans (1955, 1956) was used to stimulate the cornea. Two models of the instrument with nylon monofilament of 0.12 and 0.08 mm diameter were used to produce pressures from 1 1 to 200 mg/O.O113 mm? and from 2 to 90 mg/0.005 mm* respectively. The aesthesiometer was mounted in a holder allowing movement in x, y and z axes by means of three knobs. This set-up makes it possible to obtain reliability in stimulation of a corneal point, and stead speed of application of the monofilament at right angles to the cornea. All measurements were taken when the humidity in the room did not exceed 55 O/O because the nylon monofilament is affected by humidity (Millodot & Larson 1967). Central corneal thickness was measured by using a pachometer mounted on a slit lamp (HaagStreit 900). Five readings were taken and the mean calculated for each determnation of corneal thickness. Eleven subjects (six females, five males) between 21 and 29 years of age participated in this experiment. All were successful hard corneal contact lens wearers frce of any complaints or any discernible eye pathology. Their refraction varied between + 1.00 and - 7.50 with astigmatism not exceeding 2.75D. They had been wearing their lenses for more than 3 months and the median time was 14 months. 57 7 Acta ophthal. 53, 4
3s
Michel Millodot
Each subject reported in the morning prior to inserting his lenses and a measurement of corneal touch threshold was made subjectively (Millodot 1973) in the centre and in a peripheral point (6 o'clock) of the cornea. The measurements began with stimulation of the cornea at the lowest pressure and continued in an ascending fashion. A t each predetermined length (in increments of one half a cm) of the monofilaments, four to six contacts were made and the slightest bend of the nylon wire visible through a X 4.3 magnifier was defined as corneal contact. The subject was fixating an object on the opposite wall and he indicated when he felt the probe by pressing a bell. From these readings the touch threshold was defined as the length of the monofilament for which the subject responded for 50 O/O of the number of stimulations. This length was converted into pressure using a previously established calibration curve between length and pressure. Corneal thickness was also recorded with the pachometer following the instructions provided with the instrument. The subjects were then asked to report to the laboratory after having worn their lenses continuously for 8 hours. A measurement of corneal touch threshold was carried out within 1 min of removal. It was followed by other readings 4, 7, 10, 15, 20, 25 and 30 min after removal. The measurements of central and peripheral corneal sensitivity and of central corneal thickness were each carried out on a different day after the subject had kept his or her lenses for 8 hours.
Results The effect of continuous wear of hard contact lenses for 8 hours on corneal sensitivity (threshold-') is illustrated in Fig. 1. The upper curve (continuous line) represents the peripheral measurements, and the lower curve (broken line) the central measurements. Each data point is the mean of 1 1 subjects. The mean corneal touch threshold before inserting the lenses in the morning was 13.86 mg/ sq.mm. (s.d. 4.42) and 19.64 mg/sq.mm (s.d. 4.90) in the centre and periphery, respectively. The well-known difference between centre and periphery is again confirmed (Boberg-Ans 1955, Cochet & Bonnet 1960, Millodot & Larson 1969). After 8 hours wear the corneal touch threshold was found to be 25.91 mg/ sq.mm (s.d. 7.88) and 45.64 mg/sq.mm (s.d. 17.5) in the centre and periphery, respectively. Both these values are significantly higher (P < 0.001) than they were prior to insertion. The threshold in the corneal periphery increased on average by 1160/0 (range from 50 to 2000/0) and by 940/0 (range from 34 to 2000/0) in the centre. This decrease in sensitivity after wearing hard lenses is in qualitative accord with other studies. 578
Effect of Hard Contact Lenses
MlE"
COYIACI L I W S WEAR
I
I0
LENS
. 1
I
mrrOlE C O N I A C I
wmn
1
1
0 s T I Y E ~OL:OWWG
,
1
1
11
cormci
1
1
1s LENS
1
1
20
1
1
1
2.
1
2s
nmovhL mnn.
Fig. 1. Relationship Re1 ationship between central and peripheral corneal touch threshold (sensitivity-'), and subjiects. the wearing of hard contact lenses for 8 hours. Each data point is the mean of 1 1 subjects.
The loss of corneal sensitivity as found in this study is less in the centre than in the periphery. This fact may be accounted for by the following consideration. When testing the sensitivity in the centre of the cornea, subjects are usually apprehensive at first and therefore the threshold obtained is lower. After a few minutes of testing they relax and their threshold is less dependent on apprehension, a phenomenon which has already been demonstrated (Bonnet & Millodot 1965). This factor would contribute to the reduction of the height of the peak of the broken curve in Fig. 1. After removal of the contact lenses there is a progressive decline in corneal touch threshold (i.e. an increase in sensitivity) which is greater within the first 20 minutes and diminishes more slowly afterwards. Thirty minutes after removal the corneal touch threshold was found to be 15.82 mg/sq.mm (s.d. 4.59) and 22.73 mg/sq.mm (s.d. 5.46) in the centre and periphery, respectively. These values are significantly lower (P < 0.001) than those found immediately after removal. Although these values are close to the thresholds obtained in the morn579 38'
Miclzel Millodot
ing, when the subjects have not worn their lenses overnight for some 10 to 12 hours, the differences are nonetheless significant (P < 0.01) which means that corneal sensitivity has not fully recovered within 30 min. Yet it has regained 82 and lOOO/o (for the centre and periphery, respectively) of what it had lost during the 8 hours of wear. The effect of continuous wear of hard contact lenses for 8 hours on central corneal thickness is illustrated in Fig. 2. Each data point is the mean of 1 1 subjects. The mean corneal thickness before inserting the lenses in the morning was 0.520 mm (s.d. 0.033). This value is in good agreement with the result of other authors (see Hansen 1971). After 8 hours, corneal thickness increased to 0.556 mm (s.d. 0.041). This result is significantly higher (P< 0.001) than before inserting the lenses. Thus the corneal thickness increased on average by 6.9 O/o (range from 1 to 120/0). This finding is in good qualitative accord with the results of the investigations cited earlier. In only one subject was there only a very slight increase (1 U/O) in corneal thickness but there was nevertheless a decrease in central (34 o/o) and peripheral (50 O/O) corneal sensitivity. On the other hand, the subject displaying the greatest increase in corneal thickness had also the largest reduction of corneal sensitivity.
OW CONlACl LENS WEAR 0HOURS
-
AFIER
CONIACI LENS WEAR
0.56
t
I
BEFORE C O N l A C l LENS W E A R
0 4 0 12 16 20 24 20 TIME FOLLOWING C O N l A C l LENS R E M O V A L mtn.
I I
I 1
1
,
,
,
,
,
,
1
,
,
,
1
,
,
,
Fig. 2. Relationship between central corneal thickness and the wearing of hard contact lenses for 8 hours. Each data point is the mean of 1 1 subjects. 580
E f f e c t of Hard Contact Lenses
Thirty minutes after removal, corneal thickness decreased to 0.524 mm (s.d. 0.037) which is significantly (P 0.001) lower than immediately after removal. The decline of corneal thickness after 30 min was on average 6.3 O/O. Although the mean corneal thickness 30 rnin after removal is very close to the mean corneal thickness in the morning, it is significantly different (P 0.05),which means that corneal thickness has not fully recovered within that time. A comparison between central and peripheral measurements of corneal sensitivity obtained in the morning prior to inserting the lenses yielded a correlation coefficient of 0.78 which is significant (P 0.01). Thus it appears to be valid to evaluate the effect of contact lenses on corneal sensitivity by measuring either area of the cornea.
