In 1973, the relationship between Schlemm’s canal (SC) and intraocular pressure (IOP) was reported by Johnstone and Grant [
Previous studies have indicated that aerobic exercise can induce a reduction in IOP. And after aerobic exercise, IOP can gradually increase to the preexercise level, and this postexercise IOP recovery has been suggested to take 30–60 minutes [
With the advance of optical coherence tomography (OCT), the newly developed swept-source OCT (SS-OCT) has a higher scan speed and a higher axial resolution, leading to a more detailed and clear
The aim of this study was to investigate the recovery process of postexercise SC and its relationship with postexercise IOP recovery using SS-OCT in healthy individuals, in an effort to gain a better understanding of SC and its relationship with IOP.
This study was approved by the ethics committee of Tongji Hospital and adhered to the tenets of the Declaration of Helsinki. All subjects provided written informed consent prior to participation in the study.
Forty eyes from twenty healthy young individuals were included. They underwent measurements of best-corrected visual acuity (BCVA), refractive error, central corneal thickness (CCT), axial length (AL), slit-lamp examination, gonioscopy, visual field test, and fundus photography. The inclusion criteria were as follows: age ≥ 18 years; IOP of 10–21 mmHg (Goldmann tonometry); normal visual field; and no use of drugs affecting the circulatory system within a month prior to evaluation. Subjects with unclear ocular media, family history of glaucoma, medical history of ophthalmic diseases or surgery, or systemic diseases and low-quality OCT images (evaluated on the basis of visibility of scleral spurs and iris, continuity of anterior chamber structures, and motion artifacts) were excluded. Both eyes of each subject were selected for IOP and SS-OCT examinations, and all the examinations were performed in a right-to-left eye order [
The participants rested for 20 minutes and then performed aerobic exercise by jogging on the treadmill for 20 minutes. After jogging, they were instructed to rest (sitting quietly) for another 60 minutes. IOP and blood pressure (BP) were measured just before exercise (to evaluate the status at rest), at 0 minute (immediately) after exercise (to evaluate the status during exercise) and at 15, 30, and 60 minutes after exercise (to evaluate the status after exercise) by the same operator and instruments. IOPs were measured using a noncontact tonometer (NIDEK RT-2100; NIDEK, CO., LTD, Gamagori, Japan). Three measurements were obtained, and the average IOP was recorded. BPs were recorded using an automatic sphygmomanometer (OmronHEM-7201; Omron, Dalian, China). Heart rate (HR) was monitored throughout the exercise with an oximeter (prince-100B; Heal Force, CO., LTD, Shanghai, China) for controlling exercise intensity (aerobic exercise) by evaluating the percentage of heart-rate reserve (% HRmax) [
SS-OCT (CASIA SS-1000; Tomey Corp., Nagoya, Japan) has a 1310 nm wavelength with a scan speed of 30,000 A-scans/s and an axial resolution of less than 10
The scans were performed three times, and the best-quality image was chosen for analysis. SC was defined as observable when a thin, black, lucent space was detected in two or more consecutive horizontal B-scan images [
Blood samples were collected by each subject before and at 0, 15, 30, and 60 minutes after exercise, in vacuum with EDTA. Plasma was obtained by centrifugation of blood sample at 1000 × g (Heraeus Multifuge X1R; Thermo Scientifc, Osterode, Lower Saxony, Germany) for 10 minutes at 4°C and stored at −80°C. Plasma noradrenaline (NA) and adrenaline (A) concentration were tested using high-performance liquid chromatography (HPLC) with electrochemical detectors (Waters HPLC pump, model 515; Waters electrochemical detector, model 2465; Waters autosampler, model 717; Atlantic C18 column (4.6 mm × 150 mm); Waters, Milford, MA, USA).
All analyses were performed using the SPSS software package version 21.0 (IBM Corp., Armonk, NY, USA). Data were presented as mean ± standard deviation (SD) where applicable. The generalized estimate equations, which take into account the correlation between the measurements from two eyes of one subject, were performed to compare pupil diameter, SC and IOP before and (0, 15, 30, 60 minutes) after exercise, and linear mixed-effects models were performed to compare blood pressure and plasma catecholamine before and (0, 15, 30, 60 minutes) after exercise. After age, sex, axial length, central corneal thickness and spherical equivalent were adjusted, univariate regression analysis was used to quantify the associations of postexercise changes in SC dimensions with postexercise changes in IOP. Univariable-adjusted
A total of 40 eyes of 20 subjects (9 males; 11 females; mean age was 26 ± 3 years) were included. Mean CCT was 540 ± 29
Compared with preexercise (baseline) values, systolic blood pressure (SBP), diastolic blood pressure (DBP), plasma catecholamine (noradrenaline (NA), adrenaline (A)), and pupil diameter (PD) increased significantly during exercise (
Comparison of BP, plasma catecholamine, PD, SC, and IOP before and after exercise.
Before exercise (baseline) | 0 min after exercise | 15 min after exercise | 30 min after exercise | 60 min after exercise | |
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SBP (mmHg) | 110.7 ± 8.4 | 143.5 ± 10.2† | 113.1 ± 9.1 | 112.1 ± 12.1 | 108.7 ± 9.3 |
DBP (mmHg) | 75.3 ± 7.5 | 83.2 ± 7.6† | 76.8 ± 8.1 | 75.5 ± 5.7 | 76.0 ± 4.5 |
NA (nmol/L) | 3.15 ± 1.23 | 9.26 ± 5.46† | 3.42 ± 1.52 | 3.24 ± 1.36 | 3.31 ± 1.18 |
A (nmol/L) | 0.49 ± 0.28 | 1.18 ± 0.90† | 0.43 ± 0.23 | 0.45 ± 0.22 | 0.46 ± 0.28 |
PD (mm) | 4.50 ± 0.67 | 4.81 ± 0.94‡ | 4.58 ± 0.78 | 4.54 ± 0.87 | 4.56 ± 0.72 |
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SC area ( |
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Superior | 4056.03 ± 1608.33 | 4894.49 ± 1783.57‡ | 3990.63 ± 1967.99 | 3936.21 ± 1371.07 | 3958.88 ± 1333.92 |
Nasal | 3245.64 ± 1229.18 | 4086.96 ± 1612.77‡ | 3376.63 ± 1106.34 | 3349.13 ± 979.86 | 3219.68 ± 1087.43 |
Inferior | 3936.06 ± 1774.87 | 4802.46 ± 1697.76‡ | 3911.81 ± 1438.14 | 3854.22 ± 1445.20 | 4252.15 ± 1861.88 |
Temporal | 4015.16 ± 2199.23 | 5002.51 ± 2386.53‡ | 4254.10 ± 2015.28 | 3544.64 ± 1645.59 | 3751.98 ± 1454.45 |
Average | 3726.81 ± 1167.06 | 4660.57 ± 1284.82‡ | 3784.22 ± 1193.19 | 3598.93 ± 937.01 | 3752.92 ± 951.11 |
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SC perimeter ( |
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Superior | 347.79 ± 105.80 | 390.26 ± 126.17‡ | 342.82 ± 121.52 | 355.85 ± 103.59 | 357.08 ± 109.34 |
Nasal | 293.57 ± 69.02 | 330.20 ± 99.41 | 312.75 ± 82.78 | 320.06 ± 70.52 | 297.08 ± 72.63 |
Inferior | 320.27 ± 92.93 | 358.28 ± 96.55‡ | 318.16 ± 84.38 | 304.96 ± 79.77 | 333.45 ± 96.27 |
Temporal | 346.13 ± 107.83 | 393.32 ± 127.93‡ | 356.83 ± 122.54 | 336.62 ± 111.07 | 333.91 ± 70.83 |
Average | 324.11 ± 58.95 | 367.19 ± 73.34‡ | 328.96 ± 72.31 | 323.92 ± 61.83 | 328.26 ± 48.38 |
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IOP (mmHg) | |||||
Average | 14.02 ± 2.33 | 11.65 ± 1.90‡ | 13.16 ± 2.26‡ | 13.49 ± 2.35‡ | 14.04 ± 2.30 |
†Significance of difference between preexercise (baseline) and (0, 15, 30, and 60 min) postexercise data: linear mix-effect models. ‡Significance of difference between preexercise (baseline) and (0, 15, 30, and 60 min) postexercise data: general estimate equations. SBP: systolic blood pressure, DBP: diastolic blood pressure, NA: noradrenaline, A: adrenaline, PD: pupil diameter, SC: Schlemm’s canal, IOP: intraocular pressure.
Schlemm’s canal before exercise and 0 minutes (immediately), 15 minutes, 30 minutes, and 60 minutes after exercise: the white arrow indicated Schlemm’s canal. (a) Before exercise, (b) 0 minute after exercise, (c) 15 minutes after exercise, (d) 30 minutes after exercise, and (e) 60 minutes after exercise.
After adjusting for age, sex, axial length, central corneal thickness, and spherical equivalent, the univariate regression analysis results showed that compared with preexercise (baseline) values, changes in SC area and perimeter were not significantly associated with changes in IOP at any postexercise measurement time-points (all
Univariate regression analysis of the associations of postexercise changes in SC dimensions with postexercise changes in IOP compared with preexercise (baseline) values.
Before vs. 0 min after exercise | Before vs. 15 min after exercise | Before vs. 30 min after exercise | Before vs. 60 min after exercise | |||||
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ΔSC area ( |
ΔSC area ( |
ΔSC area ( |
ΔSC area ( | |||||
ΔIOP (mmHg) |
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−223.19 [−501.79, 55.40] | 0.116 | −276.49 [−602.93, 50.00] | 0.097 | −216.31 [−466.28, 33.67] | 0.090 | −232.94 [−520.05, 54.18], | 0.112 | |
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ΔSC perimeter ( |
ΔSC perimeter ( |
ΔSC perimeter ( |
ΔSC perimeter ( | |||||
ΔIOP (mmHg) |
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−10.98 [−24.41, 2.46] | 0.109 | −15.92 [−37.37, 5.53] | 0.146 | −4.52 [−24.78, 15.74] | 0.662 | −3.99 [−22.38, 14.40] | 0.671 |
There were no significant differences in the observable SC proportion before and after exercise in any quadrant, or in total (all
Comparison of observable Schlemm’s canal proportion before and after exercise.
Observable SC proportion | Before exercise (baseline) | 0 min after exercise | 15 min after exercise | 30 min after exercise | 60 min after exercise |
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Superior quadrant | 34/40 | 37/40 | 34/40 | 33/40 | 33/40 | 2.178 | 0.703 |
Nasal quadrant | 34/40 | 37/40 | 36/40 | 35/40 | 33/40 | 2.286 | 0.683 |
Inferior quadrant | 32/40 | 36/40 | 34/40 | 36/40 | 33/40 | 2.581 | 0.630 |
Temporal quadrant | 38/40 | 39/40 | 35/40 | 36/40 | 35/40 | 4.243 | 0.374 |
In total | 138/160 | 149/160 | 139/160 | 140/160 | 134/160 | 6.971 | 0.137 |
SC: Schlemm’s canal.
Our results showed a good interobserver agreement of SC dimensions measurements before exercise, 0 minute after exercise, and at 15, 30, and 60 minutes after exercise (Table
Interobserver agreement of SC dimensions measurements.
ICC | Difference | 95% CI | ||
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Lower | Upper | |||
Average SC area ( |
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Before exercise | 0.902 | 189.83 | 0.823 | 0.947 |
0 min after exercise | 0.923 | 229.34 | 0.859 | 0.958 |
15 min after exercise | 0.896 | 102.12 | 0.811 | 0.943 |
30 min after exercise | 0.880 | 14.82 | 0.784 | 0.935 |
60 min after exercise | 0.893 | 55.09 | 0.807 | 0.942 |
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Average SC perimeter ( |
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Before exercise | 0.874 | 1.63 | 0.774 | 0.931 |
0 min after exercise | 0.934 | 5.04 | 0.879 | 0.965 |
15 min after exercise | 0.897 | 1.68 | 0.814 | 0.944 |
30 min after exercise | 0.896 | 2.93 | 0.812 | 0.943 |
60 min after exercise | 0.895 | 10.54 | 0.810 | 0.943 |
SC: Schlemm’s canal; ICC: intraclass correlation coefficient; CI: confidence interval.
Previous studies have demonstrated that SC and IOP are inversely correlated [
With regard to the reasons for changes in SC during and after exercise, autonomic nerves may play an important role. Previous studies have found that
In the current study, BP and PD increased significantly during exercise, indicating that exercise intensity of our study participants was sufficient to activate the sympathetic nervous system [
However, after adjusting for age, sex, axial length, central corneal thickness, and spherical equivalent, postexercise changes in SC dimensions were not significantly associated with postexercise changes in IOP. With regard to IOP, there are also factors other than SC that may contribute to changes in IOP during and after exercise, including changes in aqueous humor production and changes in the trabecular outflow facility, which were caused by exercise-induced changes in ocular blood supply, colloidal osmotic pressure of plasma, and catecholamine concentration [
In the present study, we only observed a slight, nonsignificant increase in the observable SC proportions between 0 minutes (immediately) after exercise and other time-points, indicating that exercise-induced changes in SC may mainly occur in SC dimensions, not the observable SC proportion.
This study has certain limitations. First, we only recruited healthy individuals but no patients with glaucoma. Considering that previous studies have suggested that exercise might have a long-term effect on the reduction in IOP and contributed to the lower baseline IOP, indicating a role of exercise in the glaucoma prevention and management [
In conclusion, exercise induced expansion of SC and reduction in IOP. Both of these changes recovered after exercise. SC returned to baseline values first (within 15 minutes), followed by IOP (within 60 minutes). Moreover, SC may be regulated by sympathetic nerves. Thus, during exercise, the activation of sympathetic nerves may increase SC dimensions and result in the reduction in IOP. After exercise, the sympathetic nerve activity decreased, leading to the reduction in SC dimensions. The decrease in SC dimensions reduced the aqueous humor outflow facility and finally caused the elevation in IOP.
The readers could access the data related to this manuscript by contacting the corresponding author out of reasonable requests.
The authors declare that they have no conflicts of interest.