The study adhered to the tenets of the Declaration of Helsinki and was approved by our Institutional Review Board. Experimental subjects were recruited consecutively from patients referred to the Hamilton Glaucoma Center of the University of California, San Diego. Reasons for the referrals might include high IOP, family history of glaucoma, and suspected change of optic disc or visual field. Each patient underwent a complete ophthalmic examination in the Glaucoma Center that included a review of relevant medical history, best corrected visual acuity, slit lamp biomicroscopy, gonioscopy, Goldmann applanation tonometry, dilated funduscopy, and visual field test. Patients with early glaucomatous changes in at least one eye were considered for inclusion in the study.
Glaucomatous changes included abnormal optic discs and repeatable abnormal visual fields. Abnormal discs might include excavation, rim defect, hemorrhage, notching, nerve fiber layer defect, or cup-to-disc asymmetry between the eyes of 0.2 or more. An abnormal visual field was determined by clinical review and by the manufacturer’s criteria for abnormality (Statpac II; full-threshold 24-2 examination, Humphrey Field Analyzer; Zeiss-Humphrey, Dublin, CA). The corrected pattern standard deviation was outside 95% or glaucoma hemifield test outside 99% of the age-specific norms. The office IOP reading was not used as an inclusion or exclusion criterion for two reasons. First, there was no long-term follow-up of IOP in these patients. Second, we have observed in previous studies
2 3 4 that one or a few office IOP readings do not correlate well with the 24-hour IOP profile.
Individuals who had any use of glaucoma medication or systemic β-adrenergic blocking agents, a history of eye surgery, ocular inflammation or trauma, a narrow iridocorneal angle, or advanced glaucomatous changes that needed immediate treatment were excluded. Individuals who smoked or had an irregular daily sleep schedule were also excluded. Informed consent was obtained after explanation of the nature and possible consequences of the study. Fourteen patients with abnormal optic discs and abnormal visual fields, eight patients with abnormal optic discs only, and two patients with abnormal visual fields only were recruited as the glaucoma group. The group included 11 men and 13 women who were 40 to 78 years old (59 ± 12 years, mean ± SD). There were 17 whites, 4 African Americans, and 3 Asians. Their sitting IOP levels were in the range of 12 to 34 mm Hg (21.6 ± 6.0 mm Hg; mean ± SD) during office evaluations.
Experiments in the sleep laboratory took place within a few weeks after recruitment. Subjects were instructed to maintain a daily 8-hour sleep period for 7 days before the experiment. Daily wake-sleep schedules were verified using a wrist monitor for light exposure and physical activity (Actiwatch; Mini Mitter, Sunriver, OR) and by the wake-sleep log. Subjects were asked to abstain from alcohol and caffeine for 3 days and to arrive at the laboratory at approximately 2 PM on the study day. They stayed indoors for the next 24 hours. Light intensity in the laboratory was kept at 500 to 1000 lux at eye level. The 8-hour period of darkness in the subject’s room was adjusted to correspond to each individual’s sleep time. Times for measurements were also individualized to coordinate with the sleep period. Although sleep and the measurement schedules were individualized, corresponding clock times were normalized for data presentation as if each subject had a sleep period from 11 PM to 7 AM.
Over 24 hours, IOP, blood pressure, and heart rate were measured by experienced personnel every 2 hours. IOP was measured in both eyes with a pneumatonometer (model 30 classic, Mentor O&O, Norwell, MA). Topical 0.5% proparacaine was applied as the local anesthetic. The pneumatonometer was calibrated against the manufacturer’s verifier, and it was confirmed that different measurement angles produced the same IOP reading. A printout of every IOP measurement was obtained and evaluated according to generally accepted standards.
2 Blood pressure and heart rate were measured using an automated wrist blood pressure monitor (model HEM-608; Omron, Vernon Hills, IL).
5 The automated device was used to minimize the variability between researchers. A wrist model was chosen for convenience of use at night.
Before the sleep period, measurements were taken at 3:30, 5:30, 7:30, and 9:30 PM. Subjects were instructed to lie in bed for 5 minutes before the measurements of blood pressure, heart rate, and IOP. They then sat for 5 minutes before the sitting IOP measurements. Subjects were encouraged to continue normal indoor activities. Food and water were available, and meal times were not regulated. Lights in individual sleep rooms were turned off at 11 PM. Measurements of blood pressure, heart rate, and IOP were taken in supine subjects at 11:30 PM and 1:30, 3:30, and 5:30 AM. Subjects were awakened, if necessary, and the measurements were completed in a few minutes. A dim room light (<10 lux) was used to assist the nocturnal measurements. Nocturnal IOP in the sitting position was not measured because of the concern that activation of the sympathetic nervous system while changing body position to sitting (baroreflex) at night could be substantially nonphysiological. Room activities were continuously videotaped with infrared recording systems. Daytime room lighting was restored at 7 AM, and subjects were awakened, if necessary. Measurements were taken again at 7:30, 9:30, and 11:30 AM and at 1:30 PM, as previously described.
Data of IOP from the glaucomatous eyes were used for analyses. In the 22 patients in whom both eyes had glaucomatous changes (either optic disc or visual field, or both), IOPs of both eyes were averaged. Mean blood pressure was calculated as the diastolic blood pressure plus one third of the difference between the systolic and the diastolic blood pressures. Means of IOP, blood pressure, and heart rate in all subjects were calculated for each clock time point, the diurnal period, and the nocturnal period. Statistical comparisons of the means between two time points (e.g., 5:30 and 7:30 AM) and between the diurnal and the nocturnal periods were made with the paired t-test. Correlation analyses were performed to explore the association between the diurnal or nocturnal IOP and the diurnal-to-nocturnal change in IOP, blood pressure, or heart rate. The criterion for statistical significance was P < 0.05.
Estimation of the 24-hour IOP rhythm was performed for habitual body positions (sitting during the day and supine at night) and for the supine position throughout.
4 The best-fitting cosine curve was determined for each experimental subject by using IOP data obtained from the 12 time points.
6 The peak of the fitted curve (acrophase) represented the phase timing. The null hypothesis of a random distribution of acrophases around the 24 hours was evaluated statistically by the Rayleigh test.
7 Lack of significance indicated no 24-hour IOP rhythm in the group, whereas the alternative conclusion indicated a synchronized 24-hour rhythm timing. The amplitude (height of the fitted curve) estimated the magnitude of the 24-hour IOP variation.
Under similar experimental conditions, data on sitting and supine IOP, blood pressure, and heart rate were obtained from an age-matched control group of 10 men and 14 women. We assumed that potential modifications of IOP and cardiovascular parameters due to the experimental procedures, such as awakening the experimental subject during the nocturnal period, would be similar in the control and the glaucoma groups. Nonsmoking volunteers with healthy eyes were recruited from university employees and local residents for the control group. They underwent a complete ophthalmic examination, demonstrating absence of any eye disease. Normal visual fields were confirmed with a full-threshold examination (24-2 program; Zeiss-Humphrey). Other excluding criteria were similar to those used for the glaucoma group. The control subjects were 40 to 74 years of age (mean, 56 ± 9) and included 18 whites, 3 African Americans, and 3 Asians. Office IOP readings were in the range of 12 to 21 mm Hg (mean, 16.0 ± 2.4). Experimental data were analyzed as described previously. Statistical comparisons between the glaucoma group and the control group were performed using Student’s t-test, except that acrophases and amplitudes were compared using the Mann-Whitney rank-sum test, if 24-hour IOP rhythms were detected.