Craig Blackwell, MD
Ophthalmology

Santa Cruz, CA
Diplomate: American Board of Ophthalmology
Fellow: American Academy of Ophthalmology

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An Ophthalmology Practice in Santa Cruz, CA

What does Diurnal mean ?

June 28th, 2008

In studying glaucoma one frequent question that arises is whether there is a time of day when the intraocular pressure (IOP) is expected to be the highest? It is reasonable to ask if pressure follows a circadian rhythm like other functions in the body, and if pressure is predictable then we would know the best time of day to check pressure. Sounds logical, but consider the following studies.

Pressure at different times of day.

A German group led by Jost Jonas made the following measurements. In 547 Caucasian patients who had glaucoma or were glaucoma suspects the pressures were measured at 7 am, 12 noon, 5 pm, 9 pm and 12 midnight. A record was made for each patient noting the high and low readings. The following table shows the percentage of people that had their maximum pressure at a given time of day.

Time of Day

7 am

12 noon

5 pm

9 pm

12 midnight

20%

18%

21%

14%

27

Percent of Participants with Highest IOP at that Time

In this study pressure peaks are rather evenly spread throughout the day, with a possible preponderance at midnight. Jonas concluded that taking pressure from 7 am to 9 pm had a greater than 75% chance of missing the highest pressure reading of the day.

[AJO. 2005;139:1136]

Diurnal, by the way, means during the day, as opposed to nocturnal. That difference comes into play in the following report from Japan.

Pressure during Sleep

Takahashi Hara and his group measured pressures in 148 glaucoma patients in the sitting position and lying down. Pressure was measured every two hours from 6 am to midnight and at 3 am. (Results below are given as mean ± SD. Pressure units are mmHg.)

Result #1:

  • The average pressure peak while sitting was 16.0 ± 2.7.
  • The average pressure peak lying down was 18.9 ± 3.9.

At all time points the pressure was higher lying down.

Why should that be? Sitting up the heart is about 12 inches below the eyeball. Venous return is downhill. Lying down the eye and heart are at the same level so increased venous pressure causes more resistance to outflow from the eye.

Result #2:

  • The pressures measured lying down ended up being the highest pressures of the 24 hour period.

Why are pressures lying down a significant question? Because we spend 8 hours a day lying down sleeping, those are the hours when the pressure is highest.

Note, the diurnal pressure peaks were evenly spread through the day as in the Jonas study.

[Arch Ophthal. 2006;124:165-168]

Possible Effect on Treatment Choice

Do the above results affect treatment decisions regarding glaucoma? Consider the following. The production of aqueous humor is highest during the day and decreases, about 45%, during sleep. Drops like Timolol work by decreasing aqueous production. Therefore, that class is most effective during the day, but less so at night. That is why, for those taking the once-a-day form of Timolol, we recommend taking it in the morning.

Another class of glaucoma medications, Prostaglandins (Xalatan, Travatan, Lumigan), seems to be equally effective at all time periods.

However, Trusopt and Azopt supposedly work on the same part of the eye as Timolol, but they had pressure lowering equal to the Prostaglandins during sleep.

[Invest Ophthal and Vis Sci. 2006;47:2917-2923]

Various aspects of pressure fluctuation have been under study for a long time with different studies giving different results. Whether or not coming in for pressure checks at midnight, 3 am and 6 am will make a difference in treatment is not yet clear. And there are new longer lasting treatments that will be arriving soon that may render that question moot. However you answer the above questions, the principal remains that if there is glaucoma damage, then whatever pressure you have is too high and it is worth the effort to get it lower.

CB June 2008