The Intraocular Variations of IOP

By Paul Verkey, Kristina Ptochantonis, and Shetaaj Moorad

In clinic, we spend a fair amount of time measuring countless pressures and it is easy to forget what factors affect intraocular pressure (IOP) over time; these factors may affect how we manage the patient at the end of our clinical examination.
IOP values do not follow a normal distribution, however if we normalise current values the average IOP is about 16 mmHg.

So just what does affect IOP over our day? Some of the factors I have listed here:

Age
On average, IOP increases by 5mmHg between the ages of 20-70 and decreases slightly after 70 years. However, in the latter case it is only in the region of about 1-2mmHg.

Accommodation
As the crystalline lens changes its shape when we accommodate, there is initially a brief increase in IOP followed by a small continuous decrease. The initial increase is thought to be caused when extraocular muscles converge. The drop registered is about 4-5mmHg over 4 minutes for a +4.00D change; this is due to miosis linked with accommodation.

Gender
Females have been found to have a mean IOP of about 1-2mm higher compared to males.

Extra-ocular muscle action
It is believed that IOP increases with convergence by approximately 4mmHg. A varying amount of pressure is produced on Horizontal and Downward Gaze.

Posture
IOP is slightly higher lying down and therefore slightly greater when measured with a Schiotz tonometer.

This diagram, taken from an IOVS article (Reference below), shows just how IOP can vary whilst asleep, and during the day, be it standing or lying down.
IOP full size

Light levels
In the dark, the pupil dilates, hence reducing the angle of the anterior chamber. Therefore, IOP will rise slightly.

Genetic factors and Race
A variation in IOP between races shows a clear genetic influence. In Far Eastern races the IOP has been shown to decrease with age. In addition, Far Eastern races display far less IOP variation between males and females as opposed to Caucasians. Afro – Caribbean’s tend to have a greater increase in IOP with age.

Respiration
During the breathing cycle, IOP varies by as far as 2-3mmHg. It increases on expiration and decreases on inspiration.

Fluid intake, food and drugs
Keep away from the Guinness extra cold,….surprisingly alcohol may elicit a slight reduction in IOP overall.
Tobacco – causes a transient increase in IOP
Marijuana & heroin – cause a reduction in IOP
Systemic steroids may cause and increase in IOP
Beta Blockers may cause a reduction in IOP

Ocular Pulse Variations
IOP varies with the cardiac cycle in a sinusoidal manner. It can change as much as 3-5mmHg corresponding to systole or diastole.

Diurnal variations
IOP varies by about 3 - 5 mm Hg, in a 24 hour cycle. IOP is lowest at night and highest on rising in the morning. Diurnal variations may be much greater in primary open-angle glaucoma, with as much as a 20mmHg change.

Seasonal variation
The fluctuations in IOP are more significant in glaucomatous and ocular hypertensive eyes than in normal eyes. The IOP in winter is higher than in summer, with the IOP on average being 1mmHg lower in the summer.

Blinking
A blink may cause a rise of up to 10mmHg in IOP, a forced blink even higher than this. Any pressure applied to the eye, may cause fluctuations in IOP and such pressure could be caused by inappropriately lifting the patient’s eyelid.

References
Sandip Doshi & Bill Harvey. Eye Essentials: Assessment & Investigative Techniques, Pub Butterworth Heinemann