In order to complete this exercise, you are required to first read the following article:

  • Higginbotham J. The changing face of tonometry. Click here to read it.

Now consider the following points regarding IOP variation from Bill Harvey.

The measurement of intraocular pressure (IOP) by tonometry is a routinely undertaken on adult patients in practice as both a screening process and for monitoring the impact of various influences upon the eye. In community practice, tonometry is often initially undertaken using a non-contact tonometer and often as a delegated function.

It is recommended by the College of Optometrists that four readings be taken each eye and then the averaged reading recorded. This is because the instantaneous IOP measurement taken by non-contact methods will also measure any short term influence upon the IOP. If any of the four readings is an outlier, for example if the patient has blinked, then this is usually indicated by the instrument by putting brackets around the result, the result flashing, or the result being asterisked. It is important to remember that the IOP is constantly changing – even in the few seconds between readings being taken. These short-term fluctuations may be due to a number of reasons including the following;

  • Influence of the lids – a firm blink may cause a transient increase in IOP while prolonged squeezing may ‘massage’ the IOP down
  • Pupil dilation – sympathetic stress response may cause a transient rise
  • Accommodation – may cause an initial rise and the associated miosis then lead to a gradual reduction in IOP
  • Eye movement – extraocular muscle contraction may cause a transient elevation in IOP
  • Cardiac pulse – the pulsatile nature of arterial blood flow is reflected in a similar 2 to mmHg ‘pulse’ in IOP values

Other factors may further contribute to the IOP value, such as if the patient has recently drunk water, or alcohol or has to climb a flight of stairs to the consulting room and so on. If we remember the fluctuating nature of IOP, then the rule has to be to try and standardise conditions as far as possible to ensure readings taken at any one practice are as repeatable as is possible for one constantly changing.

There are other influences that might not show up between readings but certainly may influence the IOP value over hours or days. Here we can include the influence of some drugs, both legal and illegal. Perhaps the best known, though often least acted upon, of these ‘intermediate-term’ influences is the fluctuation of IOP over 24 hours.

Diurnal variation

To be pedantic, diurnal describes something throughout the daytime so, as IOP tends to fluctuate over the 24 hour period, we really are describing a circadian variation (a variation over a specific cycle of time). The diurnal variation is typically 5mmHg in normal eyes, but is higher in patients with ocular hypertension or glaucoma, with a diurnal variation of >10mmHg usually considered to be pathological. This variation over 24 hours is the reason why it is essential to note the time at which any tonometry reading was taken as it is expected to be different over time and this may be critical if the value is close to a threshold set for considering further management action. Here are a few key points regarding diurnal IOP fluctuation:

  • The most typical pattern of IOP change when measured over a complete day has a peak mid-morning, typical one or two hours after waking, and a minimum value late at night
  • Some patients show a peak mid-afternoon and occasionally some at night.
  • Some patients show no repeatable cyclical IOP change pattern
  • The characteristic mid-to-late afternoon depression in IOP might be more marked in males.

Explanations for the change tend to focus on aqueous production rate changes over the day. However, it is quite likely that the IOP is influenced by the natural circadian biorhythm process along with other aspects of metabolism. This might explain the random nature of IOP variation between patients, the observed seasonal influence (IOP is often a few mmHg higher during the winter months) and the alteration of IOP patterns with major lifestyle time management changes. Our in-built body clock regulates metabolic processes and some have linked IOP with the concentration of circulating catecholamines (such as adrenaline), and others with serum levels of cortisol.

This is a somewhat controversial so here are some useful references:

  • Fanelli J. Highs and lows of IOP. www.reviewofoptometry.com/article/highs-and-lows-of-iop This offers a useful argument about the various influences upon IOP
  • Kitazawa Y, Horie T. Diurnal variation of intraocular pressure in primary open angle glaucoma. American Journal of Ophthalmology, 1975, 79;57. A good overview often referenced in this area.
  • Costagliola C et al. IOP in a healthy population. IOVS, 1990, 67;204 A good summary of expected and varying IOP profiles in the absence of any disease
  • NICE guidance for glaucoma referral. www.nice.org.uk/guidance/cg85?unlid=469497752015922221523. The usual reference regarding IOP-based referral decisions in primary care

Before the interactive exercise we need you to complete six multiple choice questions relating to the source material. You can take these as many times as you wish until you have 100% score. You will then be able to begin the interaction.