Learning objectives To be able to assess intraocular pressure using appropriate techniques (Group 2.1.1)

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There are many factors that affect the measurement of intraocular pressure. This article explores the short,

medium and long-term factors, which can have an impact on the results, allowing the practitioner to take these

into account when they perform tonometry on their patients.

Factors affecting intraocular pressure measurementDr Kirsten Hamilton-Maxwell PhD, BOptom (Hons), FHEA

About the authorDr Kirsten Hamilton-Maxwell is a lecturer and clinical optometric supervisor in the School of Optometry and Vision Sciences at Cardiff

University, where she teaches a range of clinical techniques. Her research interests include tonometry, corneal properties and intraocular pressure.

Course code: C-35741 | Deadline: April 11, 2014Learning objectivesTo be able to obtain a full history relevant to intraocular pressure measurement (Group 1.1.1) To understand the importance of recording full detail with respect to intraocular pressure measurement (Group 2.2.4)To be able to interpret the results when undertaking tonometry (Group 3.1.6) To be able to recognise anomalies in intraocular pressure readings (Group 6.1.5)

Learning objectives To be able to explain to the patient about the implications of intraocular pressure readings (Group 1.2.4) To be able to understand the implications of intraocular pressure readings (Group 3.1.6)

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Introduction Intraocular pressure (IOP) is the primary risk

factor for the development and progression

of glaucoma, and is presently the only

modifiable one. Since the introduction of NICE

CG85 for Glaucoma in 2009,1 and subsequent

guidance from the College of Optometrists

and the Royal College of Ophthalmologists,2

optometrists have been required to refer

patients to the Hospital Eye Service or to a

specialist optometrist when the IOP is higher

than 21mmHg, even in the absence of other

glaucomatous changes.

Recent research has shown that the

requirement for IOP-based referrals has resulted

in a large increase in the total number of

patients being referred by optometrists, as well

as an increase in the proportion of patients who

do not have an IOP above the threshold level

after referral.3 Enhancing the accuracy of IOP

measurements is, therefore, vital. The purpose

of this article is to highlight factors that can

influence IOP and provide recommendations

to make measurements more reliable.

Intraocular pressure (IOP)The aqueous humour is secreted continuously

at a rate of approximately 2.75±0.63µL/min

(range 1.8 to 4.3µL/min) by the epithelia of the

ciliary body into the posterior chamber of the

eye, with a much smaller component

originating from the filtration of blood plasma

in the ciliary processes.4 It circulates through

the pupil into the anterior chamber, then drains

through Schlemm’s canal via the trabecular

meshwork, with a lesser amount passing

through uveoscleral channels.4 The aqueous

humour has several important functions within

the eye including the supply of nutrients and

the removal of waste products from anterior

ocular structures.5 It also exerts a fluid pressure

that keeps the globe of the eye inflated.

IOP is often said to be ‘normal’ if it is between

10–21mmHg; a figure calculated as the range

over which 95% of IOPs are expected to lie.6

However, IOP is a highly dynamic quantity

that changes over time. At any given moment,

the IOP depends on the combination of three

factors, which are all fluctuating independently

over differing timescales:

• The amount of fluid within the eye, which

is determined by the balance of aqueous

humour production and drainage

• External forces acting upon the eye, including

the tension within the ocular walls themselves

• The intraocular volume.

Common circumstances in which these

factors can influence IOP measurement will

now be discussed and are summarised in

Table 1.

Figure 1 Position of the GAT semi-circular mires during the highest (right) and lowest (left) parts of the ocular pulse cycle (green). The tonometer should be aligned so that the mires oscillate around an imaginary central point of alignment (grey)

Short-term Medium-term Long-term/permanent

Ocular pulse

Breath-holding

Straining

Tight clothing around the neck

Posture

Accommodation

Eye position

Lid squeezing

Opening eyes wide

Eye rubbing

Contact lens removal

Diurnal variation

Eating and drinking

Smoking

Systemic medication

Exercise

Accommodation/reading

Optometric techniques

Age

Lifestyle (e.g. smoking)

General health

Gender

Season

Ocular factors

Table 1 Common factors that affect IOP. (Note that this list is not exhaustive)

Figure 2 IOP changes induced by walking for a short distance. Adapted from Hamilton-Maxwell and Feeney (2012)45

Baseline IOP After walking After 20 min rest

Intr

aocu

lar p

ress

ure,

mm

Hg

(mea

n an

d 95

% C

I)

13

14

15

16

17

18

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or a decrease if the patient breathes in.9,10

However, a more recent study suggests that

the impact of breath-holding, of the type that

is likely to occur during tonometry, is likely to

be minimal.10 Until there is more supporting

evidence, it is still advisable to remind patients

to breathe normally during tonometry.

StrainingStraining to reach equipment is also likely

to cause an artificial rise in IOP of around

5mmHg.9,11 If it is not possible to position the

patient at the slit lamp without difficulty, then

an alternative tonometer should be used,

if available.11

Tight clothingPressure around the neck from a necktie can

cause an increase in venous pressure and

subsequently raise the IOP by 1–3mmHg.12

It has been proposed that the IOP returns

to normal levels if the necktie is left in

place.13 It is, however, still possible that tight

clothing around the neck may become an

issue when the patient leans forward into

position at the slit lamp;9 that being the case,

it is recommended that any tight clothing

around the neck be loosened prior to IOP

measurement.

PostureIOP increases when a person changes their

position from sitting to reclined; most

studies have reported a range of around

1–6mmHg in healthy eyes and up to 9mmHg

in glaucomatous eyes.14 An increase in IOP of

approximately 2mmHg has also been found

even when the patient is only slightly reclined

by 15 degrees.15 In most optometric practices,

IOP is measured in a seated patient, however

should a measurement need to be taken

when the patient is reclined, such as during a

domiciliary visit, the patient’s posture should

be recorded. It would be unwise to try to make

an adjustment based on posture because

the size of the effect varies widely between

individuals.

AccommodationAccommodation has an interesting effect on

the eye, being able to increase or decrease

the IOP depending on the circumstances. It

is likely that the accommodative effect on

Short-term fluctuations in IOPShort-term fluctuations of IOP occur over a

timescale of seconds or minutes; this is of

significance to practitioners as they can occur

during the time of measurement. Although

this means that they can have a clinical impact,

they are also among the easiest type of IOP

fluctuations to monitor or control.

Ocular pulseThe ocular pulse results from changes to

intraocular volume that occurs when the

choroid expands and contracts during the

cardiac cycle. It causes the IOP to change

by an average of 3mmHg (range 1-7mmHg)

every time the heart beats.7 Even though the

ocular pulse cannot be eliminated, it can be

controlled in several ways. When performing

Goldmann applanation tonometry (GAT),

the ocular pulse is visible as an oscillation of

the semi-circular mires around an imaginary

central point of alignment. Aligning the

upper and lower semi-circular mires, so that

they oscillate by an equal amount around

that central point, will allow for a reasonable

approximation of the IOP to be made (see

Figure 1). For tonometers that take their

measurements over a period that is shorter

than the ocular pulse cycle, such as non-

contact tonometers (NCT), it is recommended

that four readings are taken and the average

value calculated capturing snapshots of IOP at

various points across the ocular pulse cycle.8

Increased venous pressureVenous pressure can elevate the IOP via

an increase in choroidal volume, reducing

the space available for the intraocular fluid;

common examples of this are forceful

breathing, breath holding, straining and

changes in posture.

Breath holdingIt is common for patients to hold their breath

during tonometry for a variety of reasons

including anxiety, trying to be a good patient,

some may simply be unconscious they are

doing it. Many practitioners may remember

being taught that IOP increases during breath-

holding due to evidence from studies showing

that activities involving forced breathing such

as the Valsalva manoeuvre causes an IOP rise

of at least 5mmHg if the patient breathes out,

IOP is related to effort rather than the amount

of accommodation used, so will occur in

presbyopes and in cyclopleged pre-presbyopes,

as well as in patients with a full accommodative

amplitude.16,17

During accommodation, an IOP increase

of 2–4 mmHg takes place.16 Should

accommodation occur during tonometry, such

as a patient who fixates on the tonometer, slit

lamp or practitioner, the IOP will be higher

than normal. Patients should, therefore, be

encouraged to fixate on a distant object during

tonometry. However, following a period of

sustained accommodation for as little as one

minute, the IOP will decrease by 1–5mmHg, and

this effect will last for at least 15 minutes.9 It is

theoretically possible that optometric tests such

as amplitude of accommodation or near vision

testing (without an addition) will subsequently

influence the IOP; the practitioner may,

therefore, wish to consider the order of testing.

Similarly, consideration should also be given

to whether a patient may have been viewing a

near target such as a smartphone, or reading a

magazine, while waiting for their appointment.

Eye positionThe force exerted by the extraocular muscles on

the globe can cause the IOP to increase when

the direction of gaze changes from the primary

position, particularly in upgaze. Though this

effect is modest (<2mmHg) in most patients,9

those with disorders of the extraocular

muscles, for example, in thyroid eye disease,

will exhibit a much higher increase in IOP.18

Provided that the slit lamp chin rest is adjusted

so that the outer canthus is aligned with

the canthus marks, there will be no vertical

discrepancy in the position of gaze. However,

care must be taken when choosing a fixation

target that avoids the practitioner’s head (which

would induce accommodation) but is also not

positioned too far laterally. Of course, variations

to both vertical and horizontal position of

gaze can easily occur when using hand-held

tonometers, so caution should be exercised

under these conditions.

Lid squeezingLid squeezing – where a patient struggles to

keep their eyes open during measurements –

has been shown to cause an increase in IOP of

around 5mmHg.16 In its most extreme form,

Table 1 Characteristics of anterior lid margin disease

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IOPs of over 80mmHg have been recorded

during blepharospasm. This can be overcome

in most patients by encouraging the patient

to relax, trying again later (a few minutes

is often long enough), or using a different

tonometer with which the patient is more

comfortable.

‘Open your eyes wide’Contact with the eyelids can cause errors in

IOP measurements. In an attempt to avoid

this, practitioners often ask the patient to

open their eyes more widely. Research shows

that this instruction can cause the IOP to

increase by 2mmHg.9 Gentle encouragement

to refrain from blinking can often solve this,

but where the problem persists, it may be

advisable to manually position the eyelids

against the orbital bone.

49

Eye rubbing and contact lens removalApplying manual pressure to the globe will

cause the IOP to increase initially, but this

will be followed by a period of reduced IOP

once the external force has been removed.

In particular, the IOP is likely to be very high

while eye rubbing is taking place, but will

subsequently be lower by an average of

1mmHg.10 It would, therefore, be advisable

to ask a patient to avoid rubbing their eyes

immediately prior to tonometry, but it might

also be useful to ask a patient about recent eye

rubbing if there is a history ocular allergies for

patients whose IOPs are near the borderline for

referral.

Similarly, soft contact lens removal can place

transient pressure on the eye. Although the

average fluctuation in IOP is small (-0.8mmHg

if the lens is plucked or +0.5mmHg when

using a sliding technique), it can be as much

as ±4mmHg in some patients.19 This error can

be avoided completely if there is five minutes

or more between soft contact lens removal

and IOP measurement.19 There is currently no

evidence regarding the effect of rigid contact

lens removal on IOP.

Medium term fluctuations in IOPIn the context of this article, medium term

fluctuations in IOP are those that occur over a

timescale of a few hours to a few days. These

fluctuations can be more difficult to deal with

because the practitioner cannot, in most cases,

control them.

Diurnal variationOne of the most significant fluctuations in

Figure 3 Factors to consider when measuring IOP

• Use a standard tonometry protocol, paying careful attention to patient set-up and instructions

• Consider the order of your clinical routine• Avoid IOP measurements in the first two hours after waking, as this is when the IOP is

likely to fluctuate the most• If you intend to repeat IOP measurements on a different day, provide advice that will

minimise IOP fluctuations

• Identify potential problems by taking a thorough clinical history• Note any likely sources of fluctuation to aid interpretation

• For example, an asymmetric IOP can indicate pathology such as a retinal detachment or uveitis

Eliminate or minimise IOP fluctuations whenever possible

For IOP fluctuations that cannot be eliminated

Consider other conditions that may cause changes in IOP, as well as glaucoma

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IOP is the diurnal variation, where the IOP

fluctuates over a cycle lasting approximately

24 hours. The diurnal variation may be due to:

• Posture14

• Ambient illumination20,21

• Stage of sleep22

• Circulating cortisol levels23

• Suspension and resumption of eye

movements and accommodation24

• Errors in IOP measurement25

• Many others have been proposed

(Prostaglandin, aldosterone, oestrogen,

thyroxine, catecholamine, ADH, plasma pH,

serum osmolarity, hypothalamus activity,

and adrenaline).26

Diurnal fluctuation is between 2–6mmHg

in healthy eyes, but may be 10mmHg or

more in glaucomatous eyes.27,28 It may also

be higher in hypermetropes.29 Most readers

will know that it is important to record the

time of day that IOP is measured. Although

it is advisable to continue to do so, there is

a growing body of evidence that question

whether time of day is as informative as once

believed.

The statements that are up for debate are

outlined in Table 2.

Eating and drinkingConsumption of food and drink appears to

have an effect on the IOP, although there is

limited research to date. It is known that IOP

increases upon the resumption of eating

and drinking in the evenings in patients who

observe Ramadan,34 and that drinking large

volumes of fluid also raises the IOP.35 The

contents of the food and drink, e.g. caffeine,

can also play a role.36 Further research is

needed, however, before generalisations can

be made about the relationship between

food and drink consumption and IOP.

TobaccoThere is both a transient and long-term IOP

rise in patients who smoke. Eleven per

cent of non-glaucoma patients and 37% of

patients with glaucoma have an IOP spike

of at least 5mmHg following a cigarette,

although it is unknown how long the effect

may last.37 The average IOP is also higher in

smokers, although when considered across

a population, the effect is just a fraction of a

mmHg.38 The transient increase in IOP may

interfere with measurements in some patients

who have taken the opportunity to smoke a

cigarette while waiting for their appointment.

AlcoholThe relationship between alcohol

consumption and IOP is unclear. In the

short term, it can be hypothesised that IOP

would decrease following significant alcohol

consumption due to dehydration; over a

longer term, the evidence is equivocal.39 It

seems probable that other lifestyle factors

that are associated with alcohol consumption,

such as general health, body mass index

(BMI) and diet, may have confounding effects

on the IOP.

Recreational drugsRecreational drugs can have varying effects

on IOP depending on their pharmacological

actions and it is beyond the scope of this

article to discuss them in any detail. It is

worth mentioning that cannabis decreases

the IOP to such an extent that it is being

investigated as a treatment for glaucoma,

provided that the psychoactive properties

can be removed.40

Systemic and ocular corticosteroidsOral and topical corticosteroid medications

are known to raise IOP. Approximately 4–6%

of the population will have an extreme

reaction where the IOP increases by

15mmHg or more; another third will have

a more moderate increase of 6–15mmHg,

while the remainder will see no increase

in IOP.41 However, recent research has

shown that topical steroids, including those

commonly available over the counter for

hay fever, can also cause an increase in IOP.41

Importantly, the IOP increase associated with

this type of steroid is not dose dependent

but instead based on whether the patient

has ever used them. A careful history is

essential to identify current and previous

steroid use.41

Statement Debate

The IOP is always highest in the morning

Though it is true that the IOP is probably at its highest level immediately after waking,25 this has typically subsided long before most practitioners open for business. Although there is still a slight tendency for an IOP to be higher in the morning hours, there is a such a large variation in the timing of the maximum IOP that it may be more appropriate to assume that it can occur at any time27,28

The diurnal variation is the same every day

Research has shown that the diurnal pattern is not the same every day, whether the eye is healthy or glaucomatous.30 This means that the reproducibility of IOP measurements obtained on different days, even at the same time, is limited31

The diurnal fluctuation is the same in both eyes

Although it is long established that the IOP fluctuation will be different in the two eyes of patients with glaucoma,32 more recent evidence suggests that this asymmetric fluctuation can also be seen in healthy eyes33

Table 2 Uncertainties surrounding the diurnal variation of IOP

MORE INFORMATION References Visit www.optometry.co.uk/clinical, click on the article title and then on ‘references’ to download.

Exam questions Under the new enhanced CET rules of the GOC, MCQs for this exam appear online at www.optometry.co.uk/cet/exams.

Please complete online by midnight on April 11, 2014. You will be unable to submit exams after this date. Answers will be published on

www.optometry.co.uk/cet/exam-archive and CET points will be uploaded to the GOC every two weeks. You will then need to log into your CET

portfolio by clicking on ‘MyGOC’ on the GOC website (www.optical.org) to confirm your points.

Reflective learning Having completed this CET exam, consider whether you feel more confident in your clinical skills – how will you change the way you

practice? How will you use this information to improve your work for patient benefit?

are separated by five minutes or more.50 The

origin of this behaviour is uncertain, and

it has been proposed that there may be a

true reduction in IOP resulting from ocular

massage, or it may be a measurement error

resulting from corneal flow.9 A third option is

that it may be due to a reduction in patient

apprehension about the procedure.51 It is

suggested that GAT should be performed

as quickly as possible to reduce the number

and duration of contacts with the eye, and

that the first measurement in each session be

discarded.52 Repeated measurements using

NCT does not cause an IOP reduction.53

Visual fieldsIOP and visual field assessment (VFA)

are integral clinical procedures required

for the diagnosis and management of

glaucoma. Several factors that may influence

IOP are modified during VFA, including

accommodation, pupil size, posture and

anxiety levels.9 Reports have shown that IOP

increases in patients with primary open angle

glaucoma (POAG) following VFA but not in

controls.54 Others have noted, however, that

there is no mean change in IOP in either

treated POAG, suspect POAG or ocular

hypertensive (OHT) patients.55 Equivocal

findings between studies may relate to

the length of the visual field test, with

longer tests causing an increase in IOP and

short tests having no effect. It is, therefore,

recommended that IOP is measured prior to

VFA, if it is likely that the test duration will be

longer than four or five minutes.

Longer-termAge, lifestyle and general healthThe effect of age on IOP depends on two key

factors: racial origin and general health. In

a western white population, the IOP might

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Systemic anti-hypertensive medicationAnti-hypertensive medications also act upon

the eye, so it would be typical of patients

taking these medications to have a reduced

IOP; this can be useful, reducing the need for

topical IOP lowering medications in glaucoma

patients.42

Exercise Patients who have recently participated in

strenuous aerobic exercises such as running

and cycling will have an IOP that is lower than

normal. Decreases of up to 6mmHg have been

reported in healthy eyes,43 with up to 13mmHg

reduction in glaucomatous eyes.44 It is thought

that IOP returns to pre-exercise levels after

approximately 20–60 minutes, although

recovery time can vary a great deal.43

A more subtle effect is the IOP decrease that

can occur after a short walk of just two thirds

of a mile, reducing measures by 1.4mmHg with

levels not recovering even after 20 minutes

(see Figure 2).45 The fall in IOP appears to be

related to the level of exertion, rather than the

total time walked.45 Where IOP measurements

are critical, practitioners are advised to seek

information regarding a patient’s mode

of transport and their perceived level of

exertion.45 If there were any doubt, it would be

advisable to repeat the measurements after

the patient has been resting for approximately

one hour.

Optometric techniquesTonometryAn important factor to consider with IOP

fluctuations is the effect of the tonometer

itself. Repeatedly subjecting a cornea to GAT

can cause the IOP to decrease by 3–4mmHg, if

the measurements are repeated at one-minute

intervals over a period of several minutes.49

This effect disappears if IOP measurements

increase by a very small amount,56 if at

all.57 An increase of approximately 1mmHg

per decade can be expected in patients

of African descent,58 whereas a decrease

with age is found in patients of east-Asian

origin.59 Changes in IOP with age are probably

associated with systemic factors such as

hypertension, diabetes, BMI, smoking and

cholesterol.56,57 All of the aforementioned

factors can also contribute to elevated IOP

in younger individuals who have these

conditions.60

GenderGender has also been indicated as an

important factor for IOP levels in a number

of studies, with a higher IOP reported in

women,61 though this observation is not a

universal finding.56,57 Hormone replacement

therapy in menopausal women appears to

cause IOP to decrease by an average of

1.4mmHg.62

Other factorsThere is a seasonal influence on IOP, and it

is has been reported that it may be up to

2mmHg higher in winter than in summer.63

IOP tends to be higher in myopes,64 and it is

also higher in brown eyes than blue eyes in

Caucasians, although the effect is small in

both cases.65

ConclusionIn conclusion, there are many factors that can

affect IOP; these can be natural or induced,

avoidable or unavoidable, and can be over

a short or long term. It is important to

eliminate sources of IOP fluctuation wherever

possible (see Figure 3, page 49). Where it is

not possible, a good understanding of the

sources of IOP fluctuations is essential to

interpret IOP measurements with confidence.