INTRAOCULAR PRESSURE &TONOMETRY Dr. ZAW MIN HTET OPHTHALMOLOGIST MANDALAY , MYANMAR

1. INTRAOCULAR PRESSURE
AND
TONOMETRY
Dr. Zaw Min Htet
Ophthalmologist
EENT Hospital

2. Intraocular Pressure
• The fluid pressure inside the eye.
• Normal - 11-21 mmHg

3. Factors influencing IOP
Long Term Variation
• Age -increase with Age
• BP - increase with BP
• BMI - increase with BMI
• Climate - increase in winter
Short Term Fluctuations
• Alcohol consumption - a transient decrease in IOP
• Cannabis - decreases IOP
• IOP is higher in recumbent rather than upright position, predominantly
because of an increase in the episcleral venous pressure
• Caffeine - Increase IOP

4. Factors influencing IOP
Diurnal Variations
• Increase in morning
• Normal variation = 4 mmHg (> 10 mmHg in glaucoma)
• Accentuated in POAG
Eye Movement
• Clinically important in restrictive ophthalmopathy (e.g. thyroid eye
disease, pseudotumor)
Exercise
• Decrease in IOP
• Correlated to metabolic acidosis and changes in extracellular fluid
volume and osmolality

5. Factors influencing IOP
Pharmacological Effects
1. succinylcholine (muscle relaxant ) used in anaesthesia -transient increase IOP (by
around 10 mmHg for a few minutes)
2. Ketamine - increases IOP

6. TONOMETRY
Tonometry -The procedure to determine the IOP
Tonometer - a device to determine the IOP

7. APPLANATION
TONOMETRY
Goldmann applanation tonometry
GAT - Gold Standard
• based on the Imbert–Fick principle, for an ideal, dry, thin-
walled sphere, the pressure inside the sphere (P) equals the
force necessary to flatten its surface (F) divided by the area
of flattening (A)
• P = F/A

8. GAT
Type to enter a caption.

10. APPLANATION
TONOMETRY
• GAT - an accurate variable-force tonometer consisting of a double prism
• The tonometer prism should be disinfected between patients
• 2% sodium-hypochlorite (dilute bleach) - effective disinfection against
adenovirus and HSV
• disinfectants - cause the tonometer tip to swell and crack with time and lead
to disinfectant entering the tonometer tip resulting in a corneal abrasion
• 70% isopropyl alcohol wipes do not offer protection against viral infections
• Disposable tonometer prisms and caps have been introduced to address
concerns of infection from reusable prisms

11. Technique
• Topical anaesthetic (commonly proxymetacaine 0.5%) and a small amount of
fluorescein
• The patient is positioned at the slit lamp and instructed to look straight ahead
• With the cobalt blue filter and illumination of maximal intensity obliquely
(approximately 60°) at the prism, the prism is centred in front of the apex of the
cornea
• The dial is preset at 1 (i.e. 10 mmHg )
• The prism is advanced until it just touches the apex of the cornea
• 2 green semi-circular mires
• The dial on the tonometer is rotated to make the inner margins of the semi-circles
align
• The reading on the dial, multiplied by 10, gives the IOP in mmHg

12. GAT

13. SOURCE OF ERROR
1. Inappropriate fluorescein pattern
• Excessive fluorescein - too thick- overestimation of IOP
• Insufficient - the semi-circles too thin - underestimation of IOP
2. Pressure on the globe from the examiner’s fingers
• eyelid squeezing or restricted EOM (e.g. thyroid myo- pathy) - high reading
3. Central corneal thickness (CCT)
• GAT assume that CCT is 520 μm, with minimal normal variation
• If the cornea is thinner, an underestimation of IOP and if thicker, an
overestimation
• Conea - Thicker in OHT - Overestimation, Thinner in NTG and Refractive Surgery
(Underestimation)

14. SOURCE OF ERROR
4. Astigmatism if significant - distorted mires - mechanically induced errors.
5. Corneal Oedema - Artificial lowering of IOP
6. Incorrect calibration of the tonometer - a false reading
7. Wide pulse pressure
8. Repeated readings over a short period - a slight fall in IOP due to a
massaging effect on the eye.
9. Other factors - a tight collar and breath-holding - obstruct venous return- raise
IOP

15. APPLANATION
TONOMETRY
Pneumotonometry
• based on the principle of applanation, but the central part of the cornea is
flattened by a jet of air rather than a prism
• The time required to sufficiently flatten the cornea relates directly to the
level of IOP
• No contact with the eye and No need topical anaesthesia
• particularly useful for screening in the community
• The sudden jet of air can startle the patient
• Accuracy is improved if an average of at least three readings is taken

16. Pneumotonometer
Keeler Portable
Pneumotonometer

17. APPLANATION TONOMETRY
Portable applanation tonometry (Perkins)
• a Goldmann prism in conjunction with a portable light source
• hand-held and used in bed-bound or anaesthetized patients

18. Electronic indentation/applanation
Tonometry (e.g. Tono- Pen® )
• a hand-held electronic contact tonometer (a modified version
of the older Mackay–Marg tonometer)
• The probe tip contains a transducer that measures applied
force
• Besides portability, main advantage - the facility to measure
IOP reasonably accurately in eyes with distorted or
oedematous corneas and through a soft contact lens

19. TONO-PEN

20. DYNAMIC CONTOUR
TONOMETRY
Dynamic contour tonometry (DCT) (e.g. PASCAL®)
• a solid-state sensor and a corneal contour-matching surface
(instead of applanation) with the aim of measuring IOP relatively
independently of corneal mechanical factors such as rigidity
• mounted on a slit lamp like GAT and IOP shown on a digital
display
• Studies comparing DCT and GAT IOP readings with manometric
intracameral IOP seem to confirm DCT as providing a more
physiological measurement

21. PASCAL

22. Rebound Tonometry
Rebound tonometry (e.g. iCare®)
• Hand-held and portable
• involves a 1.8 mm plastic ball attached to a wire
• Deceleration of the probe upon contact with the cornea is proportional to IOP
• Anaesthesia is not required
• Especially in children
• used for self-monitoring – a tailored personal version is available – and for
screening in the community

23. I Care Tonometer
I Care 100ic I Care Home
I Care 100ic

24. Indentation Tonometry
Indentation (impression) tonometry (e.g. Schiotz)
• a seldom-used portable device that measures the extent
of corneal indentation by a plunger of known weight

25. Ocular Response Analyser
Ocular response analyser (e.g. Reichert®)
• utilizes air-puff technology to record two applanation
measurements:
• one while the cornea is moving inward and one when
the cornea returns to its normal position.
• The average of these two IOP measurements provides
a Goldmann-correlated IOP measurement.

26. Ocular Response Analyser

27. THE END