Measuring Intraocular Pressure with a Schiotz Tonometer

Measuring Intraocular Pressure with a Schiotz Tonometer

A Schiotz (Schiötz) tonometer is a contact indentation instrument that estimates intraocular pressure (IOP) by measuring how far a known plunger weight indents the anaesthetised cornea. It remains useful where slit-lamp applanation or electronic devices are unavailable, but it is technique-dependent and less accurate than modern reference methods. Students must understand both the procedure and the situations in which it should not be attempted.

Learning objectives

• Define IOP and explain the principle of indentation tonometry.
• Identify the Schiotz tonometer parts, weights, test block and conversion chart.
• Check calibration, prepare the patient and perform the measurement safely.
• Know contraindications, infection-control requirements and common sources of error.
• Convert and document readings without inventing values or mixing conversion tables.
• Interpret abnormal results in context and arrange appropriate referral.

1. Intraocular pressure and clinical meaning

IOP is the pressure within the globe, determined mainly by aqueous humour production and outflow, episcleral venous pressure, ocular rigidity and the balance of intraocular fluid. It helps maintain globe shape. A raised IOP can damage an at-risk optic nerve, but glaucoma is diagnosed through optic-nerve/retinal and visual-field changes, not pressure alone; normal-pressure glaucoma also exists.

Many clinical references describe an approximate adult IOP range around 10–21 mmHg, but the patient’s cornea, posture, time of day, instrument and conversion table affect the result. Use the local laboratory/device reference and never interpret a number in isolation.

2. Principle of Schiotz indentation tonometry

1. The patient lies supine with the cornea anaesthetised.
2. A curved footplate rests gently on the central cornea.
3. A standard plunger weight indents the cornea; the lower the IOP, the greater the indentation and the higher the scale reading.
4. Higher IOP resists indentation, so the plunger moves less and the scale reading is lower.
5. The scale reading is converted to an estimated mmHg using the conversion table supplied with that tonometer and plunger weight.

The device does not directly sense pressure. Results are influenced by scleral rigidity, corneal thickness/biomechanics, ocular rigidity, patient posture, lid pressure, instrument alignment and the specific conversion table. It is best used by trained staff and confirmed with a modern method when the result is unexpected or clinically important.

3. Parts of the instrument

4. Indications

• Assessment of suspected glaucoma or ocular hypertension when a validated applanation/electronic method is unavailable.
• Monitoring a known patient when the same instrument and technique are used consistently.
• Assessment of pressure in selected painful red-eye presentations after open globe and corneal contraindications are excluded.
• Teaching indentation tonometry and community eye screening by trained personnel.

In modern ophthalmic services, Goldmann applanation, calibrated handheld applanation, rebound or other validated methods may be preferred. A high, low or discordant Schiotz result should be confirmed.

5. Contraindications and precautions

Before measuring a traumatised eye, complete a gentle visual/structural assessment. A normal-looking small wound does not exclude an open globe.

6. Equipment and preparation

• Schiotz tonometer with footplate, 5.5-g standard weight and additional weights.
• Manufacturer-supplied test block and matching conversion chart.
• Topical ophthalmic anaesthetic prescribed/authorised by the facility.
• Clean gloves, sterile gauze/cotton swabs and approved disinfectant compatible with the instrument.
• Clean tissue/saline and a rigid eye shield when trauma is possible.
• Patient record for scale reading, weight, converted IOP, eye, time and conditions.

Calibration check

1. Inspect the footplate and plunger; confirm the plunger moves smoothly and the pointer is not bent.
2. Attach the standard weight as specified by the manufacturer.
3. Place the footplate on the test block/artificial cornea in the required position; the pointer should rest at the zero mark or within the permitted calibration range.
4. If it does not, clean and recheck once. Do not bend parts or alter the instrument; label it out of service and report for maintenance/calibration.

7. Patient preparation

1. Explain that a numbing drop will be used and that the instrument briefly touches the cornea; instruct the patient not to rub the eye afterwards.
2. Check allergy, current eye medication, contact lenses, trauma, infection and ability to cooperate.
3. Measure and document visual acuity and pupils before anaesthetic when possible.
4. Position the patient comfortably supine with the head supported and the face pointing straight upward. A level head reduces alignment and posture errors.
5. Ask the patient to relax, breathe normally and fixate on a target. Avoid breath-holding, tight collars and squeezing the eyelids.
6. Instil topical anaesthetic according to local protocol and allow adequate onset. Avoid touching the cornea or eyelashes with the dropper.

8. Step-by-step Schiotz procedure

1. Hand hygiene and gloves: clean hands; use gloves where secretions, blood or infection are possible.
2. Check the instrument: calibrated, clean, correct weight and chart available.
3. Position: stand behind the patient’s head with your hands supported and at the correct level; keep your posture stable.
4. Open the lids: gently separate the lids by holding the skin against the orbital rim. Do not press the globe or pull the lids across the cornea.
5. Align: hold the tonometer vertically between thumb and fingers. Centre the footplate over the cornea, not the sclera or limbus.
6. Lower gently: allow the instrument to rest by its own weight; do not push down or rock it. Keep the plunger perpendicular to the cornea.
7. Read: observe the pointer without touching the instrument. Record the scale reading and the attached weight.
8. Repeat when indicated: if the reading is very low (for example 2 or less in common teaching protocols), remove the instrument and use the next appropriate heavier weight according to the manufacturer/local protocol. A low scale reading means higher estimated IOP.
9. Remove: lift the instrument vertically and gently; do not drag across the cornea.
10. Second eye: repeat with a clean/disinfected instrument or according to the facility’s infection-control process. Avoid transferring infection from the first eye.
11. Aftercare: tell the patient not to rub the anaesthetised eye until sensation returns and to report pain, worsening blur or persistent foreign-body sensation.
12. Clean and store: disinfect the contact surfaces with the approved method, rinse/dry if required, check the mechanism and return to its case.

9. Conversion of scale readings

The scale number is not mmHg. Use the conversion card supplied with the same instrument, selecting the column for the attached plunger weight. Record both the raw scale reading and the converted estimate. Do not copy a conversion table from a different model or silently round a borderline result.

In general, a lower Schiotz scale reading corresponds to a higher IOP because the plunger indents less. If the pointer is at an extreme, repeat with the recommended heavier weight or use another validated method; never extrapolate beyond the chart.

10. Common errors and their effects

11. Interpretation and referral

• Compare right and left eyes, symptoms, optic-disc appearance, fields and previous measurements.
• A high reading with severe pain, halos, nausea/vomiting, cloudy cornea or abnormal pupil is an emergency; do not wait for routine glaucoma review.
• A high reading without symptoms may represent ocular hypertension, glaucoma, steroid response, inflammation, trauma or measurement error; confirm and refer.
• A normal reading does not exclude glaucoma, angle closure after intermittent symptoms, retinal disease or optic neuropathy.
• A very low reading after trauma or surgery may suggest wound leak, ciliary dysfunction or inflammation; urgent assessment is required.
• When the result conflicts with the examination, repeat carefully and confirm with applanation/handheld/electronic tonometry in an equipped eye service.

12. Infection prevention and aftercare

• Clean hands before and after each patient; use gloves when indicated.
• Follow the device manufacturer’s disinfection method; the footplate must be disinfected between eyes/patients without damaging the mechanism.
• Do not reuse contaminated anaesthetic tips or allow the dropper to touch lashes, skin or cornea.
• If infection is suspected in one eye, avoid transferring organisms to the other eye and follow local single-use/disinfection policy.
• After anaesthetic, advise the patient not to rub or touch the eye until normal sensation returns; return for pain, persistent blur or discharge.

13. Applied cases

14. Self-test

1. What does a Schiotz tonometer measure indirectly?
2. Why does a lower scale reading usually indicate higher IOP?
3. What is the standard teaching weight in many Schiotz kits?
4. When should a heavier weight be considered?
5. Name four contraindications to contact tonometry.
6. Why must the patient be supine and the instrument vertical?
7. What must be recorded besides the converted IOP?
8. Why can lid pressure falsely raise the reading?
9. Why is a normal IOP not enough to exclude glaucoma or retinal disease?
10. What is the first action when an open globe is suspected?

Answers

1. It estimates IOP from the amount of corneal indentation produced by a known weight.
2. High pressure resists plunger indentation, so the pointer moves less and the scale number is lower.
3. 5.5 g.
4. When the scale is at the low end (commonly ≤2 or another manufacturer-defined threshold) or when the standard weight cannot provide a reliable range.
5. Suspected open globe, unhealed abrasion/ulcer, active infection, severe corneal abnormality, anaesthetic allergy/intolerance, damaged/uncalibrated instrument or inability to cooperate.
6. They reduce posture/alignment variation and allow the plunger to rest vertically by gravity.
7. Eye, time, raw scale, attached weight, conversion chart/model, conditions/limitations and method.
8. External pressure compresses the globe and elevates measured pressure.
9. Pressure is only one risk factor; optic-nerve, field, retinal and neurological disease may occur at normal IOP or be missed by poor technique.
10. Stop contact examination, avoid pressure/tonometry, apply a rigid shield and obtain emergency ophthalmic care.

Key takeaways

• Schiotz tonometry is indentation estimation, not direct pressure measurement.
• Calibration, correct weight, matching conversion chart, alignment and no lid/globe pressure are essential.
• Never perform it on a suspected open globe or unsafe corneal surface.
• Record the raw scale, weight and converted value; confirm unexpected results with a validated method.
• IOP must be interpreted with symptoms, acuity, pupils, optic disc, fields and the whole examination.

References and further reading

• SlideShare: Intra-ocular pressure by tonometer
• Community Eye Health: How to measure IOP—Schiotz tonometry
• NCBI Clinical Methods: Tonometry
• StatPearls: Tonometry
• EyeWiki: IOP and tonometry