6.2 Hand-Held Applanators, Non-Contact Tonometry, Rebound Tonometry & Disinfection
Key Takeaways
- The Perkins tonometer is a hand-held Goldmann-type applanator usable supine, in theatre and in children.
- The Tono-Pen uses a Mackay-Marg style transducer and gives a mean with a confidence percentage, performing better on irregular corneas.
- Non-contact tonometry needs no anaesthetic but is less accurate at higher pressures and produces aerosol.
- Rebound tonometry requires no anaesthetic and uses a single-use disposable probe, making it well suited to children.
- Intraocular pressure varies diurnally by 3 to 6 mmHg in normal eyes and more in glaucoma, so the time of measurement must be recorded.
Hand-held applanation: Perkins
The Perkins tonometer is a hand-held, counterbalanced Goldmann-type applanator with its own cobalt blue illumination. It uses the same 3.06 mm prism and the same mire endpoint, so the technique transfers directly.
Indications: supine patients, bedside and theatre measurement, wheelchair users who cannot reach a slit lamp, children, and patients whose head position prevents slit lamp use. It requires anaesthetic and fluorescein exactly as Goldmann does.
Limitations: more operator-dependent because there is no chin rest stabilising the head, and it requires a free hand to hold the lids.
Tono-Pen and Mackay-Marg style devices
The Tono-Pen uses a small central plunger surrounded by an annular footplate. The footplate absorbs the force of corneal rigidity, so the central transducer registers something close to true intraocular pressure. The instrument samples multiple readings and reports a mean with a statistical confidence indicator (for example 5% coefficient of variation).
Advantages: portable, usable in any position, requires only a small area of cornea so it works on irregular, scarred or oedematous corneas where Goldmann mires are unreadable, and it can be used peripherally or even through a bandage contact lens in some circumstances.
Limitations: requires anaesthetic, requires a single-use latex-free cover (Ocu-Film tip cover) that must be changed between patients, tends to over-read at low pressures and under-read at high pressures relative to Goldmann, and readings drift if the tip cover is wrinkled or the instrument is not calibrated per the manufacturer's schedule.
Non-contact (air puff) tonometry
A calibrated pulse of air flattens the cornea while an optical system detects the moment of applanation; the time or force required is converted to pressure.
Advantages: no anaesthetic, no corneal contact so no direct cross-infection risk from an instrument tip, easy to delegate, useful for screening and for patients with anaesthetic allergy.
Limitations: less accurate than Goldmann, especially above about 20 to 25 mmHg; strongly affected by central corneal thickness; requires patient cooperation with a startling stimulus; readings vary with tear film and with the moment in the ocular pulse cycle, so the instrument's averaged multiple readings should be used; and it generates aerosol and tear film droplets, which is an infection control consideration for adenoviral conjunctivitis and respiratory pathogens.
Rebound tonometry
A small magnetised probe is propelled at the cornea and its deceleration on rebound is analysed. No anaesthetic is required and the probe is single-use disposable, which eliminates reprocessing.
Advantages: excellent for children and for uncooperative or needle-phobic patients, rapid, home-monitoring versions exist.
Limitations: affected by corneal thickness, requires perpendicular alignment at the corneal apex, and probes are consumable.
Choosing a method
| Situation | Preferred method |
|---|---|
| Routine adult clinic, glaucoma follow-up | Goldmann applanation — the reference standard |
| Supine patient, theatre, bedside | Perkins |
| Scarred, oedematous or irregular cornea | Tono-Pen |
| Child who will not sit at a slit lamp | Rebound or Tono-Pen |
| Anaesthetic allergy | Non-contact or rebound |
| Suspected epithelial defect or active infectious keratitis | Non-contact, or defer |
| Screening a large population | Non-contact |
When the method changes between visits, the change must be recorded, because readings are not interchangeable between instruments and an apparent pressure rise may be nothing more than a different device.
Diurnal variation and recording
Intraocular pressure is not a fixed number. Normal eyes vary by about 3 to 6 mmHg across the day, typically peaking in the early morning, and glaucomatous eyes commonly vary more. A single measurement therefore tells you the pressure at that moment only.
Practical consequences: always record the time, try to measure a glaucoma patient at a consistent time when trending, and where the clinical picture does not match the numbers, a diurnal curve with several measurements across a day may be requested. Note also that pressure falls transiently after exercise and rises with a Valsalva, a tight collar, breath-holding and, in some patients, with lying supine.
Cleaning and disinfecting tonometers
The blueprint names cleaning and disinfecting tonometers as a task at every certification level. The governing principles:
- Clean before disinfect. Organic debris shields organisms; wipe the tip free of tears, mucus and fluorescein first.
- Follow the manufacturer's validated instructions for that specific device. Disinfectants that are safe for a Goldmann prism may damage a Tono-Pen or a pachymeter probe.
- Common validated approaches for Goldmann prisms include soaking in dilute sodium hypochlorite or 3% hydrogen peroxide for the specified contact time, followed by thorough rinsing with water and drying. Residual disinfectant on a prism causes a chemical keratitis.
- Alcohol wipes alone are inadequate against adenovirus unless the manufacturer specifies a validated contact time; alcohol also degrades prism adhesive over time.
- Inspect for damage. Chipped or cracked prisms harbour organisms and can abrade the cornea. Remove them from service.
- Single-use disposables — disposable prisms, Tono-Pen tip covers and rebound probes — remove the reprocessing failure mode entirely and are preferred where transmissible spongiform encephalopathy or epidemic keratoconjunctivitis is a concern.
- Hand hygiene before and after every patient contact, and gloves where there is discharge.
A practical clinic rule: if a patient has a red, discharging eye, use a disposable-tip method and clean the slit lamp chin rest and headrest afterwards.
Which tonometer is most appropriate for a patient with a heavily scarred, irregular cornea?
What is a recognised infection control disadvantage of non-contact tonometry?
Why must the time of an intraocular pressure measurement be recorded?
After soaking a Goldmann prism in a disinfectant solution, what step is essential before using it on a patient?
A glaucoma patient's pressure was 16 mmHg by Goldmann last visit and reads 21 mmHg by Tono-Pen today. What is the most appropriate interpretation?