Tonometry Technique
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Direct answer
Goldmann applanation tonometry remains the reference standard: it flattens exactly 3.06 mm of cornea, the diameter at which corneal rigidity and tear-film surface tension forces cancel (Imbert-Fick), so the force applied, read in grams on the dial and multiplied by ten, equals intraocular pressure in millimetres of mercury — the dial at 2.0 g means 20 mmHg. Normal pressure runs 10-21 mmHg with a diurnal swing of 3-5 mmHg. The reading is calibrated to a central corneal thickness of about 520 micrometres: thick corneas over-read and thin corneas under-read by roughly 2-3 mmHg per 50 micrometres (a commonly quoted rule of thumb), which is why pachymetry accompanies any borderline interpretation. Schiotz indentation tonometry (5.5 g weight: scale 4 ≈ 21 mmHg), non-contact air-puff screening, Perkins for the supine, and rebound tonometry for children complete the toolkit.
What you must remember
- Goldmann mechanics: 3.06 mm applanation diameter; dial grams × 10 = mmHg; semicircular fluorescein mires aligned so their inner borders just touch; anaesthetic plus fluorescein first, prism disinfected between patients.
- Reading rules: mires of equal size (blink first if unequal — lid pressure falsifies), inner-edge alignment, both eyes measured, time of day recorded; astigmatism above about 3 D requires rotating the prism toward the minus axis or averaging two readings.
- Normal and suspect values: 10-21 mmHg normal range; pressures above 21 raise ocular hypertension and glaucoma suspicion — but half of open-angle glaucoma presents at or below 21 at screening, so tonometry never screens alone.
- Corneal thickness: Goldmann assumes about 520 micrometres; as a working correction, allow roughly 2-3 mmHg per 50 micrometres departure (Ehlers-derived convention), and always measure pachymetry in borderline eyes.
- Diurnal variation: 3-5 mmHg swings, highest early morning in most; a single midday reading can miss peaks — timing and repetition matter in suspects.
- Schiotz conversion: indentation tonometry with the 5.5 g plunger weight — a scale reading of 4 converts to about 21 mmHg; scleral rigidity errors plague high myopes and post-vitreoretinal-gas eyes.
- Alternatives: non-contact tonometry for screening (less exact, startle artefact), Perkins (portable Goldmann for bedridden), Tono-Pen and rebound (children, corneal scars, no anaesthetic for rebound), dynamic contour/ORA for CCT-independent research readings.
- Contraindications: suspected perforation, active infectious keratitis, recent trauma — and never on an eye about to be patched without recording the value first.
One reading, corrected for corneal thickness
A camp attendee flags 28 mmHg on air-puff tonometry. At the slit lamp, Goldmann with proper mires gives 24 mmHg; pachymetry reads 610 micrometres — a thick cornea. Applying the rule of thumb, 90 micrometres over 520 suggests the true pressure sits nearer 19-20 mmHg; the disc is healthy, angles open, fields full. Diagnosis: ocular hypertension on a mechanical artifact of thickness, observed rather than treated, with annual review — a decision that pachymetry, not the tonometer, made.
Reverse the case: a myope measures 19 mmHg with a 490-micrometre cornea; corrected, that is 20-21 — and the disc shows an inferior notch. Thin corneas both under-report pressure and carry higher glaucoma risk (the OHTS lesson), so this patient gets fields, OCT of the nerve fibre layer, and ophthalmology referral despite a "normal" number. The teaching point for exams and clinics alike: tonometry measures the force to flatten a cornea; the cornea's own thickness decides how honestly that force reflects the pressure behind it.
Where marks are lost
Procedural details sink practical candidates: mires too thick from fluorescein excess, reading the outer edges instead of the inner, or pressing the lids and calling the resultant spike "pathology". Conceptual marks go to the Schiotz-to-Goldmann comparison (indentation versus applanation, rigidity dependence), the thickness correction direction (thick over-reads — candidates routinely invert it), and the fact that half of glaucomatous eyes screen under 21 mmHg, making IOP a risk factor rather than a diagnosis.
Frequently asked questions
Why does Goldmann applanation use a 3.06 mm diameter?
At that area, tear-film surface tension and corneal rigidity forces cancel, so applanating force directly reflects intraocular pressure (Imbert-Fick principle).
How is the Goldmann dial converted to pressure?
The applied force in grams multiplied by ten gives millimetres of mercury — a dial reading of 2.4 equals 24 mmHg.
What Schiotz reading approximates 21 mmHg?
With the standard 5.5 gram weight, a scale reading of about 4 converts to roughly 21 mmHg on the Friedenwald-derived tables.
How does central corneal thickness alter applanation readings?
Thick corneas falsely over-read and thin corneas under-read by roughly 2-3 mmHg per 50 micrometres from the 520-micrometre calibration, so pachymetry accompanies borderline interpretation.
Which tonometer suits an uncooperative child?
Rebound tonometry — a brief, anaesthetic-free contact technique — or non-contact screening, reserving Goldmann or Perkins for cooperative examinations.