Goldman Applanation Tonometer

This lecture is based on post-graduate students of Ophthalmology (DO, DCO, MCPS, FCPS, MS) and optical principle of GAT has to know for a student to use the instrument friendly

1. G oldman A pplanation Tonometer (GAT) Md Anisur Rahman (Anjum) Professor & Head Ophthalmology Dhaka Medical College, Dhaka .

2. Goldman Applanation Tonometer In Goldman Applanation tonometry, slit-lamp is used to illuminate the tonometer tip and fluorescein – stained cornea. Magnification has very little importance in this procedure. Applanation tonometery is the best available procedure to record intraocular tension. It is least affected by scleral rigidity, which is an inherent deterrent of indentation tonometry (1) (PK Muk: 119)

3. Optical Principle of gat Goldmann Applanation tonometer is based on the Imbert–Fick principle, which states that for a dry thin-walled sphere, the pressure (P) inside the sphere equals the force (F) necessary to flatten its surface divided by the area (A) of flattening (i.e. P = F/A). It applies to surfaces which are perfectly spherical, dry, flexible, elastic, and infinitely thin.

4. Optical Principle of gat Theoretically, average corneal rigidity (taken as 520 μm for GAT) and the capillary attraction of the tear meniscus cancel each other out when the flattened area has the 3.06 mm diameter contact surface of the Goldmann prism, which is applied to the cornea using the Goldmann tonometer with a measurable amount of force from which the IOP is deduced.

5. Fig: Representation of forces involved in Applanation tonometry. F = tonometer force; s = surface tension of precorneal tear film; P = intraocular pressure; A = area of Applanation; b = corneal rigidity/resistance to bending.

6. Technique of measurement Plastic biprism which contacts cornea creates two semicircles Edge of corneal contact is visible after placing fluorescein into tear film & viewing with cobalt blue light Manually rotate the dial calibrated in grams, force is adjusted by changing the length of a spring within the device. Inner margins of semicircles touch when 3.06 mm of cornea is applanated.

7. Instructions to patient Press head firmly against chin and forehead rest. look straight ahead and fixate on a target (e.g. examiners opposite ear) breathe normally, do not hold your breath Blink immediately prior to measurement to moisten cornea.

8. Instructions to patient Position patient’s head with forehead rest well above eyebrows, allowing raising of eyebrows. anesthetic & fluorescein (0.25%), drop applied maximal illumination of biprism the lamp is moved toward the eye until the tip of biprism contacts the apex of the cornea stop moving forward when limbus shines with light,

9. Instructions to patient i) After contact, semicircles visible through left (or right) ocular. Center in field of view. Adjust vertically until semicircles equal in size. Tension dial adjusted so that inner edge of upper and lower semicircles are aligned. Multiply dial reading (grams of force) by 10 to obtain IOP (mmHg)

10. Instructions to patient m) Read at median over which arcs glide to control for excursions due to ocular pulsations. If slit-lamp moved too far toward patient the pressure arm will push against a spring which will press against the eye with a low inoffensive force. Mires (flattened area) too large, moving dial doesn’t alter appearance.

11. Instructions to patient p) Solution: Draw back until regular pulsation noted and appearance of mires normalizes. Blue central area represents applanated cornea, green semicircles are fluorescein-stained tears, inner border of ring is demarcation between flattened and non-flattened cornea.

12. Instructions to patient Without staining of tears, bright reflection from air-cornea interface is seen; leads to underestimation of IOP. Mires should be approximately 10% of circle width.

13. Errors in Measurement: fluorescein ring The fluorescein ring is too wide or too narrow: Too wide: occurs if prism not dried after cleaning or lids touch prism. Overestimates IOP. Solution: dry prism Too narrow: inadequate fluorescein concentration may cause hypofluorescence. Underestimates IOP. Solution: patient blinks or additional fluorescein added.

14. Errors in Measurement: Corneal astigmatism Corneal astigmatism: When regular astigmatism is present, an elliptical contact with tonometer head occurs. This results in an under estimation of IOP in with-the-rule astigmatism and an over estimation with against-the-rule astigmatism, with an error range of about -2.5 to +2.5 mmHg.

15. Errors in Measurement: Corneal curvature Steeper corneas need to be indented more to produce the standard area of contact, necessitating more force and therefore indicating a higher IOP reading. It has been suggested that over the range of corneal curvature of 40 to 49 diopters , the error in IOP reading is about 3mmHg

16. Errors in Measurement: Corneal oedema Goldmann Applanation Tonometry underestimates the IOP in eyes with moderate corneal edema. This underestimation was attributed to the observation that the epithelium of edematous corneas is easier to indent than normal epithelium.

17. Errors in Measurement: Corneal thickness Thin corneas tend to produce underestimation and thick corneas produce overestimation of IOP. Clinical implication of this fact in patients with thin corneas may be wrongly diagnosed as normal tension glaucoma and thick corneas wrongly as ocular hypertension emphasizing importance of checking central corneal thickness on a routine basis in glaucoma clinics.