Applanation tonometer

Describes the basic of applanation tonometry, the factors affecting it and also how to perform the ideal tonometry. The slide are borrowed but it gives complete idea of mastering Applanation tonometry.
If the original owner of the slides has an objection i shall take down the ppt with due apologies.

1. Basics…

2. FACTORS INFLUENCING IOP Rate of aqueous formation Resistance of aqueous outflow ( drainage) Increased episcleral venous pressure Dilatation of pupil Heredity Age Sex Diurnal variation Postural variation Blood pressure Osmotic pressure of blood

3. The concept was introduced by Goldmann is 1954. It is based on IMBERT FICKS LAW. It states that the pressure inside an ideal sphere (P) is equal to force (F) necessary to flatten its surface divided by the area of the flattening (A). P can be determined if Force F is fixed or Area A is fixed P=F/A

4. The ideal sphere is dry, thin-walled and flexible. The cornea is not ideal sphere. Two extra forces acting on cornea - Capillary attraction of tear meniscus (T), tends to pull tonometer towards cornea Corneal rigidity (C) resists flattening Thus , Modified Imbert Ficks Law F = PA , becomes F + T = PA + C , or P =( F + T - C) / A

5. These two forces cancel each other when flattened area has diameter of 3.06 mm. Applanation tonometry displaces only about 0.5 microlitre of aqueous humor, which raises IOP by about 3%. Because the volume displaced is so small, ocular rigidity, or the ‘ stretchability ’ of the globe, has little effect on the pressure readings .

6. PROCEDURE Patient is asked not to drink alcoholic beverages or large amounts of fluid (e.g., 500 ml or more) for 2 hours before the test, as the former will lower IOP and the latter may raise it. Patient is told the purpose of the test and is reassured that the measurement is not painful. The patient is instructed to relax, maintain position, and hold the eyes open wide. One drop of a topical anesthetic, such as 0.5% proparacaine , is placed in each eye, and the tip of a moistened fluorescein strip is touched to the tear layer on the inner surface of each lower lid. Tonometer tip is cleaned with a sterilizing solution, and the tip and prism are set in correct position on the slit lamp. Tension knob is set at 1g. If the knob is set at 0, the prism head may vibrate when it touches the eye and damage the corneal epithelium.

7. The 1 g position is used before each measurement. As a rule, it is more accurate to measure IOP by increasing rather than decreasing the force of applanation . 0 graduation mark of the prism is set at the white line on the prism holder. Cobalt blue filter is used with the slit beam opened maximally. The angle between the illumination and the microscope should be approximately 60°. The room illumination is reduced.

8. Heights of the slit lamp, chair, and chin rest are adjusted until the patient is comfortable and in the correct position for the measurement. Palpebral fissure is a little wider if the patient looks up. However, the gaze should be no more than 15° above the horizontal to prevent an elevation of IOP that is especially marked in the presence of restrictive neuromuscular disease such as dysthyroid ophthalmopathy . Operator sits opposite the patient, the assembly is advanced towards the patient with the tester observing from the side until the limbal zone has a bluish hue. If the tonometer tip touches the lids, the fluorescein rings will thicken , which may cause an overestimation of IOP . Clinician observes the applanation through the biprism at low power. A monocular view is obtained of the central applanated zone and the surrounding fluorescein-stained tear film .

9. Using the control stick, the observer raises, lowers, and centers the assembly until two equal semicircles are seen in the center of the field of view. If the two semicircles are not equal in size, IOP is overestimated. The clinician turns the tension knob in both directions to ensure that the instrument is in good position. If the semicircles cannot be made ‘too small,’ the instrument is too far forward. If the semicircles cannot be made ‘too large,’ the instrument is too far from the eye. Fluorescein rings should be approximately 0.25–0.3mm in thickness – or about one-tenth the diameter of the flattened area. If the rings are too narrow, the patient should blink two or three times to replenish the fluorescein. If the fluorescein rings are too narrow, IOP is underestimated. The tension knob is rotated until the inner borders of the fluorescein rings touch each other at the midpoint of their pulsations. Intraocular pressure is measured in the right eye until three successive readings are within 1 mmHg. Intraocular pressure is then measured in the left eye. Reading obtained in grams is multiplied by 10 to give the IOP in millimeters of mercury.

10. The fluorescein rings should be pproximately 0.25–0.3 mm in thickness – or about one-tenth the diameter of the flattened area. The fluorescent semicircles are viewed through the biprism and the force against the cornea is adjusted until the inner edges overlap.

11. FALSELY LOW IOP FALSELY HIGH IOP Too little fluorescein Too much fluorescein Thin cornea Thick cornea Corneal edema Steep cornea With the rule astigmatism - 1mm Hg per 4D Against the rule astigmatism - 1 m Hg per 3D Repeated tonometry Wider meniscus, Widening the lid fissure Excessively, Elevating the eyes more than 150

12. For every 3D increase in corneal curvature , IOP raises about 1 mm Hg as more fluid is displaced under steeper corneas causing increase in ocular rigidity. More than 6 D astigmatism produces an elliptical area on applanation that gives erroneous IOP. 4D with-the rule astigmatism underestimate IOP and 4D against-the rule astigmatism overestimate IOP . Goldmann applanation tonometer was designed to give accurate readings when the CCT was 550 μm . The deviation of CCT from 550 μm yields a change in applanation readings of 0.7 mm Hg per 10 μm .

13. Biprism should be rinsed and dried immediately after use . Between uses, the prism head should be soaked in a solution such as diluted bleach or 3% hydrogen peroxide . 70 % ethanol and 70% isopropanol are effective as sterilizing solutions but were shown in one study to cause mild damage to the tonometer tip after one month of immersion . Other methods of sterilization include: 10 min of rinsing in running tap water, wash with soap and water, cover the tip with a disposable film, and exposure to UV light. It is possible to transfer bacteria, viruses, and other infectious agents with the tonometer head, including such potentially serious infections as epidemic keratoconjunctivitis , hepatitis B, Jacob- Kreutzfeld and, theoretically , acquired immunodeficiency syndrome

14. Care must be taken to be sure any sterilizing solution has been completely rinsed off the tonometer tip, as some of these solutions may be toxic to the corneal epithelium , especially after LASIK or other corneal procedures . If the tonometer tip is not mechanically wiped after each use, epithelial cells may stick to the tip with the small but serious risk of transmitting Jacob- Kreutzfeld virus . Disposable tonometer tips may be an acceptable alternative to soaking in, and wiping with, antiseptic solutions .

15. PERKIN’S TONOMETER