Tonometry

Tonometry:-

Tonometry is the indirect method of objective measurement of Intra-Ocular pressure(IOP) and the instrument by which it is done, is called tonometer. If the IOP is recorded by introducing a needle either in the anterior chamber or in the vitreous cavity(the needle is connected with a suitable mercury or water manometer) i.e by direct method, it is called Manometry. There are several types of tonometer available in the market. The important one is discussed below.

1)Goldmann Applanation tonometer:-

It is the gold standard for recording of intraocular pressure(IOP) till date. It is based on the Imbert-Fick principle which states that for an ideal, dry, thin walled sphere, the pressure inside the sphere(P) equals the force necessary to flatten its surface(F) divided by the area of flattening(A) i.e P=F/A.

The IOP is proportional to the pressure applied to the globe but the human eye is, however not an ideal sphere-the cornea is rigid and resists flattening. Capillary attraction of the tear meniscus, however, tends to pull the tonometer towards the cornea. Capillary attraction and corneal rigidity cancel each other when the flattened area has a diameter of 3.06 mm; as in Goldmann tonometry. It is used with a slit lamp under a cobalt blue filter. Before the procedure, topical anaesthetic and fluorescein are instilled into the conjunctival sac. As the Goldmann prism (mounted on a tonometer) is applied axially to the corneal surface, a pattern of two semicircles will be seen. The dial of the tonometer is rotated to align the inner margins of the semicircles and that is the endpoint. The reading on the dial, multiplied by 10, equals the IOP. It can not be used in patients with abnormal corneal contour(e.g:- Scarred cornea).

2) Perkins tonometer:-

Uses the principle of Goldmanns’s tonometer. But it is portable, handheld and does not require slit-lamp. So can be used in bedridden patients.

3)Dynamic contour tonometry:-

Dynamic contour tonometry (DCT) uses the principle of contour matching instead of applanation. The tip contains a hollow the same shape as the cornea with a miniature pressure sensor
in its centre. In contrast to applanation tonometry it is designed to avoid deforming the cornea during measurement and is therefore thought to be less influenced by corneal thickness and other biomechanical properties of the cornea than other methods but because the tip shape is designed for the shape of a normal cornea, it is more influenced by corneal curvature.

So it can be used in patients having an abnormal or irregular corneal contour.

4) Electronic indentation tonometry

Electronic indentation tonometers are modified Mackay-Marg tonometers that use a free-floating transducer
to detect the transmitted pressure.It can be used to record exacct pressures in cases of ectatic corneas or post keratoplasty.

5)Tonopen:- 

It is a Mackay-Marg type tonometer but hand held, self contained,battery powered , portable, contact tonometer.Can be used in eyes with distorted or oedematous corneas, as well as through bandage contact lens.

6)Air-Puff tonometer:-

It is a non-contact tonometer based on the principle of applanation but, instead of using a prism, the central part of the cornea is flattened by a jet of air. The time required to sufficiently flatten the cornea relates directly to the level of IOP. Particularly useful for screening by non-ophthalmologists as it is easy to use. Its main disadvantage is that it is only accurate within the low to middle range.

7) Non-corneal and transpalpebral tonometry:-

Transpalpebral tonometry refers to methods of measuring intraocular pressure through the eyelid
. The tonometer is not a substitute or alternative for more established methods.

8) Indentation tonometry( Schiøtz tonometer):-

It measures the depth of corneal indentation made by a small plunger carrying a known weight. The higher the intraocular pressure, the harder it is to push against and indent the cornea. For very high levels of IOP, extra weights can be added to make the plunger push harder. A scale then gives a reading depending on how much the plunger sinks into the cornea, and a conversion table converts the scale reading into IOP measured in mmHg.As it includes ocular ridigity, this technique is obsolete now-a-days.

9) Digital tonometry:- 

It is the method of estimating intraocular pressure by gently pressing the index finger against the cornea of a closed eye. This method is notoriously unreliable.

10) Sensimed Triggerfish(for 24 hour IOP monitoring):-

The most well-known external IOP monitor is the Sensimed Triggerfish CLS (Sensimed), which provides an automated recording of continuous changes in ocular dimensions, a surrogate for IOP, over 24 hours. It is CE Mark approved but is currently pending FDA approval.