Indian Journal of Ophthalmology

: 1998  |  Volume : 46  |  Issue : 2  |  Page : 113--116

Sterilisation of tonometers and gonioscopes

D Sood, SG Honavar 
 L.V. Prasad Eye Institute, Hyderabad, India

Correspondence Address:
D Sood
L.V. Prasad Eye Institute, Hyderabad


Precautions to prevent spread of infection through tonometers and gonioscopes are described in this article. Tonometers and gonioscopes should not be used in the presence of clinically manifest conjunctivitis and keratitis. The Schiotz tonometer should be dipped in a 1:1000 merthiolate solution, and rinsed in saline/ distilled water prior to use. The Goldmann applanation prism tip can be wiped with gauze soaked in 70% isopropyl alcohol and then dried before use. Gonioscopes should be cleaned in running water, wiped with gauze soaked in 70% isopropyl alcohol, and then dried before use. Koeppes and goniotomy lenses can be sterilized with ethylene oxide, prior to use in surgery.

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Sood D, Honavar S G. Sterilisation of tonometers and gonioscopes.Indian J Ophthalmol 1998;46:113-116

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Sood D, Honavar S G. Sterilisation of tonometers and gonioscopes. Indian J Ophthalmol [serial online] 1998 [cited 2023 Jun 2 ];46:113-116
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Tonometry and gonioscopy are crucial steps in the ophthalmic work up of a patient. There is always a risk of transmitting ocular infections through the tonometer. Bacterial and viral infections can spread through patient contact with infected ophthalmic diagnostic instruments especially tonometers and gonioscopes.[1][2][3][4][5]

Epidemics of adenoviral keratoconjunctivitis have been known to occur through ophthalmic office procedures like tonometers.[6][7][8] In recent years, the presence of other viruses, including the human immunodeficiency virus (HIV), from the human conjunctiva and tears has renewed interest in the office sterlization of contact tonometers.[9],[10] The risk of hepatitis B transmission may be much more because of extreme contagiousness and increasing number of carriers. Tears may spread hepatitis B infection.[11] High risk groups include those undergoing blood transfusion, renal dialysis, and drug addicts. A clear clinical context is not always present to warn ophthalmologists that their patient may be a hepatitis B carrier.

In clinical practice, especially when screening strategies have been relegated to paramedical staff, it is best to avoid tonometry in persons presenting with a recent history of matting of eyelashes or mucopurulent discharge, superficial infections presenting as red tearing eyes, during epidemics of conjunctivitis, or presence of herpetic eruptions about the face and eye.[9]Ideally, every tonometer should be sterlized before use to prevent spread of infection from one patient to another.

The methods available to disinfect tonometers include:

1. Dry heat.

2. Mechanical cleaning with disposable wipe/sterile gauze.

3. Wipe with gauze soaked in alcohol or chemicals like hydrogen perioxide and merthiolate.

4. Soaking in chemicals like 70% isopropyl alcohol, 1:1000 merthiolate, 3% hydrogen peroxide, and 1:10 diluted household bleach (sodium hypochlorite).

5. Ultraviolet rays.

6. Gas sterlization.

Many of these methods may be unreliable (for example, mechanical cleaning), impractical, time consuming, or damaging to tonometer parts.

An ideal method of sterlization for tonometers and gonioscopes would have the following features:

1. Should be effective in removing common offending organisms, including adenoviruses, herpes simplex, and HIV from the instrument.

2. Should not damage instrument parts (for example, foot plate and plunger of Schiotz tonometer, plastic and etchings of Goldmann prism).

3. Should not affect pressure readings.

4. Should not produce iatrogenic corneal lesions.

5. Additionally, evaporation of chemicals leading to higher concentration especially in tropics should be kept in mind.

6. Above all, should be practical.

Based on these principles, the various methods in vogue and practical suggestions for office use are discussed.

 Schiotz Tonometer

In spite of the limitations of indentation tonometry, Schiotz tonometer is still in use in the primary eye care set up in developing countries.

The methods for sterilzation of such tonometers include:

1. Heating the base of the instrument with the flame of a spirit lamp for 10 seconds and allowing sufficient time for cooling before use. Repeated heating may, however, distort the curvature of the foot plate and plunger, resulting in erroneous readings.

2. Cleaning the foot plate with ether or alcohol swab (allowing sufficient time for drying of chemical).

3. Ultraviolet rays.

4. Soaking the assembled foot plate in a bowl with 1:1000 merthiolate solution.

5. Use of tonofilm.

All these methods of sterlisation may be unsatisfactory because sterility is not achieved in all parts of the tonometer.


An ideal method for sterilisation of Schiotz tonometer is:

1. Disassemble the tonometer between each use. Clean the barrel by inserting a white pipe cleaner saturated with alcohol, pulling back and forth several times and then inserting a second dry pipe cleaner.

2. Then clean the foot plate and plunger with alcohol.

3. Clean the test cornea with alcohol swab.

4. Reassemble the instrument and wait for atleast 60 seconds (after cleaning with alcohol) before placing the instrument on the cornea.

A more practical approach would involve keeping the base of the tonometer continuously dipped in a solution of 1:1000 merthiolate solution (Figure). Prior to use, the footplate can be rinsed in saline/distilled water. After usage it should be replaced in the merthiolate solution.

 Contact-type Applanation Tonometers

The prisms used in Goldmann, Perkins, Draegers and Pneumo tonometers which touch the cornea need to be sterlized appropriately to prevent cross infections. Mackay-Marg and Tonopens have sterile tonofilm covers.

The methods to sterilize tonometer prisms include:

1. Mechanical wipe with disposable kim wipe and sterile gauze. Pepose found mechanical cleaning with these did not provide any safety against lympadenopathy virus type I or herpes simplex virus type I or II.[12]

2. Wipe with wipes soaked (or pre-soaked) in 70% isopropyl alcohol or 3% hydrogen peroxide.

3. Soaking the tip of the prism in 70% isopropyl alcohol, 3% hydrogen peroxide or 1:10 household bleach.

4. Ultraviolet rays.

5. Gas sterlization.

 Wipe techniques

A prepared pad or gauze soaked in 70% isopropyl alcohol is applied to the tip of the prism for 10 seconds and the prism tip allowed to dry before use. Repeated disinfection with hydrogen peroxide or alcohol applied to the tip of the prism does not produce any damage to the plastics. Isopropyl alchohol applied to the side of the prism (which is not necessary) may erode etchings on the prism. This method effectively disinfects for adenovirus, herpes simplex, and HIV.[13] Allow the alcohol to air dry or rinse with sterile saline to wash off 3% hydrogen peroxide before application to prevent iatrogenic corneal abrasions.

 Soak techniques

The common solutions in use are 1:10 dilution of household bleach (sodium hypochlorite), 70% isopropyl alcohol, and 3% hydrogen peroxide. Soaking the tonometer head in 3% hydrogen peroxide or diluted household bleach meets the guidelines from the Centre for Disease Control and Prevention (Atlanta, USA) and the American Academy of Ophthalmology.[13],[14] Soak the prism for 5-10 minutes between use, air dry for alcohol or irrigate tip with saline and dry in case of sodium hypochlorite or hydrogen peroxide.

Some of the problems that may be encountered with these methods include:

1. Minor burns on fingers with hydrogen peroxide and minor burns of the cornea.

2. Increasing concentration of the solution by evaporation, especially during summer months in tropics.

3. Soaking the entire prism in the solution may remove the colouring of the etched calibration marks.

Removal of the applanating head is more cumbersone and time consuming. van Buskirk described a method of drilling two 11 mm holes in the cover of a perti dish.[15] The dish can be filled with 3% hydrogen peroxide, 1:1000 merthiolate or diluted household bleach, prepared fresh each morning. Goldmann applanating prisms, prism end down are placed through the hole and kept soaked for 5-10 minutes. The part of the tonometer tip outside allows easy removal. The tip is irrigated with saline, dried and put on the tonometer. After use the tip is returned to the petri dish and a second disinfected tip is placed on the tonometer. An important point to note in this method includes a fresh preparation of the solution every day.

The use of ultraviolet rays tends to damage the plastic and therefore is not practical.


For practical considerations a sterile gauze soaked in 70% isopropyl alcohol or prepad of 70% isopropyl alcohol used for wiping the tip of the prism provides safety by inactivitating adenovirusus, lymphoadenopathy virus type I, and herpes simplex virus types I and II.

 Non-contact Tonometers

These do not make contact with the cornea or tears and are perhaps ideally suited for measurement of intraocular pressure in patients suspected of having contagious viral infections or in the immediate postoperative period after glaucoma or cataract surgery, or corneal grafts.

The front surface may however be cleaned with the special soft tissue/cloth as the front surface may occassionally touch the eyelashes.


Gonioscopes are made of plastics, and the same precautions and methods for sterilization are available as for Goldmann tonometer prisms. The lens after use may be cleaned in running water and placed in disinfectant solution like 3% hydrogen peroxide or diluted household bleach or 1% formaldehyde. Subsequently it can be thoroughly rinsed with saline. The inside of the gonioscope can also be wiped for 10 seconds with a sterile swab soaked in 70% isopropyl alcohol. The sterilization of the operating direct gonioscopes (Cardona, Koeppes lenses) is usually done with ethylene oxide gas sterlization.


Spread of infections, bacterial and viral, is known through the use of diagnostic ophthalmic equipment like tonometers and gonioscopes. Spread of adenovirsus, HSV and hepatitis virus is possible. Tonometry should be deferred when there is a herpetic involvement of the face and eyes, lid infection, dacryocystitis, matting of cilia, epidemics of conjunctivitis, superficial infections presenting as tearing red eyes. In practice, simple routine procedures for ensuring adequate sterilisation should be implemented. High-risk groups such as those undergoing blood transfusion, renal dialysis, and drug addicts should be identified and appropriate measures as mentioned in this article implemented.


This work was supported in part by a grant from the Hyderabad Eye Research Foundation, Hyderabad, India.


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