Tonometers: Cleaning and Disinfection. An update


Dr. Digvijay Singh
Director, Noble Eye Care
Gurugram

Tonometers are of two types, noncontact and contact. While it is possible for both types of tonometers to spread infection, and both require disinfection and cleaning, the contact tonometers are the ones most liable to transmit infections. All tonometers come with the manufacturer guidelines for proper cleaning and disinfection and this should always be referred to when using the device.

For the biprism of the Goldmann applanation tonometer, the various forms of disinfection include wiping with 70% isopropyl alcohol or 3% hydrogen peroxide wipes. Alternatively it may be dipped/ soaked in 70% isopropyl alcohol or 3% hydrogen peroxide or 1:1000 merthiolate solution or 1:10 house hold bleach (sodium hypochlorite). Ultraviolet rays and Ethylene oxide gas sterilization may also be used.

For the Schiotz tonometer, cleaning of the barrel and the plunger should be done once a day essentially to prevent the plunger from sticking to the barrel and hinder its free movement. This may be done using an alcohol swab. The footplate too can be cleaned with an alcohol-soaked cotton swab and allowing it to dry before use. In between patients, the Schiotz tonometer should be disinfected by soaking it in 1:10 sodium hypochlorite or 1:1000 Merthiolate solution followed by rinsing with normal saline.

For the tonopen, the disposable ocufilm or the tip covers are to be changed after each and every examination. This eliminates the risk of transmission of any form of infection and is the recommended procedure. For a general cleaning of the equipment, the ocufilm has to be removed, following which the canned air is placed against the tip and blown  into the tip for approximately 3 seconds. This removes any particulate matter and enhances instrument longevity and proper measurement.

For the rebound tonometer, cleaning of the probe tip is essential in  order to reuse. It entails unscrewing the probe base and inverting the probe base over the container provided with the instrument. The container should be filled with pure alcohol and the base-tip should be allowed to soak in it for 5-30 mins. The base should then be dried by blowing compressed air before reinserting it into the instrument.

A recent study by the American Academy of Ophthalmology (http://dx.doi.org/10.1016/j.ophtha.2017.05.033 examines the efficacy of various disinfection methods for tonometer prisms. The authors examined a large number of studies but finally included 10 laboratory based studies for their review. They focused on disinfection particularly from adenovirus 8 and 19, herpes simplex virus (HSV) 1 and 2, human immunodeficiency virus 1, hepatitis C virus, enterovirus 70, and variant Creutzfeldt-Jakob disease.

Sodium hypochlorite (dilute bleach)  disinfection proved to be effective in destroying all adenovirus 8 and 19 while 70% isopropyl alcohol (e.g., alcohol wipes or soaks) did not eradicate all viable virus all the time. All 3 HSV studies concluded that both sodium hypochlorite and 70% isopropyl alcohol eliminated HSV. Ethanol, 70% isopropyl alcohol, dilute bleach, and mechanical cleaning all appeared to lack the ability to remove cellular debris completely,
and hence carried the risk of prion transmission. On the flip side, the study noted that damage to tonometer prisms can be caused by sodium hypochlorite, 70% isopropyl alcohol, 3% hydrogen peroxide, ethyl alcohol, water immersion, ultraviolet light, and heat exposure. The tonometer tips can swell and crack by dissolution of the glue that holds the hollow tip together.

The current Guideline for Disinfection and Sterilization in Healthcare Facilities from Centre for Disease control recommends that health care professionals should wipe clean tonometer tips and then disinfect them by immersing for 5 to 10 minutes in either 5000 ppm chlorine (1:20 household bleach) or 70% ethyl alcohol. Manufacturers also advise that tonometer prisms should be replaced 2 years after first use or after a maximum of 100 disinfection cycles with 1:10 dilute bleach, or immediately if damaged. If using 3% Glutaraldehyde/formaldehyde, a soak time of 15 minutes is recommended while for 3% hydrogen peroxide the soak time is 5 minutes. This should be followed by 5-second cold water wash, and then drying with a clean sterile wipe.