When Goldmann first presented his new applanation tonometer in 1955, he emphasized that cross-infection by corneal contact with the methylmethacrylate-covered biprism might occur and that appropriate disinfection should be carried out (Goldmann 1955).
Schmidt & Lebek 1971 recommended soaking the tip in 5% formaldehyde as the safest procedure for the tip with the highest efficacy of pathogen elimination. In the mid-1980s, as evidence emerged that HIV may be found in human tears and conjunctival and corneal epithelium and after the successful lacrimal isolation of HBV, HCV, HSV and adenoviruses, more aggressive disinfection methods were warranted. Guidelines were established to address the disinfection issue without paying much attention to longterm effects on the tonometer tip. Soaking the tonometer head for 5 minutes in 3% hydrogen peroxide, 0.5% sodium hypochlorite or 70% isopropyl alcohol meets the guidelines published by the Center for Disease Control and Prevention (CDC) (1985) and the American Academy of Ophthalmology (AAO) (Lichter 1989). However, wiping the tip with a 70% isopropyl alcohol swab is also described as being as effective in virus elimination as disinfectant immersion (Smith & Pepose 1999).
Our institutions are currently soaking tonometer tips once a day in 0.5% sodium hypochlorite solution for 15 minutes. Between patients, the tonometer tips are wiped with 70% isopropyl alcohol swabs. After noticing that one of our tonometer tips was cracked and distorted, we examined all remaining tips. Those that had been in use for a year or more showed some combination of sharply broken or roughened rim edges and cracked applanation surfaces. Relatively new tips showed such cracks only on the surface. As the tips aged, the cracks expanded to the rim, where a crumbled appearance was noted (Figs 1 and 2). In order to achieve a smooth alignment of the two fluorescein hemirings and an accurate pressure reading, an evenly distributed tear film along the biprism margin is crucial (Goldmann & Schmidt 1957). Obtaining appropriate fluorescein distribution around the applanation area (and thus reliable pressure readings) with a damaged tip is questionable. The accuracy of the Goldmann applanation tonometer (GAT) could be at least as dependent on the tip's shape as on corneal thickness.
In addition, formation of cracks might allow the disinfectant to get into the interior of the surface of the tip. These cracks might irritate the cornea while tonometry is taking place or even cause a corneal abrasion.
Moreover, cracked tips may harbour virulent microbes that are difficult to eradicate during disinfection and represent potential risks for cross-infection. Even a microdamaged tip may injure corneal epithelium enough to make it more susceptible to corneal infection. The damage noted could be due to mechanical abrasion from the alcohol swabs, soaking the tips in the sodium hypochlorite solution or some combination of both. Whatever the cause, these observations strongly suggest that GAT tip damage could be a source of inaccuracy in intraocular pressure readings and may present a potential safety hazard. We recommend checking the GAT tips at the slit-lamp microscope on a regular basis and replacing those that show significant damage.