By David Lunt, Consultant Eye Surgeon and Glaucoma Specialist from South Tees Hospitals NHS Foundation Trust in England
I am a consultant ophthalmologist specialising in complex glaucoma and cataract surgery at South Tees Hospital - a large tertiary referral center in Northeast England. In recent years I’ve been responsible for building many of our pathways from the ground up, including our new patient asynchronous virtual service for diagnosing and monitoring glaucoma. Patients typically enter this pathway after seeing their community optometrist, who flags potential signs of glaucoma at a routine visit and sends them to see me. There are many lower risk glaucoma and glaucoma suspect patients who are ideally suited to the virtual clinic model which is more efficient for both the patient and clinician, freeing up my face-to-face patient/ clinician time to focus on acute and complex cases.
In order for this service to be successful it is essential that we have easy, objective, and accurate instruments to assess glaucoma risk in our patients. This has been the case with instruments such as OCT and perimeters for quite some time.
However, obtaining reliable IOP readings in a virtual clinic setting can be challenging. Nurses and technicians often lack the skill required to perform Goldmann tonometry and other tonometric devices have been shown to be either less objective or less accurate than necessary for our needs. So before we moved to delegation of IOP measurements in the virtual clinic, I needed to be sure there was little to no possibility of inaccurate readings – which could be potentially visually devastating to patients who might only be seen once in 24 months.
We investigated the use of 4 different tonometers in a prospective study of patients attending a single clinician’s outpatient clinic.
Over a two-week period, IOP was measured using Tono-Pen® AVIA Handheld Tonometer, Ocular Response Analyser® G3, Goldmann Applanation Tonometry (GAT), and iCare®. All patients had their IOP measured with a Tono-Pen, then they were alternately selected to have IOP measurement using Ocular Response Analyzer G3 first followed by GAT and iCare, or by GAT and iCare first followed by Ocular Response Analyser G3. GAT measurements were obtained using standard masking techniques. Tono-Pen measurements were taken by trained healthcare assistants, Ocular Response Analyser G3 by specialist ophthalmic technicians, and GAT and iCare by the examining clinician.
A total of 54 eyes from 27 patients were included in this study. The diagnoses ranged from primary open angle glaucoma (37%, n=20), glaucoma suspect (9%, n=5), narrow angles (7%, n=4), ocular hypertension (6%, n=3), primary angle-closure (2%, n=1), primary angle-closure glaucoma (2%, n=1), and angle recession (2%, n=1). As the patient attending were a mixture of new and review patients, a portion of patients (35%, n=19) had no ocular disease.
In terms of overall correlation with GAT (remember: “a correlation coefficient of 1 means 100% correlation. The closer the number is to 1, the higher the level of correlation), correlation coefficients were 0.897 for iCare, 0.673 for Tono-Pen and 0.951 and 0.929 for Ocular Response Analyzer G3’s IOPg (Goldmann-correlated IOP) and IOPcc (corneal-compensated IOP) respectively. Tono-Pen, iCare, IOPg, and IOPcc measurements were within 2 mmHg of GAT in 61.1%, 29.6%, 85.2% and 74.1% respectively. When extended to +/- 3mm, levels of agreement were: Tono-Pen 72.2%, iCare 63.0%, IOPg 98.1%, IOPcc 90.7%. It should be noted that the Ocular Response Analyzer IOPcc measurement is designed to be less dependent on corneal properties such as thickness and biomechanics than GAT. This explains its lower correlation and agreement with GAT compared to the Ocular Response Analyzer’s IOPg value, which is intended to be a “Goldmann Equivalent”. However, numerous studies suggest that IOPcc may actually be more accurate than GAT due to it being less contaminated by corneal variables that are widely understood to cause over or under estimation of IOP by GAT.
This study suggests that Ocular Response Analyzer measurements have a high level of agreement with GAT. In all honesty, I was surprised by how closely Ocular Response Analyzer IOP readings compared to my GAT readings. Conversely, iCare tonometers appear to be less accurate than their popularity would suggest. These findings are extremely relevant to modern glaucoma care where we are already used to a clinical setting whereby visual acuity, perimetry, and imaging are carried out by technicians. But eye pressure, which is extremely important and the only modifiable risk factor for glaucoma, is typically performed by the clinician making the management decisions.
Although the present study was relatively small, it adds to the sizable body of evidence supporting Ocular Response Analyzer as a reasonable alternative to GAT. But it is really important to consider what we are comparing Ocular Response Analyzer IOP readings against: we’re comparing it against time taken for the clinician to perform Goldmann Applanation Tonometry. We’re comparing its usefulness to the IOP readings made by technicians who may not be lining up the Tono-Pen or the iCare properly for the most accurate IOP readings. Because the Ocular Response Analyzer is auto-aligning and auto-measuring, and has a Waveform Score (WS) feature (a quality index), we can be confident in the reliability of the readings when performed by staff. Ocular Response Analyzer is subject to less inter-operator variability, is not technically challenging to use, offers reduced risk of infection transmission, and does not require topical anesthesia. The lack of consumables required is conducive to cost savings for the clinic.
As such it’s something that I use now on a majority of my patients. With glaucoma prevalence expected to increase significantly in the coming years, this method of accurate IOP assessment will be invaluable to enable our departments to see more and more patients safely and effectively.
Article taken from the Reichert Technologies Quarterly Newsletter.
