Henson 9000: Technical profile

While ZATA is based upon the same principles as SITA it differs in some important ways.

1. When possible it uses prior data as a starting level. This makes it more efficient and faster especially in eyes where there is some pre-existing visual field defect. It makes better use of prior knowledge.
2. It does not use a single terminating criteria. It can be thought of as a combination of SITA Standard and SITA Fast. It uses the SITA Standard terminating criteria in locations where there is some visual field loss and in the adjacent test locations but uses a less accurate (faster) terminating criteria in other locations. This reduces test times, especially in patients who have no visual field loss.
3. It does not attempt to accurately measure thresholds in severely damaged test locations (<10dB). Accurate measures at these locations are impossible to obtain with any algorithm due to changes in the visual system that make it very variable. This again reduces test times in patients with severely damaged test locations and reduces the number of presentations in these areas and the long sequences of ‘not seen’ responses which are non-productive and can be very frustrating to the patient and perimetrist.

Threshold tests are used when there is a need to accurately measure the depth of any visual field loss.

The first threshold algorithm (Full Threshold) was developed in the 1970s. It was reasonably accurate but took in excess of 10 minutes per eye to perform. A later development (FastPac, Fast Threshold) shortened the test time but was less accurate and as such was not widely adopted. In the 1980/90s a series of papers were published by vision scientists on the more efficient Bayesian approach to obtaining thresholds. One of the algorithms promoted by these researchers (King-Smith 1994) was called ZEST (Zippy Estimate by Sequential Testing). In 1997 a Swedish group of ophthalmologists utilised this work to develop a new perimetric algorithm called SITA (Swedish Interactive Threshold Algorithm). The ZEST algorithm has also been used to develop ZATA (Zippy Adaptive Threshold Algorithm) the threshold test used on the Henson 9000.

An important characteristic of these new algorithms is what is known as their terminating criteria. This specifies how accurate the threshold measure should be. As a general rule the more accurate you want a threshold test to be the longer it will take to perform. The Swedish (SITA) group set their terminating criteria to give an accuracy similar to the 1970s Full Threshold technique (termed SITA Standard), later producing a second version that was faster, but less accurate (SITA Fast). Since the development of SITA a lot more has been learnt about how patients with glaucoma respond to stimuli and the importance of keeping test times down to a minimum. These findings have been used to develop ZATA, the fastest, most accurate threshold test available today.

• 100% sensitivity = every screening test would capture those with a visual field defect, i.e. result = fail
• 100% specificity = every screening test would not fail those with a healthy visual field, i.e. result = pass

Ideally, a screening test would be 100% sensitive and 100% specific, but in reality there will always be anomalies – i.e. ‘false negatives’ (those with the condition who pass the test) and ‘false positives’ (those without the condition who fail the test).

The sensitivity/specificity of a visual field screening test can be altered by changing the fail criteria, e.g. if the fail criteria of a supra-threshold visual field test were to be changed from a single missed point to a cluster of three missed points, then the sensitivity would go down and the specificity would go up.

So, which is the more important? To answer this question you have to answer another. What are the relative costs of a false positive and false negative? When screening for glaucoma in an unselected population, where the prevalence of the disease is low (<2%), it is generally accepted that the specificity should be ≥95%. If it falls much below this figure then we end up failing a relatively large number of people that do not have the disease. If the study population is a high risk one (higher percentage of true cases) then the sensitivity should be increased at the cost of lowered specificity.

There are 2 ways in which you can screen the visual field with the Henson 9000.

You can use the fast and accurate supra-threshold strategies, with a choice of single or multiple stimulus presentations for additional flexibility, or the rigorous and detailed ZATA threshold test.

Screening with the supra-threshold strategies is certainly the fastest option but the testing times of ZATA, compared to other threshold algorithms, make it feasible to offer a threshold strategy by default as a form of enhanced screening test.

The length of a ZATA test will be determined by the extent of a patient’s field loss.

• A normal patient with no field loss will complete the test in approximately 3 minutes (with average response times)
• A patient with significant field loss tested from previously stored data will also complete the test in under 4 minutes due to the ZATA test’s variable termination criteria.
• A patient with moderate field loss, or one who has no prior data stored, will take approximately 4-5 minutes per eye to complete the test.

ZATA was developed to counter all of the problems associated with testing patients with field loss on a full threshold type test. It runs the 24/2 test pattern but uses supra- as well as full threshold testing with different terminating criteria. Furthermore, it uses data from surrounding points as well as prior data (if available).

All of these strategies combine to make ZATA an easier test for patients to perform while offering the same level of accuracy as a standard full threshold would in monitoring glaucoma progression.

The ZATA algorithm is able to make use of prior patient data. If the Henson database contains a record for a previous threshold test then ZATA uploads the previous threshold measures to use as starting values for the new test. This reduces test times and improves accuracy.

Nor does ZATA use a fixed terminating criteria. In areas of severe loss, where thresholds estimates are very variable, and at locations near to normal values, it uses looser terminating criteria, reserving more accurate measures for locations where there is existing mild damage. This again speeds up test times by focusing on locations where change is expected to occur.

In other ways ZATA and SITA are similar. They both use the same test patterns (24/30-2). The threshold values are compatible as are the global indices.

All Hensons are static only perimeters. Isopters require kinetic testing, i.e. a stimulus that moves across the visual field. Kinetic perimetry is only available on selected HFA and Octopus perimeters.

The Henson 9000 performs static determination within the central 30° and qualitative automated threshold perimetry, including the 30-2 and 24-2 patterns of test locations. It is very fast, although exact test times are dependent upon the number of test locations and the type of visual field loss.

In a recent as yet unpublished project with 17 patients, the test time for a 24-2 ZATA Standard test with a superior arcuate defect was 3:01 mins, compared to 7:45 for a Full Threshold test with the same patient sample.

The Henson Esterman test is approved for testing Group 1 and Group 2 drivers.

Because there are certain problems with trial lens holders in perimeters. The lenses that are generally used in perimetry tend to be relatively small in diameter (38mm). This means that very careful alignment between the lens and the patient’s eye is necessary to ensure that no stimuli are occluded during the visual field test. The slightest movement from a patient away from a static, perimeter-mounted lens holder can create artefacts or cause occlusion problems.

The solution is to attach the trial lens to the patient, using a head-mounted trial lens system. The most obvious advantage to this solution is that, should the patient move, the trial lens moves with them, reducing the likelihood of artefacts. A head-mounted system also makes switching lenses easier and avoids the use of unhygienic eye patches by incorporating an occluder within the frame to cover the unused eye.

If you’d like more details about the type of trial lens system used with the Henson 9000, please contact us.

Yes, it can be operated from any PC running the MS Windows™ Professional operating system (version 7, or above).

The control device must also have two spare USB ports for interfacing with the Henson 9000.

Yes. The software is supplied on a USB memory stick and is a single file installer. Simply click on it and it will install all parts of the software.

Both Henson machines are supplied with a quick start guide that takes you through the whole process step-by-step.

This relates to a problem with the settings for your backup.

Firstly, check the location of the backup allows files to be created. If it has been set to a read-only drive, or one that is protected by Windows, then the record will save in the database successfully but the backup will fail – hence the error message you are receiving.

Such read-only locations may be: the root of the C drive; a fundus camera (these often show up in Windows as USB flash drives); a Windows system folder or a mapped drive on a computer that is not switched on.

To ensure a successful backup set the location to a mapped network drive on your PC or a removable hard drive/USB flash drive.

Yes.

The Henson software prints the test result to the default installed printer on the laptop/computer being used as the control device. Printing is done via the default installed PDF viewer (Adobe Reader in the case of Henson 8000). Essentially, as long as the installed printer is Windows-compatible it will work without a problem.