January is Glaucoma Awareness Month and to help diagnose glaucoma more efficiently we created this cheat sheet on automated threshold perimetry testing.
Primary open angle glaucoma (POAG) is a chronic optic neuropathy characterized by intra ocular pressure ( IOP) higher than tolerable for an eye, characteristic atrophy of the optic nerve head (ONH), accompanied by a loss of the retinal neural fiber layer (RNFL), and may have superior and inferior visual field loss. Glaucoma is often called a “silent” disease because central vision is not effected until later stages. However, there are early signs that a doctor might notice such as; a shallow fluctuating scotoma, depressions in sensitivity, and asymmetry in the superior and inferior visual fields.
There are 5 major aspects to pay attention to when evaluating a glaucoma suspect.
1. Determine in the level of IOP using applanation tonometry (3 mmHg difference is acceptable between eyes).
2. Assess the status of the anterior chamber angle with gonioscopy. A three or four mirror without flange is useful for indentation, while a three or four mirror with flange may deliver better optics.
3. Evaluate the health of the retina during a dilated fundus exam (DFE).
4. Evaluate the health of the optic nerve head stereoscopically during DFE.
5. Examine the visual fields using perimetry.
It is important to keep in mind that all of these aspects are of vital importance to making a diagnosis.
So, let’s talk about how we test the visual field. The three types of perimetry testing often performed include the 30-2, 24-2, and 10-2. The 30-2 tests the central 30 degrees, while the 24-2 tests the central 24 degrees, and the 10-2 tests the most central 10 degrees. Most clinicians use a 24-2 SITA Standard strategy for glaucoma and will use the 10-2 to further evaluate if they see a scotoma or an area of concern in the central or paracentral area. To receive accurate data and minimize learning artifact, the test should be run an minimum of 3 times. These perimetry tests help to diagnose glaucoma, BUT you must take the information provided and make sure it matches the other 4 areas evaluated (IOP and other risk factors, anterior chamber angle, RNFL, and ONH). The areas of the perimetry are discussed in more detail below.
Reliability parameters: The most important parameter here is the false positives. If you have a false positive higher than 10%, your patient might be “trigger happy” and the data collected is unreliable.
The number of fixation losses and false negatives should also be low. You can monitor the patient’s fixation using the gaze tracking feature.
Total deviation is the difference between the patient’s values and the average normal values.
Pattern deviation is the total deviation adjusted to correct for other optical effects such as cataracts or uncorrected refractive error. The pattern deviation should also be similar to or better than the total deviation. Glaucomatous defects that may be seen are nasal steps, arcuate defects, paracentral scotomas, and altitudinal defects. Defects will respect the horizontal midline. Darker points indicate increasing statistical significant deviations from normal. A cluster of 3 or more significant points indicates a glaucomatous defect if it is repeatable and correlates with ONH appearance and/or OCT.
Raw data: The highest value you should see is approximately 35 dB, the peak sensitivity for the human eye. The lowest value is <0 dB which is indicative of an area where no light was detected.
Gray scale: Gives you an outline of area of concern in the RNFL. While the pattern deviation plot is used more clinically, this may be good for patient education purposes.
Glaucoma hemifeild test (GHT): compares five zones in the superior and inferior visual fields to each other. Usually glaucoma is noted when the superior and inferior are asymmetrical.
There are 5 possible notations:
- Outside normal limits: less than 1% between the field are normal
- Borderline: between 1 and 3% between the fields are normal
- General reduction of sensitivity: positive value with less than 3% normality
- Abnormally high sensitivity: patient’s responses are unreliable
- Within normal limits
Visual Field Index (VFI): an adjusted parameter indication of how much vision the patient has left, recorded as a percentage.
Mean deviation (MD): overall weighted average sensitivity of the field loss. A positive value indicates a higher sensitivity than average, and a negative value indicates a lower sensitivity than average.
Pattern standard deviation (PSD): measure of how different the points are from one another in the visual field.
Special thanks to Dr. Joseph Sowka.