Detecting progression in patients with glaucoma

Visual field testing should be performed in a consistent manner with enough fields to determine the rate of progression over time, thereafter basing the frequency of testing on the rate detected, said Alfonso Anton MD, PhD at the XXXIII Congress of the ESCRS in Barcelona, Spain.

“Calculating the rate of progression is as important as identifying progression, as it allows an estimation of evolution of the disease and helps in making clinical decisions,” said Dr Anton, International University of Catalonia, Barcelona.

He presented a series of recommendations for optimal surveillance of visual field changes in patients with glaucoma. The first was that the visual fields used for comparison should be performed with the same perimeters and algorithms.

Therefore, the fields under review should have been performed with the same style perimetry, whether it be white-white, short wavelength automated perimetry (SWAP), frequency doubling technology (FDT) or others, using the same perimetry device, whether it be the Humphrey, Octopus or Topcon, and the same strategy for detecting change, whether it be Swedish interactive thresholding algorithm (SITA) – SITA Standard or SITA Fast – or tendency oriented perimetry (TOP).

He added that it is best to use strategies that are more reproducible and less variable, such as SITA Standard and TOP, while SITA Fast should be avoided. It is also important to exclude unreliable visual fields such as those with a false positive rate of more than 15 per cent, those with a learning effect and those with artefacts.

OPTIMAL FREQUENCY

He emphasised that it is essential to obtain enough fields to first detect the rate of progression, and then on that basis determine the optimal frequency with which to perform follow-up visual field testing. The baseline visual fields should include at least two similar and reliable visual fields. The recommendation for initial follow-up visual field testing should be conducted six times over two years.

A minimum of four visual field tests is necessary to reliably determine a progression rate that could cause clinically significant visual loss, and some patients require many more, Dr Anton said. More frequent visual field testing is necessary in eyes where there is suspicion of progression. Eyes with greater visual field loss also require more frequent testing. He stressed that structural images should not be used as a substitute for visual fields, which provide the true clinical picture.

Variability of visual field tests over time greatly adds to the difficulty of detecting progression and determining its rate. Variability increases towards the periphery and within deeper and in larger visual field defects.

“A greater variability increases the difficulty in detecting progression, therefore more follow-up visual fields are necessary,” Dr Anton said.

CONFIRM PROGRESSION

When there is suspicion of progression, confirmation requires repeat visual field testing, and the change must be present in at least two consecutive visual fields. The number of fields required to detect progression depends on the method used, the rate of progression, the degree of damage and variability. Once the progression has been confirmed and treatment modified accordingly, a new visual field baseline should be set, although prior visual fields should be retained in order to enable a global evaluation of the patient’s disease process.

It is also necessary to rule out other potential causes of changes in the visual field such as refractive errors, loss of transparency of the cornea, chorioretinal pathology or pathology of the visual pathway. Visual field defects not typical of glaucoma include the sudden development of large scotomas, scotomas that respect the vertical meridian and defects that cross the horizontal meridian.

At any visit, the present visual field should be compared with baseline, and the whole series of fields, at each point and with global indices. Manual methodical assessment is always available to all ophthalmologists, and the European Glaucoma Society (EGS) Guidelines has a set of criteria for the manual detection of new defects, deepening defects, extension of defects and diffuse depression.

However, although subjective assessment is useful, it is laborious, difficult and has low reproducibility. The EGS therefore recommends the use of automatic progression algorithms. For example, glaucoma progression analysis for SITA or Eye Suite, which compare observed change with expected variability and calculate rates of progression. Such rates quantify changes in the mean deviation or in the visual field index over time.

Dr Anton noted that event analysis is more sensitive in detecting progression initially. However, trend analysis is more useful in determining the rate of progression, providing the patient underwent visual field testing more than six times over a period of more than six years.

“Interpret progression in the context of each patient. Revise results critically and consider the speed of progression, to identify rates that would imply functional impairment and adjust the frequency of testing. Make therapeutic decisions based on degree of damage, the patient’s age and the rate of progression,” Dr Anton added.

Alfonso Anton: anton@icrcat.com