Optics: eye examining – vision testing and correcting – Eye examining or testing instrument – Subjective type
Reexamination Certificate
2001-08-08
2003-03-04
Manuel, George (Department: 3737)
Optics: eye examining, vision testing and correcting
Eye examining or testing instrument
Subjective type
Reexamination Certificate
active
06527391
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a method and an apparatus for performing a visual field test. It also relates to computer-readable media which store a computer program for processing a result of one or more visual field tests.
BACKGROUND OF THE INVENTION
Visual field testing is one of the most important examination methods in eye care. It is used for examining the range and the sensitivity of a subject's visual field in connection with diagnosis and treatment of different diseases of the eye, e.g. glaucoma and retinal disease, and of the central nervous system, e.g. brain tumours and inflammations of the optic nerve.
Today most visual field tests are performed with the aid of special computerised instruments (the so-called perimeters) measuring the limit or threshold of a subject's perception of light at a number of test locations in the subject's visual field.
Typically, the test is performed in the following way. The patient is seated in front of a perimeter and asked to look steadily at a centrally placed fixation target, e.g. on a screen or in a hemispherical bowl. Visual stimuli are presented successively with different intensities and at different locations. The patient is asked to press a response button every time he perceives a stimulus, whether close to or distant from the fixation target, whether faint or strong. Alternatively or additionally, the perception of stimuli can be recorded by objective methods, such as measurement of electric potentials in the brain or in the eye of the patient (VER perimetry and ERG perimetry, respectively) or recording of pupil reactions (pupil perimetry).
There are different strategies for selecting test locations and intensities of the stimuli presented at these test locations in order to establish a patient's threshold for perception of light. In one common method, the so-called staircase method, a stimulus which has an intensity close to the expected threshold value at the test location concerned is shown. If the patient does not respond to the stimulus, the intensity of the subsequently presented stimuli is thereafter increased stepwise until a response is received from the patient, i.e. until a stimulus is seen. The first intensity level at which a response is received may be defined as an estimate of the threshold of the test location concerned. The precision of the test can be increased by reversing the test process when the first response is received, and by continuing it in smaller steps with decreasing intensities until the first unseen stimulus is encountered. The threshold may then again be estimated as the intensity level of the last seen stimulus. If, on the other hand, the patient responds to the first stimulus, the intensity is decreased stepwise until no response is received, whereupon the test procedure is reversed.
There are several known strategies of how to vary the intensities of the visual stimuli depending on the response to previous stimuli in order to obtain a rapid and accurate determination of the threshold value of each test location.
In order to obtain more accurate threshold estimates, responses to the visual stimuli at all test locations can also be processed by statistical methods. See e.g. U.S. Pat. No. 5,461,435, wherein the maximum of a posterior probability function is used for calculating the best estimates of the threshold values during and after the test. See also the article “Scandinavian Journal of Statistics, Vol. 21, No 4, p 375-387, 1994, Olsson J and Rootzén H”, which describes another particular method of estimating the best threshold value of each test location from the estimated distribution of threshold values of each test location, namely MPM or Marginal. Posterior Mean which is achieved by calculating a mean threshold value from the posterior probability distribution of threshold values.
The geographical distribution of points showing reduced sensitivity is of great clinical importance, and it is, therefore, very important that this geographical information is displayed to the user in an easily comprehensible way. A common way of displaying the result is to show the numerical value of the single best threshold estimate at each test location in a map. An example of such a map is shown in FIG.
1
. Such numerical maps are difficult to use. Greyscale representation facilitate the usage to some extent.
The result of the visual field test may also be shown as a deviation map that shows the deviation of each measured threshold value from the corresponding threshold value of a mean age-corrected reference field or a significance map that shows the significances of these deviations.
FIGS. 2 and 3
show an example of a deviation map and a significance map, respectively. The deviation maps and the significance maps are advantageous in that they show deviations from expected normal values and the significances of such deviations.
The result of a visual field test can also be presented in the form of a so called visual field index, which may e.g. be obtained by averaging the calculated threshold values of all test location.
All test methods developed so far have been focused on obtaining one or two good estimates of the threshold value at each test location using as few visual stimuli as possible. The threshold values which represent these estimates are displayed, either directly or in refined versions, and used for assessing whether the measured visual field deviates from a normal field or not.
However, it has been established that threshold values from the same test location of the same eye, exhibit variability within a test and between tests often called short term fluctuation and long term fluctuation, respectively. In addition the estimation of the threshold value is always associated with a measurement error. This measurement error is influenced by many factors, e.g. the frequencies of false positive and false negative responses, loss of fixation and variations in stimulus response times. Therefore, a single threshold value estimate determined at a certain test location at a certain moment during the test does not give a complete representation of the patient's capability of perceiving light at that point of the visual field. Thus, there is a risk of erroneous conclusions with the present methods.
SUMMARY OF THE INVENTION
One object of the present invention is to suggest a method and an apparatus for performing a visual field test, which method and apparatus give a better representation of the visual field of the test subject.
Another object of the invention is to suggest such a method and apparatus, which process the responses received from the test subject during a visual field test such that the user obtains more information of the measured visual field, thereby facilitating the interpretation.
At least one of these objects are achieved by a method, an apparatus and computer-readable media having the features of claims
1
,
15
,
26
, and
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, respectively.
The present invention is based on the understanding that the estimation of the threshold values is more or less certain due to estimation errors as well as short and long-term fluctuations, that the degree of confidence varies between different test locations, with normality and with disease, and that a better representation of the measured visual field would be obtained if the confidence of the test result was estimated, used in the subsequent testing and/or conveyed to the user.
In a first aspect, the present invention relates to a method of performing a visual field test, comprising the steps of successively presenting a plurality of visual stimuli to a test subject, each visual stimulus being presented at a test location; recording, for each visual stimulus, any response thereto from the subject; and calculating, on the basis of the recorded responses and with the aid of a confidence level function, at least two visual field test values of different confidence levels of the test subject's perception of visual stimuli, said at least two visual field test values being calcul
Bengtsson Boel
Heijl Anders
Olsson Jonny
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