Method and apparatus for detecting optic neuropathy

A patient is presented with a series of sinusoidal gratings truncated at varying diameters in each quadrant of the visual field (superior nasal, inferior nasal, superior temporal, inferior temporal). The patient fixates on a central point and identifies the point in the series at which he can see the patterns. Comparison of the diameter at which the patient can first perceive the pattern with a standard value provides an indication of not only the existence of optic neuropathy but also the magnitude of the damage.

BACKGROUND OF THE INVENTION 
This application relates to a method and apparatus for the detection of 
optic neuropathy, particularly detection of glaucoma, brain tumors 
affecting the optic nervous system and other optic nerve diseases. The 
method and apparatus exploit the differential ability of normal and optic 
neuropathy patients to detect paracentrally presented sinusoidal grating 
patterns truncated at different diameters by a Gaussian aperture. 
Sinusoidal grating patterns have been used as part of various test methods 
for testing foveal vision. The grating pattern appears as a series of 
bars, the spacing of which depends on the frequency of the sine wave 
employed to generate the pattern. A generally circular outline is 
established by truncating the pattern with a Gaussian aperture. 
Foveal vision tests are in many cases inadequate for detecting the early 
signs of glaucoma, brain tumors and other optic nerve diseases. For 
example, in the early stages of glaucoma, the damage to the optic nerve is 
diffuse, or may occur in only certain types of neurons rather than being 
concentrated in individual bundles of the optic nerve. Because of this, 
discrete field defects may not be detected. 
To provide for early detection of the optic nerve damage, therefore, it 
would be highly desirable to have a mechanism which was capable of 
detecting diffuse damage to the optic nerve. It is an object of the 
present invention to fulfill this need. 
SUMMARY OF THE INVENTION 
It has now been found that sinusoidal grating patterns can be employed 
paracentrally (parafoveally) to detect optic neuropathy in a patient. In 
accordance with the invention, the patient is presented with a series of 
sinusoidal gratings truncated at varying diameters in each quadrant of the 
visual field (superior nasal, inferior nasal, superior temporal, inferior 
temporal). The patient fixates on a central point and identifies the point 
in the series at which he can see the patterns. Comparison of the diameter 
at which the patient can first perceive the pattern with a standard value 
provides an indication of not only the existence of optic neuropathy but 
also the magnitude of the damage. 
The invention is advantageously practiced with the aid of an optical 
display system which is adapted to present a series of diagnostic images 
to the patient. Each diagnostic image advantageously consists of a 
centrally located fixation point and four or more truncated sinusoidal 
grating patterns symmetrically disposed about the fixation point. The 
optical display system may take the form of a plurality of cards, each 
having a diagnostic image disposed thereon, or may be in the form of 
transparencies, video tape or other display means capable of producing a 
consistent image.

DETAILED DESCRIPTION OF THE INVENTION 
Grating patterns may be defined in terms of the spatial frequency, i.e., 
the number of dark and bright bands subtending 1 degree of Visual angle at 
the observers eye; and the spatial contrast C which is given by the 
equation 
##EQU1## 
wherein L.sub.max and L.sub.min are the maximum and minimum luminance. In 
addition, the diameter of the grating pattern can be described in terms of 
the diameter of a Gaussian aperture which is used to truncate the grating 
pattern. 
Gratings useful in the present invention are those which have spatial 
frequencies of 0.5 to 6, preferably 1 to 4 cycles/deg; contrasts of 1% to 
90%, preferably 2% to 60%; and diameters of from 1 to 16 degrees of visual 
angle, preferably 1 to 8 degrees. It will be understood that the spatial 
frequency and diameter of the spot do not fall within a specific absolute 
range since they depend on the intended distance between the eye of the 
patient and the grating being observed. Exemplary specific numbers, 
however, are indicative of the types of values considered appropriate. For 
example, if the patient is 30 cm from the diagnostic image, a pattern with 
a diameter of from 0.5 to 4 cm will span the range from 1 to 8 degrees of 
visual angle, and these spots may include from 1 to 48 bars. As will be 
recognized by the person skilled in the art, the grating patterns used in 
the invention thus coincide to what are commonly referred to a "Gabor 
patches." 
The grating patterns may be formed of black and white bands, or they may be 
colored (for example red and green or blue and yellow). The results 
observed with black and white versus color patterns may be different, for 
example, if a patient has lost luminance processing as opposed to color 
processing ability. 
To be used in the present invention, the grating pattern is presented to 
the patient in each quadrant of the visual field, with the peak contrast 
of the pattern being located from 2 to 40 degrees, preferably 4 degrees of 
visual angle from the fixation point. To determine whether there is optic 
neuropathy, the patient, with one eye blocked, may be shown a single 
series of grating patterns, all with the same contrast, and asked to 
identify what he sees in each quadrant. 
Control or "catch" cards with only three quadrants filled in or with the 
pattern in one quadrant different in contrast from the remaining patterns 
may be included in the series without departing from the scope of the 
invention. It has been found that the ability to perceive the grating 
pattern has a basically sigmoidal relationship to the size of the 
aperture. For spational frequency of 1 cycle/degree, this sigmoidal curve 
is centered (semi-saturation constant) at a diameter corresponding to an 
aperture size of about 4.5 degrees for normal individuals. For individuals 
with diagnosed glaucoma, the value is higher, about 7.7 degrees on 
average. In normal individuals, the standard deviation on the measurements 
appears to be about 0.6 degrees and this figure provides an appropriate 
measure of the difference in aperture between adjacent members of the 
series. These standard values can be used to check an individual for signs 
of optic neuropathy by presenting a series of grating patterns, having a 
constant contrast and spatial frequency but varying diameter. 
In a preferred test format, each member of the series is a diagnostic image 
comprising a central fixation point and at least four grating pattern 
disposed symmetrically about the fixation point. (FIGS. 1 and 2) The 
patient is asked to view the diagnostic image and state whether he can see 
the patterns. By comparing the diameter of the smallest grating where the 
pattern can be perceived with the established standard for that set of 
gratings, the existence of optic neuropathy can be assessed. 
In a further embodiment, the patient is exposed to two or more series of 
diagnostic images, the series differing in the contrast of the images 
presented. In this case, monocular contrast sensitivity (CS) for each 
quadrant can be determined. Contrast sensitivity is defined as the 
reciprocal of the contrast threshold, where contrast threshold is the 
lowest level of contrast at which a pattern with a guren spatial frequency 
and diameter can be detected. Contrast sensitivity also follows a 
sigmoidal relationship as a function of aperture size in normal patients. 
(See FIGS. 3 and 4) Observed contrast sensitivity as a function of 
aperture size in patients diagnosed as having glaucoma, (FIG. 5) and 
suspected of having glaucoma (FIG. 6) may have a less well defined 
sigmoidal relationship, but also has a semisaturation constant which is 
much greater. Again, comparison of the patient value for the 
semisaturation constant with a standard is diagnostic of the presence or 
absence of optic neuropathy. 
EXAMPLE 
Seven normals and 12 patients diagnosed as having various forms of optic 
neuropathy and cerebral tumors pressing on the optic nerves or optical 
radiation were presented with individual sinusoidal grating patterns 
(spatial frequency of 1 c/deg) in each quadrant with the pattern located 4 
degrees from fixation. The size of the patterns, measured as the width of 
the Gaussian aperture at 1/e.sup.4, ranged from 1 to 8 degrees. The 
stimuli were presented on an evenly illuminated 14 cm.times.14 cm 
oscilloscope screen which subtended 10 degrees. Mean luminance was 80 
cd/m.sup.2. 
As shown in FIGS. 3-6, monocular CS as a function of aperture could be 
described as an S-shaped curve. Abnormal CS function can be grouped as (a) 
a shift of the function to the right without a slope change or (b) a shift 
and a slope change. In some patients with apparently intact visual field 
quadrants, a shift of function without a change in slope was observed.