Optics: eye examining – vision testing and correcting – Eye examining or testing instrument – Subjective type
Reexamination Certificate
2002-09-23
2004-02-10
Manuel, George (Department: 3737)
Optics: eye examining, vision testing and correcting
Eye examining or testing instrument
Subjective type
C600S558000
Reexamination Certificate
active
06688746
ABSTRACT:
TECHNICAL FIELD
This invention relates to a method of assessing the integrity of the visual field by objective elecrophysiological recording with simultaneous multifocal stimulation using different stimulus sequences for each part of the field. In particular, it relates to a method for accurately diagnosing and assessing the extent of visual field loss in a glaucoma patient or any other neuro-ophthalmic disorders where there is loss of peripheral vision. It provides for more rapid collection and assessment of data during the recording than any pre-existing technology.
BACKGROUND ART
In determining the extent of damage to the visual system in ocular diseases such as glaucoma, investigation of the visual field (peripheral vision) is vital. Until recently this has relied on subjective psychophysical tests known as perimetry of which the most widely used is the Humphrey visual field analyser. This involves presentation of stimuli of varying luminance in different parts of the visual field to determine visual thresholds and relies on patient decisions. Perimetry therefore involves an element of uncertainty in interpretation of patient responses.
There is a strong demand for an objective measurement of the visual field, to supplement the variable performances seen in perimetry and other psychophysical tests in the evaluation of glaucoma—a disease which is one of the commonest causes of blindness. Recording of the electrical responses generated by the visual system in response to changing stimuli is a possible alternative. Until recently however, electrophysiological recording could only provide a summed response from the whole eye or occipital cortex, and could not assess peripheral vision.
The conventional full field visual evoked potential (VEP) provides information mostly about the central visual field. It is reported to be abnormal in about half of the population with glaucoma. Since many patients can have normal responses, this method gives poor and unreliable discriminatory power for the detection of the disease. The variable findings have previously been explained by the fact that the VEP predominantly reflects macular function and in glaucoma the damage tends to affect central vision late in the disease. With suitable recording conditions and an array of bipolar electrodes positioned overlying the visual cortex of the brain, it was shown [in Graham, Klistorner, Grigg and Billson Invest 1999 Ophthalmol Vis Sci, 40(4) ARVO abstract #318] to be possible to examine the peripheral visual field which is damaged early in glaucoma.
A major advance in stimulus and recording technology has recently been introduced which enables the presentation of a multifocal stimulus. This is now commercially available as the VERIS—Scientific system (Electro-Diagnostic Imaging, Inc., San Francisco) or Retiscan (Roland Instruments, Wiesbaden, Germany). These systems both present a similar method for topographical analysis of recordings, and utilise the orthogonal property of different phases of a special type of binary sequence, called an m-sequence, which allows stimulation of a number of sites of the visual field simultaneously. All elements of the field are stimulated with the same m-sequence shifted in time.
Due to the long VEP response time, and possible overlap of the signal between segments, the technique as disclosed in U.S. Pat. No. 4,846,567(Sutter) requires the use of long m-sequences. The method described by Sutter does not allow the observation of responses during the recording, but only displays the product of cross-correlations at the end of the test when the entire m-sequence is finished. This property is a significant limitation since in clinical testing it is desirable to observe responses constantly during the recording procedure. In cases where the signal is unsatisfactory, recording time is wasted. In cases where the subject has a limited ability to co-operate and fixate on the screen target (eg children, elderly patients), short recording sequences are essential, as are frequent checking of the quality of each segment before it is included in the data With short recording sequences it is possible to avoid unnecessarily long recording times in cases where the signal is reliable, to stop the recording without loss of data if the patient fatigues, and allow additional runs to be added in when the response is noisy.
For example, consider the case of recording a multifocal VEP with 60 segments or 120 segments of visual field stimulated. With the method described by Sutter (above) using the same m-sequences for all segments of the visual field shifted in time, it is necessary to allow at least 500 msec, preferably 1000 msec, between segments to avoid overlap and contamination of signals. With a 75Hz frame rate at least 2250 code elements are required for 500 msec, and 4500 code elements for l000 msec. Since m-sequences only come in predefined lengths of 2
n
−1, the shortest possible m-sequence required will be 2047 and 4095 elements of code respectively. This will result in recording times of approx 1 minute and 2 minutes respectively for 60 segments of visual field, 2 minutes to 4 minutes for 120 segments of field. Further increases in the number of field elements stimulated would require even greater minimum recording times before the results could be assessed in real time. Sutter alludes to the use of 255 segments or more in recording, which would be extremely useful clinically, but it would require at least 8 minutes of recording before any data could be accessed. Also, U.S. Pat. No. 5,539,482 (James & Maddess) disclosed a system whereby the contrast of each zone was modulated with a different respective temporal frequency. Each of the stimulus signals applied to each zone is orthogonal in time to the visual signals applied to all other zones such that the composition of the response into the components may be computed by Fourier transforms. The disadvantage with this method is that only a limited number of zones can be handled.
It is therefore an object of this invention to provide a method for the rapid objective measurement of the visual field based on simultaneous use of different stimulating sequences at each part of the field, where data can be readily assessed at short intervals during the recording.
Meaning of Terms
In this specification and claims the following terms have the meanings as set out:
“Cross-correlation”: Cross-correlation can be described as comparison of two sequences A=[a
0
, a
1
, . . . a
N−1
] and B=[b
0
, b
1
, . . . b
N−1
] to determine how much they correspond with one another. If sequence B is cyclically shifted by i elements, the cross-correlation r, can be calculated as follows
r
i
=
∑
n
=
0
N
-
1
a
n
b
n
-
1
“Auto-correlation”: Auto-correlation can be described as comparison of sequence A=[a
O
, a
1
, . . . a
N−1
] and it's own delayed copy to determine how much different phases of the same sequence differ one from another. If a copy of sequence A is cyclically shifted by i elements, the auto-correlation r
i
can be calculated as follows
r
i
=
∑
n
=
0
N
-
1
a
n
a
n
+
i
An acceptable level of cross-correlation for clinical testing is less than 6% of the auto-correlation peak (N−1) for a sequence of length
1023
samples or more.
“M-sequence”: M-sequence can be created by applying a single shift register with a number of specifically selected feedback-taps. If the shift register size is n then the length of the m-sequence is equal to 2
n
−1. More detailed introduction in m-sequences can be found in MacWilliams & Sloane, “Pseduo-Random Sequences and Arrays”, Proc IEEE, Vol 64, No 12, December 1976, pp 1715-1729.
“Gold family of binary sequences”: The family of the Gold sequences can be generated as a product of two m-sequences which form a “preferred pair”. So called “preferred pair” is a combination of m-sequences for which the cross-correlation shows only 3 different values: 1, −2
′m+1
1
/2
−
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