Download Vitreous fluorophotometry evaluation of xenon

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Invest. Ophthahnol. Visual Sci.
December 1978
Reports
threshold may be a useful indicator of determining
when the process of adaption to contact lenses is
complete.
Two of the subjects returned to baseline sensitivity after discontinuing lens wear overnight,
while one subject required 1 week to regain original threshold levels. Although none of these subjects developed corneal edema or any other adverse signs during lens wear, there apparently are
some individual factors which may prolong the
corneal desensitization process long after lens
wear is discontinued.
From the School of Optometry, University of California, Berkeley, Calif. This study was supported in part
by U.S.P.H.S. Grant R01 EY01755-02. Submitted for
publication July 7, 1978. Reprint requests: Dr. Kenneth
A. Poise, School of Optometry, University of California,
Berkeley, Calif. 94720.
Key words: corneal touch threshold, corneal sensitivity,
comeal swelling, corneal thickness
REFERENCES
1. Byron, H. M., and Wesley, A. C : Clinical investigation of comeal contact lenses, Am. J. Ophthalmol.
51:675, 1961.
2. Dixon, J. M.: Ocular changes due to contact lenses,
Am. J. Ophthalmol. 58:424, 1964.
3. Ko, L. S., and Tomiyama, S. K.: The influence of
contact lens application on the corneal sensitivity,
Trans. Ophthalmol. Soc. 2:1, 1963.
4. Larke, J. A., and Sabell, A. G.: A comparative study
of the ocular response to two forms of contact lenses,
The Optician 162:10, 1971.
5. Millodot, M.: Effect of hard contact lenses on corneal thickness, The Contact Lens 1:5, 1967.
6. Millodot, M.: Effect of the length of wear of contact
lenses on corneal sensitivity, Acta Ophthalmol.
54:721, 1976.
7. Cochet, P., and Bonnet, R.: L'esthesiometrie
corneene, Realisation et interet pratique, Bull. Soc.
Ophtalmol. Fr. 61(7 - 8):541, 1961.
8. Millodot, M.: Studies on the sensitivity of the
cornea, Optician 157(4067):267, 1969.
9. Millodot, M.: Objective measurement of corneal
sensitivity, Acta Ophthalmol. 51(3):325, 1973.
10. Mandell, R. B., and Poise, K. A.: Keratoconus:
Spatial variation of corneal thickness as a diagnostic
test, Arch. Ophthalmol. 82:182, 1969.
Vitreous
fluorophotometry
evaluation of
xenon photocoagulation. JAMES M. NOTH,
CHARLES VYGANTAS, AND JOSE G.
F. CUN-
HA-VAZ.*
An abnormal increase in the permeability of the outer
blood-retinal barrier was induced in the eyes of adult
pigmented rabbits after retiiial xenon arc photocoagulation. The alteration of the blood-retinal barrier, which
was assessed by vitreous fluorophotometry after systemic
administration of sodium fluorescein, followed a welldefined pattern. Higher values, which were recorded
during the first three days after photocoagulation, recovered progressively afterward. The permeability of the
blood-retinal barrier returned to near-normal levels between 10 and 14 days after photocoagulation. A direct
correlation was observed between higher initial values
and heavier photocoagulation.
A variety of retinal diseases (vascular retinopathies and macular disorders) associated with
breakdown of the blood-retinal barrier are, at
some time in their evolution, treated by photocoagulation. This appears to be somewhat contradictory, in that photocoagulation has been shown by
fluorescein angiography and histologic studies
using a variety of tracer materials1 to induce a
breakdown of the blood-retinal barrier.
Further studies were needed to measure the
degree of the alteration of the blood-retinal barrier
induced by xenon photocoagulation and its recovery to normal levels. The availability of vitreous
fluorophotometry,2 a new quantitative method of
fluorescein analysis of the vitreous that permits
the detection of very small amounts of fluorescein
penetrating into the vitreous through an altered
blood-retinal barrier, enabled us to measure at different intervals the alteration of the blood-retinal
barrier after xenon arc photocoagulation.
Material and methods. Adult pigmented rabbits, principally of the Dutch strain, were used as
the experimental animals.
Xenon photocoagulation. Xenon photocoagulation was applied to 11 eyes of seven animals. Before treatment all eyes were subjected to ophthalmoscopic examination and fundus photography. Photocoagulation (Zeiss) with the xenon arc
lamp (XBO-2007 Osram), using a 4.5° cone size
was selected for all burns. General anesthesia was
given to all animals in an ultramuscular injection of
acepromazine and ketamine. Xenon photocoagulation was applied to the posterior pole of the eye
in an area immediately inferior to the region of the
myelinated nerve fibers. A total of 20 burns were
applied to each eye. Although difficult, an attempt
was made to evaluate the intensity of the retinal
burns by following the system of grading proposed
by Tso et al.3 Five eyes received predominantly
grade I photocoagulations, whereas the other six
eyes had heavier grade II and III retinal burns
inflicted to their retinas. At the time of the sacrifice, the eyes were enucleated and the retinas
were laid flat on slides. The photocoagulated area
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Volume 17
Number 12
was then measured and its size compared with that
of the entire retina.
Vitreous fluorophotometry. The permeability of
the blood-retinal barrier was assessed by vitreous
fluorophotometry before photocoagulation and at
1, 3, 7, 10, 14, and 21 days afterward. The apparatus and techniques have been previously described.2 In this study, a fiberoptic probe 450 /am
in size was used. The posterior pole of the eye,
immediately inferior to the myelinated nerve
fiber, was selected for starting the recordings. The
recordings from the retina to the lens were divided into three equal parts, which were ascribed
to posterior, middle, and anterior vitreous.
To calculate the blood-retinal barrier permeability, a method previously reported was followed.4 The geometry of the rabbit's eye, a curved
lens surface placed 6 to 7 mm from the spherical
retina of 8 mm radius, leads to a difficult computation. For the present purpose two simple approximations were used between which the true condition might be expected to fall. In one, the retina
and lens were taken as large plane sheets 6.5 mm
apart, and in the other, the retina was assumed to
be a complete sphere 6.5 mm in radius. The distributions at any time, corresponding to these approximations, can be interpolated between the
examples plotted by Crank,5 which give curves of
D(C0-C) (Foa) plotted against r/a for different values of Dt/a2. D is the rate of movement of fluorescein through the vitreous, 6 X 10"6 cnWsec,
(Co-C) is the concentration of fluorescein at the
retinal surface, Co is background fluorescence, a —
0.65 cm, t, = 3600 sec, and Fo is the rate of
fluorescein penetration over unit area of surface.
The value of the retinal permeability is then given
by the ratio F0/Cp, where Cp is the free concentration of fluorescein in blood (approximately 25%
of the total concentration of fluorescein in the
blood).
Samples of blood, taken from a cut in the marginal ear vein, were placed in a 10 /nl "Microcap"
pipette and discharged without centrifugation into
a convenient volume of saline. Readings were
taken in the supernatant when the cells had
sedimented or been centrifuged down. The blood
was sampled from the vein on the opposite side 1
hour after the intravenous injection of 1 ml of 10%
sodium fluorescein at the time of the vitreous
fluorophotometry recordings.
Controls. All eyes were examined by vitreous
fluorophotometry before application of xenon
photocoagulation. Furthermore, three eyes from
three different rabbits were not subjected to
treatment and were similarly examined by vitre-
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Reports
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Table I. Blood-retinal barrier permeability
after heavy xenon photocoagulation*
Days after xenon
photocoagulation
Rabbit
eye No.
2
3
4
6
8
Mean
SD
10
28.0
25.0
15.0
28.0
14.0
15.0
20.8
±6.8
23.0 20.0
22.0 16.0
7.5
22.0
23.0 16.0
19.0 13.6
8.0
15.0
13.5
20.7
±3.2 ±4.9
14
21
10.0 5.0 Bt
8.5
6.5
6.5
5.5
6.0
7.1
4.5
B
B
B
B
B
B
B
B
B
±1.8
4
Values are X 10~ cm/hr.
* Heavy xenon photocoagulation consisted of 20 retinal bums of
predominantly grades II and III intensity.
t Background values in the vitreous.
Table II. Blood-retinal barrier permeability
after minimal xenon photocoagulation*
eye No.
5
7
9
10
11
Mean
SD
Days after xenon
photocoagulation
1
7
10
14
8.5
5.0
7.5
9.0
8.0
7.6
Bt
B
B
B
B
B
B
B
9.5
8.5
8.3
7.5
6.5
8.1
±1.2
3
11.0
12.0
19.0
11.0
9.5
14.5
±5.6
4.0
3.5
±1.6
Values are x 10"4 cm/hr.
•Minimal xenon photocoagulation consisted of 20 retinal burns of
predominantly grade I intensity.
t Background values in the vitreous.
ous fluorophotometry at intervals of 1, 3, 7, 10, 14,
and 21 days after photocoagulation of the contralateral eye.
Results. In the control eyes, penetration of
fluorescein across the retinal surface was not detectable. Our measurements showed that the level
of the vitreous body remained well below 10~8
gm/ml, whereas the free fluorescein in the blood
reached initial values of about 10"4 gm/ml. These
data suggested an inward permeability no greater
than 10~5 cm/hr. When a series of 20 xenon photocoagulations were applied over an area of approximately one sixth of the entire retina, fluorescein penetrated into the eye after an intravenous
injection of the dye. The entry pattern was similar
in every eye. The profile of the concentration
within the vitreous body conformed approximately
to that which would be expected if the dye were
liberated at the retinal surface at a constant rate,
diffused through the vitreous humor at the rate of
movement found in free solution (6 X 10~6 cm2/
sec), and met a barrier to further progress at the
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Invest. Ophthalmol. Visual Sci.
December 1978
Reports
l/sw
2624-
o 22ro
20-
o Lighter photocoagulation
c
18-
• Heavier
o
</>
16-
X
<D
1412-
tor
O
u.
108-
%
6-
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42-7
77^)7/7/^77/7;
10
14 Days
Fig. 1. Permeability of the blood-retinal barrier for fluorescein after xenon photocoagulation.
lens surface.4 Values of retinal permeability were
obtained at 1, 3, 7, 10, 14, and 21 days after photocoagulation (Tables I and II). The results showed
a direct correlation between the breakdown of the
blood-retinal barrier and the intensity of the photocoagulation burns. Rabbit eyes that received
lighter retinal burns showed much lower values of
retinal permeability when compared with the eyes
that had heavier photocoagulation burns (Fig. 1).
In both groups of eyes, however, the breakdown
of the blood-retinal barrier followed a similar
course. After an initial increase in the retinal permeability, which remained high during the first
three days after photocoagulation, there was a
similar progressive decline. The blood-retinal
barrier recovered to near normal values 14 days
after heavier photocoagulation and only 10 days
after grade I lesions were predominantly applied
to the retina.
Discussion. Our results show that after xenon
photocoagulation of the retina there is an abnormal increase in the permeability of the bloodretinal barrier. This alteration of the blood-retinal
barrier follows a well-defined pattern. Higher values that were demonstrated during the first 3 days
after photocoagulation recovered progressively afterward. The permeability of the blood-retinal
barrier returned to near-normal levels in the period between 10 and 14 days after photocoagulation.
A direct correlation between higher initial values and heavier photocoagulation was observed.
The permeability of the blood-retinal barrier
reached, after heavy photocogulation, very high
values (approximately 2 X 10~3 cm/hr). These
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were much higher than the values registered previously after competitive inhibition of the active
transport for fluorescein and other organic anions.4
This finding is even more significant in that only
approximately one sixth of the retina was treated
by photocoagulation, and the measurements of the
retinal permeability were made for the entire retina and vitreous.
The abnormal increase in the permeability of
the blood-retinal barrier may, under certain circumstances, be considered useful for therapeutic
purposes. A few reports in the ophthalmic literature have indicated that a breakdown of the
blood-retinal barrier induced either by cryoapplications or photocoagulation can significantly increase the concentration in the vitreous of intravenously administered drugs.6 The fact that the
recovery of the barrier becomes almost complete
by 10 days favors its use only for short-term
periods of treatment.
The anatomic substrate of the recovery of the
outer blood-retinal barrier after xenon photocoagulation is still, however, a matter for discussion. In the rabbit, the reparative tissue is composed mainly of cells from the retinal pigment epithelium. The degree of reconstitution of the tight
junctions in this newly formed epithelial layer,
however, remains to be further clarified.7
At present we are undertaking an electron microscopic examination of the eyes used in our
study. We expect that the findings will contribute
to the understanding of the cellular mechanisms
involved in the recovery of the blood-retinal barrier after breakdown. Recovery of the permeability of the blood-retinal barrier to normal values
Volume 17
Number 12
Reports
after photocoagulation is supported by clinical experience. Photocoagulation in central serous chorioretinopathy, for instance, is considered to have
a "sealing" effect upon the characteristic fluorescein leakage.
Finally, vitreous fluorophotometry may serve as
a clinical tool in evaluating the final effect of a
photocoagulation treatment.
We thank Ms. Maxine Gere for editorial assistance
and Ms. Patricia Watkins for secretarial help.
From the Department of Ophthalmology, University
of Illinois Eye and Ear Infirmary, Chicago, 111. *Professor of Ophthalmology, Clinica Oftalmologica, Universidade de Coimbra, Coimbra, Portugal. International research scholar of Research to Prevent Blindness, Inc.,
and Visiting Professor of Ophthalmology, University of
Illinois Eye and Ear Infirmary, Chicago. Submitted for
publication May 8, 1978. Reprint requests: Dr. Jose
Cunha-Vaz, Department of Ophthalmology, University
of Illinois Eye and Ear Infirmary, 1855 W. Taylor St.,
Chicago, 111. 60612.
Key words: vitreous fluorophotometry, xenon photocoagulation, blood-retinal barrier, retinal pigment epithelium, permeability
REFERENCES
1. Peyman, G. A., and Bok, D.: Peroxidase diffusion in
the normal and laser-coagulated primate retina, INVEST. OPHTHALMOL. 11:35, 1972.
2. Cunha-Vaz, J. C , Abreu, J. R. F., Campos, A. J.,
and Figo, G.: Early breakdown of the blood-retinal
barrier in diabetes, Br. J. Ophthalmol. 59:649, 1975.
3. Tso, M. O. M., Wallow, I. H. L., and Elgin, S.:
Experimental photocoagulation of the human retina:
I. Correlation of physical, clinical and pathologic
data, Arch. Ophthalmol.95:1035, 1977.
4. Cunha-Vaz, J. G., and Maurice, D. M.: The active
transport of fluorescein by the retinal vessels and retina, J. Physiol. 191:467, 1967.
5. Crank, J.: The Mathematics of Diffusion, Ed. 1, Oxford, 1956, Clarendon Press, pp. 59, 93.
6. Rober, H., Goring, W., Sous, H., and Reim, M.:
Concentration of ampicillin in the vitreous after
cryocoagulation, Albrech v. Graefes Arch. Klin. Exp.
Ophthalmol. 204:275, 1977.
7. Wallow, I. H. L., Tso, M. O. M., and Fine, B. S.:
Retinal repair after experimental xenon arc photocoagulation: I. A comparison between rhesus monkey and rabbit, Am. J. Ophthalmol. 75:32, 1973.
Cortical responses evoked by laser speckle.
WILLIAM
W.
DAWSON AND MICHAEL
C.
BARRIS.
Human cortical evoked responses elicited by patterns are
very size and focus dependent, but those produced
by moving laser speckle patterns (LASCERs) remain
similar in amplitude and subjective sharpness over a
wide (±20 D) variation in added lens power. Moreover,
cortical signal amplitudes evoked by speckle displays
with high average spatial frequencies are significantly
larger than those reported for check stimuli equivalent in
spatial frequency.
Consistent with animal data on cortical unit responses,1 patterned stimulation of the human retina produces cortical evoked signals consistent in
amplitude across normal subjects. Human visual
cortical evoked responses produced by patterned
stimuli (pattern VERs) are highly dependent upon
retinal image quality.2 Furthermore, these pattern
VERs are not linearly related to the pattern element visual angular subtense.3 Speckle patterns
produced by laser irradiation may be shifted spatially over time to produce visual stimulation. We
have found that the amplitudes of laser speckle
cortical evoked responses (LASCERs) are largely
independent of the refractive condition of the eye
and evoke signals which are several times larger in
magnitude than those reported for patterns
equivalent in mean spatial frequency.
These attributes suggest that LASCERs may be
developed to evaluate visual resolution in patients
where there are opacities. Indeed, Arden and
Sheorey4 reported that their opacity patients continued to see speckle when low-frequency gratings
produced by a laser interferometer could no
longer be seen.
The physical understanding of laser speckle has
developed largely since I960.5 Speckle is produced readily whenever coherent energy is reflected from a roughened surface. The speckle pattern (Fig. 1) is the consequence of constructive
and destructive interference of coherent wavefronts whose phase relations have been altered by
reflection from a surface. The granular pattern
may be detected in any plane where a sensor (e.g.,
photographic emulsion, retina, photodiode) is
present. An aperture in front of the sensor enhances the speckle by increasing the number of
interference fringes.6 The spatial frequency and
irradiance of the speckle is a complex interaction
between the detector-reflector distance, aperture
size, and reflector surface characteristics.7 Lenses
between the detector and the reflecting surface
produce small changes in the apparent size of the
speckle but do not alter the sharpness of speckle
borders. The spatial frequency of speckle is approximated by a Gaussian probability distribution.8
Methods. Human subjects viewed laser speckle
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