Download Vitrectomy prevents retinal hypoxia in branch retinal vein occlusion.

Investigative Ophthalmology & Visual Science, Vol. 31, No. 2, February 1990
Copyright © Association for Research in Vision and Ophthalmology
Vitrectomy Prevents Retinal Hypoxia
in Branch Retinal Vein Occlusion
Einar Srefansson, Roger L. Novack, and Diane L. Harchell
Vitrectomy has been shown to halt diabetic retinal neovascularization, but the mechanism of this
process is unknown. We propose that vitrectomy improves the oxygen supply to ischemic inner retina
by way of fluid currents in the vitreous cavity. In order to test this hypothesis, we induced branch
retinal vein occlusion in cats and measured preretinal oxygen tension before and after branch retinal
vein occlusion in ten nonvitrectomized and five vitrectomized eyes. Branch retinal vein occlusion
caused a significant decrease in preretinal oxygen tension in nonvitrectomized eyes, in which the
oxygen tension fell from 20 ± 7 to 6 ± 5 mmHg (P = 0.001). Conversely, in vitrectomized eyes the
oxygen tension was not significantly reduced after branch retinal vein occlusion. The data demonstrate
that branch retinal vein occlusion causes retinal hypoxia in nonvitrectomized eyes, whereas after
vitrectomy the hypoxic effect of branch retinal vein occlusion is reduced. The relief of retinal hypoxia
that follows vitrectomy may be responsible for halting retinal neovascularization after vitrectomy in
diabetic patients. Invest Ophthalmol Vis Sci 31:284-289, 1990
fellow eyes before and after branch retinal vein occlusion. In five additional cats the preretinal oxygen
tension was measured in normal eyes before and after
branch retinal vein occlusion.
The cats (2-4 kg, and of either sex) were anesthetized with intramuscular ketamine hydrochloride
(20-30 mg/kg) and acepromazine maleate (2-3
mg/kg), repeated as needed. The right pupil of each
animal was dilated with topically applied 0.25% tropicamide and 5% Phenylephrine, and the eye prepared for surgery under sterile conditions. A lid speculum was placed; a 180° conjunctival peritomy was
performed temporally; and two sclerotomies were
made in the pars plana area, supero- and inferotemporally, respectively. A cannula was sutured in place
in the inferotemporal sclerotomy for irrigation with
lactated Ringer's solution. The vitrectomy instrument (Minivisc, OMS) was placed through the superotemporal sclerotomy into the vitreous cavity
under direct observation through the operating microscope, using the coaxial light and a flat corneal
contact lens. A pars plana vitrectomy was performed
where the vitreous gel was completely removed except for the peripheral vitreous gel adjacent to the
vitreous base. The sclerotomies were closed with 6-0
Dexon, the conjunctiva sutured, and 10 mg Gentamicin injected subconjunctivally. The cats were observed for 2-4 weeks after vitrectomy, prior to oxygen
tension measurement.
Blankenship and Machemer' studied the long term
results of vitrectomy in patients with proliferative diabetic retinopathy. They found that retinal fibrovascular proliferations usually do not progress after vitrectomy, and so they raised the question: What is the
mechanism by which vitrectomy affects proliferative
diabetic retinopathy?
We wanted to pursue this question, and hypothesized that fluid currents in the vitreous cavity after
vitrectomy could deliver oxygen and other nutrients
from well oxygenated (perfused) areas to ischemic
(nonperfused) areas in the retina. This new oxygen
distribution relieves retinal hypoxia, reducing the
hypoxic stimulus to fibrovascular proliferation. In
order to test this hypothesis, preretinal oxygen tension was measured in vitrectomized and nonvitrectomized cat eyes before and after branch retinal vein
occlusion induced by transvitreal diathermy.
Materials and Methods
In five cats the preretinal oxygen tension was measured simultaneously in normal and vitrectomized
From The Duke University Eye Center and Durham Veterans
Administration Medical Center, Durham, North Carolina, and
The University oflceland, Reykjavik, Iceland.
Supported in part by The Veterans Administration (Career Development Award (ES) and Medical Research Funds) and by Research to Prevent Blindness, Inc., The American Federation for
Aging Research, Inc., and the National Eye Institute (grants
EY-07001 andEY05722).
Submitted for publication: April 7, 1989; accepted July 14, 1989.
Reprint requests: Einar Stefansson, MD, PhD, University oflceland, Landakotsspitali, Reykjavik, Iceland.
Oxygen Tension Measurements
Oxygen tension measurements were performed
with polarographic oxygen electrodes.2 The cats were
ORA
Downloaded From: http://iovs.arvojournals.org/pdfaccess.ashx?url=/data/journals/iovs/933576/ on 05/03/2017
VITRECTOMY AND RETINAL HYPOXIA / Srefonsson er ol
No. 2
BRVO
No Vitrectomy
Vitrecfomy
Fig. 1. The experimental set-up. The polarographic oxygen electrodes are placed transvitreally in both eyes to measure preretinal
oxygen tension. The oxygen tension was measured simultaneously
in the vitrectomized eye and the nonvitrectomized fellow eye, before and after branch retinal vein occlusion.
anesthetized with intramuscular ketamine hydrochloride (20-30 mg/kg) and acepromazine maleate
(2-3 mg/kg) and maintained under general anesthesia with alpha-chloralose (80 mg/kg), repeated as
needed. A catheter was placed in one femoral artery
for continuous measurement of arterial blood pressure and measurement of arterial blood oxygen tension, carbon dioxide tension, pH, and hematocrit.
The blood gases were measured with a blood gas analyzer (model 1302; Instrumentation Laboratories,
MA). The cat was incubated and ventilated mechanically, and the ventilation was adjusted to keep arterial
blood carbon dioxide tension between 25 and 35
mmHg. The cats normally were ventilated with 21 %
oxygen and 79% in nitrogen, and in some cases with
100% oxygen.
The pupils were dilated with 5% phenylephrine and
0.25% tropicamide. The cat was placed in a stereotaxic head-holder. In both eyes the sclera was exposed
temporally by a lateral canthotomy and conjunctival
peritomy. A sclerotomy was performed in the pars
plana, approximately 3.5 mm behind the limbus,
with a myringotomy blade. A Teflon cannula was
placed in the sclerotomy, and the polarographic oxygen electrode (model 760; Diamond Electrotech, Ann
Arbor, MI) was advanced into the vitreous cavity.
The oxygen tension was measured in both simultaneously in both eyes of each cat, 0.1 mm over the
retinal area drained by the superior vein (Fig. 1). A
silver-silver chloride reference electrode was placed
subcutaneously. The entry sites were watertight, and
no fluid was infused into the eye. All eyes maintained
normal shape throughout the experiments. The intraocular pressure was not measured, but all eyes were
soft to the touch.
After the preretinal oxygen tension had been measured for at least 20 min with polarographic electrodes, these electrodes were withdrawn for 3-5 min.
A bipolar radio frequency diathermy probe was intro-
285
duced through the sclerotomy and advanced to the
superior retinal vein under visual control through the
operating microscope and a flat contact lens. Diathermy was applied to the vein for approximately 5
sec. The application was repeated if necessary. The
vein was seen to constrict in the area of the burn and
dilate distal to that area, and the blood column became interrupted and immobile. In selected cases,
fluorescein angiography was performed by injecting
10% fluorescein sodium intravenously (0.1 ml/kg)
and to taking photographs through appropriate filters
of a fundus camera (Fig. 2).
Once the branch retinal vein occlusion had been
created, the polarographic electrodes were reintroduced into the eyes, and preretinal oxygen tension
was measured over the ischemic retina in both eyes.
Electrodes were calibrated before and after each experiment with pure N 2 , 5% O2/95% N 2 , and 21%
O2/79% N2 in a calibration cell at 37°C (calibration
cell model 1251, Diamond Electrotech). The difference in calibration before and after each experiment
was less than 20%. The oxygen electrodes were advanced to the preretinal vitreous in the area drained
by a superior vein in each eye. The electrodes were
placed approximately 0.1 mm above the retina, away
from any visible retinal vessels. The electrodes were
advanced until a subtle concave mirror effect on the
retina could be seen through the operating microscope. The electrodes were then withdrawn approximately 0.1 mm. The preretinal oxygen tension was
measured simultaneously in both eyes of each cat.
The light intensity originating from white fluorescent
ceiling lights during the measurements was 35 footcandles measured at the cornea level (Photometer I;
Quantum Instruments, Garden City, NY).
Fig. 2. Afluoresceinangiogram showing the fundus of a cat after
branch retinal vein occlusion induced with transvitreal diathermy.
The superior retinal vein is occluded and is not filled with fluorescein. The angiogram is in the arteriovenous phase.
Downloaded From: http://iovs.arvojournals.org/pdfaccess.ashx?url=/data/journals/iovs/933576/ on 05/03/2017
286
21%0 2 \i00°%\ 21%O2
Breathing Mixture
lOOr
Electrode
moved to
Non-occluded
Retina
Branch Retinal
Vein Occlusion
80
60
40
I
Vol. 31
INVESTIGATIVE OPHTHALMOLOGY & VISUAL SCIENCE / Februory 1990
20
20
40
60
80
Fig. 3. An experimental
tracing showing time versus
preretinal oxygen tension.
The breathing mixture is indicated above the tracing. A
stable oxygen tension baseline was established, and the
superior branch retinal venule subsequently was occluded. The oxygen tension
fell rapidly. Breathing 100%
oxygen raises the preretinal
oxygen tension to 100
mmHg. At 80 min the electrode was moved to an intact area of the same retina,
where the oxygen tension
was only slightly lower than
at the initial baseline.
Time (min)
The experimental animals were handled in accordance with the ARVO Resolution on the Use of Animals in Research. In addition to the ten cats reported,
three cats had to be excluded from the study due to
vitreous hemorrhages. Two of these cats had undergone vitrectomy.
Results
In nonvitrectomized eyes the preretinal oxygen
tension fell 14 ± 7 mmHg (mean ± 1 SD, n = 10)
when the branch retinal vein was occluded. A significantly smaller change was seen in the vitrectomized
eyes, in which the oxygen tension fell only 3 ± 2
mmHg (n = 5, P = 0.004) when the branch retinal
vein was occluded.
The preretinal oxygen tension in normal nonvitrectomized eyes was 20 ± 7 mmHg (mean ± 1 SD, n
= 10). After branch retinal vein occlusion, the preretinal oxygen tension fell to 6 ± 5 mmHg (n = 10, P
= 0.001; paired student t-test) in the nonvitrectomized eyes (Fig. 3).
The preretinal oxygen tension in vitrectomized
eyes was 19 ± 11 mmHg (n = 5). The oxygen tension
did not fall significantly after branch retinal vein occlusion in the vitrectomized eyes and remained at 16
± 11 mmHg (n = 5) (Fig. 4).
The preretinal oxygen tension in normal nonvitrectomized eyes and vitrectomized eyes is similar
prior to the inducing of branch retinal vein occlusion.
After branch retinal vein occlusion, the preretinal
oxygen tension in nonvitrectomized eyes (6 ± 5
mmHg) is significantly lower than in vitrectomized
eyes (16 ± 11 mmHg, P = 0.04; unpaired student
t-test) (Fig. 5). Pulling the electrode back from the
retinal surface into the vitreous cavity did not change
the measured oxygen tension in vitrectomized eyes.
In contrast, in nonvitrectomized eyes a shallow oxygen gradient existed, sloping from the retina into the
vitreous cavity. Arterial blood oxygen tension was
114 ± 18 mmHg; carbon dioxide tension was 26 ± 7
mmHg;pH was7.45 ±0.12; hematocritwas24 ±6%;
mean arterial blood pressure was 100 ± 17 mmHg;
and the body temperature was 26.6 ± 1.2°C.
Discussion
The data demonstrate that in nonvitrectomized cat
eyes, branch retinal vein occlusion causes hypoxia at
the inner retinal surface in the area of the occluded
vessel. Conversely, in vitrectomized eyes, branch retinal vein occlusion does not cause a significant decrease in preretinal oxygen tension. The preretinal
oxygen tension was significantly higher in the vitrectomized eyes than in the nonvitrectomized eyes after
branch retinal vein occlusion had been induced in
both groups. This indicates that vitrectomy reduced
the hypoxic effect of branch retinal vein occlusion.
After vitrectomy, oxygen enters the fluid in the
vitreous cavity from arterial blood in the ciliary processes and from well perfused areas of the retina. We
hypothesize that the fluid in the vitreous cavity of a
vitrectomized eye circulates freely and thus moves
dissolved oxygen from well perfused to nonperfused
hypoxic areas of the retina (Fig. 6). Since oxygen tension gradients could not be measured in the vitrectomized vitreous cavity, this indicates that mixing is
almost complete. In contrast, the vitreous gel of a
nonvitrectomized eye prevents major fluid circulation, and thus the oxygen moves mainly by diffu-
Downloaded From: http://iovs.arvojournals.org/pdfaccess.ashx?url=/data/journals/iovs/933576/ on 05/03/2017
287
VITRECTOMY AND RETINAL HYPOXIA / Srefonsson er ol
No. 2
2 1 % O,
100%
80
6
HU
•I
£
20
90
135
180
Time (Min)
NORMAL
VITRECT
BEFORE
BRVO
NORMAL
AFTER
VITRECT
BRVO
Fig. 5. Time versus the preretinal oxygen tension (mmHg) in a
vitrectomized eye (interrupted line) and in a nonvitrectomized eye
before and after the induction of a branch retinal vein occlusion in
each eye. The graph represents simultaneous recordings from the
two eyes of the same cat by means of polarographic electrodes. The
oxygen percentage in the inspired air is shown at the top.
40 -
D
>
z
30 -
UJ
z
111
O
8
9
20-
o
o
o
o
0
B
Q
8
B
D
UJ
8
cr
UJ
cc
o.
The oxygen hypothesis for the effect of vitrectomy
on proliferative diabetic retinopathy complements
our earlier theory regarding the mechanism of the
effect of panretinal photocoagulation on proliferative
(CO
10-
D
NORMAL VITRECT
BEFORE BRVO
•
NORMAL
VITRECT
AFTER BRVO
Fig. 4. Histogram (A) and scatterplot (B) showing preretinal oxygen tension (mmHg) in vitrectomized and nonvitrectomized eyes
before and after branch retinal vein occlusion (BRVO). In (A), the
mean and standard error of the mean (segment above bar) are
indicated. The oxygen tension in the nonvitrectomized eyes after
branch retinal vein occlusion was significantly lower than the value
before branch retinal vein occlusion. The branch retinal vein occlusion did not result in significant hypoxia in the vitrectomized
eyes.
sion.3 This process is too slow to supply the hypoxic
areas of retina with enough oxygen.
The finding that vitrectomy improved oxygen supply to ischemic areas of the retina suggests an explanation for the inhibitory effect vitrectomy has on retinal neovascularization.1 If we accept the hypothesis
that the hypoxic retina stimulates retinal neovascularization,4 it follows that improved oxygenation and
relief of the hypoxia would halt the neovascular process.
Fig. 6. Retinal ischemia, showing fluid fluxes in the vitrectomized eye and the diffusion of oxygen from the ciliary body and the
well perfused areas of the retina and into the vitreous cavity fluid.
Oxygen-rich fluid flows by the ischemic areas of the retina, and
oxygen diffuses from the fluid into the ischemic retina to relieve the
tissue hypoxia.
Downloaded From: http://iovs.arvojournals.org/pdfaccess.ashx?url=/data/journals/iovs/933576/ on 05/03/2017
288
INVESTIGATIVE OPHTHALMOLOGY b VISUAL SCIENCE / Februory 1990
Occluded blood
vessels
Vitreous
Retina
Choroid
Fig. 7. Oxygen delivery to the ischemic hypoxic retina after
panretinal photocoagulation or vitrectomy. After vitrectomy, oxygen diffuses from the fluid in the vitreous cavity and into the retina.
Oxygen diffuses through the photocoagulation scar in the outer
retina and into the ischemic inter retina. Both treatment modalities
improve the oxygen supply to the inner retina. PE, retinal pigment
epithelium.
diabetic retinopathy; this theory stated that after
panretinal photocoagulation, the ischemic inner retina receives oxygen from the choroid (Fig. 7).2'5 Both
treatment modalities are known to halt the progress
of proliferative diabetic retinopathy.1'6 We propose
that improved oxygenation of ischemic and hypoxic
retina is the common mechanism of action for both
vitrectomy and panretinal photocoagulation.
While retinal neovascularization is uncommon in
the posterior pole after vitrectomy,1 neovascularization may develop from the peripheral retina, which
remains covered with remnants of anterior hyaloid
vitreous gel not removed during the vitrectomy. The
observation of anterior hyaloidal fibrovascular proliferation7'8 offers additional support for our theory.
After vitrectomy, retinal neovascularization arrests in
the posterior pole. However, the peripheral retina is
still covered with vitreous gel and receives too little
oxygen from the vitreous cavity to relieve its ischemic
hypoxia.
It has been observed that the eye with total posterior vitreous detachment rarely develops proliferative
diabetic retinopathy.9"" Histopathologic studies of
the vitreoretinal relationship in proliferative diabetic
retinopathy by Tagawa and associates910 and by Foos
and associates" showed that they had either no or
partial posterior vitreous detachments. This indicates
Vol. 31
that new vessels grew onto the vitreous gel while it
was attached to the retina. Our theory of oxygen delivery from the vitreous cavity offers an explanation
for this finding. After a posterior vitreous detachment, a fluid-filled compartment is present in front of
the retina. As this fluid circulates, it can carry oxygen
in dissolved state from well perfused to poorly perfused retinal areas and thus relieve hypoxia and suppress the neovascular stimulation.
Our oxygenation theory is not the only possible
explanation for the inhibition of preretinal neovascularization after vitrectomy. It has been suggested that
the neovascularization stops because the vessels have
no "scaffold" to grow on. This theory has survived
because it has never before been challenged by an
alternative theory. It seems to be based only on the
clinical fact that absence of the vitreous gel inhibits
neovascularization. However, ocular neovascularization can grow on surfaces other than the vitreous
humor. Iris neovascularization grows on the surface
of the iris, exposed to fluids similar to those that fill
the vitreous cavity after vitrectomy. In vitrectomized
diabetic eyes with silicone oil in the vitreous cavity,
we have observed preretinal neovascularization on
the surface of the retina. Similarly, in proliferative
vitreoretinopathy, fibrous proliferation uses the retinal surface as scaffold, in the absence of vitreous.
Ernest and Archer12 measured preretinal oxygen
tension in monkeys 30 min and 3-6 months after
branch retinal vein occlusion. They did not find the
retina to be hypoxic. However, reperfusion and tissue
atrophy may have raised the preretinal oxygen tension in the months after the vein occlusion. Pournaras et al13 produced branch retinal vein occlusion
in miniature pigs. The pigs developed retinal hypoxia, a result that agrees with our findings in the cat.
The preretinal oxygen tension levels in the cats in
the current study agree well with previously reported
values.3'5 The thesis of supplying nutrients and oxygen to the retina from the vitreous cavity may also
have a role in acute retinal ischemia, perhaps by oxygenation of the vitreous14 or by vitreoperfusion.15
Over 30 years ago, Wise suggested that retinal hypoxia may stimulate neovascularization in diabetic
and other proliferative retinopathies.4 Since that
time, panretinal photocoagulation and vitrectomy
have been found empirically to suppress retinal diabetic neovascularization. We propose that both treatment modalities affect retinal neovascularization by
relieving retinal hypoxia (Fig. 7). This conclusion is a
direct extension of Wise's Theory.
Key words: diabetic retinopathy, vitrectomy, retinal vein
occlusion, oxygen, neovascularization
Downloaded From: http://iovs.arvojournals.org/pdfaccess.ashx?url=/data/journals/iovs/933576/ on 05/03/2017
No. 2
VITRECTOMY AND RETINAL HYPOXIA / Srefonsson er ol
References
1. Blankenship GW and Machemer R: Long-term diabetic vitrectomy results: Report of 10 years' follow-up. Ophthalmology
92:503, 1985.
2. Stefansson E, Landers MB, III, and Wolbarsht ML: Increased
retinal oxygen supply following pan retinal photocoagulation
and vitrectomy and lensectomy. Trans Am Ophthalmol Soc L
XXXIX: 307-334, 1981.
3. Aim A and Bill A: The oxygen supply to the retina: I. Effects of
changes in intraocular and arterial blood pressures and in arterial PO2 and PCO2 on the oxygen tension in the vitreous body
of the cat. Acta Physiol Scand 84:261, 1972.
4. Wise GN: Retinal neovascularization. Trans Am Ophthalmol
Soc 54:729, 1956.
5. Stefansson E, Hatchell DL, Fisher BL, Sutherland FS, and
Machemer R: Panretinal photocoagulation and retinal oxygenation in normal and diabetic cats. Am J Ophthalmol
101:657, 1986.
6. Diabetic Retinopathy Study Research Group: Preliminary report on effects of photocoagulation therapy. Am J Ophthalmol
81:383, 1976.
7. Lewis H, Aaberg TM, Abrams GW, Han DP, and Williams
GA: Current causes of failure following diabetic vitrectomy.
ARVO Abstracts. Invest Ophthalmol Vis Sci 29(Suppl):220,
1988.
289
8. Lewis H, Abrams GW, and Foos RY: Clinicopathologic findings in anterior hyaloidal fibrovascular proliferation after diabetic vitrectomy. Am J Ophthalmol 104:614, 1987.
9. Tagawa H, McMeel JW, Furukawa H, Quiroz H, Murakami
K, Takahashi M, and Trempe CL: Role of the vitreous in
diabetic retinopathy. Ophthalmology 93:596, 1986.
10. Tagawa H, McMeel JW, and Trempe CL: Role of vitreous in
diabetic retinopathy: II. Active and inactive vitreous changes.
Ophthalmology 93:1188, 1986.
11. Foos RY, Kreiger AE, Forsythe AB, and Zakka KA: Posterior
vitreous detachment in diabetic subjects. Ophthalmology
87:122, 1980. >
12. Ernest JT and Archer DB: Vitreous body oxygen tension following experimental branch retinal vein obstruction. Invest
Ophthalmol Vis Sci 18:1025, 1979.
13. Pournaras CJ, Ilic J, Tsacopoulos M, Leuenberger PM, and
Gilodi N: Experimental branch vein occlusion: Modifications
of preretinal PO2 in the affected territory. ARVO Abstracts.
Invest Ophthalmol Vis Sci: 26(Suppl):246, 1985.
14. Ben-Nun J, Alder VA, Cringle SJ, and Constable IJ: A new
method for oxygen supply to acute ischemic retina. Invest
Ophthalmol Vis Sci 29:298, 1988.
15. Shaw WE, Blair NP, Dunn R, and Flora C: Reversal of retinal
ischemic injury by vitreoperfusion. Invest Ophthalmol Vis Sci
30(Suppl):137, 1989.
Downloaded From: http://iovs.arvojournals.org/pdfaccess.ashx?url=/data/journals/iovs/933576/ on 05/03/2017