Oclusao de fluxo arteria maxilar

Prévia do material em texto

STRUCTURAL AND FUNCTIONAL CHANGES RELEVANT TO MAXILLARY
ARTERIAL FLOW OBSERVED DURING COMPUTED TOMOGRAPHY AND
NONSELECTIVE DIGITAL SUBTRACTION ANGIOGRAPHY IN CATS WITH
THE MOUTH CLOSED AND OPENED
PETER V. SCRIVANI, MANUEL MARTIN-FLORES, RUTH VAN HATTEN, ABRAHAM J. BEZUIDENHOUT
Some cats develop blindness during procedures with mouth gags, which possibly relates to maxillary arterial
occlusion by opening the mouth. Our first aim was to use computed tomography (CT) to describe how vascular
compression is possible based on morphologic differences between mouth positions. Our second aim was to use
nonselective digital subtraction angiography to assess whether opening the mouth induces collateral circulation.
Six healthy cats were examined. During CT, the maxillary artery coursed between the angular process of the
mandible and the rostrolateral wall of the tympanic bulla. The median distance between these structures
was shorter when the mouth was opened (left, 4.3 mm; right, 3.6 mm) vs. closed (left, 6.9 mm; right, 7.1
mm). Additionally, the distance was shorter on the side ipsilateral to the gag (P = 0.03). During nonselective
angiography, with the mouth closed, there was strong sequential opacification of the external carotid arteries,
maxillary arteries, maxillary retia mirabilia, cerebral arterial circle, and basilar artery. Additionally, there was
uniform opacification of the cerebrum and cerebellum. With the mouth opened, opacification of the maxillary
arteries (rostral to the angular processes) was reduced in all cats, the cerebral arterial circle and basilar artery
had simultaneous opacification in four of six (67%) cats, and the cerebrum had reduced opacification compared
to the cerebellum in four of six (67%). In conclusion, the maxillary arteries are situated such that they can
be compressed when opening the mouth. Opening the mouth did not consistently induce collateral circulation
sufficient to produce comparable cerebral opacification as when the mouth was closed. C© 2013 American
College of Veterinary Radiology.
Key words: angiography, blindness, cat, computed tomography, maxillary artery.
Introduction
T HE DEVELOPMENT OF TEMPORARY OR PERMANENTblindness, alone or with central neurologic deficits, has
been reported in cats recovering from general anesthesia.1–4
Blindness initially was attributed to anesthesia-related hy-
poxia and hypotension, but recently the use of mouth gags
and prolonged maximal opening of the mouth have been
implicated as contributing factors in the development of
this complication.3, 4 In cats, the maxillary arteries are the
major suppliers of blood to the eyes and brain.5–14 There-
fore, occlusion of the maxillary arteries could cause cere-
bral and retinal ischemia. The feline maxillary arteries are
at risk for vascular occlusion during maximal opening of
From the Department of Clinical Sciences (Scrivani, Martin-Flores,
Van Hatten) and the Department of Biomedical Sciences (Bezuidenhout),
College of Veterinary Medicine, Cornell University , Ithaca, NY 14853.
Authors Peter V. Scrivani and Manuel Martin-Flores contributed
equally to this work. The Feline Health Center, Cornell University,
Ithaca, NY 14853 and the Bernice Barbour Foundation, Wellington,
FL 33414 supported this report.
Address correspondence and reprint requests to Peter V. Scrivani, at
the above address. E-mail: pvs2@cornell.edu
Received May 2, 2013; accepted for publication August 25, 2013.
doi: 10.1111/vru.12119
the mouth due to compression by the mandibles or adjacent
soft tissues, or by stretching of the vascular structures.4, 5
An initial report showed that opening the mouth altered
maxillary arterial flow in some healthy cats.15 However, the
functional studies that were used did not also investigate
the morphologic basis for altered blood flow. The study also
noted differences in flow between the left and right sides in
some cats. This finding was unexpected and suggested that
unilateral or partial vision loss was possible, which could be
difficult to recognize clinically. The reason for the asymme-
try is unknown and might be due to anatomic variation or
placement of the mouth gag. Based on these possibilities,
one of the aims of this study was to investigate the morpho-
logic relationships that might mechanically alter maxillary
blood flow due to mouth position.
Earlier reports associated the development of blind-
ness with neurologic signs and suggested that cerebral is-
chemia caused blindness.3, 4 However, not all affected cats
had neurologic deficits concurrent with the development of
blindness.4 Subsequently, it was also shown in healthy cats
that opening the mouth occasionally resulted in immediate
Vet Radiol Ultrasound, Vol. 55, No. 3, 2014, pp 263–271.
263
264 SCRIVANI ET AL. 2014
loss of the electroretinographic waveform.15 Electroretinog-
raphy is extremely sensitive to retinal hypoxia.16 Therefore,
it is possible that retinal ischemia is part of the pathogenesis
in some cats: either alone or in combination with cerebral
ischemia. If the development of blindness in cats is due
to reduced flow through the maxillary arteries, then it is
reasonable to expect that hypoxic injury occurs to both the
brain and eyes because these organs are dependent on max-
illary arterial flow.5–14 However, cats also have accessory or
collateral circulation to the brain, which complicates our
understanding of how blindness actually develops.13 If this
contribution to cerebral blood flow is sufficient, then oc-
clusion of the maxillary arteries could produce blindness
because of only retinal ischemia. In this situation, cats with
concurrent signs of cerebral ischemia must have additional
factors such as systemic hypotension or hypoxia to produce
cerebral ischemia. Therefore, another aim of this study was
to examine the brain of healthy cats for accessory or collat-
eral circulation when the mouth was maximally opened.
To pursue our first aim, we used computed tomography
(CT) angiography to describe the morphologic relation-
ships among the angular process of the mandible, tympanic
bulla, and maxillary artery in different mouth positions:
contrast material was administered to easily identify the
position of the maxillary artery relative to the other struc-
tures. We specifically hypothesized that maximally opening
the mouth with a gag would reduce the distance between
the angular process of the mandible and the tympanic bulla
(the maxillary artery courses between these two structures)
and this distance would be reduced further on the same
side as the mouth gag. To pursue our second aim, we used
nonselective digital subtraction angiography to depict vari-
ous structural and functional changes relative to maxillary
artery blood flow in different mouth positions. We specifi-
cally hypothesized that opening the mouth would produce
an altered sequence of opacification of blood vessels (be-
cause blood arrived to the brain from a different path) and
no variation in the opacification of the brain (as when the
mouth was closed) if there was sufficient accessory and
collateral circulation to the brain.
Materials and Methods
The experimental design was one-group, pretest posttest
with imaging signs (defined below) as the dependent vari-
ables and mouth position (i.e., closed or opened) as the
independent variable. Closed mouth position was defined
as the natural position of the mouth when an endotracheal
tube was present. Maximal mouth opening was determined
using a spring-loaded metal mouth gag and measuring the
distance between the upper and lower gingival margins, at
the level of the canine teeth. This distance was maintained
during image acquisition by a custom piece of plastic to
avoid artifact. In all instances, the mouth gag was placed
between the right-upper and right-lower canine teeth. Our
Institutional Animal Care and Use Committee approved
this protocol.
The sample population consisted of six, presumed heal-
thy, adult cats that were part of a colony used for teaching
at our college.One cat had a mild heart murmur. All cats
underwent general anesthesia for the imaging procedures.
Computed tomographic angiography was performed dur-
ing a separate anesthetic episode about 2 weeks prior
to nonselective angiography. All images were stored and
viewed on a dedicated DICOM workstation (PACS work-
station Carestream Health, Rochester, NY) and reviewed
by a board-certified veterinary radiologist (P.V.S.) who was
aware of the mouth position.
Computed tomography angiography was performed us-
ing a 16-slice scanner (Aquilion LB, Toshiba American
Medical Systems, Tustin, CA). Helical acquisition of the
entire head and cranial half of the neck was performed
with the cat in dorsal recumbency during intravenous ad-
ministration of 2 ml/kg (700 mg I/kg) nonionic iodinated
contrast material (Omnipaque 350TM iohexol injection, GE
Healthcare, Princeton, NJ). Contrast material was injected
into a saphenous vein at a rate of 3 ml/s using an injec-
tor (MEDRAD MCT Multi-level CT Injector, MEDRAD,
Pittsburg, PA) and scanning was initiated (SUREStart) when
the average Hounsfield units (HU) in a region of inter-
est drawn around the right maxillary artery, between the
larynx or trachea and the left mandibular salivary gland,
reached 80 HU. The following acquisition parameters were
used: 0.5 s tube rotation time, 120 kVp, automatic mA
(SUREExpose), 0.5 mm slice thickness, 512 × 512 matrix,
24 cm field of view, 15 helical pitch, and 0.938 pitch factor.
Each cat was examined twice. The first three cats exam-
ined, had the mouth closed during the first injection; the
second three cats had the mouth closed during the sec-
ond injection. There was at least a 15-min delay between
injections. For both mouth positions, multiplanar recon-
structions were made using bone and soft-tissue algorithms,
standardized bone (window width, 4500 HU; window level,
1100 HU) and soft-tissue (window width, 400 HU; win-
dow level, 40 HU) display windows, and 2 mm contiguous
slices in transverse, sagittal, and dorsal planes. Addition-
ally, surface-rendered direct three-dimensional volume re-
constructions were made using a soft-tissue algorithm that
was windowed for bone. The images were evaluated sub-
jectively for any changes in morphology that related to the
maxillary artery and occurred between the different mouth
positions. The minimum distance between the angular pro-
cess of the mandible and the rostrolateral wall of the ipsi-
lateral tympanic bulla, on a line that crossed the path of
the maxillary artery, was measured in both mouth posi-
tions and on both sides (left and right). These measures
were made on transverse scans reconstructed using a bone
algorithm and window.
VOL. 55, NO. 3 MOUTH POSITION AND MAXILLARY ARTERY FLOW IN CATS 265
FIG. 1. Surface-rendered direct three-dimensional volume reconstructions of the head and cranial neck of the same cat with the mouth closed (A and C)
and opened (B and D): the bottom row contains enlarged images from the indicated areas in the upper row. The maxillary artery (m) is the rostral continuation
of the external carotid artery and a portion of this blood vessel is located between the angular process of the mandible (a) and the rostrolateral border of
the tympanic bulla (b). When the mouth is opened, the angular process moves caudodorsally, reducing the space between these two boney structures and
potentially compressing the maxillary artery. In this cat, opening the mouth did not alter the dorsoventral height of the left maxillary artery and one can
observe the anatomic relationships among the maxillary artery, angular process, tympanic bulla in the different mouth positions.
Nonselective digital subtraction angiography was per-
formed using fluoroscopy (Philips Easy Diagnostic, Philips
Medical Systems, Andover, MA) with 70 kV and 125 mA.
The cat was positioned in dorsal recumbency and a pres-
sure injector (MEDRAD Mark V, MEDRAD) was used to
administer 2 ml/kg (700 mg I/kg) contrast material (same
as for CT) into a saphenous vein at a rate of 5 ml/s and
600 psi. The passage of contrast material through the cere-
bral arterial circle and basilar artery was recorded at 8
frames/s until opacification of the jugular veins was ob-
served. Each cat was examined twice. The first three cats
examined, had the mouth closed during the first injection;
the second three cats had the mouth closed during the sec-
ond injection. There was at least a 15-min delay between
injections. The dorsoventral angiograms were evaluated
frame-by-frame and as cineloops that started at the time
of injection. Initially, the opened mouth and closed mouth
positions were evaluated separately for each cat. Next, the
opened mouth and closed mouth positions were compared
side-by-side in each cat. The angiograms were evaluated for
opacification of the external carotid arteries, maxillary ar-
teries, maxillary retia mirabilia, anastomosing branch of the
ascending pharyngeal artery, cerebral arterial circle, basi-
lar artery, ventral–spinal artery, vertebral arteries, jugular
veins, eyes, and brain. Opacification was considered present
when a difference was observed after the administration of
contrast material. The angiograms also were evaluated for
the sequence of opacification of blood vessels. Finally, sub-
jective assessments about the degree of opacification were
made by comparing structures on the same image (e.g.,
left and right maxillary arteries) or by comparing the same
structure during different mouth positions. Structures were
scored as having the same opacity or as having reduced
opacity compared to the other.
A veterinary anatomist (A.J.B.) reviewed the anatomic
information relating to image interpretation and presenta-
tion in the manuscript. Descriptive statistics were selected
and performed by one of the authors (P.V.S.) using commer-
cially available software (Microsoft Excel 2007, Microsoft
Corporation, Redmond, WA and MedCalc for Windows,
version 12.4.0.0, MedCalc Software, Mariakerke, Bel-
gium). Categorical data were reported as the frequency of
266 SCRIVANI ET AL. 2014
FIG. 2. Surface-rendered direct three-dimensional volume reconstructions of the head of a cat with the mouth opened: (A) left projection, (B) left-dorsal
projection. Note that when the mouth is opened, there is the potential for mediolateral compression of the maxillary artery (white arrow) at the level of the
angular process and rostrolateral wall of the tympanic bulla.
FIG. 3. Transverse CT scans (bone window) of the left temporal region in a cat with the mouth closed (A) and opened (B). The line indicates the distance
that was measured between the laterally located angular process of the mandible and the medially located tympanic bulla. The maxillary artery (arrow) courses
between these two structures and is susceptible to compression when the mouth is opened. There is more opacification of all blood vessels during the second
injection (B).
observation, and continuous numerical data were summa-
rized using minimum, median, and maximum values. Dot
plots were used to show the relationship between the dis-
tance between the angular process and tympanic bulla for
both sides and both mouth positions. Hypothesis testing
for numerical data was performed using a Wilcoxon signed-
rank test (two-sided, 5% significance).
Results
During CT angiography, the maxillary artery coursed
between the medial aspect of the angular process of the
mandible and the rostrolateral wall of the tympanic bulla
in all cats and both mouth positions (Figs. 1 and 2). Open-
ing the mouth resulted in reduced distance between these
two boney structures in all cats and on both sides (Figs. 3
and 4). On the left, this distance was shorter (P = 0.03)
with the mouth opened (median, 4.32 mm; range, 3.04–7.08
mm) vs. closed (median, 6.87 mm; range, 5.99–9.39 mm).
On the right, this distance was shorter (P = 0.03) with the
mouth opened (median, 3.60 mm; range, 2.33–5.29 mm) vs.
closed (median, 7.10 mm; range, 5.73–9.60 mm). Addition-
ally, with themouth opened, this distance was significantly
shorter on the same side (i.e., right) as the mouth gag than
on the contralateral side (P = 0.03). Reduced opacification
of the right maxillary rete mirabile was observed in one of
six cats when the mouth was opened (Fig. 5).
During nonselective digital subtraction angiography, a
similar sequence of opacification was observed in all six
cats when the mouth was closed. Soon after injection (arte-
rial phase), there was strong sequential opacification of the
paired external carotid arteries, paired maxillary arteries,
paired maxillary retia mirabilia, and the cerebral arterial
circle. Subsequently (arterial and venous) there was opaci-
fication of the basilar artery, paired vertebral arteries, and
one or both jugular veins. Additionally, the entire brain
and both eyes had the same degree of contrast enhance-
ment (Fig. 6). When the mouth was opened, the onset
VOL. 55, NO. 3 MOUTH POSITION AND MAXILLARY ARTERY FLOW IN CATS 267
FIG. 4. Dot plots show the distance between the angular processes of
the mandibles and the rostrolateral wall of the tympanic bullae (left and
right) in cats when the mouth was closed and opened: each of the six cats
was represented by a different symbol (square, circle, diamond, etc.). Note
that the two boney structures came into closer apposition (distance was
reduced) when the mouth was opened. Because the maxillary artery was
located between these two structures, it was prone to compression when the
mouth was opened. Also note that the distance between the boney structures
was often shorter on the right (i.e., same side as the gag) when the mouth was
opened. When the mouth was closed, minor differences between sides might
be due to placement of an endotracheal tube.
opacification was slightly delayed and there was reduced
opacification of the maxillary arteries rostral to the level
of the angular process, and maxillary retia mirabilia, in all
cats (Fig. 7). In four of six (67%) cats, opacification of the
cerebral arterial circle was observed simultaneously with
opacification of the basilar artery. In two of six (33%) cats,
opacification of the cerebral arterial circle was observed
prior to opacification of the basilar artery. All six cats had
opacification of the entire brain with four of six (67%) hav-
ing reduced opacification of the cerebrum relative to the
cerebellum, especially later during the injection (Fig. 7).
Three of these four cats had simultaneous opacification
of the cerebral arterial circle and basilar artery. The eyes
had similar opacification as to when the mouth was closed,
but one cat had reduced opacification of the left maxillary
artery, left maxillary rete mirabile, and questionably in the
left eye.
Discussion
Our primary reason for performing CT was to describe
the morphologic relationships among the angular process
of the mandible, tympanic bulla, and maxillary artery in
different mouth positions to support one theory of why
maxillary arterial blood flow is altered in some cats when
the mouth is maximally opened.15 The results of this study
showed that opening the mouth reduced the distance be-
tween the medial aspect of the angular process of the
mandible and the rostrolateral border of the ipsilateral
tympanic bulla: the maxillary artery has a sinuous path
through the head and lies between these two structures
such that it is prone to mediolateral compression when
the mouth is opened. Because most cats do not develop
blindness when mouth gags are used, we consider that par-
tial vascular compression might be only one component in
a series of events that need to be present to actually pro-
duce vision loss. For example, vascular compression may be
more profound when there is concurrent systemic hypoten-
sion. As previously reported, another potential cause for
altered maxillary arterial flow when the mouth is opened is
stretching and compression of the small-diameter vessels in
the temporal region, which supply the brain and eyes.4 We
observed elongation of the maxillary retia mirabilia when
the mouth was opened (Fig. 5) but could not evaluate the
small-diameter vessels.
Although the mouth gag affects both sides of the mouth,
placement on a particular side might account for differ-
ences observed between sides in a previous study.6, 15 Use
of a mouth gag also might be an important consideration
if unilateral or partial vision loss is shown to occur. In
this study, the distance between the angular process and
the tympanic bulla was reduced more on the same side as
the mouth gag than on the contralateral side indicating
that the gag could produce more vascular compression of
the ipsilateral maxillary artery. However, during nonselec-
tive digital subtraction angiography, one cat had reduced
opacification on the side opposite of the mouth gag suggest-
ing that there could be multiple causes of asymmetric blood
flow, such as torquing of the jaw by the gag or anatomic
differences.
The degree of vascular opacification consistently and ex-
pectantly was greater following the second injection of con-
trast material because of the close timing between the two
injections. When designing the study, this variability in at-
tenuation was not considered problematic because the rea-
son for administering contrast material was only to identify
the location of the maxillary artery relative to other struc-
tures. Other assessments of vascular attenuation should be
interpreted with caution because differences could be due
to the timing of image acquisition relative to the passage
of the bolus of contrast material. In a previous study, this
variability was accounted for by observing the passage of
the contrast material bolus through the maxillary artery us-
ing time-attenuation curves that were acquired during dy-
namic single-slice CT.15 In hindsight, a better morphologic
examination in this study might have been produced by al-
lowing the contrast material to distribute more uniformly.
Nonselective digital subtraction angiography was per-
formed to investigate for the presence of accessory or
268 SCRIVANI ET AL. 2014
FIG. 5. Transverse CT scans (soft tissue) of the paired maxillary retia mirabilia (arrows) in a cat with the mouth closed (A) and opened (B). With the mouth
opened, note that the maxillary retia mirabilia are slightly elongated or stretched when the mouth is opened, and the right maxillary rete mirabile has reduced
attenuation.
FIG. 6. Sequential (A–D), digital subtraction, dorsoventral angiograms of a cat’s head and cranial neck with the mouth closed. Left (L) is indicated. The
first image shows the location of the anatomic structures for comparison to the other images. As the bolus of contrast material passes through the head, there
is progressive opacification (A–C) of the external carotid arteries (1), maxillary arteries (2), maxillary retia mirabilia (3), and cerebral arterial circle (4) with
reduced or absent opacification of these structures in the last image (D). There also is progressive opacification (A–D) of the brain, basilar artery (5), vertebral
arteries (6), external jugular veins (7), and eyes (8): note that only the rostral half of the basilar artery is strongly opacified in (C) and the entire basilar artery
is opacified in (D) demonstrating caudal direction of flow.
collateral circulation to the brain when the mouth was
opened. This technique allowed examination of the con-
trast material bolus into and out of the head, rather than
producing only a snapshot of blood flow as during CT.
When considering collateral circulation to the brain, it is
important to know how blood flow differs among the do-
mestic animal species. In all domestic mammals, the brain is
supplied by four pairs of arteries that arise from the cerebral
arterial circle (i.e., rostral cerebral artery, middle cerebral
artery, caudal cerebral artery, and rostral cerebellar artery)
and a fifth pair that arises from the basilar artery (i.e., cau-
dal cerebellar artery): additional small arteries supply the
caudalbrainstem.10 However, species differ by how blood
is delivered to the cerebral arterial circle as well as by the
direction of blood flow through the basilar artery.10 Explic-
itly, the cerebral arterial circle may receive blood via the
internal carotid arteries, maxillary arteries, vertebral arter-
ies, or basilar artery but no domestic mammal species has
all four of these potential routes fully developed.10 Species
with vestigial or absent internal carotid arteries (e.g., ox,
pig, goat, sheep, and cat) frequently have intracranial or
extracranial retia mirabilia that convey blood to the cere-
bral arterial circle.11 Blood flow through the basilar artery
may be rostral to caudal or caudal to rostral depending on
the species.10 Based on these variances, three different pat-
terns are observed in (1) dogs and most species (including
humans), (2) sheep and cats, and (3) cattle.10 In kittens, the
proximal two-thirds of the internal carotid artery are oblit-
erated in a few weeks-to-months after birth.10 In adult cats,
the cerebral arterial circle is supplied by the paired anas-
tomosing branches of the maxillary arteries via the maxil-
lary retia mirabilia.9–13 Note that blood flow in the basilar
artery is away from the cerebral arterial circle (rostral to cau-
dal). Therefore, the entire brain of the adult cat is predomi-
nantly supplied by maxillary blood, except the caudal part
of the medulla oblongata, which is supplied by vertebral
blood.
The cat also has potential accessory or collateral blood
flow to the cerebral arterial circle.13 Part of this supply
is through the anastomosing branches of the ascending
pharyngeal arteries, which arise from the occipital arteries.
These arteries arise caudally such that blood flow would
bypass the proposed site of vascular occlusion in the max-
illary arteries (Fig. 8). Therefore, some blood flow to the
VOL. 55, NO. 3 MOUTH POSITION AND MAXILLARY ARTERY FLOW IN CATS 269
FIG. 7. Digital subtraction, dorsoventral, head and neck angiograms of a cat with the mouth closed (top row) and opened (bottom row): (A) arterial phase,
(B) mixed arterial and venous phase. Left (L) is indicated. Normal blood flow was observed with the mouth closed (compare to Fig. 5). With the mouth opened,
the approximate location of the angular process of the mandible is indicated by (*). Rostral to this level, there is reduced opacification of the paired maxillary
arteries and maxillary retia mirabilia. Additionally, there is only minimal opacification of the cerebral arterial circle. During the venous phase, note that there
was moderate contrast enhancement of the cerebellum and basilar artery, but only mild opacification of the rest of the brain.
brain must occur even when there is complete obstruction
of maxillary blood flow rostral to this level. We did not an-
ticipate seeing these vessels due to the nonselective nature
of the injection, their small size, and the superimposition
of the lower jaw that was necessary to evaluate the effect
of mouth position. Collateral circulation is possible via the
vertebral and ventral–spinal arteries with reversed flow in
the basilar artery.13 Reversed flow might occur when there
is occlusion of both maxillary arteries because direction
of blood flow is dictated by pressure gradients, and all of
these blood vessels are interconnected. However, it was un-
certain whether this source of collateral blood supply would
opacify the entire brain or only the caudal portion because
previous anatomic studies revealed contradictory results for
whether basilar arterial blood flow perfused only the cau-
dal brainstem, cerebellum, and caudal cerebrum, or also
the rostral cerebrum.12–14 It also was unknown whether the
results from those investigations would generalize to the
target population because the path of injection in those
studies was opposite to the normal direction of flow. Fur-
thermore, these studies did not evaluate the effect of mouth
position on blood flow. Nevertheless, if collateral circula-
tion was sufficient to prevent cerebral ischemia, then we
anticipated observing comparable enhancement of the en-
tire brain when the mouth was opened as when it was
closed. If collateral circulation was insufficient to prevent
cerebral ischemia, then we anticipated seeing reduced or
absent opacification of the entire brain or only the cerebral
hemispheres.
In our study, during nonselective angiography when the
mouth was closed, we observed strong opacification of the
cerebral arterial circle, basilar artery, and the entire brain
270 SCRIVANI ET AL. 2014
parenchyma. Additionally, opacification of the cerebral ar-
terial circle preceded opacification of the basilar artery indi-
cating that blood flowed from the arterial circle to the basi-
lar artery (rostral to caudal). When the mouth was opened,
we observed reduced opacification of the arterial circle and
basilar artery, simultaneous opacification of the cerebral
arterial circle and the basilar artery, and reduced cerebral
opacification compared to cerebellar opacification in five
of six (83%) cats. We interpreted the latter two findings to
indicate that blood flow either reversed direction through
the basilar artery (caudal to rostral) or flowed preferentially
caudally from the cerebral arterial circle to favorably per-
fuse the brainstem and cerebellum rather than rostrally to
perfuse the cerebrum. Unfortunately, it was difficult to pre-
cisely assess the direction of flow in the basilar artery when
the mouth was opened because the entire vessel increased
in opacity after injection rather one part prior to the other
(contributions by the ascending pharyngeal arteries likely
contributed to this observation). These observations also
might explain why some cats develop bilateral blindness
and cerebral ischemia during procedures that use mouth
gags also survive the anesthetic episode (i.e., partial maxil-
lary, accessory, and collateral flow at least spares the caudal
brainstem and cerebellum from hypoxic injury). This ob-
servation also supports the idea that the development of
bilateral blindness can be due to ischemic injury to the
visual cortex, at least in some cats. We did not observe suf-
ficient collateral circulation to the entire brain to produce
the same degree of opacification as was observed when the
mouth was closed. Therefore, we can neither conclude that
bilateral blindness is due only to retinal ischemia in some
cats, nor that obstruction of only one maxillary artery could
produce cerebral ischemia. A limitation of this study is that
the assessment of the images is necessarily subjective, but
figures are included to represent what was meant by the
subjective descriptors.
The arterial branches that supply the eyes originate from
the maxillary artery rostral (downstream) to the poten-
tial site of vascular compression.5, 7 Specifically, each eye
is supplied by branches that arise from the maxillary rete
mirabile (comparable function to the external ophthalmic
artery in the dog) and by the internal ophthalmic artery
that arises from the cerebral arterial circle (Fig. 8).5, 7 The
internal ophthalmic arteries, however, provide a smaller
contribution of the total blood flow to the eyes and one
report described that they were absent in about 50% of
their cats.6 This anatomical arrangement and the previ-
ously reported electroretinographic data support that di-
rect retinal ischemia is possible and might be part of the
pathogenesis.15 Therefore, it was unexpected that we did not
observe a greater difference in the degree of opacification of
the eyes with different mouth positions. Potential reasons
for this observation might be that the blood that bypasses
FIG. 8. Illustration of the ventral aspect of the feline cerebral arterial
circle. Note the following structures: (1) cerebral arterial circle, (2) internal
ophthalmic artery, (3) maxillary artery, (4) maxillary artery rete mirabile, (5)
tympanic bulla, (6) anastomosing branch of the ascending pharyngeal artery,
(7) vestigial internalcarotid artery, (8) external carotid artery, (9) vertebral
artery, (10) ventral spinal artery, (11) basilar artery, (12) tympanooccipital
fissure, (13) round foramen, (14) orbital fissure, and (15) superficial temporal
artery.
the partial obstruction preferentially flows to the eyes, there
are unknown sources of blood flow to the eye, the imag-
ing technique is insensitive to subtle changes in structures
smaller than the brain, or that differences in flow associated
with mouth position may be exaggerated and detectable
only when there is concurrent systemic hypotension or
hypoxia.
In conclusion, mediolateral compression of the maxil-
lary arteries between the angular process of the mandible
and the rostrolateral wall of the tympanic bulla is a likely
cause for reduced maxillary arterial flow when the mouth
is opened. Additionally, accessory or collateral circulation
to the brain when the mouth is maximally opened might
be sufficient to support the brainstem and cerebellum, but
might not fully support the cerebrum under all circum-
stances. Therefore, our results support that the develop-
ment of cerebral ischemia in some cats might involve vas-
cular compression associated with mouth position and not
VOL. 55, NO. 3 MOUTH POSITION AND MAXILLARY ARTERY FLOW IN CATS 271
only the delivery of anesthesia. The actual development of
blindness is sporadic and likely dependent on multiple vari-
ables that require further investigation such as the degree of
mouth opening, duration of mouth opening, type of mouth
gag, anatomic variation among cats, systemic oxygenation,
systemic blood pressure, degree of retinal ischemia, collat-
eral circulation to the brain, and previous condition of the
brain or eyes.
ACKNOWLEDGMENT
The authors thank Barbara Catlin, RT, for technical assistance and
Lauren D. Sawchyn, DVM, CMI, for producing the image of Fig. 8.
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