Anatomic, Radiographic and Physiologic Comparisons of the Internal Carotid and Maxillary Artery in the Horse

The Veterinary Journal 1999, 158, 182–189 Article No. tvjl.1998.0350, available online at http://www.idealibrary.com on

Anatomic, Radiographic and Physiologic Comparisons of the Internal Carotid and Maxillary Artery in the Horse *DAVE G. MACDONALD, † PETER B. FRETZ, ‡ KEITH E. BAPTISTE and §D.L. HAMILTON *Young-Crawford Veterinary Clinic, Innisfail, Alberta; †Department of Veterinary Anaesthesiology, Radiology & Surgery, ‡Department of Veterinary Internal Medicine, §Department of Veterinary Physiological Sciences Western College of Veterinary Medicine, Saskatchewan, Canada

SUMMARY The anatomy of the internal carotid and maxillary arteries was examined using angiography, subtraction angiography and arterial cast preparations in three horses. Subtraction angiography was superior to angiography in demonstrating the anatomy of the occipital, external ophthalmic, ethmoidal and palatine arteries. In three horses manipulation of the internal carotid and occipital arteries during angiography resulted in vasospasm which prevented filling of these vessels with contrast. Direct arterial blood pressure measurements of the maxillary artery impinging on the guttural pouches was measured in four anaesthetized and standing horses. Arterial pressure recordings from the maxillary artery indicate there is retrograde blood flow from contralateral vessels into the occluded arterial segment. Vasospasm prevented measurement of arterial pressure in the internal carotid artery. © 1999 Harcourt Publishers Ltd

KEYWORDS: Equine; guttural pouches; internal carotid artery; maxillary artery.

INTRODUCTION Guttural pouch mycosis can result in acute and fatal haemorrhage from the internal carotid artery (ICA) or the maxillary artery (MA) of the horse (Cook, 1968; Lane, 1989). The site of guttural pouch mycosis has been the internal carotid artery in at least 120 of 125 cases reported in the literature between 1968–87 (Lane, 1989). Conservative non-surgical treatment of guttural pouch mycosis has been unrewarding, and therefore surgical arterial occlusion techniques have been developed to prevent fatal haemorrhage (Owen, 1974; Freeman et al., 1989; Freeman & Donawick, 1989). The two most common ligation procedures are single or double ligation of the affected ICA artery proximal (cardiac) or proximal and distal (cerebral) to the mycotic lesion.

Correspondence to: Dr Peter Fretz, Department of Veterinary Anaesthesiology, Radiology & Surgery, Western College of Veterinary Medicine, University of Saskatchewan, 52 Campus Drive, Saskatoon, Saskatchewan, Canada S7N 5B4. Fax: +306 966 7174 1090-0233/99/060182 + 08 $12.00/0

The other technique involves passing a balloontipped catheter up the ICA to occlude the artery distal to the mycotic lesion (Freeman & Donawick, 1989). Complications attributable to balloon-tip catheter placement include a much longer anaesthetic time, Horner’s syndrome, excessive incisional swelling, displacement of the catheter into the lumen of the pouch and post-operative epistaxis (Lane, 1989). Both techniques carry a high rate of success (Church et al., 1986; Greet, 1987; Caron et al., 1987). The advantage of the single ligation technique is that it is simple to perform, involves less surgical dissection and requires a shorter anaesthetic time. There is a strong chance the patient is likely to be anaemic, in which case a PCV between 10–20 is not uncommon (McIlwraith & Turner, 1987). Thus, less invasive shorter surgery would be an advantage to these compromised patients. Success with single ligation of the ICA to prevent rupture can be attributed to reduction in arterial blood pressure and thrombosis of the affected artery to the level of the lesion (McIlwraith & © 1999 Harcourt Publishers Ltd

COMPARISONS OF INTERNAL CAROTID & MAXILLARY ARTERIES

Turner, 1987). Failure of this procedure can be attributed to occlusion of the wrong vessel or lack of thrombus formation or retrograde flow from the Circle of Willis (Freeman, 1992). Although there are several anatomical descriptions of the equine ICA and MA, none compares the sensitivity of angiography to gross arterial casts. Equine surgeons need basic information on the anatomy of the ICA and MA and changes in arterial blood pressure that accompany proximal and distal ligation of these arteries in order to make rational decisions on the appropriate surgical treatment for guttural pouch mycosis. The purpose of this investigation was, firstly, to compare the radiographic and gross anatomy of the ICA and MA in the horse and, secondly, to measure arterial pressures within the ICA and MA before and after occlusion. MATERIALS AND METHODS A total of seven healthy horses, ranging in age from 2–15 years of age, were used to document the radiographic anatomy of the internal carotid (ICA) and maxillary arteries (MA). All horses received phenylbutazone (4.4 mg/kg i.v. Butazone, Rogar/ STB Inc.) and penicillin G sodium (30,000 IU/kg i.v. Novopharm Ltd) 1 h before anaesthetic induction. All horses were premedicated with xylazine (1.1 mg/kg i.v. Rompun Bayer) and induced with ketamine (2.2 mg/kg; Rogarsetic, Rogar/STB Inc.). A surgical plane of general anaesthesia was maintained with a constant 2.7 mg/kg/min i.v. infusion of a 5% solution of guaifenesin (BDH Inc.), containing 500 mg xylazine/L and 1 g ketamine/L. All horses were intubated and given 100% oxygen through a Hudson demand valve (Hudson RCI).

Part I: Radiographic anatomy Carotid angiography was performed as described by McIlwraith and Turner (1987), combined with selective angiography of the left or right ICA in three horses. The ICA was accessed through a hyovertebrotomy surgical approach. A 15-cm skin incision was made immediately cranial and parallel to the wing of the atlas. The parotid salivary gland was reflected cranially, exposing the common carotid artery. The internal carotid, occipital and external carotid arteries were identified and 15-cm lengths of 6-mm umbilical tape were placed around each artery to facilitate their manipulation. An 8 F Ducor angiographic catheter (MediTech Inc.) was inserted through a midcervical common carotid

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arterotomy and advanced 3 cm proximal to the origin of the ICA. A radiograph of the region was obtained (scout film) and then 40 mL of MD-60 (meglumine diatrizoate, Bristol Myers) was injected through the catheter with a Amplatz injector (Medmac Inc.) at 827 kPa over a period of 1 s. A radiograph was taken immediately after the injection, followed by a second exposure with another cassette. The catheter was then directed 5 cm into the ICA by externally manipulating the artery while advancing the catheter. The same injection and radiographic protocol was followed, except the injection pressure was reduced to 550 kPa. Lateral and ventral-dorsal radiographs were taken until diagnostic quality images were obtained. Each scout film was mounted in a Cronex printer (E.I. DuPont, de Nemours & Co.) with an unexposed subtraction mask film and an exposure made to produce a subtraction mask. The subtraction mask and angiogram were then used to produce a subtraction print. Immediately following angiography, the horses were given heparin (200 IU/kg i.v.; Hepalean, Organon Teknika) and then humanely destroyed by lethal injection of sodium pentobarbital (Euthanyl, MTC Pharmaceuticals).

Part II: Gross anatomy The three horses from Part I were used to document the gross anatomy of the internal carotid and maxillary arteries. Immediately after euthanasia, the heads were disarticulated between the second and third cervical vertebrae and the common carotid arteries were catheterized and infused with physiological saline. The common carotid arteries were subsequently infused with Batson’s No. 17 monomer base solution (Analychem Corp. Ltd) and cured at room temperature for 48 h. The head skin was removed and corrosion casts made by placing the head in anhydrous sodium hydroxide (BDH Inc.) Arterial casts were compared with the corresponding subtraction angiograms.

Part III: Arterial pressure measurements Arterial pressure measurements in the ICA and MA were carried out on four additional horses. The previously described anaesthetic protocol was followed, but end tidal CO2 was monitored and maintained between 40 and 45 mmHg during pressure measurements.

Internal carotid artery Using previously described surgical techniques, the common carotid artery was exposed. The ICA was

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identified and a 15-cm length of 6-mm umbilical tape was placed around it. A John Hopkins ‘bull dog’ vascular clamp (Miltex Instrument Co.) was placed at the origin of the ICA, and a second one 3 cm distally. A modified 5 F Swan–Gans thermodilution catheter (Baxter Healthcare Corp.) was inserted through an arterotomy in the isolated segment of the ICA and advanced 13 cm distally to position the distal balloon at or near the foramen lacerum. The catheter was sutured in place with 5–0 polyglycolic acid using a purse-string pattern placed circumferentially around the catheter, paying particular attention not to occlude the vessel. The placement of the catheter was documented radiographically and the proximal catheter exited through the dorsal aspect of the incision. Before closure, the arterotomy site was inspected for evidence of haemorrhage and the surgery site was lavaged with sterile saline. The subcutaneous tissue was closed with 2–0 polydioxanone in a simple continuous pattern and skin closed with 2–0 NovaFil in a simple interrupted cruciate pattern.

Maxillary artery A 5 F venous thrombectomy catheter (Baxter Healthcare Corp) was introduced into the transverse facial artery through a cutdown procedure 3 cm rostral to the articular tubercule of the temporal bone, and advanced retrograde 12 cm through the superficial temporal artery to place the balloontip within the external carotid artery. The balloon was inflated with a sufficient volume of saline to make firm contact with the arterial wall. The catheter was retracted until resistance was encountered, at which time it was assumed the catheter had impacted against the origin of the superficial temporal artery. The cuff was deflated and the catheter advanced 2–3 cm in a retrograde fashion to position in the distal external carotid artery. A 5-cm incision was made 3 cm behind the corner incisor tooth, along the junction of the smooth mucosal surface with the undulating mucosa of the hard palate. Haemostasis was controlled by pressure and local application of phenylephrine (SABEX Inc.) Blunt dissection was continued until the major palatine artery was exposed against the underlying bone approximately 1–2 cm medial to the edge of the interalveolar space. Vascular clamps were placed on the proximal and distal ends of the exposed artery. A modified 5 F Swan–Gans thermodilution catheter was inserted through an arterotomy in the isolated segment of the greater palatine artery and advanced retrograde for a distance 2 cm

longer than the measured distance from the arterotomy site to the articular tubercule of the temporal bone. The balloon was inflated with saline until it made firm contact with the arterial wall and the catheter was then retracted until further withdrawal was impossible. Based on the work of Freeman et al. (1989), it was assumed that the catheter, at this point, was impacted on the wall of the maxillary artery where it enters the caudal alar foramen. Catheter placement was verified radiographically, and the catheter was then sutured in place as previously described. The mucosal incision was closed with 2–0 polydioxanone in a simple continuous pattern and the proximal catheter exited the oral cavity through a stab incision in the upper lip. Immediately following catheter placement, arterial blood pressure measurements were made using a fluid-filled Statham’s P37 transducer (Astro Med. Inc.) positioned at the level of the petrous temporal bone and recorded on a Grass model 7E polygraph (Astro Med. Inc.). Arterial blood pressure was measured in the ICA and MA individually prior to and after occlusion of the arterial segments on their cardiac side, cerebral side and both sides. Prior to recovery from anaesthesia, the proximal ends of all three catheters were secured inside a 15cm diameter stockinette hood. Following recovery from anaesthesia, each horse was confined to a box stall and monitored every 4 h. Twenty-four hours post-recovery, arterial blood pressure measurement were repeated in the standing horse as described above, and subsequently horses were humanely destroyed by lethal injection of sodium pentobarbital. Catheter location was verified at autopsy. RESULTS The anatomies of the internal carotid and maxillary arteries were consistent in horses investigated in this study. The ICA branched from the common carotid artery just caudal and ventral to the occipital artery. Horse 5 demonstrated an intertwining of the internal carotid and occipital arteries, which made identification of the ICA difficult. The ICA continued its course rostro-dorsally to the foramen lacerum. The ICA enters the cranium through the foramen lacerum and makes a tortuous S-shaped bend, after which it communicates with the contralateral ICA via the caudal intercarotid artery. The right and left internal carotid arteries then course dorsally for several millimetres and contribute to the Circle of Willis.

COMPARISONS OF INTERNAL CAROTID & MAXILLARY ARTERIES

The external carotid artery is the continuation of the common carotid artery (Fig. 1). After the external carotid artery gives off the linguofacial trunk it travels dorsally and submucosally in the lateral compartment of the guttural pouch and gives off the caudal auricular and superficial temporal arteries. After approximately a 2-cm course, the superficial temporal artery gives off the transverse facial artery, which courses around the caudal edge of the mandible to travel subcutaneously over the masseter muscle below the zygomatic process of the temporal bone (Fig. 1). After the origin of the superficial temporal artery the external carotid artery continues along the roof of the lateral compartment of the guttural pouch as the maxillary artery (Fig. 1). The MA passes above the tensor veli palatini muscle and enters the alar canal through the caudal alar foramen. In this area, the MA gives off several main branches. The inferior alveolar artery originates from the MA as it passes along the roof of the guttural pouch. It travels through the mandible and joins its fellow from the opposite side to form a complete arterial loop around the mandible. The external ophthalmic artery originates from the MA distal to the rostral alar foramen and anastomoses with the ipsilateral facial artery through several connections. After the origin of the infraorbital artery, the MA continues as the descending palatine artery, which gives off the major palatine artery at the palatine foramen. The major palatine artery joins the contralateral major palatine artery behind the upper incisor teeth to form a large arterial loop around the maxilla. Subtraction angiography was superior to conventional angiography for demonstrating the anatomy of the occipital, external ophthalmic, ethmoidal and palatine arteries (Fig. 1). The internal carotid, occipital and external carotid arteries were present in all arterial cast preparations (Fig. 2); however, due to vasospasm they were not visible on several angiograms (Fig. 3). The arterial casts provided a better anatomical illustration of arteries less than 2 mm in diameter and also provided a three-dimensional model to study the anatomy (Fig. 2). The results of MA pressure measurements are presented in Table I. We were unable to measure arterial pressures within the ICA of the horses we used. Vasospasm following external manipulation of the internal carotid and occipital arteries occurred in all horses and made it difficult to obtain suitable filling of these vessels with contrast material in subsequent injections. Attempts to

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Fig. 1 Subtraction print of lateral common carotid angiogram. (1) lingual artery; (2) facial artery; (3) linguofacial artery; (4) occipital artery; (5) internal carotid artery; (6) external carotid artery; (7) caudal auricular artery; (8) superficial temporal artery; (9) transverse facial artery; (10) maxillary artery; (11) mandibular alveolar artery; (12) external ophthalmic artery; (13) ethmoidal artery; (14) buccinator artery; (15) malar artery; (16) infraorbital artery; (17) greater palatine artery. Bar represents 5 cm.

reduce this vasospasm by bathing the arteries in lidocaine were unsuccessful. Horse 8 had systolic and diastolic arterial blood pressure recorded when the cerebral cuff was inflated, however these pressures were well below the systemic pressure recorded concurrently by the anaesthetist and were thus excluded. Although MA pressures were recorded in horse 8 under anaesthesia, they were below concurrently measured systemic pressures (Table I). The occipital artery was catheterized erroneously in horse 5. DISCUSSION Subtraction angiography was superior to conventional contrast angiography in demonstrating the anatomy of the occipital, external ophthalmic, ethmoidal and palatine arteries (Fig. 1). The subtraction technique is a photographic method used to eliminate unwanted images from a radiograph, thus highlighting diagnostically important information (Christensen et al., 1978). The principles of

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transverse facial artery 7.0 cm

common carotid a.

occipital a. internal masseteric facial a. carotid a. branch

Fig. 2. Arterial casts of the equine head.

subtraction angiography are based on the following: the scout film shows the structural detail of the skull and adjacent soft tissue; the angiogram contains exactly the same anatomical detail plus the opacified blood vessels. The information in the scout film is subtracted from the angiogram, leaving the opacified vessel pattern visible (Fig. 1). The subtraction angiograms and arterial casts produced in this study verify previous descriptions of the vascular anatomy of the internal and external carotid arteries in the horse and pony (Nanda, 1975; Orr et al., 1983; Colles & Cook, 1983) (Fig. 2). The ICA of horses in this study did not appear to supply any extra cerebral tissue prior to its entrance into the cranium. This observation is supported by Orr et al. (1983), who reported that injection of radioactive microspheres into the internal carotid artery of ponies resulted in a mean 96.6% recovered activity in the brain. The ICA communicates through the caudal intercarotid artery with its contralateral vessel, and therefore it is hypothesized that occlusion of one ICA on its cardiac side does not preclude retrograde blood flow from the contralateral ICA. This is the anatomical arrangement in the majority of horses, but in rare circumstances the internal carotid and the occipital arteries branch off the common carotid together as a single trunk (Orr et al., 1983). Freeman et al. (1993) described one case of an aberrant branch of the ICA in a horse that joined with basilar artery beneath the pons, at the usual site of union between the basilar artery and the carotidobasilar artery. In some horses, this small branch is given off from the second curve of the sigmoid flexure of one or both internal carotid arteries. Because these anatomical

Fig. 3. Lateral common carotid angiogram of a yearling horse demonstrating vasospasm of the internal carotid artery (open arrow) and occipital artery (closed arrow). Bar represents 5 cm.

variations exist, intra-operative angiography, particularly subtraction angiography, would be needed to ensure the correct vessel is occluded as well as any aberrant branches. Ultimately, a thrombosis of the affected artery, in cases of guttural pouch mycosis, to the level of the lesion provides total resolution of the problem. There are three major influences that predispose to the formation of a thrombosis, namely; (1) injury to the endothelium; (2) alterations to normal blood through the affected blood vessel; and (3) alterations to blood coagulability (Cotran et al., 1994). The mycotic plaque and subsequent aneurysm are likely to cause significant injury to the endothelium. Two other important phenomena also occur after single ligation of the ICA. Ligation creates stasis of flow and retrograde flow creates turbulence. Stasis and turbulence together

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Table I Arterial blood pressure Maxillary Artery General anesthesia Occlusion cuff Horse 5 Systolic pressure (mmHg) Diastolic pressure (mmHg) Horse 6 Systolic pressure (mmHg) Diastolic pressure (mmHg) Horse 8 Systolic pressure (mmHg) Diastolic pressure (mmHg)

Standing

Both open

Cerebral closed

Cardiac closed

Both closed

Both open

Cerebral closed

Cardiac closed

Both closed

125 90

125 90

60 55

80 70

175 120

175 120

80 70

95 85

105 65

105 65

52 45

— —

+ +

+ +

+ +

+ +

45* 25*

— —

125 75

125 75

65 50

80 70

85* 75*

90* 80*

—, pressure not measured; +, post-operative damage to catheter, not able to measure pressure; *, pressure significantly below peripheral pressure measurement.

provide four important dimensions to thrombus formation (Cotran et al., 1994), namely: (1) to disrupt laminar flow and bring platelets in contact with endothelium; (2) to prevent dilution of blood and thus concentrating activated clotting factors; (3) to retard inflow of inhibitors of clotting factors and permit build-up of thrombi; and (4) turbulence may cause damage to the endothelium, favouring platelet and fibrin deposition. Freeman et al. (1989) proposed that single ligation of the ICA on the cardiac side of the mycotic lesion does not reduce the risk of fatal haemorrhage because retrograde flow from the Circle of Willis maintains arterial pressure distal to the ligature. Freeman et al. (1994) found the blood pressure within the ICA did not change after ligation, possibly due to collateral circulation and thus may not totally prevent the risk of severe haemorrhage in horses afflicted with guttural pouch mycosis. However, a thrombus in the ICA did not form in those horses, possibly due to flushing of the measuring catheter with heparinized saline (an anticoagulant) at regular intervals. Thus, changes in ICA arterial pressure while a thrombus is forming is unknown. Therefore, double ligation (i.e. proximal and distal to the mycotic lesion) seems necessary to prevent arterial pressure from being maintained distal to the proximal ligature (Cook, 1968; Caron et al., 1987). However, placement of a ligature on the cerebral side of the lesion is especially difficult to

access and often obscured by the mycotic plaque or haematoma (Lane, 1989). Many times a ligature on the cerebral side is not possible, since guttural pouch mycosis typically involves the ICA near its point of entry into the cranium (McIlwraith, 1978). Also, there are major cranial venous lakes and cranial nerves around the site, just between the roof of the guttural pouch and base of the skull (Popesko, 1975). The clinical results of two published studies show that single ligation of the artery alone was satisfactory in 32–40 cases treated (Church et al., 1986; Greet, 1987). Of the failures, only three were the result of post-operative fatal epistaxis, while the others were from other complications (e.g. cardiac arrest, dysphagia). Despite a distinct anatomical explanation for fatal haemorrhage, in this investigation vasospasm was a repeatable phenomenon created with manipulation of the ICA and occipital arteries (Fig. 3). Manipulation of the MA did not result in the same degree of vasospasm observed with manipulation of the ICA. Vasospasm reduced or obliterated the flow of contrast media into the arteries during angiography and resulted in no arterial pressure being measured in the ICA in all horses (Table I) or the occipital artery of horse 5 standing with the cardiac balloon inflated. Partial or complete vasospasm of the ICA and/or occipital artery has been reported as a complication (Orr et al., 1983; Colles & Cook, 1983). The investigators were able to minimize the

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vasospasm by avoiding unnecessary manipulation of the ICA. The proximity of the ICA to the carotid body and carotid sinus may explain this phenomenon. The carotid body and sinus contains baroreceptors and chemoreceptors. The baroreceptors are extremely abundant in the wall of the ICA slightly above the carotid bifurcation (Guyton & Hall, 1996). These baroreceptors are very rapid, short-term, arterial pressure reflex stretch receptors that function as a blood pressure buffer system. By transmitting signals through the Herring’s nerve and the glossopharyngeal nerve, this results in feedback signals from the medullary centre of the brain to reduce arterial pressure (Guyton & Hall, 1996). These effects can last for a maximum of 1–2 days, after which they adapt to whatever new arterial pressure level they are exposed (Guyton & Hall, 1996). The behaviour of the ICA in this study would suggest that stimulation (directly or indirectly) of these baroreceptors influenced the vasospasm. Thus, manipulation and ligation of the ICA at the common carotid trifurcation may provide a sufficient and sustained arteriospasm to allow an adequate thrombus to form in the compromized section of artery, thus protecting the animal from fatal haemorrhage. Placement of the single ligatures varies between authors. Church et al. (1986) advocated siting the ligature close to the trifurcation and occluding both the internal carotid as well as the occipital arteries, whereas Greet (1987) used one or two ligatures placed rostrally to obliterate the artery close to the lesion. Clearly, prudence is required because the advantage of placing a ligature a short distance rostrally may be outweighed by the danger of subjecting a vessel which is already degenerate to undue tension in the manoeuvre (Lane, 1989). The results of this investigation would support the view of Church et al. (1986) in placing a ligature on the ICA close to the trifurcation, because of simplicity, speed and a source of constant stimulation to the carotid bodies and sinus to perpetuate vasospasms. These vasospasms could protect against elevated arterial pressure until a thrombus is formed that will resolve the problem. The balloon-tipped catheter technique is a better method to occlude the ICA at the point of the aneurysm and provides a more reliable protection against retrograde arterial pressure that may result in rupture of the aneurysm. However, many horses with guttural pouch mycosis are significantly

compromised and an anaesthetic risk. In these patients, that are uncomplicated by neurological signs (e.g. dysphagia), single ligation of the ICA may represent a viable alternative to resolve the problem. Based on the report of Freeman et al. (1994), ICA arterial pressure may not change significantly for up to 3 days. Therefore, horses that undergo this procedure may need to stay in hospital for this longer time to prevent any unnecessary stress during the time of thrombus formation. The maxillary artery is not an end artery and has numerous branches; therefore, when a portion of the artery is ligated, retrograde and collateral flow is likely to occur. Several occlusion techniques have been described in an attempt to prevent normograde and retrograde blood flow to the affected segment of MA (Caron et al., 1987, Freeman & Donawick, 1989). The technique we employed to place our catheters was reliable and straightforward. The results of our pressure measurements in horses 5 and 6 show a marked drop in arterial pressure during occlusion of the MA on the cardiac side. The reduction in arterial pressure was expected, because the majority of normograde blood flow to the artery was temporarily stopped. Occlusion of the MA on both the cardiac and cerebral sides resulted in a reduction of systolic, diastolic and mean arterial pressure, but the pressures were greater than with occlusion of the cardiac side alone (Table I). This observation may be explained by the ‘steal phenomenon’, which develops when a major artery is occluded and blood is diverted by backfill from collateral channels into the segment distal to the occlusion (Freeman et al., 1990). In the case of cardiac and cerebral occlusion, the MA could receive backfill from the inferior alveolar, caudal auricular and superficial temporal arteries, while the major efferent route through the palatine artery was occluded. Freeman et al. (1989) realized the limitations of this ligation technique, but concluded that the extent and maturity of thrombosis in the MA after surgery on two normal horses was evidence that collateral flow through the affected segment was not sufficient to prevent thrombosis. The results of this investigation support the theory that ligation on the cardiac and cerebral side of a mycotic lesion involving the MA is necessary to prevent retrograde haemorrhage. The apparent sensitivity of the ICA to manipulation documented in this study suggests a less invasive method of arterial pressure measurement is required.

COMPARISONS OF INTERNAL CAROTID & MAXILLARY ARTERIES

ACKNOWLEDGEMENTS This study was supported by the generosity of the Western College of Veterinary Medicine Equine Health Research Fund. The authors wish to thank Dr C.S. Farrow for his help with interpretation of angiograms, Joanne Parnetta, Carol Matsuda, Elmer Hackett, Richard Back and Carrie Rowat for their technical assistance and Medical Imaging University Hospital, Saskatoon, Saskatchewan. REFERENCES CARON, J.P., FRETZ, P.B., BAILEY, J.V., BARBER, S.M. & HURTIG, M.B. (1987). Balloon-tipped catheter arterial occlusion for prevention of haemorrhage caused by guttural pouch mycosis: 13 cases (1982–1985). Journal of the American Veterinary Medical Association 191, 345–9. CHRISTENSEN, E.E., CURRY, T.S. & DOWDEY, J.E. (1978). The subtraction technique. In An Introduction to the Physics of Diagnostic Radiology, 2nd Edn, pp. 296–301. Philadelphia: Lea & Febiger. CHURCH, S., WYN-JONES, G., PARKS, A.H. & RITCHIE, H.E. (1986). Treatment of guttural pouch mycosis. Equine Veterinary Journal 18, 362–5. COLLES, C.M. & COOK, W.R. (1983). Carotid and cerebral angiography in the horse. Veterinary Record 19, 483–9. COOK, W.R. (1968). The clinical features of guttural pouch mycosis in the horse. Veterinary Record 83, 336–45. COTRAN R.S., KUMAR V. & ROBBINS S.L. (1994). Robbins: Pathologic Basis of Disease, 5th Edn., pp. 105–8. Toronto: W.B. Saunders. FREEMAN, D.E. (1992). Guttural pouch. In Equine Surgery, J.A. Auer, pp. 480–8. Toronto: W.B. Saunders. FREEMAN, D.E. & DONAWICK, W.J. (1989). Occlusion of the internal carotid artery in the horse by means of a balloon-tipped catheter: Clinical use of a method to prevent epistaxis caused by guttural pouch mycosis. Journal of the American Veterinary Medical Association 176, 235–40. FREEMAN, D.E., ROSS, M.W., DONAWICK, W.J. & HAMIR, A.N. (1989). Occlusion of the external carotid and

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maxillary arteries in the horse to prevent haemorrhage from guttural pouch mycosis. Veterinary Surgery 18, 39–47. FREEMAN, D.E., ROSS, M.W. & DONAWICK, W.J. (1990). “Steal Phenomenon” proposed as the cause of blindness after arterial occlusion for treatment of guttural pouch mycosis in horses. Journal of the American Veterinary Medical Association 197, 811–2. FREEMAN, D.E., STALLER, G.S., MAXSON, A.D. & SWEENEY, C.R. (1993). Unusual internal carotid artery branching that prevented arterial occlusion with a balloontipped catheter in a horse. Veterinary Surgery, 22, 531–4. FREEMAN, D.E., DONAWICK, W.J. & KLEIN, L.V. (1994). Effect of ligation on internal carotid artery blood pressure in horses. Veterinary Surgery 23, 250–6. GREET, T.R.C. (1987). Outcome of treatment in 35 cases of guttural pouch mycosis. Equine Veterinary Journal 19, 483–7. GUYTON, A.C. & HALL, J.E. (1996). Textbook of Medical Physiology, 9th Edn, pp. 209–20, Toronto: W.B. Saunders. LANE, J.G. (1989). The management of guttural pouch mycosis. Equine Veterinary Journal 21, 321–4. MCILWRAITH, C.W. (1978). Surgical treatment of acute epistaxis associated with guttural pouch mycosis. Veterinary Medicine (SAC) 73, 67–69. MCILWRAITH, C.W. & TURNER, A.S. (1987). Equine Surgery: Advanced Techniques, pp. 228–38, Philadelphia: Lea & Febiger. NANDA, B.S. (1975). Blood supply to the brain. In: Sisson and Grossman’s The Anatomy of Domestic Animals, 5th Edn, 1, ed. R. Getty, pp. 578–84, Philadelphia: W.B. Saunders. ORR, J.A., WAGERLE, C.L., KIORPES, A.L., SHIRER, H.W. & FRIESEN, B.S. (1983). Distribution of the internal carotid blood flow in the pony. American Journal Physiology 244, H142–9. OWEN, R. (1974). Epistaxis prevented by ligation of the internal carotid artey in the guttural pouch. Equine Veterinary Journal 6, 143–9. POPESKO, P. (1975). Atlas of Topographical Anatomy of the Domestic Animals, Vol. 1, p. 139, Toronto: W.B. Saunders. (Accepted for publication 26 October 1998)