Clinical Commentaryeve 253 333..338 Computed Tomography in the Investigation of Trauma to the Ventral Cranium C

EQUINE VETERINARY EDUCATION / AE / july 2011 333

Clinical Commentaryeve_253 333..338 Computed tomography in the investigation of trauma to the ventral cranium C. S. Avella* and J. D. Perkins Department of Veterinary Clinical Sciences, The Royal Veterinary College, UK.

Keywords: horse; head; fracture; computed tomography; radiography; endoscopy; guttural pouch

The Case Report discussed in this commentary (Beccati The suture is the last to close radiographically in the et al. 2011) described a colt with a comminuted basilar skull horse at approximately 5 years of age (Butler et al. fracture diagnosed using computed tomography (CT), the 2009). fracture having not been evident on survey radiography. It The longus capitis muscle (rectus capitis ventralis major) is well established that radiography of the equine head originates in the transverse processes of C3–C5 and inserts enables limited interpretation due to the superimposition of on the tubercles at the junction of the basilar part of the the complex anatomy. Computed tomography is currently occipital bone with the body of the basisphenoid bone the gold standard imaging modality for bone pathology (Getty 1975), it flexes the head and bends it laterally. The having multiple advantages over radiography. Computed rectus capitis ventralis (rectus capitis ventralis minor) is a tomography has superior contrast resolution and enables smaller muscle located dorsal to the longus capitis (LC). It evaluation of the cross-sectional anatomy without originates from the ventral arch of the atlas and inserts on superimposition (Morrow et al. 2000) and its clinical value in the basioccipital bone (close to the insertion of LC) (Fig 3) the horse has been extensively described (Ramirez et al. and it functions to flex the atlanto-occipital joint (Getty 1998; Puchalski 2007; Kinns and Pease 2009). Computed 1975). The rectus capitis ventralis (RCV) and longus capitis tomography image acquisition is faster, less expensive than (LC) muscles can be visualised within the axial margin of MRI and is very sensitive in the detection of acute the medial guttural pouch (GP) (Fig 4). The digastricus haemorrhage, in addition to providing excellent bone muscle is evident beneath the mucosa within the lateral detail (Chalela et al. 2007). Computed tomography has compartment of the GP and originates at the jugular been used to detect intra-cranial haemorrhage in horses process of the occipital bone and inserts onto the medial but is limited in detecting lesions of the parenchyma of the surface of the ventral border of the mandible (Getty 1975). brain (Lacombe et al. 2010). Despite being inferior to MRI, soft tissue evaluation using CT is superior to that of The value of CT in the diagnosis of trauma radiography. The complex 3D anatomy of the equine skull is to the ventral cranium and adjacent a prime example whereby superimposition of structures soft tissues visualised radiographically results in the potential for pathology to be unrecognised, as demonstrated in this On endoscopic examination, horses with a history of head clinical case where the comminuted basilar fracture was trauma may have pharyngeal swelling and blood not evident radiographically. The ability to perform post emanating from the external ostia of the GPs (Fig 5). processing CT reconstruction facilitates the precise Thorough examination of the GPs may be precluded due localisation and evaluation of pathology. to compression of the GPs due to inflammation and trauma to adjacent tissues or due to the presence of haemorrhage within the GPs. If there is trauma to the LC The normal anatomy of the and RCV muscles then swelling and/or haematoma basisphenoid-basioccipital bone and formation may be evident within the axial wall of the surrounding soft tissues medial GP, whereas trauma of the digastricus muscle results in swelling and compression of the lateral As described by Beccati et al. (2011), the open suture compartment. If the mucosal lining of the GP is torn between the basisphenoid bone and basioccipital following RCV, LC or digastricus trauma, haemorrhage into bone in young horses can mimic a fracture (Figs 1 and 2). the GP may occur. Within 24 h after cessation of bleeding, free blood will have drained from the GPs; however, the *Corresponding author email: [email protected] presence of haematomas and persistent soft tissue swelling

BS BO BS

b) Fig 2: A CT sagittal reconstruction of the cranium of a normal Thoroughbred yearling filly demonstrating an open basisphenoid- basioccipital suture which can mimic the appearance of a fracture line (multiplanar reconstruction using Visbion software). BS: basisphenoid bone, BO: basioccipital bone.

predisposes to fracture of the basioccipital and basisphenoid bone as a consequence of hyperextension of the head whereas in older horses when the growth plate has closed, avulsion of the RCV and LC more commonly results in avulsion without fracture as there is greater stability of the ventral skull. Computed tomography BO enables evaluation of the nature of the soft tissue trauma that may not be evident endoscopically; for example, CT BS can aid differentiation between muscle rupture and haematoma formation. Computed tomography can enable evaluation of the LC and RCV muscles and basisphenoid-basioccipital bone to help distinguish between the potential differentials for GP bleeding. Fig 1: Lateral radiographs of a) A normal 3-month-old Warmblood Figure 7 demonstrates a case of RCV muscle tearing and filly demonstrating a radiographically open basisphenoid- partial avulsion diagnosed using CT; endoscopy could not basioccipital suture and b) a normal 4-year-old Thoroughbred be used to evaluate the RCV and LC muscles due to gelding demonstrating a radiographically partially open compression of the GP. In this case CT ruled out an avulsion basisphenoid-basioccipital suture which can mimic a fracture in horses less than 5–6 years of age. The superimposition of rostral fracture of the insertion of the RCV and LC muscles which borders of the stylohyoid bone adjacent to the suture may also was a likely differential in this horse. The value of CT in this impede interpretation. BS: basisphenoid bone, BO: basioccipital case was the exclusion of the presence of a fracture; bone. however, the use of MRI would be superior in evaluation of the muscle trauma. The use of intravenous contrast during may still limit endoscopic examination of the GP CT can also be used to improve the definition between soft precluding evaluation of the LC and RCV muscles tissue structures including muscles; however, in cases of (Fig 6). acute head trauma, this is rarely indicated. Overextension of the head can result in rupture of LC In the case presented by Beccati et al. (2011), the and RCV resulting in muscle or tendon disruption alone or authors were uncertain of the presence of a fracture in conjunction with an avulsion fracture of the based on their survey radiographs. There are various basisphenoid-basioccipital bone. If a ventral skull fracture cranial fractures which may not be detected on is present, then the horse is likely to show concurrent radiography but are evident on CT. In man, skull neurological signs, whereas in cases of head flexor muscle radiography is rarely used in the evaluation of head rupture alone, neurological deficits are rare. In younger trauma, with CT being used as the imaging modality of animals, the persistence of the open growth plate choice due to the high sensitivity in both fracture detection

a) c) A

RCV LC RCV

GP GP LC

E b)

RCV SC GP

RCV

LC GP

Fig 3: CT 3D reconstructions (a and b) and 2D transverse images (c and d) of the normal CT appearance at the level of the guttural pouches (GP). Image c) is at the level of the red line in image (b) and image d) is at the level of the yellow line in image (b). RCV: rectus capitis ventralis, LC: longus capitis, ON: optic nerve, T: trachea, E: epiglottis, C: cricoid cartilage, SC: Spinal cord (3D reconstructions created using Osirix software).

D LC & RCV

M AAA L SH

b) Fig 4: The normal endoscopic appearance of the rectus capitis ventralis and longus capitis muscles on the axial border of the medial compartment of the left guttural pouch (arrow).

Fig 6: A 10-year-old Thoroughbred x Irish Draught mare following severe head trauma. a) Endoscopy of the right guttural pouch (GP) Fig 5: Endoscopic image of the pharynx with blood emanating from revealed compression of the lateral compartment (L) secondary to the ostium of the right guttural pouch (arrow). Right sided trauma of the digastricus muscle. The medial compartment (M) is pharyngeal compression is also evident. also compressed. b) CT transverse image (soft tissue window) at the level of the GPs showing a large haematoma present in the right GP virtually obliterating the entire lumen of the GP. Marked soft tissue swelling is evident axial to the right ramus of the and intra-cranial haemorrhage. Radiography has been mandible resulting in displacement of the right stylohyoid bone described as obsolete in the investigation of head trauma and larynx to the left. in man. Examples of fractures in 2 horses that presented with a history of head trauma, that were also not evident radiographically but were apparent on CT, are shown in Figure 8; a coronoid process fracture of the mandible The use of standing CT vs. CT under and a fracture of the hamulus of the pterygoid (the general anaesthesia hamulus forms a pulley around which the tensor veli palatine is reflected [Hillman 1975]). In man coronoid In the case described by Beccati et al. (2011), the colt process fractures have been reported as the rarest was anaesthetised because it was recumbent and had occurrence of all facial fractures (Delantoni and been seizuring thus CT was performed under general Antoniades 2010) and have not previously been anaesthesia. The development of standing CT now documented in the horse. negates the need for general anaesthesia for CT of the

a) a)

M L

b) b)

Fig 7: A 14-year-old Thoroughbred gelding presented with a history of being trapped in a stable in lateral recumbency Fig 8: a) CT transverse image (bone window) at the level of the during an episode of colic. a) An endoscopic image of the left ethmoids. There is a simple transverse fracture of the base of the guttural pouch revealed marked compression when visualised hamulus of the pterygoid process on the left side. The ventral endoscopically. b) CT transverse image (soft tissue window) portion of the hamulus is displaced slightly axially. b) CT at the level of the basisphenoid-basioccipital bone showed transverse image (bone window) at the level of the caudal partial avulsion and disruption of the left rectus capitis ventralis border of the mandible. A fracture of coronoid process of the muscle. right mandible was evident which was not visible on survey radiographs. equine head in conscious patients. Standing CT avoids Conclusions complications that have been encountered during CT under general anaesthesia. In our experience, the As described by Beccati et al. (2011), CT can be critical in presence of central neurological signs in cases of head the precise diagnosis of fractures in cases of head trauma trauma subsequently anaesthetised are associated with a and, in addition, provides additional information regarding poor recovery and survival post general anaesthesia. soft tissue trauma which may not be apparent on Intra-cranial bleeding in a horse has also has been endoscopy and plain radiography. Beccati et al. (2011) described during general anaesthesia in a case of head describe the cost of CT as a major disadvantage; however, trauma (Leblanc and Bunson 1986) and may present an CT scanners have and will become more affordable and additional risk when anaesthetising horses with head the ability to perform standing CT significantly reduces the trauma. cost of the procedure (Fig 9).

J.A. Butler, C.M. Colles, S.J. Dyson, S.E. Kold and P.W. Paulos, Wiley-Blackwell, Oxford. pp 413-504. Chalela, J.A., Kidwell, C.S., Nentwich, L.M., Luby, M., Butman, J.A., Demchuk, A.M., Hill, M.D., Patronas, N., Latour, L. and Warach, S. (2007) Magnetic resonance imaging and computed tomography in emergency assessment of patients with suspected acute stroke: a prospective comparison. Lancet 369, 293-298. Delantoni, A. and Antoniades, I. (2010) The iatrogenic fracture of the coronoid process of the mandible. A review of the literature and case presentation. Cranio 28, 200-204. Getty, R. (1975) Equine myology. In: Sisson and Grossman’s The Anatomy of the Domestic Animal, 5th edn., Eds: C.E. Rosebaum, N.G. Ghoshal and D. Hillman, WB Saunders, Philadelphia. pp 255-348(a). Hillman, D.J. (1975) Equine osteology. In: Sisson and Grossman’s The Anatomy of the Domestic Animal, 5th edn., Ed: R. Getty, WB Saunders, Philadelphia. pp 255-348(a). Fig 9: Standing computed tomography enables thorough Kinns, J. and Pease, A. (2009) Computed tomography in the evaluation evaluation of the equine head and cranial cervical region in cases of the equine head. Equine vet. Educ. 21, 291-294. of pericranial trauma, negating risks associated with general anaesthesia. Lacombe, V.A., Sogaro-Robinson, C. and Reed, S.M. (2010) Diagnostic utility of computed tomography imaging in equine intracranial conditions. Equine vet. J. 42, 393-399. Leblanc, P.H. and Bunson, D.B. (1986) Anesthetic management of References equine head trauma: a case report. Vet. Surg. 15, 279-282. Morrow, K.L., Park, R.D., Spurgeon, T.L., Stashak, T.S. and Arceneaux, B. Beccati, F., Angeli, G., Secco, I., Contini, A., Gialletti, R. and Pepe, M. (2000) Computed tomographic imaging of the equine head. Vet. (2011) Comminuted basilar skull fracture in a colt: Use of computed Radiol. Ultrasound 41, 491-497. tomography to aid the diagnosis. Equine vet. Educ. 23, 327- Puchalski, S.M. (2007) Computed tomography in equine practice. 332. Equine vet. Educ. 19, 207-209. Butler, J.A., Colles, C.M., Dyson, S.J., Kold, S.E. and Poulos, P.W. (2009) Ramirez, O., Jorgensen, J.S. and Thrall, D.E. (1998) Imaging basilar skull The head. In: Clinical Radiology of the Horse, 3rd edn., Eds: fractures in the horse: a review. Vet. Radiol. Ultrasound 39, 391-395.

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