Comminuted Basilar Skull Fracture in a Colt: Use of Computed Tomography to Aid the Diagnosis F

EQUINE VETERINARY EDUCATION / AE / july 2011 327

Case Reporteve_158 327..332 Comminuted basilar skull fracture in a colt: Use of computed tomography to aid the diagnosis F. Beccati*, G. Angeli, I. Secco, A. Contini, R. Gialletti and M. Pepe Ospedale Veterinario Universitario Didattico, Università degli Studi di Perugia, Dipartimento di Patologia, Diagnostica e Clinica Veterinaria, Sezione di Chirurgia e Radiodiagnostica, Perugia, Italy.

Keywords: horse; head trauma; computed tomography; diagnosis; skull fracture

Summary horse, a fall and a kick from another horse are the most common causes of head trauma with consequent The case of a one-year-old colt with acute onset of neurological signs (Tietje et al. 1996; Feary 2007; McSloy neurological dysfunction and epistaxis after a traumatic et al. 2007; Mayhew 2009). Brain injury is usually a result of event is presented. After initiating emergency treatment, fracture of the skull bones, such as the stylohyoid and the the colt was anaesthetised for diagnostic imaging. basisphenoid-basioccipital bone (Tietje et al. 1996; Nout Radiographic examination of the head was suggestive of and Reed 2005), and these represent the most severe brain soft tissue opacity in the area of the guttural pouches, but injuries (Feary 2007). The degree of brain injury and was inconclusive about osseous involvement. A computed neurological dysfunction is variable and depends on the tomography (CT) scan, used to obtain further details, inertial load of the impact, the direction of the trauma and showed a comminuted basilar skull fracture with 2 the shape of the contusive object. displaced fragments that were not detected by Basisphenoid-basioccipital bone fractures are radiography. Because of the poor prognosis for survival frequently avulsion fractures and affect the weakest and return to athletic function, the horse was subjected areas, such as the insertion area of primary flexor muscles to euthanasia. CT imaging provided the most useful of the head (rectus capitis ventralis major and minor, and diagnostic information about type, localisation, extension longus capitis ventralis); the weakness of this area is due and severity of the basilar skull fracture. to the lack of lateral support of the foramen lacerum in horses (Ramirez et al. 1998; Feary 2007; McSloy et al. 2007). Introduction Skull fractures have a poor prognosis, and basilar skull fractures are associated with high morbidity and mortality Traumas of the central nervous system (CNS) are the most rates (Sweeney et al. 1993; Tyler et al. 1993; Ramirez et al. common causes of neurological diseases in horses; the 1998; McSloy et al. 2007). majority of cases show spinal trauma but the brain is only The diagnosis of skull fractures by radiographic et al et al rarely involved (Tyler . 1993; Feige . 2000). examination is difficult, especially if there is no displaced Contusion trauma of the skull can cause bone lesions and bone fragment, and radiographs are easily misinterpreted primary injury (mechanical) of the brain tissue; mechanical because the suture line between the basioccipital and injuries occur at the time of the impact and develop as basisphenoid bones remains open in young horses (<5 a consequence of the disruptive forces associated with years) (Ackeman et al. 1974; Feige et al. 2000; Johnson local and diffuse neuronal depolarisation. A primary and Kellam 2001; McSloy et al. 2007). neurological injury can be complete and nonreversible at The good resolution and sensitivity of computed the time of impact (Feary 2007). Only about half of the tomography (CT) makes it the optimal imaging technique horses that suffer head trauma present with CNS for straightforward diagnosis of skull fractures in horses and involvement, the others present with fractures and do not it should be used whenever possible in cases of suspicious et al show neurological clinical signs (Tietje . 1996; Feary fractures of the head. 2007). Collision with stationary objects or with another This case report describes an unusual comminuted fracture of the basisphenoid-basioccipital bone in which diagnosis was obtained with CT because the radiographic *Corresponding author email: [email protected] evidence was inconclusive.

Case details

History

A one-year-old 200 kg Anglo-Arabian colt was presented to the Veterinary Teaching Hospital for head trauma. During basic training, the colt reared up, fell backwards and hit its head on the ground. The owner, who was a farm animal veterinary surgeon, reported that the colt had seizure-like activity and showed severe epistaxis and mouth bleeding. After the accident, the colt was unable to adopt the normal standing position and the owner immediately referred the colt to the hospital. The colt was sedated with xylazine hydrochloride (Megaxilor 20%)1 (0.6 mg/kg bwt i.v.) during transport to the hospital. * Clinical findings and clinical pathology

At the time of admission, the colt was recumbent and Fig 1: Lateral view of the cranium revealing some irregular radiolucent lines over the basioccipital-basisphenoid bone showed moderate bilateral epistaxis and abnormal (arrow), and an increased soft tissue opacity in the area of the mentation; it was unresponsive to stimuli and had guttural pouches (asterisk). numerous abrasions and swellings located on the zygomatic arch, the zygomatic process of the frontal bone and around the orbit regions. The animal was sedated with xylazine hydrochloride (Megaxilor 20%)1 (0.5 mg/kg bwt i.v. 3 times in 2 h) for further evaluation of its clinical status. The colt’s mental status made a neurological examination difficult and evaluation of the cranial nerve and spinal cord reflexes was incomplete. The colt had absent pupillary light response (direct and consensual), reduced eyelid and corneal (blink) reflex and anisocoria, and the clinical laboratory findings showed only mild dehydration. The colt was fitted with protective facial shield apparatus2 and confined to a recovery box where it was maintained in pharmacological coma by administration of phenobarbital (Luminale)3 (bolus 20 mg/kg bwt i.v. over 30 min, then 9 mg/kg bwt i.v. q. 8 h for 1 day) for convulsion control and to perform primary treatment and diagnostic evaluation. Initial medical therapy consisted of Lactated Fig 2: Ventrodorsal view of the cranium. No abnormalities were 4 Ringer’s solution and 0.9% (w/v) sodium chloride detected. administered i.v. at the rate of 50 ml/kg bwt/h, and oxygen therapy (100%) at the rate of 15 l/min through insufflation. (Fig 2), the basioccipital-basisphenoid suture line was Dexamethasone 0.2 mg/kg bwt i.v. (Desashock)5 and identified and no abnormality was detected. Given the 10% (v/v) dimethyl sulphoxide 1 g/kg bwt i.v., were inconclusive radiographic findings, a CT scan was used in administered to reduce oedema and to control an attempt to obtain further details. The animal was laid inflammation and free radical scavenging, and the use of on a custom-made CT table and placed in dorsal mannitol was avoided. recumbency with the head positioned fully extended in After primary treatment and stabilisation, the colt the gantry, so that the longitudinal axis of the head and was placed under general anaesthesia to undergo the occlusal surface of the cheek teeth were parallel with radiographic examination. Radiographs of the lateral view the table and perpendicular to the gantry. of the skull were taken with the animal in the right lateral Computed tomography images were obtained with a position and ventrodorsal views of the skull were obtained fourth-generation helical CT scanner (Siemens Somatom in the dorsal recumbency position. In the lateral view Volume Zoom)6. The imaging parameters were 120 kV and (Fig 1), some small radiolucent lines over the basisphenoid 380 mA, with a pitch of 2.5 mm and slice thickness of bone and increased soft-tissue opacity in the area of the 2.5 mm. The scan length was 30 cm to ensure the inclusion guttural pouches was detected. In the ventrodorsal view of the nuchal crest and the last maxillary cheek teeth

Fig 3: CT image (bone window) at the level of the temporomandibular joints revealing multiple longitudinal Fig 5: CT transverse image (bone window) at the level of the hypoattenuating lines within the basisphenoid bone (arrows). temporomandibular joints. One bone fragment is detached from the dorsal aspect of the basisphenoid bone (arrow). A hyperattenuating mass of soft tissue density is detached between the guttural pouches compressing the medial wall of the left guttural pouch (asterisk).

Fig 4: CT transverse image (bone window) at the level of the inner ear revealing one fragment detached from the lateral aspect of the basisphenoid bone (arrow). A dense mass is detached Fig 6: CT transverse image (soft tissue window) at the level of the between the guttural pouches compressing the medial wall of the temporomandibular joint revealing haemorrhage compressing the left guttural pouch (asterisk). medial wall of the left guttural pouch (asterisk).

(Triadan 11s). The scan direction was caudorostral and the recognised on survey radiographs and were located close images were evaluated using Osirix v3.5.1 DICOM view to the basioccipital-basisphenoid bones. Using a soft tissue software. Using a bone window, the CT scan showed window, a large soft tissue density between the guttural several irregular hypoattenuating linear gaps arcuate in pouches was detected extending from the tympanic bulla lateral and dorsoventral directions within the basisphenoid to the most caudal portion of the sphenopalatine sinus. This bone (Fig 3). At the level of the stylohyoid bones, CT mass was located in the area corresponding to the rectus evaluation revealed a large osseous fragment (17 ¥ 24 ¥ capitis ventralis and lungus capitis ventralis muscles and 3 mm) along the basisphenoid bone slightly displaced was consistent with haemorrhage between guttural laterally (Fig 4). In addition, at the level of the pouches. The roof of the medial compartment of the left temporomandibular joints, a smaller osseous fragment (13 guttural pouch was compressed by this mass (Fig 6). The ¥ 27 ¥ 3 mm) lying on the floor of the brain stem was use of a soft tissue window allowed detection of a mildly detected (Fig 5). These displaced fragments were not hyperdense area within the left lateral aspect of the

mid-brain in correspondence with the most caudal portion pharmacological coma was induced to allow further of the fracture. This area was compatible with diagnostic testing without causing pain to the horse or haemorrhage, but was not confirmed on histopathological endangering the medical staff. Phenobarbital sodium was examination. Unfortunately, contrast-enhanced CT was chosen in this case because it has a slightly longer onset not performed. A diagnosis of comminuted fracture of of action and prevents the recurrence of seizures. A the basioccipital-basisphenoid bone and haemorrhage radiographic examination can be useful to define skeletal between guttural pouches was made on the basis of the trauma involving basilar skull fracture, but the use of CT findings. radiography is limited by the superimpositon of the The owner elected euthanasia on account of the complex anatomy of this region and the clinician’s inability severity of the lesion and the poor prognosis. to identify small differences in tissue density. In some horses, a few radiographic findings can be identified, such as Post mortem examination increased soft tissue opacity in the guttural pouch area, irregularity or widening of the suture lines, ventral deviation Post mortem evaluation confirmed a comminuted fracture of the dorsal pharyngeal wall, dorsoventral attenuation of of the basioccipital-basisphenoid bones and detected the the nasopharynx and the presence of bone fragments presence of clotted blood in the right guttural pouch. The ventral to the basisphenoid bone (Ramirez et al. 1998; overlying brain tissue was very oedematous but no sign of Butler et al. 2008). Some authors report that displacement is haemorrhage or damage of its architecture was noted. an important diagnostic criterion and a considerable The meninges (dura mater, arachnoid mater and pia dislocation has to occur before a fracture diagnosis can mater) were only mildly oedematous and no lesion was be detected radiographically (Ramirez et al. 1998). detected. The tendon of the longus capitis muscle had In the present case, a partial obliteration of the normal areas of contusion and small areas of laceration at the air-filled guttural pouches with a soft tissue opacity and point of insertion on the basisphenoid bone. small suture irregularity were identified by survey radiography. In some cases, at palpation it is possible to Discussion find a haematoma as a swelling in the retropharyngeal region within Viborg’s triangle (Stick et al. 1980; McSloy Traumatic injuries of the CNS are not very common in et al. 2007), which can be painful during manual horses and brain tissue damage is seen in only a minority of ventroflexion of the neck (McSloy et al. 2007) and can be cases, e.g. 22% (Feige et al. 2000) and 26% (Mayhew 2009). detected also by ultrasonography (Sweeney et al. 1993); Neurological signs vary from a mild deficit to a complete unfortunately, no sonographic examination was done in inability to move, to recumbency (Sweeney et al. 1993; this case. In the lateral view, some radiolucent linear areas Tietje et al. 1996; Feige et al. 2000) and these depend on in the basisphenoid bone were suspected but no the severity and location of the lesion. Almost all clinical displacement fragment was identified. However, oblique cases of basilar skull injury have a history of a traumatic views were not done. Instead, a CT scan was preferred due event; most commonly, horses fall over backward and to the severity of the neurological signs. This imaging land on the poll of the head (Stick et al. 1980; Sweeney technique has a better resolution of bone structures and et al. 1993; Tietje et al. 1996; Ramirez et al. 1998; Johnson differentiation of soft tissues compared with radiography. and Kellam 2001; Alexander et al. 2002; Feary 2007; McSloy This decision allowed us to detect an unusual comminuted et al. 2007). In horses, the biomechanics of basilar skull basilar skull fracture with 2 large fragments of displaced trauma often result in the most severe brain injury (Reed bone, but without displacement of these on radiographs. 2007). Some cases with brain stem damage are able to The fragment slightly displaced laterally (Fig 7) was not walk but generally show abnormal mentation and strange noticed on radiographic evaluation because of behaviour, eventually associated with cranial nerve superimposition of basisphenoid bone in both lateral and deficits (i.e. anisocoria, head tilt) (Sweeney et al. 1993; ventrodorsal views, as a consequence of the triangular Feige et al. 2000; Reed 2007; Lacombe and Furr 2008; shape of the basioccipital-basisphenoid bone. Instead, Mayhew 2009). In the case described here, the horse the dorsally displaced fragment was superimposed on showed typical signs of a basilar skull fracture, including basisphenoid bone only on ventrodorsal view, but the neurological deficit and epistaxis (Stick et al. 1980; minimal displacement did not allow its identification on the Sweeney et al. 1993; Ramirez et al. 1998; Alexander et al. lateral view (Fig 8). Radiographic examination of the 2002; McSloy et al. 2007; Butler 2008). The neurological basioccipital and basisphenoid bone is difficult to evaluate dysfunctions observed were blackout, coma, seizures, and easy to misinterpret (McSloy et al. 2007), but oblique anisocoria, loss of normal pupillary light responses and views could potentially have helped to gather further reduced corneal reflex, which represent the most severe information in this case. In contrast, the transverse images signs in cases of head trauma, and a severe brain injury obtained with CT allow the creation of highly detailed can be hypothesised when these clinical signs are cross-sectional images without superimposition of assessed. The colt was sedated because of a history of anatomic structures. The use of CT provides information seizure-like episodes. After the first clinical examination, about type, localisation, extent and severity of bone

basioccipital bones at the level of the suture line (Stick et al. 1980; Darien et al. 1991). Longitudinal fracture of the basioccipital bone has been described in one case (Stick et al. 1980) and a comminuted fracture has been reported in only one case (Solano and Brawer 2004). In young horses, transverse fractures are generally known as the separation of the junction (Butler 2008). In most cases, the fractured area is stable and minimal displacement occurs, but the cerebral parenchyma is more usually severely injuried, being exposed to rapid acceleration and deceleration forces (Nout 2008). Most of these cases show ataxia as the principal neurological finding, and in a few cases there were severe neurological signs, such as coma, blindness with or without reduced pupillary light reflex, seizures, recumbency and unconsciousness (Stick et al. 1980; Feige et al. 2000). The longitudinal fracture described by Stick et al. (1980) was identified at post mortem examination but not on the radiographic views; the horse showed severe neurological signs. In most cases, avulsion fracture is identified quite easily on radiographic views but in cases of severe neurological signs and normal or unclear Fig 7: CT sagittal image reconstruction showing the fragment radiographic findings, a CT scan should be taken into displaced laterally (arrows). consideration to give detailed cross-sectional images providing accurate information about the fracture and soft tissue involvement whenever possible. CT is very useful to differentiate basilar skull fractures and rupture of the longus capitis muscle; in both cases, haemorrhage occurs from rupture of the flexor muscles and their associated vascular supply, or from vasculature and venous sinuses associated with the roof or the disrupted medial wall of the guttural pouches (Sweeney et al. 1993). In fact, survival and a return to athletic function depend on the severity and location of the injury. For horses that survived the acute phase, complications were a more or less prolonged epistaxis, a variable grade of ataxia that can be transient or permanent (Sweeney et al. 1993), inhalation pneumonia as a consequence of dysphagia (Knight 1977; Sweeney et al. 1993) and recurrent colic (Alexander et al. 2002). In this case, because of the severity of the neurological signs and the CT findings, the prognosis was considered poor and the owner elected to have the horse subjected to euthanasia. The disadvantages of a CT scan are related to the cost of the equipment, facilities and general anaesthesia. The

Fig 8: CT sagittal image reconstruction showing the fragment advantages, which include the provision of detailed displaced dorsally (arrow). images without superimposition of anatomical structures and the ability to provide a high degree of soft tissue lesions and the involvement of brain tissue or soft tissue. It is contrast resolution, make this the optimal technique in important, especially in cases of basilar skull fracture, to cases of traumatic brain injury and skull fracture. obtain specific and thorough information for correct diagnosis, therapy and prognosis. Acknowledgements Basilar skull fractures are usually avulsion fractures of the rectus capitis ventralis major and minor muscles from The authors acknowledge Dr Maria Teresa Mandara DVM, the basioccipital and basisphenoid bones (Knight 1977; PhD, for undertaking the gross and histopathology of the Stick et al. 1980; Sweeney et al. 1993; Tietje et al. 1996; skull and the brain. The authors would like to thank all intern Alexander et al. 2002; McSloy et al. 2007); in severe cases students for their collaboration in the management of the there is a transverse fracture of the basisphenoid and horse. Continued on page 368