Epihyoid bone fracture associated with tongue deviation in an adult dog

Abstract

An 8-month-old male Rhodesian ridgeback dog was evaluated for right lingual deviation, mild dysphagia, and inability to retract the tongue. Transverse and three-dimensional computed tomography reconstruction images revealed a transverse fracture of the left epihyoid bone. After 4 months of conservative management, that included assisted feeding of a semi-liquid diet or small volumes of food and analgesics, the dog recovered.

Introduction

The hyoid bone (os hyoideum) or apparatus is a group of osteochondral structures at the base of the skull that provides a flexible suspensory function to the tongue and larynx (1). The hyoid bone comprises the basihyoid (corpus ossis hyoidei), thyrohyoid (cornu majus), ceratohyoid (cornu minus), epihyoid, and stylohyoid bones and the tympanohyoid cartilages (1) (Figure 1). Injuries or abnormalities of the hyoid apparatus can be caused by traumatic fractures or dislocations secondary to car accidents, dog bites in the ventral cervical area, strangulation by training collars or abuse, birth injury, congenital structural abnormalities such as hypoplasia or aplasia, and infiltrative neoplastic diseases (2–6).

Figure 1.

Plastinated anatomical preparation of the hyoid bone from an adult dog. a — tympanohyoid, b — stylohyoid, c — epihyoid, d — ceratohyoid, e — basihyoid, f — thyrohyoid

Lateral tongue deviation is an uncommon sign normally associated with dysfunction of the ipsilateral hypoglossal nerve, nerve roots (infranuclear segment of cranial nerve XII), or medulla oblongata (nuclear segment of cranial nerve XII) (7). Additional signs of neurogenic tongue impairment include lack of retraction in response to manual stretching, flaccid paralysis, muscular fasciculations, muscular atrophy, and asymmetry of the tongue (7,8). Functional problems associated with tongue deviation and paralysis include complications with deglutition, prehension, swallowing, mastication, and vocalization (8). Tongue deviation results from contraction of the ipsilateral genioglossus and intrinsic lingual muscles that protrude the tongue toward the affected side (8). However, other possible causes of unilateral tongue deviation include trauma, lingual ulcers, lingual abscesses, neoplasia, generalized neuropathies, inflammatory myopathy, muscular dystrophy, neuromuscular disorders (botulism), and hyoid bone lesions (8–12).

The purpose of this report is to describe the clinical and imaging characteristics of an adult dog with a unilateral epihyoid fracture associated with unilateral lingual deviation.

Case description

An 8-month-old male Rhodesian ridgeback dog was referred to our veterinary hospital for right lingual deviation and inability to retract the tongue. The animal was able to eat and to drink water, but with difficulty. Neurological examination revealed slight atrophy of the right side of the tongue, with decreased muscle tone and right lingual deviation (Figure 2). No alterations were observed in the dog’s mental status, posture, gait, or spinal reflexes. Possible neuroanatomical diagnoses included an alteration of the right peripheral hypoglossal nerve, hypoglossal nerve roots, hypoglossal canal, or hypoglossal nucleus in the brain stem; a unilateral lingual lesion; or an alteration in the hyoid apparatus.

Figure 2.

Right lingual deviation and mild atrophy in a Rhodesian ridgeback dog observed during the physical examination.

A complete blood (cell) count (CBC) and biochemistry analysis were unremarkable. Conventional radiographic examination revealed no abnormalities. A computed tomography (CT) scan of the head and cranial cervical region was performed using a fourth-generation CT scanner (Picker PQ 6000; Picker International, Cleveland, Ohio, USA). For this scan, the patient was premedicated with 2% xylazine (Xilazina 2%; Centrovet, Santiago, Chile), 0.5 mg/kg body weight (BW), IM, and general anesthesia was induced with propofol (Diprofol; Biosano, Santiago, Chile), 4 mg/kg BW, IV and maintained with an isoflurane and oxygen mixture. The animal was placed in sternal recumbency with the thoracic limb extended caudally. The technical parameters used in the image acquisition were a 2-mm interval, 2-mm thickness, 120 kV, 85 mA, and 178 mAs. Computed tomography images were obtained from the cribriform plate to the external occipital protuberance. Transverse CT images showed no abnormalities in the left hypoglossal foramina or evidence of masses in the base of the cranium. Three-dimensional (3D) reconstructions of the hyoid apparatus were obtained using OsiriX software (version 3.9.2; OsiriX Foundation, Geneva, Switzerland). Transverse and three-dimensional reconstruction images revealed a transverse fracture in the central portion of the left epihyoid bone and mild craniodorsal displacement of the proximal portion of the epihyoid bone (Figures 3A,B). After the examination, the dog underwent supportive care that included assisted feeding of a semi-liquid diet or small volumes of food and analgesics. Follow-up 1 mo later revealed no significant recovery; however, 4 mo later the owner described significant recovery of function with continuing improvement (Figure 3C).

Figure 3.

A — Transverse computed tomography (CT) scan (soft tissue window) of the cranium at the level of the nasopharynx in an adult Rhodesian ridgeback dog. The black arrow indicates a transverse fracture of the left epihyoid bone. B — Craniolateral view of a 3D bone reconstruction image showing the linear fracture and mild cranial displacement of the left epihyoid bone. C — Photograph of the Rhodesian ridgeback dog 4 months after the initial evaluation showing no signs of lingual deviation.

Discussion

Few cases of hyoid fracture in small animals have been reported (2,13,14). Traumatic luxation between the epihyoid and ceratohyoid associated with a car accident and hyoid fracture and displacement associated with a gunshot lesion have been described in dogs (14). Traumatic fracture of the epihyoid bone associated with incorrect training procedures was also reported in 2 dogs (13). The cause of the hyoid injury in this report was unknown, but unintentional harm associated with a collar was hypothesized. Fractures of this group of bones are rare because these bones are protected by the mandible and the cervical muscles. Sublingual hematoma, tongue swelling, and injury to the laryngeal area were reported in 2 cases (13). Hyoid fractures are also rare in humans, accounting for only 0.002% of all fractures (15). In humans, the junction of the body and greater horns is the region of the hyoid most prone to injury (16). For diagnosis of hyoid lesions, digital palpation of the cranial cervical area may indicate no abnormalities or lesions with the exception of mild soft tissue swelling (13). Additional clinical signs associated with hyoid fracture include pain, dysphagia, and partial loss of tongue mobility caused by the loss of stability at the base of the tongue. The hyoid apparatus in dogs may develop a series of injuries that produce similar signs such as stylohyoid bone dislocation (3) or, as described by Kang et al (5), a congenital structural abnormality involving right stylohyoid bone aplasia and left stylohyoid bone deviation that compromises articulation with the left epihyoid. In horses, fractured hyoid bones can also produce tongue protrusion and dysphagia, but these problems resolve upon repair of the hyoid bone (17). Laterolateral radiography of the cranial cervical region can be used; however, overlying structures can lead to an inaccurate diagnosis (18). Ultrasound allows for inspection of some components of the hyoid apparatus in motion, such as the basihyoid (19). Computed tomography is the preferred imaging method for a definitive diagnosis because it provides a detailed view of the hyoid components, allows for 3D reconstruction, and avoids superimposition (5,6); and is helpful in detecting hyoid bone fracture associated with road traffic accidents, gunshot wounds, or shearing injuries secondary to shaking by the neck (14,20). Computed tomography imaging findings of hyoid bone fracture/luxation usually include discontinuity of the hyoid apparatus, resulting in asymmetry and soft tissue swelling (14).

There is one report documenting surgical treatment for hyoid fractures, and reconstruction of adjacent tissues such as muscles has only been described in cases of dog bites (2,21). In cases of dislocation, surgical fixation between the stylohyoid bone and tendinous origin of the digastric muscle can be performed using non-absorbable suture (3). Conservative therapy is the preferred treatment approach for hyoid bone fractures and is usually based on rest, pain management with analgesics, and assistance with feeding, which is more liquid than solid to facilitate swallowing (22).

In the present case, the dog had right tongue deviation and mild atrophy contralateral to the hyoid lesion. The lingual paralysis and deviation could be explained by compression or stretching of the ipsilateral hypoglossal nerve, indicating neuropraxia or axonotmesis (8). Neuropraxia is a functional disturbance of the nerve without anatomic discontinuation and the recovery of nerve function usually takes 1 to 6 wk (8). Axonotmesis is disruption of the axons with intact endo-, epi and peri-neural sheaths and partial or total recovery can be achieved after several months (8). Additionally, lack of continuity and alignment of the left epihyoid bone may contribute to a lack of support of the left extrinsic musculature of the tongue, so that when the right styloglossus muscle contracts, the tongue will protrude and deviate to the right side (8). The main differential diagnosis of tongue deviation is injury to the hypoglossal nerve, by inflammatory, traumatic, or neoplastic causes (7,23). The hypoglossal nerve provides pure motor innervation to both the intrinsic and extrinsic (styloglossus, hyoglossus, geniohyoid, and genioglossus) muscles of the tongue (1). The nerve pathway is divided into 6 segments and can be damaged anywhere along its course. These segments include the medullary (nuclear), cisternal (extramedullary intracranial), skull base (the segment that passes through the hypoglossal nerve canal), nasopharyngeal, oropharyngeal and carotid (in close vicinity to the glossopharyngeal and vagus nerves as well as to the internal carotid artery), and sublingual space (where its branches terminate innervating lingual muscles) (24,25). In cases of chronic unilateral hypoglossal nerve injury, the tongue deviates to the ipsilateral side because of asymmetric lingual atrophy, and contraction of the intrinsic tongue muscles and contralateral genioglossus muscle results in lingual deviation to the same side affected (26,27). The weight of the paralyzed half of the tongue also contributes to this deviation (8). An acute unilateral lesion of the hypoglossal nerve produces contralateral deviation of the lesion, while bilateral lesions are characterized by an inability to retain and voluntarily retract the tongue into the oral cavity (7). Other signs observed in the paralyzed tongue include muscle twitching of the hemi-atrophied side of the tongue during panting, diminished lingual retraction force, an inability to catch food and drink water, alteration of the oral phase of swallowing, difficulty chewing, and dysphagia (7). In cases of hypoglossal nerve injury, CT imaging can identify the integrity of the cranial foramina, but magnetic resonance imaging provides better detail of nerve structures because of the higher contrast resolution (14,25).

Fractures of the hyoid bone in dogs are uncommon and should be included as a differential diagnosis in dogs with dysphagia or tongue movement impairment. Computed tomography 3D reconstruction was useful in confirming the diagnosis of hyoid bone fracture in this case, and conservative treatment was successful in revolving the symptoms. CVJ